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SHAFT ALIGNMENT

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Alignment

Accurate alignment is vital to the operating life of rotating equipment. Bearings, mechanical seals, packing, and couplings are all directly affected by the alignment of shaft center lines. The goal of the alignment process is to create a straight line through the coupling, as shown in Figure 1. The two coupled shafts are considered to be perfectly aligned when their center lines are coaxial at the operating condition.


Figure 1: Coaxial Alignment

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It has been found that 50 to 70% of all vibration problems in machines are caused by misalignment. A thorough understanding of what conditions create misalignment will help us understand what needs to be done to correct it.

Types of Misalignment

There are two basic types of misalignment: parallel (or offset) and angular. Both types can be found in the vertical and horizontal planes. Typically, a combination of offset and angular misalignment is found in both directions, as shown in Figure 2. To achieve our goal, we must correct both types of misalignment in each direction.


Figure 2: Combination of Offset and Angular Misalignment

Two additional conditions must be addressed. The axial location of the machines and the baseplate are important to the operation. The proper shaft-to-shaft distance must be maintained, particularly when a limited end float coupling is being used, as shown in Figure 3.


Figure 3: Maintain Proper Shaft-to-Shaft Coupling

Torsional effect, or machine torque, may also need to be considered when aligning the equipment, as shown in Figure 4. Machines may move horizontally during start-up and operation. These are not entirely alignment subjects, but we should be aware of their importance when performing alignments.


Figure 4: Torsional Effect

Causes of Misalignment

The basic causes of misalignment are:

  • Movement of one piece of equipment relative to another due to thermal growth in one or both machines.
  • Piping strain or strain induced by electrical connections.
  • Torsional movement taking placeat start-up or while operating.
  • Movement or settling of the foundation or baseplate.
  • Inaccurate or incomplete alignment procedures (human error).
  • Misboredcouplings.

Any one of the above conditions will dramatically affect the alignment of equipment. If more than one of the conditions exists, the odds are highly against a machine running smoothly, quietly, or for any appreciable amount of time. Only after all of the situations have been examined and corrected can a craftsperson be assured of an accurate alignment job being achieved and maintained.

Effects of Misalignment

The effects of misalignment are all around us in a facility. High noise levels or constantly vibrating floors are strong indications of possible misalignment of machinery. Some of the other effects can be:

  • Lost production
  • Poor-quality products
  • Higher than normal repair orders
  • Increased spare parts purchases and inventory on hand
  • Reduced profits

In addition to the financial impact on the company, the direct effect on the various machine components can be considerable. Bearings will run hot, causing them to fail prematurely. Mechanical seals, seal rings, and packing will leak. Loss of product and lubrication can occur. Couplings will fail due to excessive strain on the hubs. In severe cases, shafts can break, causing extensive damage to machines.

Indications of Misalignment

Misalignment in rotating machinery can be detected in many different ways. Some methods are incorporated into the plants preventative maintenance program. Others are inspections that could be used on a regular basis but usually are performed after the equipment has failed. Some of the indications of misalignment are:

  • Wobbling shafts
  • Excessive vibration
  • Excessive bearing temperature
  • Noise
  • Bearingwear pattern
  • Coupling wear

Wobbling of the shafts can be observed without any instruments or tools. It indicates that the shafts are improperly lined up and need adjustment. Misalignment is one of the leading causes of equipment vibration. In spite of "self-aligning" bearings and flexible couplings, it is difficult to align two shafts and their bearings so that no forces exist that will cause vibration. The significant characteristic of vibration due to misalignment is that it will be in both the radial and axial directions. If a bearing is found to have an abnormally high temperature and proper lubrication is present, then misalignment is probably the cause. Turbines, pumps, and other plant equipment have bearing temperature monitors or indicators. It is best to refer to the manufacturers literature for the correct bearing temperature parameters. Some pumps and smaller equipment do not have any monitoring devices. For those, placing a hand on the bearing is a good indicator of excessive bearing temperatures. If the bearings on a particular pump are hotter than the bearings on similar pumps, misalignment could be the cause. Due to the possibility of the presence of high temperatures, care must be taken when touching bearings. Like vibration, noise can be detected simply by noticing a change in the equipment sounds during operation. All running equipment produces a certain normal amount of noise. Only if an operator is familiar with normal equipment noise will they be able to detect abnormal sounds. Possible misalignment may be detected through bearing and coupling inspections. If bearings show signs of excessive wear, misalignment usually is the cause. The bearings will have to be replaced and the alignment corrected to prevent further bearing damage. The coupling parts should also be inspected. Any sign of excessive wear, especially if the wear is uneven, is a good indication of misalignment.

Alignment Tools

There is a multitude of methods available to perform accurate alignment, any of which can deliver the desired result. However, several precision tools are commonly used in alignment work. dial indicators, parallel blocks, taper gauges, feeler gauges, a tape measure, a 6-inch rule, and a small mirror are all useful. Each one has a part to play in doing alignments.

Dial Indicators

Dial indicators are probably the most widely used precision tools. They are available in various styles, sizes, and range. A back plunger type, shown in Figure 5, is often used to take rim and face readings on couplings, to measure soft foot, and to monitor accurate machine moves. Their small size makes them easy and convenient to use.


Figure 5: Bottom and Back Plungers

Bottom plunger style indicators are used for taking runout readings on couplings and shafts, and for measuring misalignment. They come in two styles: balanced or continuous reading. Examples of both are shown in Figure 6. The dials are 1 inches in diameter or 2 1/8 inches in diameter. Their usable range is from 0.250 inch to as much as 12 inches. Typically, a 0.0250-inch or 0.500-inch travel indicator is used in alignment work.


Figure 6: Balanced and Continuous Reading Plunger

Parallels

Adjustable, or sliding, parallels, shown in Figure 7, are used to measure gaps or holes. They usually are available in sets. Sliding parallels vary in length from 1 to 5 1/16 inches, are 9/32 inch thick, and can measure ranges such from 3/8 inch to 1 to 2 inches. To check the size of a gap, the sliding parallel is inserted and expanded to the proper size. The parallel then is measured with an outside micrometer to determine the gap size. Sliding parallels can be used to take coupling hub face readings.


Figure 7: Sliding Parallels

Thickness Gauges

The standard thickness gauge, also called a feeler gauge is a compact assembly of high-quality, heat-treated steel leaves of various thicknesses, as shown in Figure 8. The leaves usually vary in thickness by .001 inch, and the exact thickness of each leaf is marked on its surface.


Figure 8: Thickness Gauge

A thickness gauge is the measuring instrument commonly used to determine the precise dimension of small openings or gaps, such as those that must be measured in the course of aligning a coupling. To determine the dimension of an opening or gap, the steel leaves are inserted singly or in combination until a leaf or combination is found that fits snugly. The dimension is then ascertained by the figure marked on the leaf surface or, if several leaves are used, by totaling the surface figures. Another type of thickness gauge, not as widely known or used but ideally suited for coupling alignment, is the taper gauge, shown in Figure 9. It is sometimes referred to as a gap gauge. Its principal advantage for coupling alignment is that it gives a direct reading and does not require trial-and-error "feeling" to determine a measurement. The tool end is inserted into an opening, or gap, and the opening size is read on the graduated face. Two measurement systems inch and metricare shown.


