HYDRAULIC COMPONENTS II
To accomplish work, hydraulic systems require some means to provide flow of fluid. Pumps are used to provide this requirement in hydraulic systems. A hydraulic pump is a device that converts mechanical force and motion into hydraulic energy. Many different sources are used to provide the mechanical power to the pump. Electric motors, air motors, and gasoline engines are all used as a source of power to operate hydraulic pumps. Some pumps are manually operated.
The purpose of a hydraulic pump is to supply a flow of fluid to a hydraulic system. It should be emphasized that a pump does not create pressure, since pressure can only be created by a resistance to flow. As it provides flow, it transmits a force to the fluid. As a result, a pump can produce the flow and force necessary for the development of pressure. Since it cannot provide resistance to its own flow, a pump alone cannot produce pressure.
Resistance to flow is the result of a restriction or obstruction in the path of flow. This restriction is normally the work accomplished by the system, but can also be restrictions in lines, fittings, and valves within the system. Thus, the load imposed on the system or the action of a pressure-regulating device controls the pressure.
For a pump to supply a flow of fluid to the pump outlet and into the system, it must have a continuous supply of fluid available to the inlet port. Atmospheric pressure plays an important role in the supply of fluid to the inlet port. If the pump is located at a level lower than that of the reservoir, the force of gravity supplements atmospheric pressure. However, in most systems the pump is located above the reservoir.
As the pump forces fluid through the outlet port, a partial vacuum or low-pressure area is created at the inlet port. This low-pressure area contains a pressure lower than the surrounding atmospheric pressure. When this condition exists the atmospheric pressure acting on the fluid in the reservoir forces the fluid into the pump.
Many different methods are used to classify pumps. Terms such as non-positive displacement, positive displacement, fixed displacement, variable displacement, fixed delivery, and variable delivery are used to describe pumps. The first of these terms, non-positive and positive displacement, describe the fundamental division of pumps.
Positive displacement pumps are the most widely used in hydraulic systems. The term positive displacement can be defined as "those pumps whose discharge volumes are separated by a period of no discharge." This type of pump delivers a definite volume of fluid for each cycle of pump operation, regardless of the resistance offered. If the outlet of the pump was closed, the pressure would increase until the driver stalled or something broke.
Positive displacement-type pumps as classified as either rotary or reciprocating.
All rotary pumps operate by means of rotating parts that trap the fluid at the inlet port and force it through the discharge port into the system. Gears, screws, lobe, vanes, and pistons are all commonly used as elements in rotary pumps.
Rotary pumps are designed with very small clearances between rotating parts and stationary parts to minimize slippage from the discharge side back to the suction side. Pumps of this type are designed to operate at relatively moderate speeds to maintain these clearances. Operation at higher speeds cause erosion and excessive wear, resulting in increased clearances.
Rotary pumps can be further classified as one of the following types:
A gear-type pump is a power-driven unit having two or more intermeshing gears or lobed members enclosed in a suitably shaped housing. Gear-type pumps are positive delivery, and their delivery rate can be changed only by changing the speed at which the pump shaft revolves. The efficiency of this pump is determined largely by the accuracy with which component parts are machined and fitted.
Gear pumps are available in a wide range of ratings, from less than one gpm to over a hundred gpm for pressures less than 100 psi to over 3,000 psi.
Figure 1 shows an external gear pump. The two gears fit closely inside the housing. The hydraulic fluid is carried around the periphery of the two gears and it is then forced through the outlet port by the meshing of the two gears at their point of tangency.
Figure 1: External Gear Pump
A typical internal gear pump is shown in Figure 2. In the internal design, the pumping chambers are also formed by the gear teeth. A crescent seal is machined into the valve body between the inlet and the outlet where clearance between the teeth is at a maximum.
Also included in the gear pump family is the lobe or rotor pump. This pump operates on the same principle as the external gear pump, but has a higher displacement. Figure 3 shows a lobe pump used in hydraulic systems.
Figure 3: Hydraulic Lobe Pump
In the rotary vane-type pump, operation is based on the principle of increasing the size of the cavity to form a vacuum, allowing the space to fill with fluid, and then forcing the fluid out of the pump under pressure by diminishing the volume. Figure 4 shows the internals of a typical rotary vane-type pump.
