Talk Page



Belt drives for power transmission are classed as frictional drives. The belt transmits power by friction contact between the belt and the driving and driven sheave.

Power transmission belts are available in several types: flat belts, V-belts, synchronous belts, and multi-ribbed belts.

To obtain the best service from any particular belt application, remember:

  1. Select the correct belt for the job.
  2. Ensure that the belt is installed correctly and used properly.
3.2.2 Undercord
3.2.3 Overcord
3.2.4 Cover

Belt Selection Considerations

Environmental conditions in which the belt will operate, such as: exposure to oil and grease, range of operating temperatures, abrasive dust and chemical conditions, sunlight, and other weather conditions.

Other considerations include:

  • Type of drive required
  • Driver/Driven Revolutions Per Minute (RPM)
  • Horsepower requirements
  • Pulley diameters and center distance
  • Take-up allowances and take-up design
  • Space limitation for operation
  • Pulsating or shock load conditions
  • Static dissipation problems
  • Belt availability and inventory considerations
  • Belt construction and service life

Belt Drive Advantages

  • Wide range of speeds available.
  • Belts permit flexibility ranging from high horsepower drives to slow speed and high speed drives.
  • Belt drives are less expensive than chain drives for low horsepower and low ratio applications.
  • Belts require no lubrication.
  • Single belt drives will accept more misalignment than chain drives.
  • Flat belts are best for extremely high speed drives.
  • Belt drives cushion shock loads and load fluctuations.
  • Belts will slip under overload conditions, preventing mechanical damage to shafts, keys, and other machine parts.

Belt Drive Disadvantages

  • Belts cannot be used where exact timing or speed is required because slippage does occur (only timing belts can be used).
  • Belts are easily damaged by oil, grease, abrasives, some chemicals, and heat.
  • Belts can be noisy; also loose or worn belts can be a major cause of machinery vibration.

Belt Drive Principles

Flat belts and V-belts transmit power by their grip on the pulley or sheave.

Three major factors determine the potential of the grip:

  1. Area of contact
  2. Belt tension
  3. Friction between the belt and pulley or sheave surface (coefficient of friction)

Area of Contact

The area of contact is determined by width and the arc of contact. The arc of contact with pulleys of equal diameters is 180 degrees on each pulley, as shown in Figure 1.

Figure 1: Area of Contact

Pulleys of equal size are not always used. With pulleys of unequal diameter, the arc of contact is less than 180 degrees on the smaller pulley. Under most conditions, this small pulley is the driver. An example is shown in Figure 2.

Figure 2: Unequal Pulleys

An arc of contact greater than 180 degrees can be obtained three ways:

  1. A crossed belt drive.
  2. Moving the input and output shafts farther apart.
  3. Using an idler or snub pulley.

Crossed Belt Drive

A crossed belt drive, as shown in Figure 3, is not usually recommended for V-belts. In the crossed position, the center-to- center distance between the pulleys must be long enough to limit the internal stress in a belt. Crossed belt drives make the pulleys rotate in opposite directions to each other.

Figure 3: Crossed Belt Drive

Pulley Center-to-Center Distance

For maximum power transfer on the belts and pulleys, the pulley ratio should be 3 to 1 or less as shown in Figure 4 Top. Higher ratios, as in Figure 4 Bottom, lessen the arc of contact, causing slippage and loss of power.

Figure 4: Pulley Center to Center Distance

The arc of contact on the critical smaller pulley may be increased if the shafts are moved farther apart as shown in Figure 5. Where a high ratio is required, a two-step drive (counter-shaft) can be used to avoid excessive single-step ratios or undersize pulleys.

Figure 5: Increase Arc of Contact

Idler Pulleys

A properly designed V-belt drive does not require an idler to deliver fully rated horsepower if proper belt tension and area of contact are maintained. Idlers put an additional bending stress on the belt, which reduces belt life. Also, the smaller the idler pulley, as shown in Figure 6, the greater reduction in belt life.

Figure 6: Idler Pulley

The best location for an inside idler is on the slack side of the drive. Figure 7 Top shows a backside idler that is commonly used to help increase the arc of contact on both pulleys. This idler forces a backward bend in the belt, which decreases belt life. The idler puts additional strain on the bottom portion of the belt, which may crack that section.

The diameter of the flat idler pulley should be at least 1.5 times the diameter of the smallest sheave located as close as possible to the small sheave.

Figure 7 Middle shows an inside idler. An inside idler reduces the arc of contact but the amount of take-up is unlimited. The smaller arc of contact will decrease the horsepower rating of each belt.

Figure 7 Bottom shows a backside idler, which is located as close as possible to the driven pulley. In this example, the idler helps to increase the arc of contact on the large diameter pulley, which reduces belt slippage problems that may be encountered on the driven side.

Figure 7: Idler Positions

Belt Tension

The best tension for a V-belt is the lowest tension at which the belts will not slip under full load. Other belt tension recommendations are:

  • Check the belt tension frequently during the first 24 to 48 hours of run-in operation.
  • Maintain sheave alignment while tensioning the drive.
  • Make V-belt drive inspections periodically checking belt tension.
  • Keep all belts free from foreign material, which can cause slippage.
  • Over-tensioning belts will lead to reduced belt and bearing life.

To tighten a belt drive unit, take up the drive until the belts are snug in the grooves. Run the drive for a few minutes to allow the belts to seat. If the belts slip, tighten them until slippage is minimal when a full load is applied.

Many experienced maintenance people develop a feel for belt tension, and often it may not be critical to have extremely accurate tensioning.

If an accurate tension is required, a numerical method for determining and setting belt tension can be used. This method provides accurate tension values using a tension tester or a spring scale. This method should only be applied if the belt drive has been selected from stock drive tables of belt manufacturers and the number of belts on the drive conforms to manufacturer's recommendations.