Figure 9: Taper Gauge

Micrometers

Another precision measuring instrument used for coupling alignment is the outside micrometer caliper shown in Figure 10. As its name implies, it is used to measure outside dimensions.


Figure 10: Micrometer Caliper

Outside micrometers are available as single units or as complete sets. A complete set of micrometers gives you the advantage of being able to quickly choose the micrometer appropriate for a specific situation. Two types of micrometer sets are generally available. One contains many micrometers of different sizes with a variety of frames. Such sets may contain anywhere from 3 to 24 micrometers and have ranges varying from 0 to 3 inches up to 0 to 24 inches. Different combinations of a ratchet stop or friction thimble and lock nut may be provided on micrometers in these sets. Micrometer sets of this type may be graduated in thousandths of an inch, ten-thousandths of an inch, or hundredths of a millimeter. The second type of outside micrometer set, illustrated in Figure 11, contains an outside micrometer with interchangeable anvils and a set of standards. Micrometers of this type range from 0 to 4 inches up to 20 to 24 inches and are also available with Metric calibrations. An outside micrometer with interchangeable anvils is frequently used in the field to measure objects of varying sizes. The micrometer has an adjustable stop on the anvil to alter the overall anvil dimensions. Both types of micrometer sets are capable of measuring within the same size range and producing results with equal accuracy.


Figure 11: Outside Micrometer with Anvils

Miscellaneous Tools

Another useful and necessary tool needed when doing alignment work is a tape measure. This is a common item to most craftspeople, so a detailed description is not needed here. Your 6-inch pocket scale is very likely the alignment tool you are most familiar with. Many of your alignment jobs will be performed with this device. This type of alignment has its place, but it is not a precision method. A small mirror is another very helpful item to have in your toolbox. It is very handy when trying to read an indicator that may be positioned in a location that is inaccessible. Shims are very likely the most important tool used when performing alignments. Proper placement and accurate thickness are the most crucial elements of using shims. Using precut stainless-steel shims is rapidly becoming the preferred way of making vertical elevation changes. They are very easy to use, quick to install, and usually accurate in their exact thickness.

Alignment Methods

It is obvious there are many methods available today to align machinery. Almost all of these methods originated over 30 years ago. The concept of alignment is not new; it is just not understood by most people who actually perform the alignment. The purpose of this section is to familiarize the technician with the most common methods used today. All of these methods will be described in detail later in this manual. As with any method, there are potential sources of error as well as advantages. This section points out some of the more salient aspects for each method. The methods we will cover are:

  • Visual Line-Up
  • Straightedge/Feeler Gauge
  • Rim and Face
  • Cross Dial
  • Reverse Dial
  • Laser

Visual Line-Up

The visual line-up method, shown in Figure 12, is the most common method of alignment. Used in initial installations, visual line-up allows technicians to analyze the working conditions and feasibility of installation.


Figure 12: Visual Line-Up Method

Straightedge/Feeler Gauge

Straightedges are used to determine the offset between coupling halves; this is shown in Figure 13. Corrections are made under all four of the machines feet. Feeler gauges or taper gauges measure the gap between coupling halves at the bottom and top of the coupling.


Figure 13: Straightedge/Feeler Gauge

Rim and Face

This method is similar in principal to using a straightedge and feeler gauge, but more accurate since dial indicators are used. The rim reading measures the offset between the coupling halves. The face reading measures the angular difference between the faces of the coupling, as shown in Figure 14. Changes are calculated with the same formula as the straightedge/feeler gauge method.


Figure 14: Reading Angular Difference Between Faces

Advantages:

  • Used when onlyone shaft can be rotated.
  • Given the correct precautions, precision alignment is attainable with this method.

Disadvantages:

  • Endfloat affects face reading.
  • Indicator bracket (bar) sag affects readings.
  • Eccentric, skewedcouplings or damaged surfaces will cause errors.
  • Fixture looseness causes errors.
  • Indicator stems not perpendicular to shaft causes errors.

The indicators should be checked to ensure that:

  • The plungers are level, parallel to shafts, and depressed about half their total travel.
  • The indicators are same distance from the shaft axis and exactly opposite each other when two indicators are used.
  • The contact points are midway between coupling halves in the axial direction.

If sag-free brackets are not available, sag greater than .001 inch must be compensated for.

Cross Dial

This method uses two dial indicators mounted exactly 180 apart to take shaft-to-shaft readings. Both parallel and angular misalignment may be compensated for at the same time. This method allows the couplings to remain attached, as the shafts must move together. Figure 15 show a typical cross dial setup.


Figure 15: Cross Dial Setup

Advantages:

  • Very accurate method of using dial indicators
  • Easy and fast to use
  • Simple:

- Graphical calculations for misalignment are non-technical

- Computer or pocket calculators can also be used Sources of error are:

  • Indicator stems must be perpendicular to the shaft
  • Looseness
  • Indicator bracket (bar)sag
  • Coupling backlash
  • Extreme axial float
  • Indicators that are not exactly opposite each other

Reverse Dial

This method uses two dial indicators that take shaft-to-shaft readings and is almost the same as the cross dial method, except that the indicators are in the same plane with each other. Both the offset and angularity are combined in the alignment calculation. This method, shown in Figure 16, determines the misalignment by taking two rim readings at different points along the shaft.


Figure 16: Reverse Dial Method

Advantages:

  • Most accuratemethod of using dial indicators
  • Easy and fast to use
  • Simple:

-Graphical calculations for misalignment are non-technical

- Computer or pocket calculators can also be used

- Requires only 180 rotation Sources of error are:

  • Indicator stems not perpendicular to the shaft
  • Looseness
  • Indicator bracket (bar) sag
  • Coupling backlash
  • Extreme axial float

Laser

The laser method of alignment is similar to the rim and face method, but it uses light to span the shaft-to-shaft distance. As both shafts are rotated, the misalignment is determined by the movement of the laser beam on the detector surface. This is shown in Figure 17.


Figure 17: Laser Method

Advantages:

  • Most accurate measuring device available
  • Speed: with practice, alignment calculations can be made quickly
  • Wired to a computer
  • Only requires 180 shaft rotation
  • Horizontal move capabilities

Sources of error are:

  • Heat/cold - air can distort the laser and affect alignment calculations
  • Looseness in brackets or fixtures
  • Coupling backlash

Proper shaft alignment is required to maintain operational longevity of all rotating machinery. The two basic types of misalignment are parallelism and angularity. Both types can be found in the vertical and horizontal planes. To achieve our goal of proper shaft alignment, we must correct for both types of misalignment. It has been noted in this section that 50 to 70% of all vibration problems in rotating equipment are caused by misalignment. The overall effect of this misalignment is lost production, poor quality of products, higher repair costs, and increased spare parts, all of which lead to reduced profits. We also discussed the tools used in alignment. It is very important that we know how to use these tools. In the sections that follow, we will learn how to use the information as well as the tools to properly align a piece of equipment. Also, each of the alignment methods will also be discussed in detail later in this manual.

