selection principle and key points of hydraulic motor

Created on 2025.05.08

selection principle and key points of hydraulic motor

(l) Selection principle and basis: as the hydraulic motor is essentially the same as the hydraulic pump with similar structure, the selection principle is also the same. But there are many differences between hydraulic motor and hydraulic pump in working (table below). Especially considering that the function of the hydraulic motor is to convert the hydraulic energy into mechanical energy to drive the load to rotate, the output torque (including the starting torque) and speed become the most important factors in the selection of the hydraulic motor. In addition, the basis or issues to be considered for selecting hydraulic motor include efficiency, low speed stability, service life, speed regulation ratio, noise, shape and connection size, weight, price, source of goods, convenience of use and maintenance, etc.

The difference between hydraulic pump and hydraulic motor

Serial number	Hydraulic pump	Hydraulic motor
1	Provide pressure and flow, emphasizing volumetric efficiency	Generate torque to drive the load, emphasizing mechanical efficiency
2	Typically operates under relatively constant high-speed conditions	Most motors have a wide range of speed variations, with some requiring operation at very low speeds.
3	The pump shaft usually operates in one direction, but the flow direction and pressure may change.	Hydraulic motors mostly require operation in both forward and reverse directions, and some hydraulic motors also require the ability to operate in pump mode (pump condition) to achieve the purpose of braking load.
4	In most systems, it operates continuously, and the temperature change of the working fluid is relatively small.	According to the working conditions, operation may be intermittent and will be subjected to frequent temperature shocks.
5	Most hydraulic pumps are installed together with the prime mover, and the spindle does not bear additional loads.	Many hydraulic motors are directly mounted inside the wheels or connected to pulleys, sprockets, or gears, and sometimes the spindle will bear higher radial loads.

(2) There are many types of hydraulic motors with different characteristics, so we should choose the appropriate hydraulic motor according to the specific use and working conditions. See table 1-18 for applicable conditions and application scope of various hydraulic motors. Low speed motor or high speed motor acceleration and deceleration device can be selected for low speed operation. The choice of the two schemes should be based on the structure and space conditions, equipment cost, and whether the driving torque is reasonable.

(3) The selection of specification (displacement) displacement is the main specification parameter of hydraulic motor, and the selection is mainly based on the working load characteristics of the motor.

The working load characteristics can be represented by the speed time cycle diagram (N-t diagram) and torque time cycle diagram (T-T diagram) through the working condition analysis (motion analysis and dynamic analysis) of the main engine (Fig. J). The load torque of the motor can be determined by calculation or test according to the working mechanism of the main engine and its technological purpose. From T-T diagram and N-t diagram, we can clearly understand the changes of load torque and load speed of the hydraulic motor in the whole working cycle from start-up to normal operation and to stop, that is, the peak load torque of the motor in actual operation and the load torque value of long-term continuous operation, as well as the related maximum load speed and load speed of long-term operation, so as to provide reference for calculation and confirmation Set the displacement specification of hydraulic motor to lay the foundation.

Before selecting the displacement, the reference value of nominal displacement should be calculated according to the above working load characteristics. According to the use of different focus, the reference value of the nominal displacement of the hydraulic motor has the following two calculation methods.

① When the main purpose of the motor is to drive the load, the reference value VG (ml / R) can be calculated according to the maximum load torque Tmax (n · m), the preselected working pressure P (MPA) [or differential pressure △ P (MPA)] and the mechanical efficiency η m (η m = 0.90 ~ 0.95), i.e.

Vg≥（2πTmax）/pηm                   (1-45)

② When the main purpose of the motor is speed and its transformation, the reference value VG (ml / R) can be calculated according to the minimum speed Nmin (R / min), the known input flow QV (L / min) and the volumetric efficiency of the motor η V (which can be selected according to the product sample or between η v = 0.85-0.9), that is, the motor speed can be adjusted according to the motor speed

Vg≥（1000qvηv）/nmin                   (1-46)

According to the calculated displacement and the product samples, the nominal displacement is determined by the principle of proximity.

(4) The calculation of the actual working pressure (or pressure difference) should be based on the peak torque and continuous working torque to calculate the peak pressure and continuous working pressure. If the calculated value is in the range of the performance parameters of the motor, the displacement selection is reasonable. In general, the actual continuous working pressure should be 20% - 25% lower than the recommended rated pressure in the product sample, so as to improve the service life and working reliability. When the peak torque appears at the moment of starting, the maximum pressure can be 80% of the maximum pressure provided in the sample, so that 20% reserve is ideal.

(5) After the displacement and actual working pressure are determined, the output power of the motor can be checked according to the power calculation formula.

(6) Life evaluation or checking calculation after determining the model and specification of the motor, refer to the sample data provided by the manufacturer to evaluate or check the possible life of the hydraulic motor under the actual working conditions, so as to determine whether the above selection can meet the requirements of the main engine. If the service life is not enough, the products with larger specifications must be selected.

(7) Others

① The hydraulic motor is usually allowed to work under the pressure of 20% - 50% of the rated pressure in a short time, but the instantaneous maximum pressure and the maximum speed cannot appear at the same time. The back pressure of the oil return circuit of the hydraulic motor is limited to a certain extent. When the back pressure is large, the leaking oil pipe must be set.

② In general, the maximum torque and maximum speed of the hydraulic motor should not occur at the same time. The actual speed should not be lower than the minimum speed of the motor to avoid crawling. When the speed required by the system is low, and the motor speed, torque, and other performance parameters are not easy to meet the working requirements, a speed reduction mechanism can be added between the motor and its driving host. In order to run smoothly at a very low speed, the leakage of the motor must be constant, with a certain return oil back pressure and an oil viscosity of at least 35mm2gs. If the motor needs to run at low speed, its minimum stable speed should be checked.

③ In order to prevent cavitation or loss of braking capacity of the brake motor working as a pump, it is necessary to ensure that there is enough oil replenishing pressure at the oil suction port of the motor at this time, which can be realized through the oil replenishing pump in the closed circuit or the back pressure valve in the open circuit; when the hydraulic motor drives a large inertia load, a bypass check valve in parallel with the motor should be set in the hydraulic system to replenish oil to avoid shutdown in the process, the motor with inertia motion is short of oil.

④ For the hydraulic motor that cannot bear additional axial and radial forces, or the hydraulic motor can bear additional axial and radial forces, but the actual axial and radial forces of the load are greater than the allowable axial or radial forces of the hydraulic motor, the elastic coupling should be considered to connect the motor output shaft and working mechanism.

⑤ When the motor needs to be locked for a long time to prevent the load movement, the mechanical brake with spring-up brake and hydraulic release brake on the motor shaft should be used (Fig. K).

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