Figure 4: Vane-Type Rotary Pump
The sliding vanes or blades fit into the slots in the rotor. Ahead of the slots and in the direction of rotation, grooves admit the liquid being pumped by the vanes, moving them outward with a force or locking pressure that varies directly with the pressure the pump is operating against. The grooves also serve to break the vacuum on the admission side. The operating cycle and the alternate action of centrifugal force and hydraulic pressure hold the vanes in contact with the casing.
Rotary piston-type pumps are either radial or axial in design. Each of these pumps may be designed for either constant displacement or variable displacement. The pistons are arranged radially around the rotor hub in the radial pump. As shown in Figure 5, the slide block is at the right-hand side of the centerline of the cylinder barrel. Reciprocating motion is imparted to the pistons, so that those pistons passing over the lower port of the pin deliver oil to that port while the pistons passing over the upper port are filling with oil. The delivery of the pump can be controlled accurately from zero to maximum capacity, because the piston and movement of the slide block can be controlled accurately.
Figure 5: Radial Piston-Type Rotary Pump
Axial piston pumps are available in two basic designs: the in-line style and the bent axis style. The simplest type is the in-line style. In this type, the drive shaft and cylinder block are on the same centerline, and the pistons reciprocate parallel to the drive shaft. Figure 6 shows an in-line axial piston pump.
Figure 6: In-Line Axial Piston-Type Pump
The cylinder block in this pump is turned by the drive shaft. Pistons fitted to bores in the cylinder are connected through piston shoes and a retracting ring, so that the shoes bear against an angled swash plate. As the block turns, the pistons follow the swash plate, causing the pistons to reciprocate. The ports are arranged in the valve plate so that the pistons pass the inlet as they are being pulled out and pass the outlet as they are being forced back in.
In a bent axis piston pump, the cylinder block turns with the drive shaft, but at an offset angle. The piston rods are attached to the drive shaft flange by ball joints, and are forced in and out of their bores as the distance between the drive shaft flange and cylinder block changes. A universal link keys the cylinder block to the drive shaft to maintain alignment and assure they turn together. The link does not transmit force except to accelerate and decelerate the cylinder block and to overcome resistance of the block revolving in the oil filled housing. Figure 7 shows a bent axis axial piston pump.
Figure 7: Bent-Axis Axial Type Piston Pump
The term reciprocating is defined as back and forth motion. In the reciprocating pump, it is this back and forth motion of pistons inside of cylinders that provides the flow of fluid. Reciprocating pumps, like rotary pumps, operate on the positive principle; that is, each stroke delivers a definite volume of liquid to the system.
The master cylinder of an automobile brake system is an example of a simple reciprocating pump. Most manually operated pumps are of the reciprocating type. Radial and axial piston rotary pumps are sometimes classified as reciprocating pumps because the actual pumping is performed by sets of pistons reciprocating inside sets of cylinders.
There are two types of reciprocating pumps: single-acting and double-acting. A single-acting pump provides flow during every other stroke, while the double-acting provides flow during each stroke.
In a single-acting pump, the fluid is forced out of the cylinder by means of a piston or plunger working against a pressure that corresponds to the head or elevation above the inlet valve to which the fluid is being pumped. In its simplest form, the pump consists of an inlet valve, a discharge valve, and a single acting plunger. Figure 8 shows an example of a single-acting pump.
Figure 8: Single-Acting Reciprocating Pump
When the system is cleared of air and in operation, the working cycle is completed in two strokes of the piston, a downward or transfer stroke and an upward or discharge stroke.
In a double-acting piston-type pump, the piston discharges fluid on one side of the piston while drawing fluid into the cylinder on the other side, without a transfer stroke. Thus, fluid is discharged on every stroke, rather than on every other stroke as in the single-acting pump. Therefore, the capacity of a single-acting pump can be doubled in a double-acting pump having an identical cylinder displacement. The basic construction of a double-acting piston-type pump is shown in Figure 9.
Figure 9: Double-Acting Reciprocating Pump
Control valves that are used in hydraulic systems can be divided into three different categories:
Directional controls are used to direct the hydraulic fluid to various passages in the system. Flow controls regulate the speed at which the hydraulic fluid is permitted to flow. This, in turn, can control such things as piston speed, movement of valve spools, and actuation speeds. Pressure controls regulate the pressure intensity in various portions of the system.
