Category Archives: Hydraulic Actuators

Rack and Pinion Rotary Actuator

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referred to as limited rotation cylinders, of the single or multiple, bidirectional piston are used for turning, positioning, steering, opening and closing, swinging, or any other mechanical function involving restricted rotation. Figure 10-10 shows a typical rack-and-pinion double piston actuator.

The actuator consists of a body and two reciprocating pistons with an integral rack for rotating the shaft mounted in roller or journal bearings. The shaft and bearings are located in a central position and are enclosed with a bearing cap. The pistons, one on each side of the rack, are enclosed in cylinders machined or sleeved into the body. The body is enclosed with end caps and static seals to prevent external leakage of pressurized fluid.

Only a few of the many applications of actuating cylinders were discussed in the preceding paragraphs. Figure 10-11 shows additional types of force and motion applications.

In addition to its versatility, the cylinder-type actuator is probably the most trouble-free component of fluid power systems. However, it is very important that the cylinder, mechanical linkage, and actuating unit are correctly aligned.
Any misalignment will cause excessive wear of the piston, piston rod, and seals. Also, proper adjustment between the piston rod and the actuating unit must be maintained.

Hydraulic Single Acting Piston Cylinders

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The single-acting piston-type cylinder is similar in design and operation to the single-acting ram-type cylinder. The single-acting piston-type cylinder uses fluid pressure to provide the force in one direction, and spring tension, gravity,
compressed air, or nitrogen is used to provide the force in the opposite direction. Figure 10-5 shows a single-acting, spring-loaded, piston-type actuating cylinder. In this cylinder the spring is located on the rod side of the piston. In some spring-loaded cylinders the spring is located on the blank side, and the fluid port is on the rod side of the cylinder.

A three-way directional control valve is normally used to control the operation of the single-acting piston-type cylinder. To extend the piston rod, fluid under pressure is directed through the port into the cylinder (fig. 10-5). This
pressure acts on the surface area of the blank side of the piston and forces the piston to the right. This action moves the rod to the right, through the end of the cylinder, thus moving the actuated unit in one direction. During this action, the spring is compressed between the rod side of the piston and the end of the cylinder. The length of the stroke depends upon the physical limits within the cylinder and the required movement of the actuated unit.

To retract the piston rod, the directional control valve is moved to the opposite working position, which releases the pressure in the cylinder. The spring tension forces the piston to the left, retracting the piston rod and moving the actuated unit in the opposite direction. The fluid is free to flow from the cylinder through the port, back through the control valve to the return line in hydraulic systems or to the atmosphere in pneumatic systems.

The end of the cylinder opposite the fluid port is vented to the atmosphere. This prevents air from being trapped in this area. Any trapped air would compress during the extension stroke, creating excess pressure on the rod side of the piston. This would cause sluggish movement of the piston and could eventually cause a complete lock, preventing the fluid pressure from moving the piston.

The spring-loaded cylinder is used in arresting gear systems on some models of carrier aircraft. To raise (retract) the arresting hook, fluid pressure is directed through the arresting hook control valve to the rod side of the cylinder. This force moves the piston, which, through the rod and mechanical linkage, retracts the arresting hook. The arresting hook extends when fluid pressure is released from the rod side of the cylinder, allowing the spring to expand.

Leakage between the cylinder wall and piston is prevented by adequate seals. The piston in figure 10-5 contains V-ring seals.

Hydraulic Piston Cylinders

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An actuating cylinder in which the cross sectional area of the piston is less than one-half the cross-sectional area 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. The piston type cylinder is the most common type used in fluid power systems.

The essential parts of a piston-type cylinder are a cylindrical barrel, a piston and rod, end caps, and suitable seals. The end caps 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 seals are used between the hole and the piston rod to keep fluid from leaking out and to keep dirt and other contaminants from entering the barrel. The opposite end cap of most cylinders is provided with a fitting for securing the actuating cylinder to some structure. This end cap is referred to as the anchor end cap.

