# What are the main performance parameters of hydraulic motors?

Updated: Aug 26, 2018

The main performance parameters of __hydraulic pump__ and __hydraulic motor__ are pressure, speed, displacement, flow and efficiency.

Pressure

The pressure can be divided into working pressure, rated pressure and maximum pressure.

The working pressure refers to the pressure at which the hydraulic pump (or __motor__) outputs (inputs) the oil during actual operation, and the working pressure is determined by the external load.

Rated pressure refers to the maximum pressure that a hydraulic pump (or __travel motor__) can continuously operate under normal working conditions according to the test standards. The size is limited by the life of the hydraulic pump (or __final drive motor__). When the working pressure is greater than the rated pressure, it is called overload.

The highest pressure refers to the reliability life of the hydraulic pump (or motor) and the maximum intermittent pressure allowed by the leakage. The action time does not exceed 1% to 2% of the total working time. The pressure is normally set by the relief valve. The working pressure of the hydraulic pump (or __final drive motors__) is not equal to its rated pressure.

2. Speed

The speed can be divided into working speed, rated speed, maximum speed and minimum stable speed, etc., r/min. The working speed refers to the actual rotational speed of the hydraulic pump (or motor) during operation. The rated speed is the maximum speed at which the hydraulic pump (or motor) can operate continuously for a long time at rated pressure. That is, when the rotational speed exceeds the rotational speed, the hydraulic pump (or motor) will cause insufficient oil absorption, generate vibration and noise, suffer from cavitation damage, and have a reduced life. The maximum speed is the maximum speed limit that cannot be exceeded when the hydraulic pump (or motor) is not damaged by abnormal conditions. The minimum stable speed is the minimum speed allowed for normal operation of the motor.

The speed capability of the hydraulic pump (or motor) is affected by the flow and the mechanical load of the rotating component. It is a function of displacement and pressure. In general, the speed of the hydraulic pump (or motor) is reduced when the pressure is reduced or the displacement is reduced. improve.

Under the same pressure conditions, the speed increases with the decrease of the displacement, and the limit value does not increase until a certain displacement between the minimum displacement (not necessarily the zero displacement) and the full displacement. At the maximum displacement of the small displacement, the inertial force of the rotating component of the hydraulic pump (or motor) has a large additional load, which may cause the hydraulic pump (or motor) to break or cause the lubrication to form a limit lubrication state and aggravate the wear.

Below the rated speed, the service life and transmission efficiency of the hydraulic pump (or motor) are not as sensitive to the change in speed as the pressure change. Therefore, from the perspective of improving the power utilization of the hydraulic pump (or motor) and reducing the cost, the rated speed is selected as the match. The speed is suitable.

3. Displacement

Displacement refers to the volume of liquid discharged (or inhaled) obtained by changing the geometry of the sealed cavity per revolution of the hydraulic pump (or motor), ml/r. The displacement can be adjusted by changing the variable pump (motor), and the displacement cannot be changed, called the metering pump (motor).

In order to determine the displacement of the hydraulic pump (motor), the concept of angular power is introduced. Angular power is a description of the limit state. It is not the power normally available, but it effectively reflects the transmission capacity of the transmission, which reflects the power capacity and transformation capability of the transmission. The angular power is equal to The product of the maximum output torque of the transmission and the maximum speed. When the torque is large, the rotation speed is low, and the two cannot reach the maximum value at the same time, so the angular power is impossible to realize, but if the hydraulic system has the angular power capability required by the special working device, the two parameters of the torque and the speed are adjusted and changed. In the process, you can always find a maximum torque point that meets the requirements and another maximum speed point.

The hydraulic pump and hydraulic motor can be selected based on the determined hydraulic pump (or motor) pressure, speed and displacement.

4. flow

The flow rate is equal to the product of displacement and speed. The actual flow refers to the flow at the outlet (or inlet) of the hydraulic pump (or motor). Due to the internal leakage of the hydraulic pump (or motor) itself, the actual flow is less than the theoretical flow, and the specified speed of the motor is to be compensated. The amount of leakage, the actual flow input must be greater than the theoretical flow.

5. effectiveness

The efficiency of a hydraulic pump (or motor) is divided into volumetric efficiency and mechanical efficiency. Volumetric efficiency refers to the ratio of actual flow to theoretical flow for hydraulic pumps; for hydraulic motors, the ratio of theoretical flow to actual flow. The theoretical calculation formula is as follows:

Mechanical efficiency refers to the ratio of the theoretical torque to the actual input torque for a hydraulic pump; for a hydraulic motor, the ratio of the actual output torque to the theoretical torque. The theoretical calculation formula is as follows:

The total efficiency is the ratio of the output power of the hydraulic pump (or motor) to the input power, which is equal to the product of volumetric efficiency and mechanical efficiency.

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