Directional control valves are designed for the specific purpose of directing the flow of fluid, at the desired time, to the point in a fluid power system where the fluid is applied to accomplish work. It may be desired, for example, to perform a work operation by driving a piston or ram back and forth in its cylinder. A directional control valve, which functions alternately to admit fluid to and from each end of the cylinder, is used to make this operation possible.
Directional control valves may be operated by differences of pressure acting on opposite sides of the valving element, or they may be positioned manually, mechanically, or electrically. Often two or more methods of operating the same valve will be used in different phases of its action.
A sliding spool is probably the most common type of valving element used in directional control valves. The valve is so named because the shape of the valving element resembles that of a spool and because the valving element slides back and forth to block and uncover ports in the housing. Some manufacturers refer to this element as a piston-type. The inner piston (lands) areas are equal. Thus, fluid under pressure, which enters the valve from the inlet ports, acts equally on both inner piston areas regardless of the position of the spool.
Sealing is usually accomplished by a very closely machined fit between the spool and the valve body or sleeve and O-rings and backing rings. For valves with more ports, the spool is designed with more pistons or lands on a common shaft.
Like all classes of directional control valves, various methods are used for positioning the sliding spool valve.
Most actuating devices require system pressure for operation in either direction. A four-way directional control valve, which contains four ports, is used to control the operation of such devices. A four-way valve is also used in some systems to control the operation of other valves. With the exception of the check valve, a four-way valve is the most widely used directional control valve in fluid power systems.
A typical four-way directional control valve has four ports - a pressure port, a return or exhaust port, and two cylinder (or working) ports.
The pressure port is connected to the main system pressure line and, in hydraulic systems, the return port is connected to the reservoir. In pneumatic systems, the return port is usually vented to the atmosphere and, therefore, is referred to as the exhaust port. Lines to the actuating units connect the two cylinder ports.
The four basic configurations for the four-way directional control valves are:
In the following explanations of four-way directional control valves, the inlet cavity is referred to as the center and the two cavities that lead to the cylinder are called ports. One of the cylinders is called the base and the other the rod end. Figure 10 shows the parts of a four-way directional control valve.
Figure 10: Parts of a Four-way Directional Control Valve
Figure 11 shows an open center, closed port four-way directional control valve. It is called open center because when it is in the neutral position, pump flow is allowed to pass through the center of the valve and return to the tank. It is called closed port because, in neutral, oil cannot leave or enter either side of the cylinder.
Figure 11: Four-Way Directional Control Valve (Open Center, Closed Port)
When the spool is moved to the right, pump flow can no longer travel directly to the tank. Instead, it is directed to the base end of the cylinder, causing the rod to extend. As the rod extends, oil is exhausted from the rod end of the cylinder. This exhaust oil flows to the control valve where it is directed to the tank.
When the spool is moved to the left, pump flow is sent to the rod end of the cylinder. Oil exhausting from the base end of the cylinder is returned to the valve where it is directed back to the tank.
In Figure 12, the flow path for an open center, open port four-way directional control valve. It is called open center because in neutral, pump flow is allowed to flow through the center of the valve and return to the tank. It is open port because in neutral, both cylinder ports are open to the tank.
Figure 12: Four-way Directional Control Valve (Open Center, Open Port)
When the spool is moved to the right, pump flow cannot travel directly to the tank. Instead, it is directed to the base end of the cylinder, causing the rod to extend. Oil exhausting from the other end of the cylinder returns to the valve where it is directed to the tank.
When the spool is moved to the left, pump flow is directed to the rod end of the cylinder. The base end is exhausting oil through the valve back to the tank.
The closed center, closed port configuration is referred to this way due to the fact that in neutral, pump flow is blocked at the entrance to the valve (Figure 13). Oil cannot enter or leave the cylinder.
Figure 13: Four-way Directional Control Valve (Closed Center, Closed Port)
If the spool is moved to the right, oil is directed to the base end, extending the rod. At this time, oil from the rod end is directed to the tank.
If the spool is moved to the left, oil is directed to retract the rod while oil in the base end is directed to the tank.