The piston rod may extend through either or both ends of the cylinder. The extended end of the rod is normally threaded so that some type of mechanical connector, such as an eyebolt or a clevis, and a locknut can be attached. This threaded connection of the rod and mechanical connector provides for adjustment between the rod and the unit to be actuated. After the correct adjustment is made, the locknut is tightened against the connector to prevent the connector from turning. The other end of the connector is attached, either directly or through additional mechanical linkage, to the unit to be actuated.

In order to satisfy the many requirements of fluid power systems, piston-type cylinders are available in various designs.

Hydraulic Vane Motors

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Figure 4-11 shows a vane-type motor. Flow from the pump enters the inlet, forces the rotor and vanes to rotate, and passes out through the outlet. Motor rotation causes the output shaft to rotate. Since no centrifugal force exists until the motor begins to rotate, something, usually springs, must be used to initially hold the vanes against the casing contour. However, springs usually are not necessary in vane-type pumps because a drive shaft initially supplies centrifugal force to ensure vane-to-casing contact.

Vane motors are balanced hydraulically to prevent a rotor from side-loading a shaft. A shaft is supported by two ball bearings. Torque is developed by a pressure difference as oil from a pump is forced through a motor. Figure 4-12 shows pressure differential on a single vane as it passes the inlet port. On the trailing side open to the inlet port, the vane is subject to full system pressure. The chamber leading the vane is subject to the much lower outlet pressure. The difference in pressure exerts the force on the vane that is, in effect, tangential to the rotor. This pressure difference is effective across vanes 3 and 9 as shown in Figure 4-13. The other vanes are subject to essentially equal force on both sides. Each will develop torque as the rotor turns. Figure 4-13 shows the flow condition for counterclockwise rotation as viewed from the cover end. The body port is the inlet, and the cover port is the outlet. Reverse the flow, and the rotation becomes clockwise.

In a vane-type pump, the vanes are pushed out against a cam ring by centrifugal force when a pump is started up. A design motor uses steel-wire rocker arms (Figure 4-14) to push the vanes against the cam ring. The arms pivot on pins attached to the rotor. The ends of each arm support two vanes that are 90 degrees apart. When the cam ring pushes vane A into its slot, vane B slides out. The reverse also happens. A motor’s pressure plate functions the same as a pump’s. It seals the side of a rotor and ring against internal leakage, and it feeds system pressure under the vanes to hold them out against a ring. This is a simple operation in a pump because a pressure plate is right by a high-pressure port in the cover.

Hydraulic Gear Motors

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Figure 4-10 shows a gear-type motor. Both gears are driven gears, but only one is connected to the output shaft. Operation is essentially the reverse of that of a gear pump. Flow from the pump enters chamber A and flows in either direction around the inside surface of the casing, forcing the gears to rotate as indicated. This rotary motion is then available for work at the output shaft.

Hydraulic Cylinders Construction and Application

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A cylinder is constructed of a barrel or tube, a piston and rod (or ram), two end caps, and suitable oil seals. A barrel is usually seamless steel tubing, or cast, and the interior is finished very true and smoothly. A steel piston rod is highly polished and usually hard chrome-plated to resist pitting and scoring. It is supported in the end cap by a bushing or polished surface.

The cylinder’s ports are built into the end caps, which can be screwed on to the tubes, welded, or attached by tie bolts or bolted flanges. If the cylinder barrel is cast, the head-end cap may be integral with it. Mounting provisions often are made in the end caps, including flanges for stationary mounting or clevises for swinging mounts.

Seals and wipers are installed in the rod’s end cap to keep the rod clean and to prevent external leakage around the rod. Other points where seals are used are at the end cap and joints and between the piston and barrel. Depending on how the rod is attached to the piston, a seal may be needed. Internal leakage should not occur past a piston. It wastes energy and can stop a load by a hydrostatic lock (oil trapped behind a piston).

