HYDRAULIC PUMPS

The hydraulic pump creates the flow of fluid within the hydraulic system. The pressure in a hydraulic system is caused by a restriction placed in the path of the fluid as it leaves the pump. Because of the resulting mechanical drive and positive displacement, the pump merely moves the fluid regardless of the restriction. When enough pressure is built up, movement of the restriction occurs or a relief valve placed in the system opens, allowing the fluid to return to the reservoir or the suction side of the pump.

When the pump operates, hydraulic fluid is trapped between the gear teeth and the pump housing and is carried to the outlet side of the pump. As the teeth mesh, a seal is freed by the mating surfaces that prevent the oil from leaking back to the inlet side of the pump. The sealing action causes the oil to be forced out of the pump and into the system.

CONTROL VALVES

Control valves are valves accessible to the operator for directing the flow of fluid within the system to operate the machine or its attachment. By skillful use of the control valves, the operator can regulate the speed and operation of the hydraulic cylinders.

NOTE: Hydraulic controls should be operated smoothly to eliminate the jerking motion that causes rapid wear and failure of the mechanical parts of the machine.

HYDRAULIC CYLINDERS

Hydraulic cylinders are used to transmit motion in relation to the volume of fluid directed into the cylinder. The force created by the cylinder is determined by the pressure of the fluid and the area of the piston contacted by the fluid. Thus the larger the piston, the more force generated.

Hydraulic cylinders used on heavy equipment are either single- or double-acting cylinders.

Single-Acting Cylinders

Single-acting cylinders, similar to the one shown in figure 4-11, view A, are used to exert force in only one direction. This means the weight or resistance moved must be located so it causes the cylinder to return to its original position when pressure is relieved from the piston. A common use of this type of cylinder is in a hydraulic jack.

Double-Acting Cylinders

Double-acting cylinders are used on equipment where force is needed in two directions. Unlike the single-acting cylinder, the double-acting cylinder contains seals at both ends of the piston where the piston rod passes through the end of the cylinder. With the use of this cylinder, fluid can be directed to either side of the piston and cause the piston rod to extend or retract under pressure. The double-acting cylinder shown in figure 4-11, view B, is called an unbalanced cylinder. This means that the cylinder can exert more force in one direction than in the other. This is due to the piston rod preventing fluid from acting on the full area of the piston on one side.

NOTE: Wipe off all foreign material from hydraulic rams with a clean rag during pre- and post-operations to prevent damaging seals and wiper seals. Before you store equipment, a very important procedure to remember is that the exposed hydraulic rams on the equipment and attachments should be coated in grease. This action protects the surface of the hydraulic ram and is critical in storage locations where corrosive environmental conditions, such as salty air, strong winds, or blowing sand, exist.

Remember: Wipe off the grease before using the equipment when it is removed from storage.

FLEXIBLE HOSES

Flexible hoses are used in a hydraulic system to allow movement between mechanical parts of the

Figure 4-11.-(A) Single- and (B) double-acting hydraulic cylinders.

equipment. Hoses are manufactured in layers (fig. 4-12). The inner layer is made of synthetic materials that resist deterioration from the fluid in the system. The middle layer or layers are made of either fabric or rubber for low-pressure systems or wire braid for highpressure applications. These layers give the hose its strength.

Part of your pre- and post-operational inspections is to inspect hoses for cracking or splitting, pinhole leaks, improper hose length, rubbing, heat, twisting, and so forth. Any problems with hydraulic hoses should be repaired before use.

QUICK-DISCONNECT COUPLERS

Quick-disconnect couplers (fig. 4-13) are used where hydraulic lines must be connected or disconnected frequently; for example, in the NCF, quick-disconnect couplers used on front-end loaders

Figure 4-12.-Hydraulic hose construction.

allow quick changing of loader buckets, backhoes, and forklift attachments.

The quick-disconnect couplers are self-sealing devices that accomplish the work of two shutoff valves and a tube coupler. They are easy to use and keep hydraulic fluid loss at a minimum. More importantly, you do not have to drain or bleed the system each time a hookup is made.

Quick-disconnect couplers consist of two halves: the body contains a spring-loaded poppet or seal, while the other half is inserted to open the poppet when the two halves are connected. A locking device holds the two halves and seals them.

When quick-disconnect couplers are disconnected on attachments, you must remember that it is very important that dust plugs are inserted in the coupler ports. If dust plugs are unavailable, a common practice is to use a plastic bag to wrap the couplers in for protection from foreign matter.

CAUTION

Hydraulic systems can create up to 3,000 pounds of pressure per square inch and the fluids may reach temperatures above 200F. Wear protective gloves and use extreme care when disconnecting and reconnecting quick-disconnect couplers.

ACCUMULATORS

Accumulators are sometimes placed in a hydraulic system to absorb shock. These are frequently used on

Figure 4-13.-Quick-disconnect couplers.

tracked front-end loaders and other equipment containing hydraulic systems that are subjected to severe shock.

The accumulator is a large cylinder that contains compressed gas or a coil spring separated from the hydraulic fluid by a piston, rubber bladder, or diaphragm. When a heavy shock is placed on the hydraulic system, fluid enters the cylinder and causes the gas or spring to compress. Once the shock load stabilizes within the hydraulic system, the fluid is forced back to the operating portion of the system.

HYDRAULIC MOTORS

The hydraulic motor provides power to winches on cranes, drives conveyors on ditching machines, and is used in other applications where mechanical drives are impractical.

The hydraulic motor is turned by fluid under pressure supplied by the pump. The fluid enters the housing and acts on the rotating members. It then discharges and returns to the reservoir or pump.