The last configuration is the closed center, open port (Figure 14). When this valve is in neutral, pump flow is blocked at the entrance to the valve and both ports are open to allow oil to enter or leave the cylinder. When the spool moves to the right, pump flow is directed to the base of the cylinder, while exhaust oil from the rod end is directed to the tank. Moving the spool to the left directs pump flow to the rod end and exhaust flow from the base to the tank.
Figure 14: Four-way Directional Control Valve (Closed Center Open Port)
Check valves are sometimes classified as flow control valves. However, since they permit flow in one direction and prevent flow in the other direction, they are usually classified as a one-way directional control valve. Check valves are probably the most widely used valve in hydraulic systems.
For the most part, check valves are installed independently in a line to allow flow in only one direction. Figure 15 shows this type of check valve.
This type of check valve is nothing more than a spring-loaded disc, which opens to allow the fluid to flow in one direction when fluid pressure overcomes spring tension. When flow comes from the other direction, the spring seats the disc and prevents fluid flow. This is referred to as a blocked flow condition.
Another commonly used check valve in hydraulic systems is the pilot-operated check valve. This is used in applications when flow is desired to be in both directions at specific moments but only allowed to flow in one direction at other times. This valve sequence is shown in Figure 16.
Figure 16: Pilot Operated Check Valve
When there is no pressure directed to pilot the piston, the check is closed and flow is blocked. When pressure activates the pilot piston, it opens the check and free flow is allowed.
Flow control valves are used to limit the amount of flow to a particular component. Figure 17 shows a needle-type flow control valve, which is the simplest type and most common flow control valve.
Flow is restricted as the fluid moves through the narrow space between the needle and the seat. The space can be increased, thus increasing flow, by turning the needle valve out. Flow is infinitely variable from fully closed to the fully open position.
Unlike manually and mechanically operated controls, only a limited amount of power is available to actuate the valve mechanism of electrically operated valves, and this power must be used to the best possible advantage. The operating means in an electrically operated valve may be an electric motor or a solenoid. An electric motor is seldom used in industrial hydraulics as an operating means.
Solenoids are often used to operate valves. A solenoid is an electrical device that converts electrical energy into straight-line motion and force, as shown in Figure 18. A solenoid consists of a coil of wire mounted on a soft-iron spool. The force developed is different at various points along the solenoid plunger travel.
Figure 18: Solenoid
Solenoids are also used to operate a mechanical operator which, in turn, operates the valve mechanism. As indicated in Figure 19, solenoids may be connected directly to the valve mechanism. Solenoids may be of the push-type, pull-type, or push-pull type. The push-type solenoid is one in which the plunger is pushed when the solenoid is energized with electricity, and the pull-type solenoid is one in which the plunger is pulled when the solenoid is energized.
Figure 19: Solenoid Plunger Connections
Figure 20: Typical Two-Way Solenoid Valve
The solenoid coil is made of copper wire. Insulating paper separates the layers of wire. The entire solenoid coil is covered with a varnish that is not affected by solvents, moisture, cutting oil, or other fluids. Coils are rated in various voltages.
A solenoid frame serves several purposes:
The solenoid plunger is the moving mechanism of the solenoid. The plunger is made of steel laminations that are riveted together under high pressure, so that there will be no movement of the laminations with respect to one another. At the top of the plunger, a pinhole is placed for making a connection to some device. The solenoid plunger is moved by a magnetic force in one direction, and is usually returned by spring action.
Solenoid-operated valves are usually provided with a cover over either the solenoid or the entire valve. This protects the solenoid from dirt and other foreign matter, and protects the actuator.
Most solenoid valves are of the smaller types, and the solenoid plunger is connected directly to, or is a part of, the valve operating mechanism. Frequently, these valves are used for operation of pilot-operated controls.
An actuating cylinder is a device that converts hydraulic power to linear or straight-line force and motion. Since linear motion is a back and forth motion along a straight line, this type of actuator is sometimes referred to as a reciprocating or linear motor. The cylinder consists of a ram or piston operating within a cylindrical bore. Actuating cylinders are normally installed in such a manner that the cylinder is anchored to a stationary structure and the ram or piston is attached to the mechanism to be operated.
Although the terms ram and piston are often interchangeable, a ram-type cylinder is usually considered one in which the cross-sectional area of the piston rod is more than one-half that of the movable element.