Figure 4-8, shows force-and-motion applications of cylinders. Because fluid power systems have many requirements, actuating cylinders are available in different shapes and sizes. A cylinder-type actuator is versatile and may be the most trouble-free component of fluid-powered systems. A cylinder and a mechanical member of a unit to be actuated must be aligned correctly. Any misalignment will cause excessive wear of a piston, a piston rod, and the seals. Also, a piston rod and an actuating unit must stay properly adjusted. Clean the exposed ends of the piston rods to ensure that foreign matter does not get into the cylinders.

Hydraulic Cylinders

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A cylinder is a hydraulic actuator that is constructed of a piston or plunger that operates in a cylindrical housing by the action of liquid under pressure. Figure 4-1 shows the basic parts of a cylinder. A cylinder housing is a tube in which a plunger (piston) operates. In a ram-type cylinder, a ram actuates a load directly. In a piston cylinder, a piston rod is connected to a piston to actuate a load. An end of a cylinder from which a rod or plunger protrudes is a rod end. The opposite end is a head end. The hydraulic connections are a head-end port and a rod-end port (fluid supply).

a. Single-Acting Cylinder. This cylinder (Figure 4-1) only has a head-end port and is operated hydraulically in one direction. When oil is pumped into a port, it pushes on a plunger, thus extending it. To return or retract a cylinder, oil must be released to a reservoir. A plunger returns either because of the weight of a load or from some mechanical force such as a spring. In mobile equipment, flow to and from a single-acting cylinder is controlled by a reversing directional valve of a single-acting type.

b. Double-Acting Cylinder. This cylinder (Figure 4-2) must have ports at the head and rod ends. Pumping oil into the head end moves a piston to extend a rod while any oil in the rod end is pushed out and returned to a reservoir. To retract a rod, flow is reversed. Oil from a pump goes into a rod end, and a head-end port is connected to allow return flow. The flow direction to and from a double-acting cylinder can be controlled by a double-acting directional valve or by actuating a control of a reversible pump.

c. Differential Cylinder. In a differential cylinder, the areas where pressure is applied on a piston are not equal. On a head end, a full piston area is available for applying pressure. At a rod end, only an annular area is available for applying pressure. A rod’s area is not a factor, and what space it does take up reduces the volume of oil it will hold. Two
general rules about a differential cylinder are that—

• With an equal GPM delivery to either end, a cylinder will move faster when retracting because of a reduced volume
capacity.
• With equal pressure at either end, a cylinder can exert more force when extending because of the greater piston area. In fact, if equal pressure is applied to both ports at the same time, a cylinder will extend because of a higher resulting force on a head end.

d. Nondifferential Cylinder. This cylinder (Figure 4-3) has a piston rod extending from each end. It has equal thrust and speed either way, provided that pressure and flow are unchanged. A nondifferential cylinder is rarely used on mobile equipment.

e. Ram-Type Cylinder. A ram-type cylinder is a cylinder in which a cross-sectional area of a piston rod is more than one-half a cross-sectional area of a piston head. In many cylinders of this type, the rod and piston heads have equal areas. A ram-type actuating cylinder is used mainly for push functions rather than pull.

Figure 4-1, shows a single-acting, ram-type cylinder. A single-acting ram applies force in one direction. This cylinder is often used in a hydraulic jack. In a double-acting, ram type cylinder, both strokes of a ram are produced by pressurized fluid. Figure 4-2 shows this cylinder.

Figure 4-4 shows a telescoping, ram-type, actuating cylinder, which can be a single- or double acting type. In this cylinder, a series of rams are nested in a telescoping assembly. Except for the smallest ram, each ram is hollow and serves as a cylinder housing for the next smaller ram. A ram assembly is contained in a main cylinder housing, which also provides the fluid ports. Although an assembly requires a small space with all of the rams retracted, a telescoping action of an assembly provides a relatively long stroke when the rams are extended.

f. Piston-Type Cylinder. In this cylinder, a cross-sectional area of a piston head is referred to as a piston-type cylinder. A piston-type cylinder is used mainly when the push and pull functions are needed.