Alignment Preparation

Before the alignment process can begin, several things must be examined and corrected. If a precision alignment is attempted while any of these conditions has yet to be rectified, accurate results will be unobtainable. If any one of these items is overlooked or ignored, the effect on the machine could be an unscheduled or emergency shutdown. The machine could even be heavily damaged. A pre-alignment checklist is also designed to save time in performing alignments, as 90% of the problems involved with alignment can be avoided by following the pre-alignment checklist. These checks should be completed every time an alignment is performed. You do not need to follow the order of the list, but each point needs to be checked.

  • Before shutting the machine off, take temperature readings in the planes of the feet for both machines to determine if either machine is subject to thermal growth.
  • Check the service history for any information that may be useful.
  • Lockout/tagout the machine to be worked on. Ensure the safety of all individuals. For pumps, close the suction/discharge valve to protect against pump backspin.
  • Clean up around the machine.
  • Loosen the mechanical seals or packing.
  • Before the rotating shafts, make sure that the bearings are properly lubricated with the correct type and amount of grease or oil. If an auxiliary oil system is used, make sure it has been serviced.
  • Rotate the shafts slowly. Listen/feel for binding/roughness. Always rotate in the direction of equipment rotation to prevent backlash in couplings and gearboxes.
  • Check the machine for worn or defective bearings.
  • Check the coupling for the following:

- Looseness (grids, teeth, disks or elastomers, etc.)

- Fit on shaft (taper or straight bore)

- Eccentricity (runout)

- Worn grid/teeth members

- Correct lubricant type and amount

- Setscrew tightness

- Proper key length

- Match marks in the correct place

- Proper bolts and washers: note length, machining, weight

  • Check shafts on both machines for:

- Concentricity (runout)

- Movement in the axial, horizontal, and vertical directions greater than the manufacturers allowable limits

- Smooth fixture to mounting surface (pipewrench footprints)

  • Inspect machine base and foundation for cracks, warped surfaces, and corrosion.

Clean base (near feet) of rust and other foreign matter.

  • If carbon-steel shims are used, remove and replace with pre-cut stainless-steel shims. Remove and replace any shims that may be cracked, bent, folded, rusted, handcut, brass, or otherwise defective.
  • Whenever possible, start with 1/8 inch (0.125 inch) of shim under each foot to allow for vertical adjustment.
  • Ensure the axial position of the machine is correct and that the coupling will allow both machines to run in their respective axial positions.
  • Find and mark the magnetic center on motors that have axial end float (sleeve bearings).
  • Check for pipe and/or electrical connection strain if possible (rough-in stage). :
  • Ensure both the vertical and horizontal jack bolts are loose. Lubricate for smooth operation.
  • Remove dowel pins from both machines.
  • Check and remove any soft foot for both machines.
  • Ensure all bolts on both machines are torqued. Note the bolt lubrication and remove any "bell/cupped" washers.
  • Determine alignment method to be used.
  • Assemble fixtures and check for accuracy and working condition.
  • Take machine dimensions.

This list is not meant to be a complete systematic procedure. There may be more specific checks required at different facilities, while others may not apply. The objective is a thorough examination of the condition of the machine before attempting a precision alignment. Without these checks, precision is not obtainable.

Soft Foot

Soft foot is a term that many people have heard, yet is widely misunderstood. Historically, soft foot has been considered insignificant and only a minor cause of machine vibration. Not until recently have specialists begun to understand that a considerable amount of machinery vibration is caused by presence of a soft foot condition. This subject can be considered separately from alignment, yet the effects of soft foot are so prevalent in the alignment process that it must be eliminated before making any alignment corrections. The pre-alignment checks and procedures include eliminating soft foot; however, it is advised that soft foot be checked in each stage of the alignment process. The reason soft foot is difficult for many technicians to eliminate is that the causes of soft foot are not clearly understood. Common measurement practices do not give a complete picture of the actual soft foot condition. What exactly is soft foot? The term has several other names, such as soft leg or rubber leg, but there is no adequate definition that accurately describes the situation. Perhaps a description in non-technical terms will be helpful in clarifying the problem. Have you ever sat down at a table in a restaurant and found that the table rocks when you lean on it One leg is shorter than the other three, and the table does not rest evenly on the floor. The table has a soft foot. When this occurs, the logical solution is to find some type of shim pack, which in most restaurants is usually a book of matches, and place it under the foot not touching the floor to correct the rocking motion. Similar situations occur with rotating equipment where the feet of a machine do not rest evenly on the base. It is identical to the table; however, the movement is measured with dial indicators or feeler gauges. Tightening the hold-down bolt can create enough force to close the gap between the foot and the base and appears to correct the problem, but this is where problems actually begin. As shown in Figure 18, the term "soft foot" does not necessarily describe the actual condition of the foot because the foot is not actually soft. Soft foot is defined as a condition where the machines feet do not lie in the same plane as the base.


Figure 18: Soft Foot

Effects of Soft Foot

For the purpose of our discussion, we will use the term "soft foot," understanding that the condition can be called by several other names.

Machine Frame Distortion

Machine frame distortion is the result of changes that occur internally or externally to the frame of the machine. This often causes shaft deflection or movement as the hold-down bolt is tightened when a soft foot condition exists. Machine frame distortion is most easily explained with a simple soft foot example. Consider a machine with a soft foot condition. With the hold-down bolts loose, one foot does not rest on the base. There is no internal stress on the machine in this state. This is shown in Figure 19 by the straight horizontal lines across the machine.


Figure 19: No Internal Stress

Tightening the hold-down bolt will draw the foot down to the base, closing the gap. However, this creates stress in the machines frame, as the curvature of the lines on the machines frame now show in Figure 20.


Figure 20: Stress Induced by Tightening the Hold-Down Bolt

The distortion of the machines frame can have multiple effects on the shaft position. The shaft position can change when the hold-down bolts are loosened and tightened if a soft foot condition exists. Using proper torque values and patterns for hold-down bolts, as shown in Figure 21, is helpful, but it cannot completely eliminate the variation in shaft movement.


Figure 21: Proper Torque Pattern for Hold-Down Bolts

Distorted Bearing Housing

In other situations, machine frame distortion can distort the bearing housing. This can result in excessive wear on the top and bottom of the outer race. Forces due to soft foot usually will act 180 apart, creating a preloading condition on either side of the bearing. Changes in the shaft position due to soft foot can create other problems internal to the machine. The distortion may change the position of one of the bearings, causing increased wear. Machine frame distortion can also create internal misalignment between the bearings. Tightening the hold-down bolt causes a distortion of the machines frame, creating an offset between the bearings. The angle the shaft makes between the inboard and outboard bearings actually causes internal misalignment between the bearings. Figure 22 shows a distorted bearing.