Accurate Belt Tensioning

Refer to Figure 8 for reference to the following tensioning steps:

  1. Measure the span "T".
  2. At the center "T" apply force with the tension tester perpendicular to the span, sufficient to deflect one belt of the drive 1/64th inch per one inch of span length from its normal position.

Figure 8: Belt Tension Measurement

  1. Determine the amount of deflection distance on the lower linear scale of tension tester, as shown in Figure 9, by sighting straight across the tops of the belts. A straight edge laid across the belts can provide accurate readings.

Figure 9: Tension Tester

  1. Find the amount of deflection force on the upper scale of the tester. The sliding rubber O-ring collar slides down scale as the tester compresses. The collar stays down to give a pressure reading.
  2. Compare the deflection force reading with the general range of forces listed in Table 1. If less than normal recommended deflection force exists, belts should be tightened. If more than maximum deflection force is found, the drive may be tighter than needed.
Table 1: Recommended deflection Forces
Recommended Deflection Forces (in lbs.)
BeltNormalMaximumNew Belts
  1. A V-belt manufacturer’s manual will provide a proper deflection force figure to suit specific belt types.

Example:Find the deflection required for a new C-section V-belt installed on sheaves with 32-inch centers and a required pull of 14 pounds.

T=32 inches

Deflection=T/64=32"/64"=1/2"(12.7 mm)

The drive will be tightened up until the deflection of the belts is 1/2 inch with a 14-pound push.

Figure 10 shows how a spring scale can be used to obtain the required deflection force for accurately tensioning a belt. A ruler can be used to measure the belt after the required deflection force is applied.

Figure 10: Spring Scale

Establishing Belt Tension

The drive powers the driven pulley by the pull, which results in increased tension and stretch on the tight side of the unit as it overcomes the load resistance. The slack side has no tension increase, it simply returns to the driven pulley. As shown in Figure 11, belts should run with a distinct tight and slack side.

Figure 11: Establishing Belt Tension

Keep take-up guides, rails and motor base area free of dirt, moisture and grit. Keep the take-up screws clean and periodically apply a light lubrication. This makes for easier adjustments when belts have to be tightened or replaced.

If one or more belts are too loose (Figure 12) or too tight (Figure 13), one of the following problems exists:

  • Worn sheaves
  • Improper belt tension
  • Damaged belts
  • Improperly matched belts
  • Angular sheave misalignment

Figure 12: Too Loose

Figure 13: Too Tight

Coefficient of Friction

Figure 14 can be used to help define the coefficient of friction for belt drives. If a body of weight (W) rests on a horizontal plane surface and a force (P) parallel to the surface is sufficient to cause the body to be at a point of slipping, then the ratio of P to W is the coefficient of friction (F) between the two surfaces.

Coefficient of Friction (F)= P/W

Figure 14: Determining COF

Friction between sliding surfaces, as in belt and pulley surfaces, is not influenced by the area of surface. The friction is solely dependent on the character and condition of the faces, and the total pressure normal to the faces.


V-belts are designed to operate in V-shaped grooves in the sheaves used for power transmission. V-belts have a major advantage over other types of belt friction drives; as the wedging effect of the belt pushing into the sheave results in lower belt take-up tension being required.

For the same horsepower, sheave diameter, and sheave speeds, V-belts will operate with lower tension and, therefore, lower bearing load than other friction-type belt drives.

V-Belt Construction

Industry standards exist which control sheave groove details for V-belts. Due to manufacturing differences, mold details and various belt materials, the belt of one manufacturer may differ slightly in shape, stretch and friction characteristics from belts of the same cross-section made by another manufacturer. Belt manufacturers meet the standards and tolerances as set by the Rubber Manufacturers Association (RMA). Each manufacturer’s belt must operate at the same speed in the standardized sheave groove.

V-Belt Components

A V-belt consists of five inter-related components. Refer to Figure 15 for reference to typical V-belt construction.

Figure 15: V-Belt Components

Tensile Member

The tensile member or pitch line purpose is to withstand the tension or pull that is imposed to transmit the desired power. The tensile member materials commonly used are rayon, nylon, polyester, steel, fiberglass, and Kevlar.


The undercord materials commonly used are: natural or synthetic rubber compounds, fiber-loaded rubber compounds, woven natural or synthetic cords, or piles.


The overcord locates the tensile member correctly in relation to other belt components, and it also assists in preventing the tensile member from sagging in the center under load.


The cover protects the internal belt components from weather and environmental conditions. It also provides the wearing surface for the belt. The cover must remain flexible, and may be oil and heat resistant. The cover material meets RMA standards for static conductivity, and most belt covers are flame-resistant; they do not catch fire from heat build-up if the belt is subjected to severe slippage.

Adhesion Resin

The adhesion resins or gums act as a cushion to prevent tensile members from rubbing together as well as fully bonding all of the belt components together. Continual flexing of the belt tends to loosen the cords from the surrounding bonding material. To prevent excess cord separation, the adhesive resins must completely saturate the tensile cords. Through a hot vulcanizing process, both the adhesion gums and tensile members are joined in a chemical bond, which is flexible and permanent.

V-Belt Length

Table 2 has been designated by the RMA as the tolerance range for matched belts up to 63 inches in length. All belts in a set up to 63 inches must not vary more than the recommended RMA tolerance; otherwise, the load will not be distributed and the belts will wear out faster.

V-Belt Performance

Some exact length V-belts may appear to hang unevenly when installed. It is normal for belts that are only hundredths of an inch apart in length to create noticeable differences in deflection (within RMA tolerances).

The sag is more noticeable on longer length drives, but does not affect the drive performance or the belt’s ability to equally share the load. This condition may also be referred to as the catenary effect.

If an 8V2500 V-belt is in use, the allowable length tolerance is approximately .45 inch. This means that in any set 8V2500 belts, the difference between the shortest and longest belt cannot exceed .45 inch. Therefore, over the belt length span each belt will hang differently because of the allowable length tolerance set by the RMA.

