Swash plate inline piston pump

In this type, the axial reciprocating motion of the pistons is obtained by a swash plate that is either fixed or variable in its degree of angle. As the piston barrel assembly rotates, the pistons rotate around the shaft, with the piston shoes in contact with and sliding along the swash plate surface. Since there is no reciprocating motion when the swash plate is in vertical position, no displacement occurs. As there is an increase in the swash plate angle, the pistons move in and out of the barrel as they follow the angle of the swash plate surface. The pistons move out of the cylinder barrel during one half of the cycle of rotation thereby generating an increasing volume, while during the other half of the rotating cycle, the pistons move into the cylinder barrel generating a decreasing volume. This reciprocating motion results in the drawing in and pumping out of the fluid. Pump capacity can easily be controlled by altering the swash plate angle, larger the angle, greater being the pump capacity. The swash plate angle can easily be controlled remotely with the help of a separate hydraulic cylinder. A cross-sectional view of this pump is shown in Figure 3.16.


The cylinder block and the drive shaft in this pump are located on the same centerline. The pistons are connected through shoes and a shoe plate that bears against the swash plate. As the cylinder rotates, the pistons reciprocate due to the piston shoes following the angled surface of the swash plate. This operation of drawing in and drawing out of the fluid is illustrated in the Figure 3.17.


The outlet and the inlet ports are located in the valve plate so that the pistons pass the inlet as they are being pulled out and pass the outlet as they are being forced back in.

These types of pumps can also be designed to have a variable displacement capability. In such a design, the swash plate is mounted in a movable yoke. The swash plate angle can be changed by pivoting the yoke on pintles.

The positioning of the yoke can be accomplished by manual operation, servo control or a compressor control and the maximum swash plate angle is usually limited to 17.5° (Figure 3.18).


Hydraulic Piston Motors

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.