Figure 22: Distorted Bearing

Types of Soft Foot

The various types of soft foot are described, in detail, below.

Air Gap or Parallel Soft Foot

The most common explanation of soft foot is parallel or straight soft foot. When the hold-down bolt is loose, the foot simply does not reach the base, leaving a gap between the foot and the base. The bottom of the foot is parallel to the baseplate though. This condition is easiest to detect using either a feeler gauge or dial indicator, as shown in Figure 23.


Figure 23: Parallel Soft Foot

Downward Bent Foot

A common situation is known as a bent foot. Here the foot is touching the base on the outside portion, but the inside of the foot is bent, creating an angle between the base and the bottom of the foot. When the hold-down bolt is tightened, the foot will deflect and distort the machine frame, depending on what portions of the frame will flex, as shown in Figure 24.


Figure 24: Downward Bent Foot

Upward Bent Soft Foot

The foot may also be bent upward so that the outside edge is not touching the base and the deflection occurs along the outside of the foot. Figure 25 shows that tightening the hold-down bolt will result in a distortion of the machines frame as the inside of the foot is drawn down to the base. Either of these situations can cause frame distortion; however, an inconsistent or warped base will have the same result as a bent foot.


Figure 25: Upward Bent Soft Foot

Squishy/Spring Foot

A new deck of cards stacks shorter than an old deck, not because the cards are thicker, but because of the buildup of oil and dirt on each card and the bending and creasing of the cards. The same holds true for shims. Grease, dirt, rust, paint, metal filings, and other substances can build up the thickness of a shim. Bending and creasing can cause a stack of shims to give under pressure, as illustrated in Figure 26.


Figure 26: Squishy/Spring Foot

If these factors are multiplied by several shims, the shim pack can actually have a spring effect. The foot will move when the hold-down bolt is tightened or loosened. A soft foot condition exists even though there is no gap because the foot moves when the bolt is tightened.

Stress-Induced Soft Foot

Perhaps the most difficult soft foot condition to detect is caused by forces that are external to the machine. This is referred to as stress- or force-induced soft foot, illustrated in Figure 27. It can be the result of pipe strain or stresses induced by the electrical connections as well as drastic misalignment. Binding at the coupling can also induce external forces that create a soft foot condition.


Figure 27: Stress-Induced Soft Foot

Stress-induced forces can be created during any stage of the alignment process; therefore, eliminating this kind of soft foot may require more than one check. Soft foot can be a major problem. If alignment is attempted on a machine with a soft foot condition, inconsistent readings will make the alignment calculation difficult regardless of the alignment method used. Throughout industry, most alignments are performed without checking for soft foot. The main reason for this is incomplete analysis and the results are hours of frustration, compromised alignments, and machines that do not run as smoothly as possible. If soft foot is corrected after alignment is complete, any shim changes may change the alignment. To have smooth-running machines, soft foot must be eliminated on both the drive and driven machines before performing the alignment. Understanding the different types of soft foot is essential in examining the various methods of measuring and correcting soft foot deflection causing machine frame distortion. Some methods do not always accurately measure soft foot, and if the craftsperson does not completely understand the proper procedure, a soft foot condition may go undetected.

Measuring and Correcting Soft Foot

In ideal circumstances, the baseplate and feet are machined flat to within one mil when the machine is installed. In most situations, however, the base is not a level surface, and the amount of soft foot can change depending on the location of the machines feet on the base. Since any movement of the machine on the base may change the soft foot, a rough-in alignment must be performed before checking for soft foot. It is possible for soft foot to change during the alignment process, so there will be some going back and forth between correcting the misalignment and checking for soft foot during an alignment. Soft foot changes, which occur during the moves of the adjustment and precision alignment stages, will be checked at that time. Once the machines are in their approximate final positions, a rough-in soft foot can be performed. The procedure for an initial soft foot determination is as follows:

  1. Loosen all the hold-down bolts and check for any gaps under the feet using a feeler gauge, as shown in Figure 28.
  2. Eliminate the gap under the foot by placing the largest single shim that will close the gap under each foot without raising the machine.
  3. If a gap still remains, place additional shims under each foot to close the remaining gap.


    Figure 28: Checking for Soft Foot

    For residual soft foot, simply slide the shim into position under the foot until stops. Do not force the shim into place, as this will raise the foot and can create soft foot at other feet. The purpose of the rough-in soft foot is to eliminate any gap under the foot. It has been found that a rough-in soft foot check can eliminate as much as 90% of the soft foot present in a machine. Once the rough-in soft foot is complete, along with all of the other pre-alignment checks, the technician is ready to move on to the adjustment stage of the alignment. We want to check for soft foot a second time after all of the adjustments have been made to be certain that we have not introduced any stress or forces on the machines frame that will create a soft foot condition. With the rough-in soft foot complete, all of the hold-down bolts should be tightened to the proper torque specification once the alignment has been checked in the adjustment stage. It is common practice at this point to check for soft foot by loosening one bolt at a time and measuring the deflection using a feeler gauge or base-mounted dial indicator. The procedure for checking each foot is to loosen each hold-down bolt one at a time, leaving the others tight. Place a base-mounted dial indicator on the foot so the dial is perpendicular to the top of the foot for an accurate reading. Be sure that the stem of the dial is not in the way of the bolt or wrench. If the dial is bumped with the wrench when loosening the bolt, the readings will be inaccurate. With the dial in position, adjust it to zero and then loosen the hold-down bolt. The amount of soft foot deflection will be measured on the dial indicator. Notice this will always be a positive number, as the foot will move away from the base, pushing the stem of the dial in and giving a positive reading. If the dial shows a negative number, check for deflection of the base or movement of the indicator support. In many situations, it is difficult to position the dial indicator on the foot because there is little clearance between the foot and the machines frame. When this occurs, a feeler gauge can be used to measure the gap under the foot, as shown in Figure 29.


    Figure 29: Checking for Clearance with a Feeler Gauge

Adding the shim thickness equal to the amount read on the indicator or by the feeler gauge will not always eliminate the soft foot. In many cases, the whole foot does not deflect by the same amount; as we have seen, there are several types of soft foot deflection. Indicators can give erroneous readings if the machine is mounted on a thin baseplate that moves when the foot hold-down bolt is tightened and loosened. Therefore, it is recommended that both indicators and feeler gauges be used if they are available. We want to check every foot on both machines for a soft foot deflection before making corrections. If you simply check a foot and then add shims, you may be creating more problems than you are eliminating. This process will eventually accomplish the goal of removing the soft foot, but the entire process is much faster and easier if the worst foot can be identified first. Quite often, eliminating the largest soft foot will remove the soft foot at the other feet. Therefore, we want to check all of the feet individually before making any shim corrections. In the previous section, we examined the effect on the shaft position due to a soft foot deflection. This was shown to have a considerable impact on the movement at the coupling. Therefore, once the initial soft foot is eliminated in the rough-in stage, we want to determine if there is any shaft movement caused by the remaining soft foot.

