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.

in line axis piston motor Hydraulic Piston Motors

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.

swash plate Hydraulic Piston Motors

(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.

bent axis piston motor Hydraulic Piston Motors

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.

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