Created on:2021-05-28 09:24

Notes on no load displacement of hydraulic pump and motor

Notes on no load displacement of hydraulic pump and motor

Measurement accuracy level

(1) Measurement accuracy level. According to the required accuracy requirements, the test shall be carried out in one of the three measurement accuracy levels a, B and C as agreed by the parties concerned.

Note 1: Class A and class B are suitable for special occasions requiring very accurate performance measurement.

Note 2: note that class A and class B tests require very accurate instruments and methods, which will increase the cost of such tests.

(2) Error. Any measuring instrument and method used shall be calibrated or compared with the international standard to prove that it has the measurement capability not exceeding the systematic error given in Schedule 6.


Allowable system error of measuring instrument

Controlled parameters





























Appendix B (normative appendix)

Calculation of no load displacement by zero pressure interception method

(1) General. The displacement is nominally constant over the entire pressure range and the flow rate should be recorded. If the initial calculation result of no-load displacement is inconsistent with the nominal no-load displacement within the whole continuous rated pressure range specified by the manufacturer, the test should be conducted again, and the inlet pressure of the pump or the outlet pressure of the motor should be increased during the test.

It should be confirmed that the deviation of no-load displacement is not caused by temperature control error of test medium, measurement error or any parameter control error. The suspicious value should be reevaluated under appropriate conditions to determine whether the data is within the error or abnormal in the expected performance under specified test conditions. If the test results in suspicious values, the test shall be repeated under appropriate conditions.

(2) Calculation method of VI in no-load platoon. Under the pressure P of several pump outlets or motor inlets, the no-load displacement of the hydraulic pump or motor is determined by measuring the flow Q of the hydraulic pump outlet or motor inlet (see Figure d).


Firstly, the obvious random error is eliminated, and then the best linear relationship between Q and P is established by the least square method within the specified pressure range. The no-load displacement VI can be determined by intercepting the characteristic curve of Q relative to △ P with zero pressure, because the no-load displacement is nominally constant in the whole pressure range (see Fig. d). According to each group of test conditions (i.e. shaft speed, test medium temperature and displacement of the test piece), the no-load displacement VI can be determined by the least square method of the equations shown in formula (annex-1) and formula (annex-2), and the ml / R is used express.

VI = Q0 / N (when △ P = 0) (attached-1)


Where Q0 -- flow rate when △ P = 0, L / min;

Qi -- flow rate measured at API for the first time, L / min;

N -- shaft speed, R / min;

Δ p - pressure difference, that is, the pressure difference between the outlet and the inlet, MPa (bar);

Δ PI -- pressure difference at specific pump outlet or motor inlet, MPa (bar);

K -- the measured quantity of pump outlet pressure or motor inlet pressure during the test.

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