Soft Foot Analysis

With the information gathered using indicators and feeler gauges, it is possible to analyze any situation and determine the best way to eliminate soft foot from the entire machine. All the information should be gathered before any shim changes are made. Taking a few minutes to examine the soft foot on the entire machine can save hours of tedious soft foot correction. The best method for eliminating soft foot is to have the base and bottom of the machines feet as flat and smooth as possible. This requires more time in preparation, but the time saved in eliminating soft foot problems later makes this the best approach.

Soft Foot Tolerance

We have reached the point in our discussion where we must talk about the real world. We all know that things look much better on paper than they do in the field and there are always going to be those things that we just cannot fix. Eliminating all of the soft foot is our goal, but when we have time restrictions or encounter difficult problems, we need to have some criteria that can be used as a tolerance guideline for removing soft foot. Many tolerances suggest 2 or 3 mils as an acceptable reading, but is this close enough With the measuring devices available today, every attempt should be made to achieve soft foot corrections as close as possible. If proper soft foot readings are taken on a solid base, we should be able to eliminate any readings above 1 mil. Keep in mind that this is an art, not a science; we want to remove as much soft foot as possible. If, for some reason, we cannot get the soft foot below 1 mil, this should be recorded. There may be practical limitations that will not allow you to achieve tight tolerances even after thorough measuring and analysis. If the base, feet, or legs of the machine are too flexible, final adjustments may cause more frustration than results. Reinforcing the base may be necessary before precision soft foot correction can be accomplished. Any soft foot that can be removed with reasonable time and effort will make the alignment process faster, easier, and more effective.

Bar Sag

Bar sag is simply the effect of gravity on a fixture. This effect can be measured accurately. As it affects the final accuracy of the alignment, it must be accounted for in your readings or eliminated from your fixtures before taking indicator readings.

Measuring Bar Sag

A certain amount of sag exists in most brackets. This amount must be known before attempting to align the couplings so that the sag can be calculated into the indicator reading. A procedure for determining the amount of sag in a bracket is as follows:

1. Assemble the bracket on a shaft or a lathe, as shown in Figure 30, then set up the indicator on top and zero the dial.


Figure 30: Bracket Assembles on a Shaft

2. Turn the complete assembly 180 and take a reading on the bottom. This reading is the amount of sag for that particular setup. Remember, the test should be as near as possible to the actual setup because a change in the distance between the shaft and the bracket may change the amount of sag. :Where two indicators are to be used, both indicators must be assembled on the brackets before zeroing on top.

3. Keep a record of the amount of sag for each setup for reference when calculating the actual indicator readings. If the amount of sag is greater than .001 inch, it is subtracted from the vertical readings.

Compensating for Bar Sag

Before we can determine the amount of bar sag, we must first be able to get repeatability from the fixtures. The best way to do this is to set the dial to zero at the 12 o'clock, or top, position, rotate the machine or bar one full 360o, and see if the indicator still reads zero at the top. If there is more than one mil or 0.001inch showing on the indicator, there may be some looseness in the fixtures. You may need to tighten up the fixture parts or make additional changes in the fixture configuration until you can get repeatable readings. This is very important to the accuracy of your alignment calculations. Once the amount of bar sag has been determined for the fixtures you are using, it can be handled in one of two ways. The first possible method is to simply add the amount of bar sag to your indicator readings. Because the sag is always a negative value, it can be added to the total indicator readings. The exception to this is that indicators starting at the 6 o'clock position and moving to the 12 o'clock position must have the bar sag subtracted from the final reading. This would be the case for one dial in the cross dial method. A second method is to simply dial the amount of bar sag determined at the 6 o'clock location into the 12 o'clock position as a positive number, or set the dial at the 6 o'clock position to a negative value. For example, if you have assembled the fixtures on a pipe or piece of bar stock that represents two shafts in perfect alignment, as shown in Figure 31, and adjusted the indicator to zero at the top, or 12 o'clock position, then rotated the fixtures so that the indicators are now at the 6 o'clock position, you should get a negative reading on the indicator, say a minus 8. Now rotate the fixture back to the 12 o'clock position. Dial the minus 8 into the indicator as a positive value, or as plus 8. To confirm that you have properly compensated for the bar sag, rotate the fixtures back down to the 6 o'clock location. The indicator should read zero.


Figure 31: Compensating for Bar Sag

When you place the fixtures back on the shafts of the machines being aligned, set the dial at the top to positive 8. When you rotate the fixture down to the 6 o'clock position, the number you read is the total indicator reading of the amount of misalignment. The bar sag has been compensated for, and the indicator now gives you the amount of difference between the two machine shaft centerlines. This section covered items that must be corrected and/or accounted for before performing a precision alignment. Items such as casing strain, cracked foundations, and soft foot will defeat the purpose of performing an alignment in the first place. Bar sag, when unaccounted for, will cause errors during the alignment process or lead us to believe a machine train is aligned, when it actually is misaligned.

Moving the Machine

Any instruction on precision alignment of rotating equipment must include methods for making accurate, controlled moves of the machine. There is a multitude of ways to move a machine. Some are better than others with possibilities ranging from using large hammers or pry bars to using hydraulic wenches or fine-threaded, smooth-operating devices. Any alignment method, regardless of its accuracy in measuring the misalignment, is useless if we do not take the proper precautions and procedures in order to achieve precision movement of the machine. All of the work in taking accurate readings can be lost in a split second if the proper techniques are not applied. In this section, we will examine many of the procedures and methods used to ensure an accurate alignment. There has been some discussion among alignment specialists about which move should be made first: the horizontal or the vertical move. Once the machine is adjusted to its proper vertical orientation, a slight move in the horizontal direction should not change the vertical alignment. This is the procedure once the rough alignment and adjustment stages are complete. However, certain situations may require a horizontal move be made first in order to attain accurate indicator readings if the alignment has been changed during the adjustment stage. In any alignment process, there may be some going back and forth from the vertical to the horizontal until the alignment is completed and precision is achieved.