Conventional Belts

The conventional V-belt is the most common belt in use. These belts are sized as to cross-section and there are five cross-section sizes (A, B, C, D, E) as shown in Figure 16.

Figure 16: Conventional V-Belt Sizes

Conventional V-belts are made with either a flat or a concave sidewall. Bend a V-belt as if it were running around a sheave. One can feel the concave sides fill out and become straight as in Figure 17. This precise fit ensures full contact with the sides of the sheave and the belt grips the sheave evenly, distributing the wear uniformly across the side of the belt.

Figure 17: V-Belt on a Sheave

Table 3 identifies conventional V-Belt length ranges, angles, maximum cross-section width, and belt thickness

Table 3: Conventional V-Belts
SectionLength Range (in.)Angle Top Width (in.)Thickness (in.)




















(1 1/4)






(1 1/2)





Narrow V-Belt

Narrow cross-section V-belts transmit up to three times the horsepower of conventional V-belts in the same drive space, or the same horsepower.

Three cross-sections of narrow V-belts are available, as shown in Figure 18; again all sizes are nominal.

Figure 18: V-Belt Cross-Section

Narrow V-belts provide savings in drive space with narrower sheaves, shorter centers, smaller sheave diameters, and reduced sheave weight which may help decrease bearing loads. Greater speeds can be handled by this type of V-belt; up to 6,500 FPM.

Narrow V-belts have a narrow cross-section, but they sit deeper in the sheave groove than a conventional V-belt. Concave sides are commonly used which makes for more uniform belt wear.

The radius relief minimizes corner wear and the arched top helps prevent dishing and distorting of the tensile member.

The belt number identifies the belt cross-section and effective length. The number preceding, such as 3V, indicates the top width of the belt in 1/8ths of an inch. The number following indicates the effective outside circumference.

Example: 3V400 = 3/8 inch (9.5 mm) cross-section and 40 inch (1,016 mm) effective outside circumference

Table 4 identifies narrow V-belt length ranges, angles, widths, and thicknesses.

Table 4: Narrow V-Belts
SectionLength Range (in.)Angle (0)Top Width (in.)Thickness (in.)
3V25-15038.375 (3/8).313 (5/16)
5V50-35540.625 (5/8).531 (17/32)
8V100-560401.000 (1).875 (7/8)

Molded Notched V-Belts

Notched V-belts provide higher horsepower rating than conventional cross-section belts. They are suited for drives with smaller sheave diameters where conventional cross-section V-belts would not be practical. Notched V-belts, as shown in Figure 19, can be used on some heavy duty A, B, C, and D drives. The molded notch in the belt’s bottom surface helps to reduce bending stress and provides uniform distribution of load. Notches also help to dissipate the heat of rapid flexing.

Figure 19: Notched V-Belt

Double V-Belts

Double V-belts are used on serpentine drives, as shown in Figure 20, transmit power to two or more sheaves through both the top and bottom of the belt.

Figure 20: Double V-Belt

Figure 21 shows two styles of a double V-belt cross-section.

Figure 21: Two Styles of Double V-Belt Cross Section

Double V-belts are manufactured in four cross-sections, identified as AA, BB, CC, and DD. See Figure 22 for correct double V-belt cross-sections and nominal sizes.

Figure 22: Double V-Belt Cross Sections and Nominal Sizes

The length designation number for double V-belts is separated by the cross-section designation.

Example: 3BB5 is a double B cross-section belt with a 35-inch effective outside circumference length.

Table 5 identifies the various length ranges for double V-belts.

Table 5: Double V-Belts
SectionLength Range (in.)

Power Band V-Belts

On some V-belt drives, fluctuating loads induce vibration, causing the belts to whip particularly on the slack side. This severe belt whipping can result in the V-belts rolling over in the groove or jumping off the sheave. In either case, the belts are quickly damaged. It is often impossible to eliminate the cause of the vibration. Therefore, to solve the problem of rolling or jumping belts, power band V-belts are recommended for the drive.

The power band V-belt is made by adding a common back or hand to the top of two or more belts. The belts and backing are vulcanized together to form a complete unit. The overcord section is thicker than on a normal V-belt, thus the backing rides well above the sheave. The same wedging action in the sheave groove is obtained as with a set of individual V-belts. The backing provides for increased transverse rigidity. Figure 23 shows a cross-section view of a power band narrow V-belt. These belts do not prevent vibration, they merely restrict it to an up and down motion, and prevent the belts from rolling over in the groove or jumping off the sheave.

Figure 23: Cross-Section View of Power Band Narrow V-Belt

The cross-section and spacing of a power band is such that standard multiple groove sheaves can be used. Cross-sections are also available in 3V, 5V, or 8V.

Power band belts are also available in B, C, or D cross-sections. A and E cross-sections are available in production lots only. Figure 24 shows a conventional cross-section power band belt.

Figure 24: Conventional Cross-Section Power Band Belt

Table 6 identifies the length range for both conventional and narrow cross-section power band V-belts.

Table 6: Power Band V-Belt Length Range
SectionLength Range (in.)SectionLength Range (in.)

Light Duty V-Belts

Light duty V-belts are used in the fractional horsepower range and are often referred to as fractional horsepower or FHP V-belts. They are commonly used singly on small pumps, compressors, lawn mowers, garden tractors, home appliances, small fans, and other light equipment. They generally transmit less than one horsepower.

The RMA standard nominal cross-sections for light duty belts are identified as 2L, 3L, 4L and 5L. Figure 25 identifies the cross-section and nominal size of FHP V-belts.

Figure 25: Cross-Section and Nominal Size of FHP V-Belts

Table 7 indicates the length range, angle, cross-section width, and the belt thickness for FHP V-belts.