A ram-type actuator is used primarily for push functions rather than pull. Most applications require simply a flat surface on the external part of the ram for pushing or lifting the unit to be operated. The design of ram-type cylinders will vary to satisfy the requirements of different applications. The two types most commonly used in hydraulic systems are single-acting ram and double-acting ram.
A single-acting ram applies force in only one direction. Fluid directed into the cylinder displaces the ram and forces it outward. Since there is no provision for retracting the ram by the use of hydraulics the retracting force can be gravity, or some mechanical means such as a spring. This type of cylinder is often used in the hydraulic jack. Figure 21 shows a single-acting ram-type cylinder.
Figure 21: Single-Acting Ram-Type Cylinder
In this type of actuating cylinder, fluid pressure is used to force the ram outward and lift the object. When fluid pressure is released, the weight of the object and gravity force the ram into the cylinder. This action will force the fluid back to the reservoir.
In the operation of the double-acting ram, both strokes of the ram are produced by pressurized fluid. There are two fluid ports, one located at or near each end of the cylinder. Fluid under pressure is directed to the closed end to extend the ram and apply force. To retract the ram and reduce force, fluid is directed to the opposite end of the cylinder. A four-way directional control valve is normally used to control the flow of fluid in this type of ram. Figure 22 shows a double-acting ram-type cylinder.
Figure 22: Double-Acting Ram-Type Cylinder
An actuating cylinder in which the cross-sectional area of the piston rod is less than half that of the movable element is referred to as a piston-type cylinder. This type of cylinder is normally used for applications that require both push and pull functions. For this reason, the piston-type serves many more requirements than the ram-type, and is the most commonly used in hydraulic systems.
The housing consists of a cylindrical barrel that usually contains either external or internal threads on both ends. End caps with mating threads are attached to the ends of the barrel. These end caps usually contain the fluid ports. The end cap on the rod end contains a hole for the piston rod to pass through. Suitable packing must be used between the hole and the piston rod to prevent external leakage and the entrance of dirt and other contaminants.
The piston rods may extend through either or both end of the cylinder depending on application. To satisfy the many requirements of hydraulic systems, piston-type cylinders are varied in their design. The most common for use in hydraulic systems are:
The single-acting piston-type cylinder is similar in design and operation to the single-acting ram-type. It uses fluid pressure to apply force in only one direction. In some designs of this type, gravity moves the piston in the opposite direction; however, most cylinders of this type apply force in both directions. Fluid pressure provides the force in one direction and spring tension provides force in the opposite direction. In some applications, compressed air or nitrogen is used to provide this force.
Figure 23 shows the typical arrangement for a single-acting piston cylinder using a spring for the necessary operating force. In most applications, the spring will be located on the rod end, although according to the particular use, it could be located on the piston end.
Most piston-type actuating cylinders are double-acting, meaning that fluid under pressure can be applied to either side of the piston to provide movement and apply force in the corresponding direction.
As shown in Figure 24, the cylinder contains one piston and piston rod assembly. The stroke of the piston in either direction is produced by fluid pressure. The two fluid ports alternate as inlet and outlet depending on the direction of flow from the directional control valve.
This cylinder is referred to as an unbalanced or differential pressure cylinder because of the difference of the effective working areas on the two sides of the piston.
This type of actuating cylinder is used in applications where it is necessary to move two mechanisms at the same time. This eliminates the need for using two separate control valves and cylinders. The cylinder contains two pistons and two rod assemblies. Figure 25 shows a double-acting, three-port cylinder.
Fluid under pressure is directed through port A by a control valve and moves the pistons outward. The fluid on the rod side of each piston is forced out of the cylinder through ports B and C, which are connected by a common line to the control valve.
When fluid under pressure is directed into the cylinder through ports B and C, the two pistons move inward. Fluid between the two pistons is free to flow from the cylinder through port A and through the control valve to the reservoir.
Figure 26 shows a double-acting balanced-type cylinder. The piston rod extends through the piston and out through both ends of the cylinder. One or both ends of the piston rod may be attached to a mechanism to be actuated. In either case, the cylinder provides equal areas on each side of the piston so that the amount of fluid and force required to move the piston in one direction is exactly the same as the amount required to move it an equal distance in the opposite direction.
Some hydraulic system applications require two or more independent systems to be able to operate the same mechanism for increased reliability. A tandem actuating cylinder is commonly used in this type of system.