Vertical Moves

As with all vertical moves, using a dial indicator is not necessary to determine the change in the vertical direction, but it can be helpful to check for any soft foot created when moving the machine. By using a micrometer, the exact thickness of the shims being installed can be determined for the required alignment correction. Be sure to mic every shim that is installed. Do not believe what is printed, stamped, or etched on the shim. Always check the shim to be sure. When loosening the hold-down bolts, the first precaution is to loosen only two bolts at a time when making any shim change. If you loosen all the hold-down bolts at the same time and then raise the machine, the entire configuration at the feet may change. This is particularly true if any of the feet have been corrected for soft foot conditions using a tapered or step shim. Leaving two hold-down bolts tight reduces the chances of an uncontrolled move. Any uncontrolled movement of the machine may change the horizontal, axial, and even the vertical positioning of the machine, thereby negating all your previous work. Loosen the bolts on either the left or the right side of the machine, but not both sides. Raise one side of the machine just enough to make the shim change. If the machine is raised too high, it can bend the feet on the side of the machine where the hold-down bolts are tight. Make the necessary shim change and tighten down the bolts to their proper torque value. Repeat the process for the other side of the machine. Each tightening of any bolt on the machine should be treated as though it is the last time to work on that particular bolt. For all vertical moves, be sure to raise the machine just enough to add or remove the desired amount of shims. Any excess movement of the machine in the vertical direction can result in a bent foot. Once you have the machine raised enough to make the shim change, remove the shims from underneath each foot and add or subtract the amount of shims you have determined. Always use the least number of shims under a foot as is practical. The more shims under a foot, the more likely a spring effect or soft foot is to occur. If possible, make or use a single piece of shim the total thickness required under the foot. If this is not possible, a maximum of three to four shims should be the limit. This gives the foot a solid place to rest on and reduces the possibility of anything getting between individual shims. When making shim changes for the vertical alignment, it is important to remember the shims must be placed beneath those shims that are present to correct for soft foot. If the shims used for correcting the vertical alignment are placed on top of the soft foot shims, it can create more soft foot problems. When inserting the shims under the feet, insert the shims slot all the way in until the shim has "bottomed out" in the slot. You should then pull the shim back about a quarter of an inch before tightening the bolt. If you leave the shim inserted all the way into the threads, tightening the bolt will pinch the end of the shim and may affect the accuracy of the shim change. After completing the shim changes and tightening the hold-down bolts, another set of readings should be taken with the method used in the alignment calculation. If the move is within the specified tolerance, proceed with the horizontal moves. If a second move is required, determine the necessary shim change and make the correction.

Horizontal Moves

If the base plate has jackbolts installed, the task of moving the machine is made much easier. In many situations, it is best to have jack screws installed at the time of the alignment rather than trying to move the machine some other way, which is often more difficult and requires more time. Figure 32 shows a foundation with jacking screws installed.


Figure 32: Foundation with Jacking Screws Installed
Before any of the hold-down bolts are loosened, all of the jackbolts must be loosened on the machine. Any unequal pressure of the jackbolts could result in an uncontrolled move and require the technician to take another set of alignment readings. When loosening the hold-down bolts, back them off just enough to allow the machine to slide sideways. Since it takes very little pressure on the jackbolt to move the machine horizontally, there is no real need to completely back off or remove the hold-down fasteners. If a graph is constructed determining the position of the misaligned shaft, you will know exactly which direction to move the machine. Another way to avoid confusion is to always look from the machine toward the machine.

  • Everything to the right, or toward the 3 clock position, is in the positive direction.
  • Everything to your left, or toward the 9 clock position, is in the negative direction.

This should help you visualize the proper direction for the move. The preferred and most accurate method for measuring the horizontal move is to place dial indicators around the machine in the planes of the feet, as shown in Figure 33.


Figure 33: Place Dial Indicators Around the Machine
It is important that the indicators are positioned on the machine in the location used in the alignment calculation. As shown in Figure 34, this usually is the center of the hold-down bolt and at the approximate shaft height.


Figure 34: Place Indicators in the Location Used in the Alignment Calculation
The procedure for performing a horizontal move of a machine is as follows:

  1. Position the base-mounted dial indicators in the location of the machines feet or at the point determined from the alignment calculation.
  2. Adjust the dials to zero.
  3. Move the machine in the proper direction by the determined amount, making sure all of the dial indicators agree. '

Figure 35 shows the proper dial indicator setup.


Figure 35: Proper Dial Indicator Setup
No matter what device or method is used to move the machine, it is very important to have control of the machine. The most common problem encountered when moving the machine horizontally is that there are no jackbolts on the machine to maintain this control. If no jackbolts are installed, small hydraulic jacks can be used to move a machine if they are backed up to a solid structure, such as a beam, a solid wall, an adjoining pedestal, or a base. A chain fall or come-along can also be used. Again, they must be secured to something solid or immovable and operated one click at a time. Another very good idea is to use pony clamps or pipe clamps, as shown in Figure 36, attached and operated to smoothly move the machine in the desired direction.


Figure 36: Using Pipe Clamps

Bolt Binding

At this point in the alignment, you may be thinking, "Ive got it now." There is; however, another problem you may encounter. For those of you who have been in this business for any time at all, you know that being bolt-bound happens quite often. This happens when the required horizontal move is greater than the amount of clearance in the foot. The most common solution, although not the preferred solution, is to mill down the bolt or "Chicago" the bolt. By removing the excess material along the shank portion of the bolt, you can gain many thousandths of an inch, which may allow you to reach the proper horizontal position for the machine, as shown in Figure 37.


Figure 37: Bolt Binding
The only precaution that must be pointed out is: do not turn the diameter of the shank down to less than the diameter of the root of the threads on the bolt. If too much material is removed, it will greatly reduce the holding strength of the bolt, possibly allowing it to break when tightened. Another possible option is to install the next size smaller bolt in the place of the bolt-bound foot if a through hole is present. For situations where the base has a threaded or blind hole, a helicoil can be installed. Before doing this, it is a good idea to check whether the next smaller bolt will provide enough holding force without creating a problem for the machine. Usually, one bolt of a smaller size on one hold-down point of a machine will not create a safety hazard, but it is a good idea to check with an engineer, supervisor, or foreman before proceeding with this option. Another common practice is drilling out the hole in the foot just enough to allow the machine to move over further. If you use this method, be careful to make sure that the hole does not exceed the size of the washer. If the washer is too small for the hole, it will bell or cup out of shape when the bolt is tightened, as shown in Figure 38. To avoid this, you can place the next size larger washer below the correct-sized washer for the bolt being used or use a piece of steel plate. Usually a piece with inch thickness is large enough to cover the enlarged hole in the foot. Drill a hole in the plate slightly larger than the bolt diameter.


Figure 38: Belled Washer
A bent bolt will have the same effect. When the bolt is tightened, it will have an elliptical or cam action and can move the machine out of alignment, as shown in Figure 39.


Figure 39: Bent Bolt

Precision Alignment

There are three methods currently used to calculate the amount of shim changes and horizontal moves required to achieve accurate machine alignment. They are each capable of arriving at the same results by use of mathematical formulas and/or graphical solutions. There are various tools that are available to help in the process. The first is a simple pocket calculator. With the proper formula and a basic understanding of the alignment process, accurate machinery movement can be achieved. The second method available is the graphical solution. This method uses simple 10 x 10 piece of graph paper that gives a pictorial representation of the machines and the amount of movement needed to correct the misalignment. The third method is the use of a computer, either a desktop or a special computer designed specifically for the alignment task. In this section, graphical and mathematical solutions to misalignment are discussed. We mentioned the computer method only to make you aware of its existence. The following are the methods of alignment that are discussed and their methods of solution:

1. Rim and Face (Mathematically Only)

2. Cross Dial (Mathematically and Graphically)

3. Reverse Dial (Mathematically and Graphically)

Rim and Face

The rim and face alignment method is commonly used where space considerations would prevent the use of the cross dial or reverse dial methods. It also is the only method that can be used when rotation of both shafts cannot be accomplished. The results of the misalignment can only be calculated mathematically, and parallel and angular misalignment must be calculated separately. After the rough alignment is done, the angular misalignment should be removed before solving for the parallel misalignment. For this reason, the rim and face method is often more time consuming than the other methods available.