A single-acting, piston-type cylinder uses fluid pressure to apply force in one direction. In some designs, the force of gravity moves a piston in the opposite direction. However, most cylinders of this type apply force in both directions. Fluid pressure provides force in one direction and spring tension provides force in the opposite direction.

Figure 4-5 shows a single acting, spring-loaded, piston type cylinder. In this cylinder, a spring is located on the rod side of a piston. In some spring loaded cylinders, a spring is located on a blank side, and a fluid port is on a rod end of a cylinder.

Most piston-type cylinders are double-acting, which means that fluid under pressure can be applied to either side of a piston to provide movement and apply force in a corresponding direction. Figure 4-6 shows a double acting piston-type cylinder.

This cylinder contains one piston and piston-rod assembly and operates from fluid flow in either direction. The two fluid ports, one near each end of a cylinder, alternate as an inlet and an outlet, depending on the directional-control valve flow direction. This is an unbalanced cylinder, which means that there is a difference in the effective working area on the two sides of a piston. A cylinder is normally installed so that the head end of a piston carries the greater load; that is, a cylinder carries the greater load during a piston-rod extension stroke.

Figure 4-6 shows a balanced, double-acting, piston type cylinder. The effective working area on both sides of a piston is the same, and it exerts the same force in both directions.

g. Cushioned Cylinder. To slow an action and prevent shock at the end of a piston stroke, some actuating cylinders are constructed with a cushioning device at either or both ends of a cylinder. This cushion is usually a metering device built into a cylinder to restrict the flow at an outlet port, thereby slowing down the motion of a piston. Figure 4-7 shows a cushioned actuating cylinder.

h. Lockout Cylinders. A lockout cylinder is used to lock a suspension mechanism of a tracked vehicle when a vehicle functions as a stable platform. A cylinder also serves as a shock absorber when a vehicle is moving. Each lockout cylinder is connected to a road arm by a control lever. When each road wheel moves up, a control lever forces the respective cylinder to compress. Hydraulic fluid is forced around a piston head through restrictor ports causing a cylinder to act as a shock absorber. When hydraulic pressure is applied to an inlet port on each cylinder’s connecting eye, an inner control-valve piston is forced against a spring in each cylinder. This action closes the restrictor ports, blocks the main piston’s motion in each cylinder, and locks the suspension system.

Hydraulic Double Acting Piston Cylinders

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Most piston-type actuating cylinders are double-acting, which means that fluid under pressure can be applied to either side of the piston to apply force and provide movement.

One design of the double-acting cylinder is shown in figure 10-6. This cylinder contains one piston and piston rod assembly. The stroke of the piston and piston rod assembly in either direction is produced by fluid pressure. The two fluid ports, one near each end of the cylinder, alternate as inlet and outlet ports, depending on the direction of flow from the directional control valve. This actuator (fig. 10-6) is referred to as an unbalanced actuating cylinder because there is a difference in the effective working areas on the two sides of the piston. Therefore, this type of cylinder is normally installed so that the blank side of the piston carries the greater load; that is, the cylinder carries the greater load during the piston rod extension stroke.

A four-way directional control valve is normally used to control the operation of this type of cylinder. The valve can be positioned to direct fluid under pressure to either end of the cylinder and allow the displaced fluid to flow from the
opposite end of the cylinder through the control valve to the return line in hydraulic systems or to the atmosphere in pneumatic systems.

There are applications where it is necessary to move two mechanisms at the same time. In this case, double-acting piston-type actuating cylinders of different designs are required. See figures 10-7 and 10-8.

Figure 10-7 shows a three-port, double-acting piston-type actuating cylinder. This actuator contains two pistons and piston rod assemblies. Fluid is directed through port A by a four-way directional control valve and moves the pistons outward, thus moving the mechanisms attached to the pistons’ rods. 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 directional control valve. The displaced fluid then flows through the control valve to the return line or to the atmosphere.

