How Does a Hydraulic Drive Motor Work?

Figure 1: A blue five-star design hydraulic motor.

A hydraulic motor is an actuating element of the hydraulic system, which converts the hydraulic pressure energy provided by the hydraulic pump into the mechanical energy (torque and rotational speed) of its output shaft. Liquid is the medium that transmits force and motion.

Hydraulic motors can be divided into gear type, vane type, piston type and other forms according to their structure types. The following will introduce the working principle and characteristics of these three types of hydraulic motors in detail.

1. Hydraulic Vane Motor

The working principle of the vane type hydraulic motor is that due to the action of the pressure oil, the unbalanced force causes the rotor to generate torque. The output torque of the hydraulic vane motor is related to the displacement and the pressure difference between the inlet and outlet of the hydraulic motor, and its speed is determined by the flow rate input to the hydraulic motor.

1.1 Working Principle of Hydraulic Vane Motor

The commonly used hydraulic vane motor is a double-acting type. Now, take the double-acting hydraulic vane motor as an example to illustrate its working principle.

Figure 2: Working principle diagram of hydraulic vane motor.

1, 2, 3, 4, 5, 6, 7, 8 - Vanes; 9, 13 - Oil return cavity; 10, 14-Pressure chamber
11, 15 - Oil inlet; 12, 16 - Oil return port

Figure 2 shows the working principle of the vane motor. When the pressure oil enters the pressure chamber from the oil inlets 11 and 15, the sealing volume between the two adjacent vanes is filled with the pressure oil. Vane 2 and 6 on both sides of the pressure chamber are under the action of the pressure oil, and do not generate torque.
One side of vane 1 and 3 is subject to pressure oil, while the other side is in the return chamber, subject to low pressure oil. Because the extension length of vane 3 is greater than that of vane 1, the bearing area of vane 3 is larger than that of vane 1. So a torque that makes the rotor rotate counterclockwise is generated.

In the same way, there is also a torque between vanes 5 and 7 that rotates the rotor in a counter-clockwise direction. The oil pressure in the oil return chamber is low, and the torque generated by the action on the vanes is negligible. As can be seen from the figure, if the oil delivery direction is changed, the vane motor rotates clockwise.

1.2 Features of Hydraulic Vane Motor

Figure 3: Structure diagram of hydraulic vane motor.

1. There is a spring at the bottom of every vane to ensure that the vane can stick close to the inner surface of the stator under the initial condition to form a sealed working cavity. Otherwise, the oil inlet and the oil return cavity are connected, then the oil pressure cannot be formed, and the torque cannot be output.

2. Since the hydraulic motor generally requires forward and reverse rotation, the vanes of the hydraulic vane motor should be placed radially.

3. Two one-way valves are installed in the casing in order that the bottom of the vane can always have pressure oil to make the inner surface of the vane and the stator have a close contact without being affected by the rotation direction of the vane motor.

The vane motor features small size, small moment of inertia, sensitive action, and allows for higher frequency commutation, but has large leakage during operation and cannot work at low speed. It is suitable for occasions with high speed, small torque and sensitive action.

2. Hydraulic Gear Motor

Gear type hydraulic motors include internal gear motors and external gear motors.

2.1 Working Principle of Hydraulic Gear Motor

Figure 4: Working principle diagram of hydraulic gear motor.

Figure 4 is the working principle diagram of the external gear motor. P is the meshing point of the two gears, h is the tooth height of the gear, the distances from the meshing point to the tooth roots of the two gears are a and b respectively, and the tooth width is B.
When the pressure oil enters the high pressure chamber of the motor, all the gear teeth in the high pressure chamber are affected by the pressure oil, and only part of the tooth surface of a pair of gear teeth meshing with each other is affected by the pressure oil.

Since both a and b are smaller than h , there is a force pM (h - a) B and p M (h - b) B on each of the two gears that make them generate torque. Under the action of the above-mentioned force, the two gears rotate in the direction shown in the figure, and carry the oil to the low-pressure chamber for discharge.

2.2 Features of Hydraulic Gear Motor

Figure 5: External gear motor diagram.

1. In order to meet the requirements of bidirectional rotation, the hydraulic gear motor has the same size of oil inlet and outlet and is symmetrical.
2. There is a separate oil drain port to lead the leaking oil from the bearing part to the outside of the pump.
3. In order to reduce the friction torque when starting, rolling bearings are used.
4. In order to reduce the output torque pulsation, the number of gear teeth of the hydraulic gear motor is more than that of the gear pump.
5. In order to improve the mechanical efficiency, the tooth tip clearance of the hydraulic gear motor is larger than that of the gear pump.

Due to structural reasons, the gear motor has poor sealing performance, low volumetric efficiency, the input oil pressure cannot be too high, and the output torque is small, so the gear motor is only suitable for high-speed, low-torque occasions.

3. Hydraulic Piston Motor

The hydraulic piston motor (piston type hydraulic motor) is divided into radial piston motors, axial piston motors, etc. The following takes the axial piston hydraulic motor as an example to explain its working principle.

3.1 Working Principle of Hydraulic Piston Motor

Figure 6: Working principle diagram of axial piston motor.

1-Swash plate; 2-Cylinder block; 3-Piston; 4-Oil distribution plate; 5-Motor shaft; 6-Oil inlet; 7-Oil return port

The working principle of the axial piston motor is shown in Figure 6. The swash plate 1 and the oil distribution plate 4 are fixed, and the cylinder block 2 and the motor shaft 5 are connected and rotate together. When the pressure oil enters the piston hole on the cylinder block 2 through the oil inlet 6 of the oil distribution plate 4, the pressure oil pushes out the piston 3 in the hole and presses it tightly on the swash plate.

The reaction force F of the swash plate to the piston is perpendicular to the surface of the swash plate, and the force can be decomposed into a horizontal component (axial component) Fx and a vertical component Fy. The horizontal component Fx balances the hydraulic pressure on the piston, while the vertical component Fy causes the piston to generate a torque to the center of the cylinder, driving the cylinder and motor shaft to rotate counterclockwise. If the input direction of the motor pressure oil is changed, the motor shaft will rotate clockwise.

3.2 Features of Hydraulic Piston Motor

Low-speed and high-torque hydraulic motors are mostly radial piston types. They have large displacement, large output torque, low rotational speed, and good low-speed stability. Some low speeds reach a few revolutions per minute or even a few tenths of a revolution, so it can be directly connected to the working mechanism without a reduction device, which greatly simplifies the transmission mechanism, widely used in heavy equipment.

Figure 7: Swash plate piston pump diagram.

4. Working Characteristics of Hydraulic Motor

The working principle of the hydraulic motor is to achieve energy conversion by the volume change of the sealed working chamber. The working of hydraulic motor has the following characteristics:

1. The hydraulic motor should be able to rotate forward and backward. Therefore, it requires a hydraulic motor in the design with structural symmetry.
2. When the hydraulic motor has large inertial load, high rotational speed, and is required for rapid braking or reverse rotation, a high hydraulic shock will occur, so a necessary safety valve or buffer valve should be set in the system.

3. Since internal leakage is unavoidable, there will still be a slow slip when braking with the motor's oil discharge port closed. Therefore, when precise braking is required for a long time, a separate brake to prevent slipping is needed.
4. Certain types of hydraulic motors must have sufficient back pressure at the oil return port to ensure normal operation.

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