Tandem is defined as a group of two or more arranged one behind the other. The tandem cylinder consists of two or more cylinders arranged one behind the other but designed as a single unit. As shown in Figure 27, this is actually two double-acting, balanced cylinders with two pistons connected two a common shaft.
To slow the action and prevent shock at the end of the piston stroke, some cylinders incorporate a cushioning device to slow the movement of the piston during part of its stroke. This cushion is usually a metering device and/or a check valve built into the cylinder to restrict flow at the outlet port so as to slow down the movement of the piston. Figure 28 shows a cushioned cylinder.
Figure 28: Cushioned Actuating Cylinder
This section discusses the shear line, MEP machine, and EVG machine system operation and maintenance, and some common causes of hydraulic system failures.
The purpose of the shear line hydraulic system is to provide hydraulic power to move the shear line table back and forth during shearing operations.
The shear line hydraulic power unit consists of a one-horsepower, 1,800 RPM motor, hydraulic pump, and double solenoid valve. Flexible hoses connect the valve to the positioning cylinder. An over-pressure relief valve is installed to prevent damage to hydraulic components. Filters are installed at the suction of the pump and in the oil return line to filter out foreign matter. A variable flow restrictor is installed to adjust the time of cylinder retraction. Cylinder retraction time and cylinder extension time should match (approximately 15 seconds).
Figure 29 shows the shear line hydraulic system diagram. Oil from the reservoir flows into the pump. The pump, driven by an electric motor, increases the pressure of the oil and sends the high-pressure oil to the solenoid control valve. Based on the signal received from the control panel, the solenoid control valve is positioned so that oil flows to the cylinder, and moves the cylinder to the desired position.
Figure 29: Shear Line Hydraulic System
Oil that enters the cylinder acts on the piston. This pressure action over the area of the piston develops a force on the piston rod. The force of the piston rod is used to position the shear line table. Oil from the opposite side of the cylinder leaves the cylinder and returns to the solenoid control valve which, in turn, sends the oil back to the return filter. As the oil passes through the filter, dirt and foreign matter are removed from the oil. The oil then returns to the reservoir.
The purpose of the MEP hydraulic system is to provide hydraulic power to feed, bend, and cut the rebar.
The MEP hydraulic power unit consists of a three-phase, asynchronous electric motor that drives two hydraulic pumps. One hydraulic pump supplies oil to the bend, feed, and shear units while the other pump supplies oil for the center form/tool and castle. This system also uses two hydraulic motors. One hydraulic motor is used for the wire feed unit and the other is used for the bending unit. There are three hydraulic cylinders in this system. These cylinders are used for the shear unit, center form/tool unit, and the bending castle unit.
There are five double-solenoid valves, one for each hydraulic motor and one for each hydraulic cylinder. There are two overpressure relief valves installed in this system, one in each pump discharge line, to prevent damage to the hydraulic components. Filters are installed at the suction of each pump and in the oil return line to filter out foreign matter. This system uses two oil flow adjustment valves, one for each hydraulic motor, to control the flow of oil to each hydraulic motor. Figure 30 shows the MEP hydraulic system diagram.
Oil from the reservoir flows into both pumps. The hydraulic pump for the bend-feed and shear units increases the pressure of the oil and sends the high-pressure oil to the solenoid control valve for the wire feed hydraulic motor. Based on the signal received from the control panel, the solenoid control valve is positioned so that oil flows to the wire feed hydraulic motor, allowing it to feed wire as needed.
Oil then flows back to the solenoid control valve and then to the solenoid control valve for the bending unit hydraulic motor. Based on the signal received from the control panel, the solenoid control valve is positioned so that oil flows to the Bending unit hydraulic motor, so that the desired bend can be achieved. Oil then flows back to the solenoid control valve and then to the solenoid control valve for the shear cylinder.
Based on the signal received from the control panel, the solenoid control valve is positioned so that the shear cylinder enables the machine to make the desired cut. Oil then flows back to the solenoid control valve, through a filter, and returns to the oil reservoir. An overpressure valve senses pressure from the discharge line of the pump to prevent overpressurizing the system.
