# Main parameters of hydraulic motor

Main parameters of hydraulic motor

1.7.1 main parameters

The main parameters of hydraulic motor are pressure, speed, displacement and flow, power and efficiency.

(1) Pressure

① Working pressure p the input pressure of the hydraulic motor is called the working pressure of the hydraulic motor. The working pressure of hydraulic motor depends on the load torque and oil discharge pressure (back pressure).

② Rated pressure PN under normal working conditions, according to the test standard can run continuously the highest pressure, known as the rated pressure of hydraulic motor, rated pressure is also known as the nominal pressure. The rated pressure of hydraulic motor is restricted by the structural strength, leakage and other factors of the motor itself.

③ According to the test standard, the maximum allowable pressure Pmax, which exceeds the rated pressure for short-term operation, is called the maximum allowable pressure of hydraulic motor (referred to as the maximum pressure).

④ Pressure difference △ p the difference between the oil inlet pressure and the oil outlet pressure of the motor is called the pressure difference of the hydraulic motor. When the discharge pressure of the motor is close to zero, the inlet working pressure P of the motor can be used to replace △ P.

⑤ Minimum oil return back pressure in order to avoid void phenomenon, the minimum pressure must be maintained in the oil discharge chamber of hydraulic motor.

The measurement unit, pressure series and pressure classification of hydraulic motor pressure are the same as those of hydraulic pump.

(2) Rotational speed

① The rated speed NN is the highest speed that can run normally for a long time under the rated pressure, which is called the rated speed of hydraulic motor.

② The maximum speed nmax under the rated pressure, which exceeds the rated speed to allow short-term operation, is called the maximum speed of hydraulic motor.

③ The minimum speed Nmin is the minimum speed allowed for normal operation, which is called the minimum speed of hydraulic motor.

(3) Displacement and flow

① Displacement V the volume of liquid flowing into the shaft of a hydraulic motor calculated by the geometric dimension change of its sealing cavity is called the theoretical displacement of the hydraulic motor (displacement for short), that is, the volume of liquid that can be entered when the motor shaft rotates one revolution without leakage.

② Theoretical flow QT the input liquid volume of the hydraulic motor calculated by the change of its sealing cavity geometry in unit time, that is, the liquid volume that can be input in unit time without leakage, is called the theoretical flow of the hydraulic motor, which generally refers to the average theoretical flow without considering the pulsation of the liquid input by the hydraulic motor.

The measurement unit of motor flow is the same as that of hydraulic pump.

The relationship between theoretical flow QT and displacement V of hydraulic motor is as follows:

qt=(vn/60) ×10-6(m3/s) (1-20)

Where V -- displacement of hydraulic motor, ml / R;

N -- speed of hydraulic motor, R / min.

③ The instantaneous flow of a motor is called the instantaneous flow of a hydraulic motor. It generally refers to the instantaneous theoretical (geometric) flow, which has a certain pulsation.

④ Average flow QAV the flow calculated by average time is called average flow of hydraulic motor.

⑤ When the rated flow QN operates under rated pressure and rated speed, according to the test standard, the input flow of the hydraulic motor must be guaranteed.

⑥ Actual flow Q the actual input flow when the hydraulic motor is working is called the actual flow of the hydraulic motor.

When the hydraulic motor works, it will lose part of the flow due to leakage, so the actual flow Q of the hydraulic motor is greater than the theoretical flow QT of the motor. The actual flow can be expressed as

q=qt+q1 (1-21)

Where Q1 -- flow loss caused by leakage of hydraulic motor to the outside or low pressure chamber.

(4) Power and efficiency

The hydraulic motor is used to drive the load to realize the rotary movement. The input is hydraulic energy, which is expressed as the pressure P and flow Q of the liquid; the output is mechanical energy, which is expressed as torque and speed n (or angular velocity W). The energy conversion process of hydraulic motor in operation can be represented by the block diagram shown in Figure W.