Table 7: Light Duty (FHP) V-Belts
SectionLength Range (in.)SectionLength Range (in.)

Correct V-Belt Selection

The A, B, C, D, and E belts, the narrow 3V, 5V, and 8V belts, and the FHP 2L, 3L, 4L, and 5L belts are used in many applications. Know which type of belt to install on the machine's sheaves. If there is any uncertainty, measure the top width of the old belt or use a manufacturer's sheave and belt gage.

The nominal dimensions of conventional, narrow, or FHP V-belts may appear confusing. For example, top widths of B and 5V belts are very close (1/32nd of an inch). The 5V belt is considerably thicker, and the groove angles of the sheave are quite different. Using the one cross-section type on a drive designed for the other leads to short term belt life. FHP V-belts should never be used on any heavy-duty industrial applications, even if they seem to fit the conventional or narrow V-belt sheave grooves.

Poly V-Belts Ribbed

A poly V-belt is a single unit with a longitudinally ribbed traction surface. As shown in Figure 26, the ribs mate with sheave grooves of the same shape. These belts offer power transmission capabilities of standard V-belts and the flexibility of flat belts. Uniform engagement of the belt into the sheave grooves and complete support of the tensile member eliminates differential driving and equalizes belt stresses.

Figure 26: Poly V-Belt (V-Ribbed)

Poly V-belts have a greater area of belt sheave contact than either flat or standard or high capacity V-belts. They also take less space than standard V-belts, as shown in Figure 27.

Figure 27: Poly V-Belt

Poly V-Belt Advantages

  • Reduced belt thickness permits use of smaller sheaves.
  • Lighter, more compact drives are available.
  • Speed ratios of up to 40 to 1 are available.
  • Center distances are reduced; space is saved with no loss in horsepower.
  • Even distribution of pressure over all parts of the drive surface provides uniform loading.
  • Smooth running, good response to shock loads.
  • No belt turnover, smooth tracking.

Poly V-Belt Cross-Section

Table 8 identifies the cross-section of the three common poly V-belts. J, L, and M cross-section sizes cover a broad range of applications including appliances, automotive accessories, agricultural equipment, as well as light- and heavy-duty industrial drives. H and K cross-sections are available but they are limited to specialized drives. H is intended for miniature drives and K for automotive accessory drives.

Table 8: Poly V-Belt Cross-Section
FHP V-Belts
SectionLength Range (in.)No. of RibsRib Width (in.)Power Range (hp)
J18-982-145.094 (3/32)up to 15
L50-145.54-90.188 (3/16)5-50
M90-3613-40.375 (3/8)25-1,700

Figure 28 is a cross-section selection chart for poly V-belts based on design horsepower.

Figure 28: Cross-Section Selection Chart for Poly V-Belts

In reference to Figure 28:

  • Along the horizontal axis of the chart, find the design horsepower of the drive.
  • On the left side of the chart, along the vertical axis, find the RPM of the faster shaft. The proper poly V-Belt cross-section is found where the two lines intersect.

Variable Speed Belt Construction

Variable speed belts are molded into an arch construction shown in Figure 29. A strong compression section gives these belts excellent crosswise rigidity that resists squashing or distorting. Abrasion resistant compounds assure that the belt grips both faces of the sheave uniformly.

Figure 29: Variable Speed Belt

The cog of construction on the belt’s underside, as shown in Figure 30, helps the belt to achieve extreme flexibility over small sheaves without loss of gripping action or cross rigidity.

Figure 30: Variable Speed Belt Flexibility

Variable Speed Cross-Sections

The RMA has established twelve variable speed belt cross-sections and sheave groove sizes for standard industrial drives. Standard nominal dimensions of the twelve belt cross-sections are indicated in Table 9 and standard belt lengths are identified in Table 10.

Table 9: Normal Variable Speed Belt Cross-Section

Refer to Figure 29 for reference to the dimensions of twelve cross-sections of variable speed belts on Table 9. The variable speed belts produced by various manufacturers may differ form the nominal dimension indicated in the tables, but all standard variable speed belts will operate interchangeably in standard sheave grooves designated by the same number.

The twelve selected cross-sections of variable speed belts, ranging in top width from 7/8 inches and four sheave groove angles (22, 26, 30, and 36 degrees) will provide the necessary speed variation and power capacity for many industrial variable speed drives.

Table 10: Standard Variable Speed Belt Lengths

Variable Speed Belt Sheaves

Standard variable speed belt sheave designs conform to the dimensions and tolerance indicated in Table 11 and Figure 31. The included groove angle of the sheave, top width, and clearance are also identified.

Figure 31: Closed and Open Sheaves

Table 11: Variable Sheave Groove Dimensions

The sides of the sheave’s grooves should be smooth with a surface finish of 125 micro-inches or less. The groove surfaces should be free of defects, scratches, and the edges of the groove should be rounded.

Variable speed sheaves should have a maximum TIR (Total Indicator Reading) of .010 inch eccentricity. Sheaves over 10 inches in diameter can have allowable eccentricity of .0005 inch per inch of additional diameter. Side wobble and run-out on the sheave should be held to within .001 inch TIR per inch of outside diameter. Most variable speed sheaves are designed for maximum rim speeds of 6,500 FPM (Feet Per Minute). Dynamic balancing is recommended where high speeds and vibration are present.

Single Variable Sheave

The single variable sheave drive is the simplest of the three variable speed drives. This type uses one variable pulley and one fixed pulley. The drive mechanism consists of one spring-loaded variable sheave and a V-grooved or flat companion. The spring-loaded variable sheave is usually mounted on the drive or motor (Figure 32). An adjustable motor base is commonly used to vary the center distance of the drive, thereby changing the speed ratio.

Figure 32: Single Variable Sheave

Variable sheaves for this type of drive can be designed with either one or both flanges moveable. Either way, the spring mechanism of this sheave will keep the flanges up against the variable speed belt and automatically supplies the required belt tension for the drive.

