Figure 37 Hydraulic Actuator
Initially, with no hydraulic fluid pressure, the spring force holds the valve in the closed position. As fluid enters the lower chamber, pressure in the chamber increases. This pressure results in a force on the bottom of the piston opposite to the force caused by the spring. When the hydraulic force is greater than the spring force, the piston begins to move upward, the spring compresses, and the valve begins to open. As the hydraulic pressure increases, the valve continues to open. Conversely, as hydraulic oil is drained from the cylinder, the hydraulic force becomes less than the spring force, the piston moves downward, and the valve closes. By regulating amount of oil supplied or drained from the actuator, the valve can be positioned between fully open and fully closed.
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.
Rotary Actuators- Hydraulic Motors.
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. 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.
b. Vane-Type Motors. 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.
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