Created on：2021-06-19 10:48

# Main parameters and commonness of hydraulic motor

Pulsatility and its harm

(l) Instantaneous displacement and instantaneous flow of pulsating hydraulic motor in operation, the displacement changing with the angle at each instant is called the instantaneous displacement of hydraulic motor; the flow changing with the angle at each instant is called the instantaneous flow of hydraulic motor. In theory, the instantaneous displacement and flow of most motors are pulsating.

The pulsation of instantaneous displacement is evaluated by displacement non-uniformity coefficient δ V (%)

(1-34)

The fluctuation of instantaneous flow is evaluated by the coefficient of flow non-uniformity δ Q (%)

(1-35)

Where, (VInst) max, (qinst) max -- maximum instantaneous displacement and maximum instantaneous flow of hydraulic motor;

(VInst) MIM, (qinst) MIM -- Minimum instantaneous displacement and minimum instantaneous flow of hydraulic motor.

The smaller the flow non-uniformity coefficient δ V and δ Q, the smaller the pulsation of the displacement and flow, or the better the quality of the instantaneous displacement and flow.

(2) The harm of pulsation when the input flow of hydraulic motor is constant, the output speed of the motor will pulsation according to a certain law due to the constant change of the instantaneous displacement of the motor. Because the output torque of the motor is proportional to the displacement, under constant input pressure, when the friction is ignored, the output torque of the motor will change with the instantaneous displacement according to the same law. When the load torque is fixed, because the pressure is inversely proportional to the displacement, with the change of the instantaneous displacement of the motor, the pressure will also pulse according to a certain law.

The pulsation of hydraulic motors with different structures and parameters is different. This kind of periodic pulsation is mainly determined by the structure of the hydraulic motor itself. When the speed is high, the output pulsation is not obvious for the external load with large inertia, but it makes the whole hydraulic system produce vibration and noise. When the vibration frequency is consistent with the inherent vibration frequency of the system, resonance will occur, which will cause serious vibration and howling of the pipeline system, affect the stability of the system and hydraulic components, and reduce the service life. When the wheel is running at high speed, the pulsation will also be one of the causes of low speed crawling.

1.7.5 starting performance and braking performance

(l) Starting characteristics in most mechanical equipment, hydraulic motor often starts, stops, rotates forward and reverses with load. Under the condition of frequent alternation, the starting performance of hydraulic motor should meet the requirements of reliable starting at any angle when the load is full torque or allowable torque. Starting characteristics are measured by starting torque and starting mechanical efficiency.

The torque on the output shaft when the hydraulic motor starts from the static state under the rated pressure is called the starting torque of the hydraulic motor, that is, the output torque of the shaft after overcoming the friction loss in the starting process of the hydraulic motor. After the pressure oil is injected, the static friction must be overcome when the hydraulic motor starts from static to moving. That is to say, after the pressure oil is injected, the output shaft will rotate through a small pretightening angle to overcome the gap between the movable parts and the elastic deformation of the parts, so that the load of the hydraulic motor is in the pretightening state before starting. At this time, Coulomb friction is generated between the relative sliding surfaces, and then the friction gradually increases, and the output torque gradually decreases. When the friction is fully established, the output torque tends to increase This is the starting torque. The hydraulic pressure makes the hydraulic motor overcome the friction and realize the start under load.

Starting mechanical efficiency η MS, also known as starting torque efficiency, refers to the ratio of starting torque ts and theoretical torque TT of the motor when the hydraulic motor starts from the static state, namely

ηms=Ts/Tt (1-36)

The starting torque and mechanical efficiency of hydraulic motor are affected by internal friction and torque pulsation. When the output shaft starts at different positions (phase angle), the starting torque is slightly different. In practical work, the starting performance is expected to be better, that is, the starting torque and starting mechanical efficiency are expected to be as large as possible. For different hydraulic motors, the starting mechanical efficiency (starting torque efficiency) is different.

(2) Braking performance when the hydraulic motor is used to drive the hydraulic winch to lift heavy objects, or to drive the walking mechanism of the excavator and other construction machinery to work, in order to prevent the heavy objects from falling or the walking mechanism from sliding on the slope, there are certain requirements for the braking performance of the hydraulic motor.

After the oil inlet and outlet of the hydraulic motor are cut off, the output shaft should not rotate at all in theory, but the hydraulic motor turns into "hydraulic pump working condition" due to the action of load torque. The oil outlet of the pump is the high pressure chamber. The high pressure oil leaks from this chamber, which makes the hydraulic motor rotate slowly (slip). This speed is called slip speed.

The sliding speed under rated torque is usually used to evaluate the braking performance of hydraulic motor. Sometimes the leakage at zero speed is used to express the braking performance. The better the sealing performance of the hydraulic motor, the lower the slip speed, the better the braking performance.

The hydraulic motor with end face distribution has the best performance. For the same structure of the hydraulic motor, when the load torque and oil viscosity are different, its braking performance is not the same.

There is always a gap between the relative moving parts in the hydraulic motor, so leakage slip is inevitable. Therefore, the hydraulic motor should be equipped with other braking devices for the machinery that needs long-time braking.

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