The limits of the drive speed variation are usually determined by the selection of the mating sheave. This is because the output speeds are a function of the ratio of the pitch diameter of the mating sheave to a maximum and minimum diameter of the spring-loaded variable sheave.

Speed Variation Pitch Diameter x Minimum Pitch Diameter

Various speeds can be designed into a drive such as the single variable sheave unit. It is recommended that maximum speed ratio variation be kept to 3:1; this provides longer belt life.

Dual Variable Sheaves

Dual variable sheaves are generally used when larger ranges of speed variations are needed. In this type of drive, both the driver and driven sheave are variable. These are referred to as compound drives. While both sheaves are variable; one, usually the driven, is a spring-loaded sheave that automatically provides the required belt tension.

The movable flange on the driver sheave is mechanically actuated to provide the overall speed control. With this design, the center distance is usually fixed.

Speed variation on this drive is accomplished by axial movement of the movable flange of the mechanically actuated sheave. Movement of this flange to the open position of the sheave permits the spring on the driver sheave to pull the variable speed belt to a minimum pitch diameter position on this sheave. Automatically, the pitch diameter increases on the driven sheave to its designed maximum position. This ratio of minimum driver pitch diameter to the maximum driven pitch diameter results in the minimum drive output speed. This is shown in Figure 33.

Figure 33: Dual Variable Sheaves

Actuation of the driver flange to the closed position on the sheave forces the variable speed belt to the maximum driver pitch diameter, with the axial actuating force being enough to compress the spring on the driven sheave to pull the variable speed belt to a minimum driven pitch diameter. The ratio of maximum driven pitch diameter develops the maximum drive output speed.

The limits of the drive speed variation are determined by selecting the correct operating pitch diameters for both of the variable speed sheaves. Maximum speed variation should be kept to a 9:1 ratio for an overall balance of performance.

To maintain belt alignment of this type of variable speed drive, the variable pitch sheaves are arranged so that the movable flanges are on opposite sides of the variable speed belt. Any movement of the flange on one sheave will result in a simultaneous opposite movement of the flange on the other sheave.

Countershaft Dual Variable Sheaves

The least common of the basic variable speed belt drives is the countershaft dual variable sheave drive. This drive is used when there is a need to span large distances from the input shaft, or for a large speed variation.

This drive has two fixed-pitch (non-adjustable) pulleys at the driver and driven ends, with a single intermediate adjustable sheave. The intermediate sheave has dual grooves to accept belts from both the driven and driver pulleys. Movement of a pivoting control on the center (intermediate) sheave is such that an increase in pitch on one side of the sheave causes a corresponding decrease in pitch on the other side. Complete movement of the dual variable sheave toward the driver sheave will result in the slow speed position, and a reversal of this movement will put the drive in high speed. As with a compound drive, a maximum speed variation of 9:1 is recommended.

Variable Speed Sheave Alignment

Alignment of sheaves can be done with either a straight edge or a tight line, provided suitable checks are taken on the sheave widths.

Variable Sheave Alignment

If the driver and driven sheaves are checked and found to be the same width, a straightedge or line can be used without any offset allowances. The belt will run in the center of both sheaves.

When the driver and the driven sheaves are of different widths, the alignment check should be made using a straight edge to measure the offset. Figure 34 shows how the belt does not run parallel to the sheave nor the straight edge.

Figure 34: Incorrect Alignment

Figure 35 shows how the belt runs in the center of each sheave. The alignment check is done by determining if the belt is parallel to the straightedge.

Figure 35: Correct Alignment

Variable Sheave Maintenance

Lubrication is needed because of the sliding action of the flanges. Excess lubrication can cause oil or grease to leak onto the belt and result in surface damage.

Keys or splines required for axial movement and to transfer rotary motion must be free from tight spots and burrs. In addition, if the sheave operates in one position for an extended period of time, the profile of the key or spline may become worn, which may make shifting difficult. Some variable speed sheaves use non-metallic keys, which can shear on overloads. Any sheave with one fixed flange will change the alignment of the belts as they are shifted up or down the groove. It may be necessary to align the sheaves in the position of greatest use to reduce side wear on the belt.

Never allow the belt to bottom on the sheave.

Take caution when dismantling spring-loaded sheaves. The sheave can break apart easily due to the compressed spring. Position the sheave in a press or clamp arrangement before disassembly begins.

Positive Drive Belts Synchronous

Positive, or timing, belts are used in applications where slippage cannot be tolerated. Input and output shafts of the drive unit must be synchronized. These belts have a tooth profile which mates with corresponding grooves in the pulleys, thereby providing the same positive engagement as chain or gear drives.

Positive Drive Advantages

  • Full, non-slip power transmission.
  • Speed ranges from fractional to 6,000 FPM.
  • Light, compact drive as compared to chain or gear drives.
  • Less bearing loads.
  • No lubrication is required.
  • Provides accurate timing.
  • No metal to metal contact.
  • Less noise, no chatter, reduced vibration.

Table 12 identifies the standard sections for positive drive belts; XL, L, H, XH, and XXH, the belt pitch, belt length and common belt widths.

Table 12: Positive Drive Belts
SectionPitch (in.)Length(in.)Width (in.)
XL1/56.0-26.01/4, 5/16, 3/8
L1/812.4-60.01/2, 3/4, 1
H1/224.0-170.03/4, 1, 1 1/2, 2, 3
XH7/850.7-175.02, 3, 4
XXH1 1/470.0-180.02, 3, 4, 5

Timing Belt Pulleys

Pulley teeth are generated with an involute profile to ensure correct belt mesh and minimum belt wear. Figure 36 shows a positive drive belt in contact with a timing belt pulley.

Figure 36: Positive Drive Belt Pulley

Positive Drive Pitch Sizes

Belt Pitch

The distance between the tooth centers, measured on the belt’s pitch line that corresponds to the centerline of the belt's tension member.