When fluid under pressure is directed into the cylinder through ports B and C, the two pistons move inward, also moving the mechanisms attached to them. Fluid between the two pistons is free to flow from the cylinder through port A and through the control valve to the return line or to the atmosphere.

The actuating cylinder shown in figure 10-8 is a double-acting balanced type. 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 operated. In either case, the cylinder provides equal areas on each side of the piston. Therefore, the same amount of fluid and force is used to move the piston a certain distance in either direction.

Hydraulic Piston Motors

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Piston type motors can be in-line-axis or bent-axis types.

(1) In-Line-Axis, Piston-Type Motors. These motors (Figure 4-15) are almost identical to the pumps. They are built-in, fixed- and variable-displacement models in several sizes. Torque is developed by a pressure drop through a motor. Pressure exerts a force on the ends of the pistons, which is translated into shaft rotation. Shaft rotation of most models can be reversed anytime by reversing the flow direction.

Oil from a pump is forced into the cylinder bores through a motor’s inlet port. Force on the pistons at this point pushes them against a swash plate. They can move only by sliding along a swash plate to a point further away from a cylinder’s barrel, which causes it to rotate. The barrel is then splined to a shaft so that it must turn.

A motor’s displacement depends on the angle of a swash plate (Figure 4-16). At maximum angle, displacement is at its highest because the pistons travel at maximum length. When the angle is reduced, piston travel shortens, reducing displacement. If flow remains constant, a motor runs faster, but torque is decreased. Torque is greatest at maximum displacement because the component of piston force parallel to a swash plate is greatest.

(2) Bent-Axis, Piston-Type Motors. These motors are almost identical to the pumps. They are available in fixed- and variable-displacement models (Figure 4-17), in several sizes. Variable-displacement motors can be controlled mechanically or by pressure compensation. These motors operate similarly to in-line motors except that piston thrust is against a drive-shaft flange. A parallel component of thrust causes a flange to turn. Torque is maximum at maximum displacement; speed is at a minimum. This design piston motor is very heavy and bulky, particularly the variable- displacement motor. Using these motors on mobile equipment is limited.

Although some piston type motors are controlled by directional-control valves, they are often used in combination with variable-displacement pumps. This pump-motor combination (hydraulic transmission) is used to provide a transfer of power between a driving element, such as an electric motor, and a driven element. Hydraulic transmissions may be used for applications such as a speed reducer, variable speed drive, constant speed or constant torque drive, and torque converter. Some advantages a hydraulic transmission has over a mechanical transmission is that it has—

• Quick, easy speed adjustment over a wide range while the power source is operating at constant (most efficient) speed.
• Rapid, smooth acceleration or deceleration.
• Control over maximum torque and power.
• A cushioning effect to reduce shock loads.
• A smooth reversal of motion.

Hydraulic Motors

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Hydraulic motors convert hydraulic energy into mechanical energy. In industrial hydraulic circuits, pumps and motors are normally combined with a proper valving and piping to form a hydraulic-powered transmission. A pump, which is mechanically linked to a prime mover, draws fluid from a reservoir and forces it to a motor. A motor, which is mechanically linked to the workload, is actuated by this flow so that motion or torque, or both, are conveyed to the work. Figure 4-9 shows the basic operations of a hydraulic motor.

The principal ratings of a motor are torque, pressure, and displacement. Torque and pressure ratings indicate how much load a motor can handle. Displacement indicates how much flow is required for a specified drive speed and is expressed in cubic inches per revolutions, the same as pump displacement. Displacement is the amount of oil that must be pumped into a motor to turn it one revolution. Most motors are fixed-displacement; however, variable- displacement piston motors are in use, mainly in hydrostatic drives. The main types of motors are gear, vane, and piston. They can be unidirectional or reversible. (Most motors designed for mobile equipment are reversible.)

a. Gear-Type Motors.
b. Vane-Type Motors.
c. Piston-Type Motors.