HYDRAULIC SYSTEM – ACTUATORS 01

INTRODUCTION

A hydraulic actuator receives pressure energy and converts it to mechanical force and motion. An actuator can be linear or rotary. A linear actuator gives force and motion outputs in a straight line. It is more commonly called a cylinder but is also referred to as a ram, reciprocating motor, or linear motor. A rotary actuator produces torque and rotating motion. It is more commonly called a hydraulic motor or motor.

Hydraulic Cylinder :   This converts the energy of pressure into straight line motion.

Hydraulic Motor :   This converts the energy of pressure into rotational motion.

Linear actuators- Hydraulic cylinders.

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.

The single-acting ram-type actuating cylinder is often used in the hydraulic jack. The elevators used to move aircraft to and from the flight deck and hangar deck on aircraft carriers also use cylinders of this type. In these elevators, the cylinders are installed horizontally and operate the elevator through a series of cables and sheaves. Fluid pressure forces the ram outward and lifts the elevator. When fluid pressure is released from the ram, the weight of the elevator forces the ram back into the cylinder. This, in turn, forces the fluid back into the reservoir.

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.

A double-acting ram-type cylinder is illustrated in figure. In this cylinder, both strokes of the ram are produced by pressurized fluid. There are two fluid ports, one at or near each end of the cylinder. Fluid under pressure is directed to the closed end of the cylinder to extend the ram and apply force.

A four-way directional control valve is normally used to control the double-acting ram. When the valve is positioned to extend the ram, pressurized fluid enters port A (fig. 10-2), acts on the bottom surface of the ram, and forces the ram outward. Fluid above the ram lip is free to flow out of port B, through the control valve, and to the return line in hydraulic systems or to the atmosphere in pneumatic systems.

Figure 10-2.—Double-acting ram-type actuating cylinder.

Normally, the pressure of the fluid is the same for either stroke of the ram. Recall that force is equal to pressure times area (F= PA). Notice the difference of the areas upon which the pressure acts in figure 10-2. The pressure acts against the large surface area on the bottom of the ram during the extension stroke, during which time the ram applies force. Since the ram does not require a large force during the retraction stroke, pressure acting on the small area on the top surface of the ram lip provides the necessary force to retract the ram.

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.

A series of rams is nested in the telescoping assembly. With the exception of the smallest ram, each ram is hollow and serves as the cylinder housing for the next smaller ram. Although the assembly requires a small space with all the rams retracted, the telescoping action of the assembly provides a relatively long stroke when the rams are extended.

An excellent example of the application of this type of cylinder is in the dump truck. It is used to lift the forward end of the truck bed and dump the load. During the lifting operation, the greatest force is required for the initial lifting of the load.

Figure 10-3.—Telescoping ram-type actuating cylinder.

As the load is lifted and begins to dump, the required force becomes less and less until the load is completely dumped. During the raise cycle, pressurized fluid enters the cylinder through port A (fig. 10-3) and acts on the bottom surface of all three rams. Ram 1 has a larger surface area and, therefore, provides the greater force for the initial load, As ram 1 reaches the end of its stroke and the required force is decreased, ram 2 moves, providing the smaller force needed to continue raising the load. When ram 2 completes its stroke, a still smaller force is required. Ram 3 then moves outward to finish raising and dumping the load. Some telescoping ram-type cylinders are of the single-acting type. Like the single-acting ram discussed previously, these telescoping ram-type cylinders are retracted by gravity or mechanical force. Some hydraulic jacks are equipped with telescoping rams. Such jacks are used to lift vehicles with low clearances to the required height. Other types of telescoping cylinders, like the one illustrated in figure 10-3, are of the double-acting type. In this type, fluid pressure is used for both the extension and retraction strokes. A four-way directional control valve is commonly used to control the operation of the double-acting type. Note the small passages in the walls of rams 1 and 2. They provide a path for fluid to flow to and from the chambers above the lips of rams 2 and 3. During the extension stroke, return fluid flows through these passages and out of the cylinder through port B. It then flows through the directional control valve to the return line or reservoir.

To retract the rams, fluid under pressure is directed into the cylinder through port B and acts against the top surface areas of all three ram lips. This forces the rams to the retracted position. The displaced fluid from the opposite side of the rams flows out of the cylinder through port A, through the directional control valve to the return line or reservoir.

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.

Typical piston-type hydraulic actuator is shown in Figure 37. It consists of a cylinder, piston, spring, hydraulic supply and return line, and stem. The piston slides vertically inside the cylinder and separates the cylinder into two chambers. The upper chamber contains the spring and the lower chamber contains hydraulic oil.