Angular Misalignment Corrections

The face dial is used to measure the distance between the coupling faces. This measures the angular misalignment in both the horizontal (3 o'clock to 9 o'clock position) and the vertical (12 o'clock to 6 o'clock position) planes. The total indicator reading gives the actual difference in distance between the coupling faces. Because of the dial position relative to the face of the coupling, bar sag will not have an effect on face readings. It will still be an issue to consider when taking rim readings though. Figure 40 shows a typical setup for taking the face readings.


Figure 40: Face Readings

Procedure

  1. Zero the dial indicator at the 12 o'clock position (3 o'clock position for horizontal moves).
  2. Rotate the indicator 180 and read the error from the difference in reading.
  3. Measure the coupling diameter of indicator travel.
  4. Measure the distance between the coupling face and the front foot and the rear foot.
  5. Calculate proper shim movement (or horizontal movement) with the following formula:

Example 1:Rim and Face Angular Misalignment Calculation

Given the following information and Figure 41, calculate the required shim moves to achieve perfect angular alignment.

Face Reading @ 12 o'clock: .000"

Face Reading @ 6 o'clock: -.072


Figure 41: Example One

Solution: These calculations tell us that, to achieve perfect angular alignment, it will be necessary to remove .216 inch from the front foot and .360 inch from the rear foot. Negative numbers will always indicate that shims need to be removed, while positive numbers are an indication that shims will need to be added.

Parallel Misalignment Corrections

The dial indicator positioned to take the rim readings will measure the amount of parallel offset misalignment. The total indicator reading is always double the actual offset. Therefore, any shim moves to correct parallel offset misalignment will always be half the total indicator rim reading. Bar sag will need to be accounted for during vertical adjustments but will be negligible for horizontal adjustments. Figure 42 shows a typical setup for taking rim readings.


Figure 42: Taking Rim Readings

Procedure

1. Zero the dial indicator at the 12 o'clock position (3 o'clock position for horizontal moves).

2. Rotate the indicator 180 and read the error from the difference in reading. This is the TIR.

3. Calculate proper shim movement by dividing the TIR by two; this will be the shim adjustment for all four feet.

Example 2:Rim and Face Parallel Misalignment Calculation Given the following information from Figure 43, calculate the required shim moves to achieve perfect angular alignment.

Rim Reading @ 12 o'clock: .000

Rim Reading @ 6 o'clock: +.038

Bar Sag: .010


Figure 43: Example Two

Solution: Perfect parallel offset alignment may be achieved by adding .024 inch shims under each foot of the movable machine. Negative numbers indicate shims to be removed, while positive numbers indicate shim addition.

Cross Dial

Cross dial alignment is another method of achieving the same results as the rim and face method. Although it is not any more accurate, it is a much faster method of alignment. The reason for this is that both angular and parallel misalignment can both be corrected for at the same time. The shafts must be able to rotate together to perform this alignment, which will make it a better choice if you are shutting down a piece of equipment to check the alignment. The solutions for cross dial may be calculated mathematically or graphed out. Figure 44 shows a typical dial indicator setup for the cross dial alignment method.
Figure 44: Typical Dial Indicator Setup for Cross Dial Alignment

Mathematical Solution

The mathematical formula for doing a cross dial alignment follows a basic rise-over-run geometric principle. By following this principle, the alignment of machinery can be easily accomplished using the following formulas:

where:

A=Distace Between Dial Indicators

B=Distance from Stationary Machine Indicator and Front Foot

C=Distance from Stationary Machine Indicator and Rear Foot

SM=Sationary Machine Indicator Reading

MM=Movable Machine Indicator Reading

For ease of use, this formula is incorporated into a shaft alignment data form, shown in Figure 45. This is a fill-in-the-blank representation of the preceding formula. By filling in the appropriate information into the proper boxes, the calculation of the required shim changes for correcting both the angular and offset misalignments can be determined. The steps to completing this form are as follows:

  1. Enter the total indicator reading (TIR) for the stationary machine (SM) and the movable machine (MM) indicators in the blocks labeled ASM TIR and AMM TIR . Ensure bar sag has been accounted for.
  2. Enter distance between SM and MM indicators in the block labeled AA .
  3. Enter distancebetween SM indicator and the MM front foot in the block labeled AB .
  4. Enter distance between SM indicator and the MM rear foot in the block labeled AC .
  5. Enter previously recorded data in the calculation area of the form and calculate the MM front and rear foot moves.

    Figure 45: Shaft Alignment Form

    Example 3:Cross Dial Mathematical Misalignment Calculation

    Given the following information, calculate the required shim moves to achieve perfect parallel and angular alignment. Assume the MM dial starts in the 12 o'clock position and the SM dial starts in the 6 o'clock position.

    SM Indicator TIR = +.030

    MM Indicator TIR = -.026

    Bar Sag = .006

    AA Dimension = 6

    AB Dimension = 14

    AC Dimension = 32

    Solution: You may notice that the SM and MM TIRs are different from the given. This is because bar sag must be accounted for. In this example, .006 inch must be added to the MM TIR, and .006 inch must be subtracted from the SM TIR for correct calculations, which makes the SM TIR+.024 inch and the MM TIR -.020 inch.

    Figure 46 shows the calculation using the cross dial alignment form. When using this form, you may find it convenient to use mils instead of thousandths of an inch. This allows the use of whole numbers and avoids confusing decimals. For example, if the TIR was .027 inch, you would enter 27 on the form. When using mils, remember the solution will also be in mils; that is,+32.5 would actually be+.0325 inch. The example in Figure 46 uses mils and the same given data as above.

    Figure 46: Calculation of Example Three Using Cross Dial Alignment Form

Graphical Solution

The graphical solution for cross dial alignment problems is a method that will give you an actual visual indication of the misalignment. The results will be the same as if it were calculated mathematically though.

Procedure

  1. Lay out a horizontal line at the approximate middle of the graph paper. This is the running alignment line (RAL).
  2. Lay out a vertical line near the left edge of the graph paper. This represents the SM dial indicator position.
  3. Lay out another vertical line scaled from the SM indicator line that represents the MM indicator. If the distance is 6" and you are using a 1:1 scale, the line will be six blocks from the SM indicator line.
  4. Lay out another vertical line scaled from the SM indicator line that represents the MM front foot.
  5. Lay out the last vertical line scaled from the SM indicator line that represents the MM rear foot.
  6. Determine the SM dial indicator TIR (remembering to account for bar sag) and divide the reading by two. This is your plot point on the SM indicator line. Positive readings are above the RAL, and negative readings are below the RAL.
  7. Determine the MM dial indicator TIR (remembering to account for bar sag) and divide the reading by two. This is your plot point on the MM indicator line.
  8. Using a straight edge, extrapolate these plotted points across the vertical lines for the MM front and rear foot. This is the misalignment line.
  9. On the vertical lines for the MM feet, count the number of blocks either up or down from the misalignment line to the RAL. If the misalignment line is above the RAL, shims must be removed. If the misalignment line is below the RAL, shims must be added. Assuming a 1:1 scale is being used, each block equals .001 inch.