The center form/tool and castle hydraulic pump takes suction from the reservoir, independent of the bend, feed, and shear hydraulic pump. The hydraulic pump increases the pressure of the oil and sends the high-pressure oil to the solenoid control valve for the center form/tool cylinder. Based on the signal received from the control panel, the solenoid control valve is positioned so that oil flows to the center form/tool cylinder, so that the wire is held before bending the wire.
Oil then flows back to the solenoid control valve and then on to the solenoid control valve for the Bending castle cylinder. Based on the signal received from the control panel, the solenoid control valve is positioned so that oil flows to the Bending Castle cylinder to position the turret for bending (this is taking place simultaneously to the Bending unit hydraulic motor bending the wire).
Oil then flows back to the solenoid control valve and joins with the oil return line from the other pump. Next, the oil flows through the same return filter and returns to the oil reservoir. An overpressure valve senses pressure from the discharge line of the pump, to prevent over-pressurizing the system.
The actual operation of each component happens in the following order:
The oil filter placed on the hydraulic unit contains an external obstruction indicator that shows the internal condition of the cartridge and whether it is necessary to clean it or replace it.
General maintenance for the MEP hydraulic system is as described below.
The purpose of the EVG hydraulic system is to provide hydraulic power to perform the following functions:
Figure 31 shows the EVG hydraulic system diagram. For a list of EVG hydraulic components, refer to the EVG hydraulic system diagram legend for Figure 31.
The following numbered components represent the EVG hydraulic system components shown in Figure 31.
EGV Hydraulic System Operation
The hydraulic power unit consists of an electric motor, which drives a hydraulic pump. The hydraulic pump draws suction from the oil reservoir, and supplies oil to the large accumulator, the directional control valve for the operating MOOG servo valves, and into Flow line #1. The oil from the directional control valve is then directed to the following components (which will be called Flow Lines # 2 and # 3):
Oil is directed into this line just prior to the directional control valve, supplies the small accumulator, and continues to the regulating valve for the transport wheels up/down line. From there, it goes through the solenoid valve and to the transport wheels up/down cylinder. The oil discharged from the cylinder returns to a common line. Continuing with the oil supply line, the next component supplied is the regulating valve for the constant and stop brakes. From there, it goes through the solenoid valve and to the constant brake and stop brake units. The oil discharged from these units return to the same common line.
Oil from the common line is then directed two ways. The first direction is to the solenoid valve for determining high/low speed straightening motors. Once this is determined, the oil goes to four separate solenoid control valves (one for each hydraulic motor) and then to the four hydraulic motors themselves. Oil returning from the hydraulic motors returns to the oil reservoir.
The second direction the oil takes goes to the solenoid valves for the bending cylinder, cover cylinder, straightener unit, and hydraulic motors. The oil goes from each solenoid control valve to its respective component and returns to a common line. In this common line, there is another accumulator to keep pressure on the straighteners. The oil continues to the solenoid control valve for the shear cylinder and to the shear cylinder itself. Oil returning from the shear cylinder returns to the oil reservoir.
Oil flows to the MOOG servo valves for the bending cylinder and the up/down bending cylinders. From one of the MOOG servo valves, oil is directed to bending cylinder and, from the other MOOG servo valve, the oil is directed to the two up/down bending cylinders. The oil returning from the cylinders is directed back to the oil reservoir.
Oil from the directional control valve is delivered to the solenoid valve and the MOOG servo valve for the transport system high/low speed hydraulic motors which, in turn, goes to the motors themselves. Oil returning from use in these components is also directed back to the oil reservoir.
Aging of oil is dependent on several operating parameters such as temperature, pressure, humidity, and pollution.
Oil condition and aging and its continued usability can be roughly judged by a visual check. Table 1 gives guidelines for visually checking hydraulic oil. Therefore, periodically check hydraulic oil for dirt, aging, and moisture.
The maximum of 60&176;C is recommended for mineral oil-based fluids since higher temperatures accelerate the aging of the fluid and shorten the life of seals and hoses. The best working temperature for the oil is 50&176;C.
The oil temperature in the reservoir must be monitored continuously. A gradual rise in temperature is a possible indication of contamination or gumming, or of wear at seals or metal components, and should be taken as a sign to examine all components that might be affected. Sudden sharp increases in temperature are an alarm signal, and the system should be shutdown immediately for inspection.