Pulley Pitch

The distance between the groove centers, measured on the pitch circle of the pulley. The pitch circle of the pulley is larger than the actual pulley circle, and coincides with the pitch line of the belt.

In any positive drive, the belt must have the same pitch as both pulleys to mesh and drive properly.

The action between the belt and the pulley is positive as there is no slippage under load conditions. Because this drive does not rely on friction, belt tension does not have to be as high as with flat or V-belts. Shaft-bearing loads are also low because of the low-tension requirements.

Positive Drive Belt Pulleys

Figure 37 shows a typical positive drive pulley. During inspection, check for worn tooth sections on the pulley. The correct fit for the pulley contact is shown. A small amount of clearance should exist between the root section of the pulley teeth to prevent the belt from bottoming out.

Figure 37: Left Running Variable Belt

Generally, positive drive belt pulleys are flanged to help keep the belt in position. On long center drives, the belt may have a tendency to run to one side of the pulley. Stock or standard positive drive belts will ride toward the left flange, as indicated in Figure 38. If only one flange is located on the pulley, it should be placed on the left.

Figure 38: Left Running Variable Belt

Minimum Pulley Diameters

Table 13 identifies minimum recommended pulley diameters for each positive drive belt cross-section. Smaller diameter pulleys could be used other than what is recommended, but the belt life decreases.

Table 13: Minimum Pulley Diameters

Selecting Positive Drive Belts

Positive drive belt size nomenclature indicates:

  • Pitch length
  • Tooth pitch
  • Belt width

The actual length of the belt is multiplied by ten to avoid use of decimals in belt codes. For example, a 22.5-inch long belt is a code 225. The width of the belt is multiplied by 100; a 3/4-inch is referred to as a code 075. Figure 39 provides examples of positive drive belt codes.

Figure 39: Selecting Positive Drive Belts and Pulleys

Matching is not required on positive drive belts. No matching code number appears on the belt. Additionally, two 2-inch wide positive belts will operate satisfactorily on a 4-inch wide pulley.

Positive Drive Idlers

Neither inside or outside idlers are recommended on positive drive belts. If a positive drive belt is tensioned with a snug fit, the result is long belt life, less wear on bearings, quiet operation, and therefore no need for an idler.

If the torque load is unusually high at power take off, an idler may be required to help prevent the belt from jumping teeth on the pulley. Idlers may also serve a functional duty, acting as another drive for an integral part of the machine.

If an idler has to be used, it should be mounted on the slack side. Flat idlers should not be crowned; a flange idler is recommended. Idler diameters should exceed the diameter of the small pulley and idler arc of contact must be minimal.

Figure 40 shows a twin tooth timing belt. This positive drive belt is recommended for synchronized drives where load transmission is required from both sides of the belt.

Figure 40: Twin Tooth Timing Belt

Linked V-Belts

Linked V-belts are used where it is very difficult to use endless V-belts because of equipment design. Figure 41 shows an example of a linked V-belt. The flexible links consist of laminated fabric and rubber and each is held together by metal studs secured with washers. The belt can be installed or removed easily without removing outboard bearings, tilting, resetting, or moving motors or drive shafts.

Figure 41: Linked V-Belt

Linked V-belts are available in conventional V-belt sizes (A, B, C) and they may be used to replace fractional horsepower V-belts in the 4L and 5L series.

Flat Belts

The use of flat belts has decreased as most modern industrial equipment has built-in drives or uses V-belts.

The main disadvantage of flat belts is their reliance on belt tension to produce frictional grip over pulleys. The high belt tension required to transmit power often shortens bearing life. Another problem with flat belt drives is their failure to track properly.

If equipment bearings are adequately sized, tensioning requirements should present little problem to the drive. Because flat belts are quite thin, they are not subjected to high centrifugal loads and thus operate well over small pulleys at high speeds in ranges exceeding 9,000 FPM.

Flat belts are constructed in a variety of ways. Three common designs are:

  • Fabric Ply Belts
  • Fabric Cord Belts
  • Synthetic Flat Belts

Fabric Ply Belts

These belts consist of several plies or layers that are made of cotton or synthetic fiber with or without rubber impregnation. The number of plies determines the belt thickness that will help determine the minimum pulley diameter for the drive.

Fabric Cord Belts

These belts are constructed with multiple cords made from cotton or synthetic fibers such as rayon, nylon, plastic, or Kevlar. They are encased in rubber and covered with a fabric/rubber covering. This type is generally classed as a heavy-duty flat belt, used for high speeds, small pulley diameters and shock loads. Steel cables can also be used as they have higher capacity and lower stretch than fabric cord flat belts.

Synthetic Flat Belts

These belts are made from nylon. Nylon offers flexibility, extremely high tensile strength and operates very effectively at high rim speeds. The belts are thin, and they may consist of several plies of thin nylon bonded together to form a tough, but flexible flat belt.

Flat Belt Pulleys

Figure 42 shows a flat belt pulley. The narrowest pulley on the machine's drive will usually determine the selection of the width of a new belt. The pulley face should be one inch wider than the width of a new belt. The pulley face should be one inch wider than the belt for pulleys of up to 6 inches and 2 inches wider for pulleys over 6 inches.

Figure 42: Flat Belt Pulley Width

Crowned Pulleys

Pulley crown is defined as "the difference, in inches or millimeters, between the diameter of the pulley at its center and its edge." The standard crown, as supplied by pulley manufacturers, is 1/8 inch for each foot of pulley face width.

It is important to avoid excessive crown as this over tensions the center of the belt, causing premature failure. The reason for crowning a flat belt pulley is to help the flat belt track in the center of the pulley. Under normal conditions, belts will run on the largest diameter of the pulley, and a crowned pulley uses this factor. Usually the higher the belt speed, the smaller the crown required.