In the next example, we will use the same dimensional data and indicator readings as used in cross dial Example 3. This will let you compare the two methods.

Example 4:Cross Dial Graphical Misalignment Calculation

Given the following information, calculate the required shim moves to achieve perfect parallel and angular alignment. Assume the MM dial starts in the 12 o'clock position and the SM dial starts in the 6 o'clock position.

SM Indicator TIR = +.030

MM Indicator TIR = -.026

Bar Sag = .006

AA Dimension = 6

AB Dimension = 14

AC Dimension = 32

Solution:

When comparing the graph in Figure 47 to the mathematical solution from Example 3, you can see that the results are very similar. Accuracy is very dependent on the scale chosen for the graph. Remember: the neater and more precise the graph, the more precise your solution will be. Results will also become more accurate as the movable machine becomes nearer to the RAL. Whenever possible, you should use a vertical scale of one block equals .001" or, even better, one block equals .0005" (space permitting).


Figure 47: Graph for Example Four

Reverse Dial

Reverse dial alignment is very similar to cross dial both in theory and misalignment calculation. They both can either be calculated mathematically or graphically. Although there are slight differences in the formula and the plotting, the process is practically the same. The two major advantages to using reverse dial over cross dial are that many pre-manufactured rigs are set up for reverse dial, and you may achieve alignment with only three points. Although a cross dial alignment only requires three points to read, you still need the space for the indicator setup, whereas the reverse dial setup, with both indicators in the same plane, allows alignment of machines that are space-limited. By zeroing the indicator at the 12 o'clock position and reading the 3 and 9 o'clock positions, the 6 o'clock position may be determined. The characteristics of a circle tell us that the sum of the side readings, when read with a dial indicator, must equal the sum of the top and bottom readings. This is shown in Figure 48.


Figure 48: Unknown Position Using a Circle
The ability to only read the shaft a three points can be a major advantage, but there are some flaws to using this method all the time. In order for this to work, you must read at exactly the 12, 3, and 9 o'clock positions. Generally, this can lead to some inaccuracies when calculating the 6 o'clock position. One way of ensuring the readings are taken at the correct points is to use a combination bubble level attached to the shaft. When the bubble is exactly in the center, you will be at the correct position to read the indicator. This is sometimes called a four-point indicator. Figure 49 shows a typical reverse dial setup.


Figure 49: Typical Reverse Dial Setup

Mathematical Solutions

For ease of use, the reverse dial formula has been incorporated into a shaft alignment data form, shown in Figure 50. By filling in the appropriate information into the proper boxes, the required shim changes for correcting both the angular and offset misalignments can be determined. The steps to completing this form are as follows.

  1. Enter the total indicator reading (TIR) for the stationary machine (SM) and the movable machine (MM) indicators in the blocks labeled ASM TIR and AMM TIR . Ensure bar sag has been accounted for.
  2. Enter distance between SM and MM indicators in the block labeled AA .
  3. Enter distance between SM indicator and the MM front foot in the block labeled AB .
  4. Enter distance between SM indicator and the MM rear foot in the block labeled AC .
  5. Enter previously recorded data in the calculation area of the form and calculate the MM front and rear foot moves.


    Figure 50: Reverse Dial Shaft Alignment Form

    Example 5:Reverse Dial Mathematical Misalignment Calculation Given the following information, calculate the required shim moves to achieve perfect parallel and angular alignment.

    SM Indicator TIR = .024

    MM Indicator TIR = .010

    Bar Sag = .006

    AA Dimension = 5

    AB Dimension = 8.5

    AC Dimension = 14

    Solution:Figure 52 shows the calculation using the reverse dial alignment form. When using this form, you may find it convenient to use mils instead of thousandths of an inch. This allows the use of whole numbers and avoids confusing decimals. For example, if the TIR was .027 inch, you would enter 27 on the form. When using mils, remember the solution will also be in mils; that is,+32.5 would actually be+.0325 inch. The example in Figure 52 uses mils and the same given data as above.

    You may notice that the SM and MM TIRs are different from the given. This is because bar sag must be accounted for. In this example, .006 inch must be added to the TIRs for correct calculations, which makes the SM TIR+.030 inch and the MM TIR -.004 inch.

    Note: This alignment form applies to and should be used for the reverse dial method of alignment only.


    Figure 52: Calculation Using Reverse Dial Alignment Form

Graphical Solution

The graphical solution for reverse dial alignment problems is a method that will give you an actual visual indication of the misalignment very much like that used for cross dial alignment. The results will be the same as if they were calculated mathematically. The only difference in this type of alignment is that the MM indicator reading must have its sign changed before plotting.

Procedure

  1. Lay out a horizontal line at the approximate middle of the graph paper. This is the running alignment line (RAL).
  2. Lay out a vertical line near the left edge of the graph paper. This represents the SM dial indicator position.
  3. Lay out another vertical line scaled from the SM indicator line that represents the MM indicator. If the distance is 6 inches and you are using a 1:1 scale, the line will be 6 blocks from the SM indicator line.
  4. Lay out another vertical line scaled from the SM indicator line that represents the MM front foot.
  5. Lay out the last vertical line scaled from the SM indicator line that represents the MM rear foot.
  6. Determine the SM dial indicator TIR (remembering to account for bar sag) and divide the reading by two. This is your plot point on the SM indicator line. Positive readings are above the RAL, and negative readings are below the RAL.
  7. Determine the MM dial indicator TIR (remembering to account for bar sag) and divide the reading by two and then change the sign (positive readings become negative and negative readings become positive). This is your plot point on the MM indicator line.
  8. Using a straight edge, extrapolate these plotted points across the vertical lines for the MM front and rear foot. This is the misalignment line.
  9. On the vertical lines for the MM feet, count the number of blocks either up or down from the misalignment line to the RAL. If the misalignment line is above the RAL, shims must be removed. If the misalignment line is below the RAL, shims must be added. Assuming a 1:1 scale is being used, each block equals .001 inch.

In the next example, we will use the same dimensional data and indicator readings as used in reverse dial Example 5. This will let you compare the two methods.

Example 6:Reverse Dial Graphical Misalignment Calculation Given the following information, calculate the required shim moves to achieve perfect parallel and angular alignment.

SM Indicator TIR = .024

MM Indicator TIR = .010

Bar Sag = .006

AA Dimension = 5

AB Dimension = 8.5

AC Dimension = 14

Solution:

Note: The graph scale for this example is as follows:

Horizontal Scale 1 Block = .5

Vertical Scale 1 Block = .001


Figure 53: Reverse Dial Graphical Calculation