# Performance parameters of hydraulic pump

Performance parameters of hydraulic pump

1. Pressure of hydraulic pump

The pressure parameters of hydraulic pump mainly refer to working pressure and rated pressure.

(1) Working pressure P. The working pressure refers to the actual pressure of the output liquid when the pump works. Its size is determined by the external load. When the load increases, the working pressure of the hydraulic pump increases; When the load decreases, the working pressure of the hydraulic pump decreases.

(2) Rated pressure PN. The rated pressure of the hydraulic pump refers to the maximum allowable working pressure of the pump during normal operation. During normal operation, it is not allowed to exceed the rated pressure of the hydraulic pump. Exceeding this value is overload; The maximum working pressure of hydraulic pump is limited by the structural strength and leakage degree of pump parts.

Due to different uses of hydraulic transmission, the pressure required by the system is also different. In order to facilitate the design, production and use of hydraulic components, the pressure is divided into the following levels, as shown in the table below.

Pressure classification

Pressure class |
低 压 |
中 压 |
中高压 |
高 压 |
超高压 |

压力p(MPa) |
≤205 |
＞2.5～8 |
＞8～16 |
＞16～32 |
＞32 |

2. Displacement of hydraulic pump

The displacement of the hydraulic pump refers to the volume of discharged liquid calculated from the geometric dimension change of the sealing cavity according to each revolution of the pump shaft. The displacement can be expressed in V. The unit of displacement is L / R or ml / R.

3. Flow of hydraulic pump

The flow of hydraulic pump is divided into theoretical flow, actual flow and rated flow.

(1) Theoretical flow QT. The theoretical flow of hydraulic pump refers to the volume of discharged liquid calculated by the geometric size change of sealing cavity in unit time. The theoretical flow is expressed by QT, which is independent of the working pressure of the hydraulic pump. It is equal to the product of the displacement V of the pump and its speed n, i.e

qt=Vn （3-1）

(2) Actual flow Q. The actual flow of the hydraulic pump refers to the actual output flow when the pump is working, which can be expressed by Q. Due to the leakage problem of the hydraulic pump, its actual flow is always less than the theoretical flow. If the leakage is △ Q, there is

q = qt - △q (3-2)

(3) Rated flow QN. The rated flow of hydraulic pump refers to the output flow that must be guaranteed according to the test standard under normal working conditions.

4. Power of hydraulic pump

The hydraulic pump inputs the mechanical energy of the prime mover, which is expressed as torque T and speed n; Its output is liquid pressure energy, expressed as pressure P and flow QT (QT - VN). When the pressure output by the hydraulic pump can drive the hydraulic cylinder to overcome the load resistance F and move at a uniform speed at speed V (if energy loss is not considered), the theoretical power of the hydraulic pump and the hydraulic cylinder is equal, i.e

Pt - Tt2πn = Fv = pAv = pVn = pqt (3-3)

Then TT = PV / 2 π (3_4)

Where n -- rotational speed of hydraulic pump;

TT -- Theoretical torque of driving hydraulic pump;

P -- working pressure of hydraulic pump;

V - displacement of hydraulic pump;

A - effective working area of hydraulic cylinder.

If the actual torque T driving the hydraulic pump is used to replace the theoretical torque TT in the formula, the actual transmission power a p of the hydraulic pump can be obtained; By replacing the theoretical flow QT with the actual flow Q of the hydraulic pump, the actual output power P0 of the hydraulic pump can be obtained.

(1) Pump input power pi

Pi = T2πn (3-5)

(2) Pump output power P0

P0 = pq (3-6)

5. Efficiency of hydraulic pump

The output power of the hydraulic pump is always less than the input power, and the difference between the two is the power loss. Power loss can be divided into volume loss (flow loss caused by leakage) and mechanical loss (torque loss caused by friction). Volumetric efficiency is usually used for volumetric loss η V, mechanical loss is expressed by mechanical efficiency η M.

Volumetric efficiency refers to the ratio of actual flow and theoretical flow of hydraulic pump, i.e

η v=q/qt (3-7)

The leakage of hydraulic pump increases with the increase of pressure, and its volumetric efficiency decreases with the increase of pressure. Mechanical efficiency refers to the ratio of theoretical torque and actual torque of driving hydraulic pump, i.e

η m=Tt/T (3-8)

Since TT = PV / 2 π, there is

η m=pV/2πT (3-9)

Total efficiency of hydraulic pump η Is the ratio of the actual output power P0 to the actual input power, i.e

η= P0/Pi=pq/2πnT=(pVn/2πnT)(q/Vn)= η m η v (3-10)