Figure 43 shows a crowned pulley, which is curved. This uniform curve is recommended as it equalizes the loading across the width of the belt. Figure 43 also shows the crown height and crown taper on a flat belt pulley.

Figure 43: Crowned Pulleys

Flat Belt Idler Pulleys

Idler pulleys are used to increase the arc of contact on the driver pulley, or to act as a belt take-up adjustment.

Idlers should be located on the slack side of the belt and should be larger than the small pulley in the drive. Idlers do not require a crown. They put a reverse bend in the belt and this shortens belt life.

Cone Pulleys

Cone pulleys (Figure 44) are used in pairs to obtain speed variations in a driven machine. By using a narrow flat belt and moving it across the pulley faces by means of a shifter fork, variable speed ratios are developed. This service puts extra strain first on one edge of the belt, and then on the other, as the belt passes over pulleys tapered in opposite directions. The shifting mechanism can also damage the belt edges.

Figure 44: Coned Flat Pulleys

Flat Belt Joining

Flat belts can be ordered from belt manufacturers as endless belts. The following factors must be considered when ordering endless belts:

  • Provisions for initial belt tension
  • Account for belt stretch
  • If the load is a normal rated load or an overload
  • Amount of take-up available

Vulcanized or chemically bonded splices are recommended for joining belt ends when endless belts are not supplied. Vulcanized splices have several advantages over mechanical fasteners.

Vulcanized Advantages

  • Long lasting, dependable
  • Freedom from tear and snagging hazards
  • Smooth, free from noise and vibration
  • More strength in the splice with possible reduction in number of plies
  • Less bending stresses
  • Less opportunity for belt deterioration and moisture entrance

Flat Belt Fasteners

The most important requirement of a mechanically fastened joint is that the ends of the belt are cut square, and the belt ends are aligned properly before fastening procedures begin. Use a long straight edge on the belt’s edge to help keep the two belt ends in alignment. Two types of flat belt fasteners are wire lacing and steel hinge.

Wire Lacing

Figure 45 shows wire lacing used for joining flat belts. This type consists of many wires pressed into the belt, forming a series of wire loops extending beyond the ends.

The belt ends, each containing one half of the fastener, are meshed together and a pin is inserted to make the joint. Special presses are normally used to push the wires into the belt.

Figure 45: Wire Lacing Fastener

Steel Hinge

Figure 46 shows a steel hinge fastener, commonly referred to as alligator lacing. The ends of the prongs are driven into the belt. They are applied in sections running across part of the belt and a steel hinge pin is used in assembling.

Figure 46 also shows another example of steel hinge fasteners. This hinged plate consists of individual U-shaped clips, and each clip is bolted to the end of the belt. A special template is used to lay out and space the holes for each section.

Figure 46: Steel Hinge Fastener

Plate-Type Fasteners

Figure 47 shows a single plate fastener riveted to the top-side of the flat belt. The curve in the plate partially conforms to pulley curvature.

Figure 47: Plate Fastener

Also shown in Figure 47 is a plate fastener which consists of matching pairs of plates that are bolted to the belt ends, one plate on top and one on the pulley side. Special templates are used for proper hole spacing. The bolts are tightened sufficiently to compress the belt between the plates to derive maximum holding power. The protruding bolt ends are either broken or ground off.

The third example of a plate fastener shown in Figure 47 is a steel pronged hook plate, where only one plate is used on the top side and the prongs are driven through the belt and hammered over on the pulley side.

Standard Dimensions

V-belt sheaves are made to standard dimension to permit the use of various belt manufacturer’s products on the same sheave. Conventional V-belts can be identified by their standard diameter and groove dimensions.

For example, Table 14 identifies the industry standards in groove dimensions for narrow V-belt sheaves.

Table 14: Industry Standard Groove Dimensions for Narrow V-Belt Sheaves

Routine Sheave Maintenance

Worn sheaves will reduce belt life. If the grooves are worn, the belt will shift. Belts will slip and burn out, and if the sidewalls of the sheave groove are dished, as indicated in Figure 49, the bottom shoulder of the sheave will wear the bottom corner of the belt.

Figure 49: Correct Sheave Grooves

Select the proper groove gage and template for checking sheave size and groove wear. Figure 50 indicates how a conventional "D" size groove is worn.

Figure 50: Sheave Groove Gage (Conventional)

Figure 51 shows a groove gage used to check the pitch diameter and groove wear on an 8V narrow V-belt. Figure 51 also shows a belt gage used to check the size of a narrow V-belt.

Figure 51: Narrow V-Belt Sheave/ Belt Gage

Figure 52 shows how the grooves are checked for size and wear on a poly V-belt, in addition to indicating how a bell gage is used to identify the size and pitch of the V-belt.

Figure 52: Poly V-Belt Sheave Belt Gage

Checking Belt Alignment

Proper alignment is essential to maintain long V-belt and sheave life.

Figure 53 (Top) shows how a straight edge is used. Touching the driver and driven sheaves at four points indicates good alignment. Use a square positioned from the base up to the sheave face to check if the sheave is perpendicular to the base.

Figure 53 (Bottom) indicates how a tight cord can be used to check for pulley alignment. Tie the cord to the driven pulley shaft and bring it around the pulley face, bringing the cord to the driver pulley face, a quick check can be performed to see if the tight cord contacts the driver and driven pulley at four points.

Figure 53: V-Belt Sheave Alignment

Sheave Balancing

Most conventional V-belt sheaves are statically balanced and are satisfactory for rim speeds up to 6,000 FPM or 6,500 FPM for narrow V-belt sheaves.

Dynamic balancing is necessary for rim speeds beyond these, or in any application where vibration is of concern.

Sheaves for V-Belt Drives

Sheaves can be ordered single or multiple groove and they can be either cylindrical or tapered bore. Tapered bore hubs accept standard sized taper lock bushings.