Dirty oil substantially reduces the function and lifetime of all hydraulic components. Oil should be changed once a year or after 3,000 hours of service. However, it is necessary to observe that the maximum working temperature is 60°C and that the filters must be changed periodically.
The intervals of oil changes may be extended if the oil is serviced according to the technical manual instructions.
After the oil change, the contents of the tank must be filtered by flushing. This is done through an 8-hour flushing process.
If the maximum service life of the filter cartridges has not been achieved and the dirt indicator does not respond, the filter cartridges may be used further.
These elements filter the air in and out of the oil reservoir as the level changes. The frequency of inspection and element changes depends on the environmental conditions in which the machine operates. As dusty conditions exist in the rebar fab shop, they need to be checked periodically. The vent filter must be replaced at least once a year or after every 3,000 hours of service.
The filter cartridges must be replaced once a year or after every 3,000 hours of service at the latest. The cartridges must also be replaced if the dirt indicator signals when the oil is warm.
Clogging of the filter is indicated by the warning signal "filter clogged," and by the mechanical indicator found directly on the filter.
When the oil is cold, the electric indication may respond in spite of perfect filtering. However, if the oil is at normal operating temperature, the filter must be replaced.
Clean dirty cooling ribs weekly by means of compressed air or an appropriate cleaning solution.
As with any mechanical system, failures can occur in hydraulic systems. These common causes of failure must be kept in mind when servicing system to prevent possible damage or improper operation.
Without a doubt, dirt causes more hydraulic components to fail than any other single cause. Dirt also includes foreign substances. In a hydraulic system, dirt and foreign matter may cause excessive damage to the components, since the fits between the parts are held to very close limits. Dirt not only scores the parts but often causes valve spools to stick and become inoperable. Dirt sometimes becomes lodged between the piston, piston ring, and tube of the hydraulic cylinder, causing the piston ring to be broken. This, in turn, can cause the tube to become badly scored.
Dirt clogs small orifices in valves, causing them to malfunction. Dirt tears the rod packing and causes external leakage of the fluid. Foreign matter also causes pitting of the piston rods and valve stems. Foreign matter, such as hydro-carbons, may clog intake strainers and cause carbons or cavitation in the pump. Intake strainers have been known to collapse due to collection of foreign matter. Dirt can cause a pump to seize and the driving means twist off the pump shaft.
Cutting oils and coolants sometimes get into the hydraulic oil, causing considerable corrosion within the system and failure of the components. Every precaution should be taken to keep these solutions out of the system.
Heat causes considerable trouble to the components of a hydraulic system. It may cause valve spools to stick, packing to deteriorate, oil to break down, deposits to cling to the finished surfaces, excessive external and internal leakage, and inaccurate feeds in the system.
Hydraulic systems must be protected from hot blasts. If heat is caused by internal conditions, install after-coolers and, if possible, correct the condition causing the heat. Some causes of high heat are ambient temperatures, restrictions in lines and components, high pressures, and high pressures being spilled through relief valves.
Misapplication causes many failures of hydraulic system components. The selection of the improper component as to capacity, ability to withstand shock loads, or ability to withstand certain other operating conditions may cause failure. The use of a pneumatic valve for high-pressure oil service is likely to cause trouble. The use of a cylinder with thin cast iron covers for heavy-duty mill applications is another example.
Care should be used in selecting the fluid for use in a hydraulic system. Check the pump manufacturers recommendations. If the oil is satisfactory for the pump, it is likely that it will be fine for other components of the system.
As discussed previously, certain hydraulic fluids have detrimental effects on seals, packing, paint, and strainers. If these fluids are used, provisions must be made accordingly. Mixing of hydraulic fluids is not recommended as one of the fluids may have a property that is detrimental to the other. Fluids that cause deposits or corrosive action should not be used in hydraulic systems.
Poor maintenance can often be a cause for hydraulic system failures. A regular maintenance program can reduce these failures. The oil should be changed at regular intervals. Then the system should be cleaned and checked for leaks. Change packing and seals when necessary and never allow dirt to accumulate around the system.
Faulty installation may contribute to many hydraulic failures. Some more common installation problems are:
Careful planning and the ability to produce good workmanship is a prerequisite to satisfactory installation. Remember that precision equipment, often very costly for one single component, is being installed.