Hydraulic Piston Motor: Types, Working Principle and Pros and Cons (2)

Video 1: Axial piston hydraulic motor animation from YouTube.

1. Axial Piston Hydraulic Motor

The structure of the axial piston motor is basically the same as that of the axial piston pump. However, in order to meet the requirements of the forward and reverse rotation of the hydraulic motor, the structure of the valve plate and the size and shape of the flow channel of the inlet and outlet are generally completely symmetrical.

The classification method and specific types of the axial piston hydraulic motor are basically the same as those of axial piston pumps. For example, according to the positional relationship between the movement direction of the piston and the axis of the drive shaft, there are two types: straight axis type (swash plate type, in-line piston motor) and bent axis type (bent axis hydraulic motor, bent axis motor, angled axis motor).
According to the form of the flow distribution mechanism, it is divided into two types: end-face distribution and valve distribution. At present, most axial piston motors use end-face distribution.

1.1 Types of Axial Piston Hydraulic Motors

Swash plate type piston motor: every time the cylinder body rotates, each piston reciprocates once to complete one oil suction and oil discharge action. By changing the inclination angle of the swash plate, the sealed working volume can be changed to realize the variable displacement of the pump.
The drive shaft of the bent axis axial piston motor is inclined at an angle to the axis of the cylinder block, so it is also called an angled axis motor.

Table 1: Types of axial piston motor/pumps.

1.2 How Axial Piston Hydraulic Motors Work

The following takes the swash plate axial piston motor as an example to introduce the working principle of the axial piston motor.

Figure 1: Working principle diagram of axial piston high-speed motor.

1—Swash plate; 2—Piston; 3—Cylinder; 4—Oil distribution plate

The figure above shows the working principle of the swash plate axial piston motor. The swash plate 1 and the oil distribution plate 4 are fixed, and the cylinder block 3 and the piston 2 on it can rotate around the horizontal axis of the cylinder block. When the pressure oil enters the cylinder hole through the oil distribution plate and reaches the bottom of the piston, the piston is pushed out by the oil pressure, and is tightly pressed on the swash plate surface. At this time, the reaction force of the swash plate to the piston is F.

Since the swash plate has an inclination angle γ, F can be decomposed into two components: one is the axial component Fx, which is parallel to the axis of the piston and balanced with the oil pressure at the bottom of the piston; the other is Fy, which is perpendicular to the piston axis. The component force Fy produces a torque on the axis of the cylinder block, which drives the cylinder block to rotate. The cylinder body then outputs torque and rotational speed through the main shaft, and outputs mechanical energy to become a hydraulic motor.

When the hydraulic motor is continuously supplied with pressure oil, the hydraulic motor will drive the load to rotate continuously. If the oil inlet and outlet of the hydraulic motor are reversed, the hydraulic motor will rotate in the opposite direction, and the working principle is the same as above. When the inclination angle γ of the swash plate is decreased, the displacement of the hydraulic motor decreases.
Most straight axis (swash plate) end-face distribution axial piston motors can be used interchangeably with axial piston pumps of the same structure.

1.3 Advantages and Disadvantages of Axial Piston Motors

The main advantages of axial piston motors are compact structure, high power density, high working pressure, easy realization of variable displacement, rich variable methods, and high efficiency. The disadvantages are more complicated structure, expensive price, poor anti-pollution ability, and high use and maintenance requirements.

Figure 2: Bent axis axial piston motor in blue.

Advantages of in-line axial piston motor / pump:
①It is reversible, and generally can be used as both a pump and a motor.
②Using the swing of the swash plate to achieve the flow and direction of change.
③High pressure, up to 50MPa; large displacement, usually 500mL/r, and some up to 1400mL/r.
④The variable dynamic characteristics are good, and the response time can reach 0.2s.
⑤High efficiency. The volume efficiency can reach more than 95%, and the total efficiency can reach more than 90%.
⑥Compared to the bent axis type, the noise is small.
⑦Simple structure, small size and high power density.

Disadvantages:

① The structure is generally more complicated than the gear type and vane type.
②Poor pollution resistance.

Advantages of bent axis axial piston motor /pump:
①The lateral force of the piston is smaller than that of the straight axis type, so the friction loss is small.
②The angle between the drive shaft and the cylinder block is large, generally 25°, and the largest is 40°.
③Because of the large sealing gap between the spherical surface of the cylinder block and that of the valve plate, it is not sensitive to oil pollution.
④The moment of inertia of the rotating parts is small, so the starting characteristics are good and the starting efficiency is high.
⑤Due to the large inclination angle, the variable range is large.

Disadvantages:
①The main disadvantage of the bent axis axial piston motor/pump is that the processing of the piston, connecting rod and valve plate (when spherical surface is used) is difficult.
②Relying on the swing cylinder to realize the variable displacement, so the external size and mass are also larger and the structure is more complicated.

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