Figure 54 shows a spoked cast sheave with an integral hub. Figure 55 shows a formed steel sheave, used for light duty applications. Both sheaves are locked to the shaft with set-screws.

Figure 54: Integral Hub V-Belt Sheaves

Figure 55: Integral Hub V-Belt Sheaves

Figure 56 shows how a cast-type sheave uses a removable taper lock bushing in its hub to locate the sheave on the shaft.

Figure 56: Removable Hub Cast Sheave

Figure 57 STANDARD and Figure 57 REVERSE shows two methods for positioning a taper lock bushing in a sheave.

Figure 57: Taper Lock Bushing Positioning

Taper Lock Bushing Installation

Use caution in the last step as over-tightening the bushing could cause bursting pressures to be created in the hub of the mating sheave if a lubricant is used.

Usually, lubricants are not used when installing these bushings.

  1. Thoroughly inspect the tapered surface of the bushing and the tapered bore of the sheave and remove dirt, oil, paint, and grease.
  2. Assemble the bushing. Either the standard or the reverse assembly can be mounted so the bushing flange is toward or away from the motor.
  3. Carefully align the cap screw holes of the bushing to the threaded holes in the sheave.
  4. Loosely insert the cap screws into the assembly. Do not lubricate the threads.
  5. Install the key into the shaft key-seat and slide the sheave-bushing assembly to its desired position with the cap screw heads to the outside. If it is difficult to slide the bushing onto the shaft, insert a screwdriver blade into the bushing’s saw cut to help overcome the tightness.
  6. Position the assembly on the shaft so that the belts are aligned.
  7. Tighten each cap screw evenly and progressively until obtaining the torque value recommended for the bushing by the manufacturer.
  8. A gap between the bushing flange and the mating hub should exist when installation is complete.

Taper Lock Bushing Removal

  1. Relieve the drive tension by shortening the center distance between the driver and driven sheaves.
  2. Lift off the belts.
  3. Loosen and remove the cap screws.
  4. Insert cap screws into the tapped removal holes on the bushing face and progressively tighten each one until the mating sheave is loose on the bushing. An exception to this is if the sheave is installed with the cap screw heads next to the motor.
  5. With insufficient room to insert the cap screws in the tapped holes, loosen the cap screws off and use a wedge between the bushing flange and the sheave’s hub.
  6. Remove the mating part from the bushing and, if necessary, the bushing from the shaft. Inserting a screwdriver blade in the bushing’s saw cut will help to overcome the tightness of the bushing on the shaft.
  7. Never hammer on the face of the sheave as many sheaves are cast iron and, therefore, brittle. Cracks may develop and, under high rim speeds, pieces may break off. Pieces of metal missing from the sheave only cause greater vibration problems for the drive, because imbalances are present.

Belt Installation

  1. Follow all required safety procedures regarding equipment shutdown and lockout requirements.
  2. Check to see if there are correct replacement belts in stock.
  3. Shorten the center distance between the driven and driver sheaves.
  4. Check the sheaves for wear, oil, grease, and rough spots. Use a groove gage to check for dished grooves.
  5. Lay the belts into the sheave grooves without using force or prying them on.
  6. While the belts are loose on the drive, rotate the drive manually until the slack is evenly distributed on both sides.
  7. Increase the center distance until the belts are snug.
  8. Tension the belts to proper specifications.

Worn Belt Removal

  1. Lockout the equipment and make sure that all drive components are in a safe position.
  2. Shorten up the center distances between the driver and driven unit. If the drive uses an idler for belt tensioning, loosen it off and swing it out of the way.
  3. Remove each belt from the sheave one at a time.
  4. Do not pry belts off the drive or attempt to roll off by rotating the sheaves. Fingers or hands can be severely pinched or cut by attempting to do this.

Table 15 identifies slack-off and take-up allowance recommendations for poly V-Belts, conventional, and narrow V-Belts. Less slack-off and take-up allowances are required for poly V-Belts. Conventional V-Belts require the greatest amount of slack-off and take-up movement.

Table 15: Poly V-Belt Slack-off and Take-Up Allowance

Belt Tensioning Motor Bases

Motor bases for belt drives must be sturdy and able to be mounted in any position. Figure 58 shows an adjustable sliding motor base.

Figure 58: Adjustable Motor Base

When adjustment is performed on this base, belt alignment is maintained. The screw threads and the slide ways should be kept clean.

Figure 59 A is also a sliding method for belt alignment. On this example, the motor base bolts are loosened and the motor is pushed or pulled by hand, or by using jacking bolts mounted to the unit’s base.

Figure 59 B uses a pivoting cradle for belt adjustment, while Figure 59 C uses a pivot point at one end of the motor frame. Figure 59 D uses a spring tension take-up for belt adjustment.

Figure 59: Tensioning Motor Bases

Troubleshooting Belt Drives

Causes of Shortened Belt Life

  • Excess belt stretch results from misaligned sheaves or greatly overloaded and under-designed drives.
  • Damaged or broken tensile members due to improper belt installation.
  • Worn sheave grooves.
  • Oil or grease on belts makes the belt sidewalls sticky or swells the belt’s cross-section.
  • Deterioration of the rubber compounds caused by the softening effects of belt dressings.
  • Belts rubbing against guards and frames often results in heat and abrasion damage to the cover section.
  • Belts slip under starting loads, resulting in visible spin burn marks on the belt.
  • Sheaves that are too small may cause the bottom of the belt to crack.
  • Excess lateral whip causes belts to turn over.
  • Misaligned sheaves may cause early belt wear, belts to turnover, and wear on sheave grooves.
  • Dirty and dusty sheave grooves cause belts to track poorly, whip, and vibrate. Squeaking and chirping noises are usually associated with dirty sheave conditions. Do not apply belt dressing or oil to the belt to eliminate the noise.