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Method for Calculating the Theoretical Flow Rate of the Hydraulic System in a Truck-Mounted Pump
Release time:
2026-05-13
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Summary:
In the design, fault diagnosis, and performance evaluation of vehicle-mounted pump hydraulic systems, theoretical flow rate is a fundamental and critical core parameter. It represents the volume of hydraulic fluid that the pump can deliver per unit time under ideal conditions—i.e., when no internal leakage, friction losses, or fluid compressibility are taken into account. Mastering the method for calculating theoretical flow rate facilitates an understanding of the system’s operational capability and provides a quantitative basis for pump selection, motor power matching, and systematic problem analysis.
The calculation of theoretical flow rate hinges on understanding the concept of hydraulic pump displacement. Displacement refers to the volume of fluid that is theoretically discharged from the pump’s outlet chamber for each complete revolution of the pump shaft, typically expressed in milliliters. / Transfer ( ml/r ) as the unit. This is a geometric parameter determined by the pump’s structural dimensions—such as the plunger diameter and stroke, gear module and tooth width—and represents an intrinsic characteristic of the pump itself, independent of rotational speed and pressure.
Based on the definition of displacement, the calculation formula for the theoretical flow rate of a hydraulic pump is straightforward and unambiguous:
Theoretical flow (Q_t)= Displacement (V_g) × Rotational Speed (n)
Among them:
Q_t This is the theoretical flow rate, typically expressed in liters. / minute (L/min) 。
V_g It is the pump displacement, with units in milliliters. / Transfer (ml/r) . When performing calculations, ensure that units are consistent. 1 Equals to 1000 Milliliter.
n It is the input rotational speed of the pump shaft, with units in revolutions. / minute (revolutions per minute) 。
Therefore, the calculation formula can be specifically expressed in practical applications as:
Q_t (L/min) = [V_g (ml/r) times n(r/min)]/1000
This divided by its 1000 The operation is precisely to convert milliliters to liters, thereby obtaining the result in liters. / Traffic value in minutes.
Let’s demonstrate the calculation process through an example:
Assume a main hydraulic pump that supplies oil to an on-board pump, with a displacement of 100ml/r , it is driven by the engine via a power take-off, with a rated operating speed of 1500r/min Therefore, the theoretical flow rate of this pump is calculated as follows:
Q_t = (100 ml/r times 1500 r/min)/1000 = 150 L/min
This means that, under ideal conditions, this pump operates at per minute 1500 At the given rotational speed, it can output 150 Hydraulic oil for the lift.
It is crucial to understand the relevant concepts and considerations regarding theoretical flow:
1. Theoretical flow vs. Actual Flow: It is essential to clearly distinguish between theoretical flow and actual flow. Actual flow refers to the flow rate measured at the pump outlet, which is always lower than the theoretical flow. The difference between the two is primarily caused by internal leakage (volumetric loss) within the pump. When the pump operates under pressure, the sealing clearances between the high-pressure and low-pressure zones give rise to internal leakage; the higher the pressure, the greater the internal leakage, and consequently the lower the actual flow. Volumetric efficiency ( eta _v) It is the parameter used to quantify this loss: * Actual flow = Theoretical flow × volumetric efficiency *。
2. Variable displacement: The above calculations are based on fixed-displacement pumps. For variable-displacement pumps, which are widely used in vehicle-mounted pumps, their displacement ( V_g ) can vary between zero and a certain maximum value. Therefore, the theoretical flow rate of a variable-displacement pump is not a fixed value; rather, it changes in accordance with the setting of the displacement adjustment mechanism. To calculate the theoretical flow rate at a given instant, one must use the currently set displacement value.
3. Rotational speed stability: the pump’s rotational speed ( n ) is directly proportional to the flow rate. For vehicle-mounted pumps driven by diesel engines, fluctuations in engine speed will directly result in variations in the hydraulic pump’s output flow. Therefore, maintaining stable engine speed is crucial for ensuring smooth operation of the hydraulic system.
Conclusion
Calculating the theoretical flow rate of an on-board pump hydraulic system hinges on mastering the “theoretical flow rate.” = Apply the fundamental formula “displacement × speed,” ensuring consistent unit conversion. Recognize that theoretical flow represents the ideal upper limit of system performance, whereas actual flow is reduced due to internal leakage. This understanding is essential for accurately analyzing system operating conditions and determining whether hydraulic pump volumetric efficiency has dropped excessively due to wear, thereby necessitating maintenance. Such calculations constitute a foundational skill for system design, commissioning, and maintenance.
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