to the inlet port. (For example, in a nine piston motor, four cylinder bores are receiving fluid while four are discharging.) The fluid acting on the pistons in those bores forces the pistons to move away from the valve plate. Since the pistons are held by connecting rods at a fixed distance from the output shaft flange, they can move away from the valve plate only by moving in a rotary direction. The pistons move in this direction to a point around the shaft axis, which is the greatest distance from the valve plate. Therefore, driving the pistons axially causes them to rotate the drive shaft and cylinder block. While some of the pistons are being driven by liquid flow from the system, others are discharging flow from the outlet port.

This type of motor may be operated in either direction of rotation. The direction of rotation is controlled by the direction of flow to the valve plate. The direction of flow may be instantly reversed without damage to the motor. This design is found mainly on construction equipment as an auxiliary drive motor.

The speed of the rotation of the motor is controlled by the length of the piston stroke in the pump. When the pump is set to allow a full stroke of each piston, each piston of the motor must move an equal distance. In this condition, the speed of the motor will equal that of the pump. If the tilting plate of the pump (normally called a swash plate or hanger assembly) is changed so that the piston stroke of the pump is only one half as long as the stroke of the motor, it will require the discharge piston one full stroke; therefore, in this position of the plate, the motor will revolve just one half as fast as the pump. If there is no angle on the tilting plate of the pump, the pumping pistons will not move axially, and liquid will not be delivered to the motor; therefore, the motor will deliver no power.

If the angle of the tilting plate is reversed, the direction of flow is reversed. Liquid enters the motor through the port by which it was formally discharged. This reverses the direction of rotation of the motor.

An additional benefit to this axial-piston pump/axial-piston motor configuration is the dynamic braking effect created when the motor, in a coasting situation, in effect, becomes a pump itself and attempts to reverse-rotate the hydraulic pump. In this situation the pump now becomes a motor and attempts to reverse-rotate the prime mover. The degree of reverse angle on the tilting plate in the pump determines the effectiveness of the dynamic braking.

VALVES

Once the pump has begun to move the fluid in a hydraulic system, valves are usually required to control, monitor, and regulate the operation of the system. While the pump is recognized as the heart of the system, the valves are the most important devices for providing flexibility in today's complex hydraulic systems.

Valves are included in a hydraulic system to control primarily (1) the direction of fluid flow, (2) the volume of fluid going to various parts of the system, and (3) the pressure of the fluid at different points in the system.

It is beyond the scope of this training manual to cover all of the many different valves in use today; however, since most of these valves are almost always combinations and elaborations of basic types, an understanding of their operation can be obtained by a review of the basic types.

The basic valves are those designed to do one of the primary functions mentioned above; that is, control direction of flow, control volume, and regulate fluid pressure.

Valves, like pumps, are precision made. Usually, no packing is used between the valve element and the valve seat since leakage is reduced to a minimum by machined clearances. (Packing is required around valve stems, between lands of spool valves, etc.) Here again is an important reason for preventing system contamination. Even the most minute particle of dirt, dust, and lint can do considerable damage to hydraulic valves.

Relief Valves

A relief valve is a simple pressure-limiting device. It is incorporated in most hydraulic systems and acts as a safety valve, used to prevent damage to the system in case of overpressurization.

A simple two-port relief valve is shown in figure 10-17. An adjustment is provided so that the valve may be regulated to any given pressure; therefore, it can be used on a variety of systems. Before the system pressure can become high enough to rupture the tubing or damage the system units, it exceeds the pressure required to overcome the relief valve spring setting. This pushes the ball off its seat and bypasses excess fluid to the reservoir. If the system pressure decreases, the spring setting reseats the ball; the ball then remains seated until the pressure again reaches the predetermined maximum.