① Theoretical power (theoretical input power and theoretical output power) does not consider the loss of hydraulic motor in the process of energy conversion. The theoretical input hydraulic power of hydraulic motor is called theoretical input power; the theoretical mechanical power is called theoretical output power.

The theoretical input power is equal to the theoretical output power, so it can be expressed by the same symbol Pt, and its expression is

(1-22)

Where △ P -- pressure difference between inlet and outlet of hydraulic motor, PA;

QT -- theoretical flow rate of hydraulic motor, m3 / S;

N -- speed of hydraulic motor, R / min;

V -- displacement of hydraulic motor, ml / R;

TT -- theoretical output torque of hydraulic motor, n · m;

ω - rotation angular velocity of hydraulic motor, rad / s.

② Actual power (actual input power PI and actual output power P0) the actual input hydraulic power of the hydraulic motor is called the actual input power of the hydraulic motor, and its expression is

Pi=△pq (W) (1-23)

Where △ P -- pressure difference of hydraulic motor, PA;

Q -- actual input flow of hydraulic motor, m3 / s.

The actual output power of the hydraulic motor shaft is called the actual output power of the hydraulic motor, and its expression is

Po=Tω=T（2πn/60）(W) (1-24)

③ When the theoretical torque TT and the actual torque t do not account for the energy loss of the hydraulic motor in the process of energy conversion, the output torque of the hydraulic motor (theoretical output torque) is called the theoretical torque of the hydraulic motor. According to the formula (1-22) reflecting the energy conservation relationship, there are

△pqt=△pVn/60×10-6=Ttω=Tt2πn/60 (1-25)

So TT = △ PV / 2 π × l0-6 (n · m) (1-26)

The meanings and units of the symbols in the formula are the same as those in formula (1-22).

In the working process, the actual output torque of the hydraulic motor is called the actual torque. Because the torque loss T1 is caused by friction when the hydraulic pump works, the actual torque T is less than its theoretical output torque TT, that is, the actual output torque of the hydraulic motor is

T=Tt-Tl (1-27)

① Efficiency because there are flow loss (volume loss) and mechanical friction loss in the process of energy conversion of hydraulic motor, the output power of the motor is less than the input power, and the difference between the two is the power loss. The power loss consists of flow loss and torque loss, and the power loss is also expressed by efficiency.

a. The volumetric efficiency η V is the theoretical flow rate Q of the hydraulic motor. The ratio of Q to the actual flow rate is called the volumetric efficiency of the motor

ηv=qt/q (1-28)

Substituting formula (1-21) into formula (1-28) has

ηv=qt/q=qt/(qt+q1) (1-29)

Where Q1 -- the flow of multiple inputs to ensure that the output speed of the hydraulic motor meets the requirements due to the volume loss.

Volumetric efficiency represents the ability of a hydraulic motor to resist leakage.

b. The ratio of the actual output torque T to the theoretical torque T is called the mechanical efficiency of the hydraulic motor

ηm=T/Tt (1-30)

Substituting equation (1-27) into equation (1-30) has

ηm=T/Tt=(Tt-T1)/Tt (1-31)

Substituting formula (1-26) into formula (1-31) has

ηm=(2πT/△pV) ×106 (1-32)

C -- total efficiency η the ratio of the output mechanical power of the hydraulic motor to the input hydraulic power is called the total efficiency of the hydraulic motor, i.e

(1-33)

That is, the total efficiency of hydraulic motor is equal to the product of volumetric efficiency and mechanical efficiency.

According to GB / T According to 7935-1987 "general technical conditions for hydraulic components", the name, model and graphic symbol of the motor shall be marked on the product name plate of the hydraulic motor (installed on the obvious part of the product), and the main technical parameters such as displacement, rated pressure, maximum pressure, rated flow, rated speed, volumetric efficiency, total efficiency and output torque of the hydraulic motor shall also be marked.