How To Calculate Hydraulic Pump and Motor Efficiency
Hydraulic pumps and motors are essential components of many fluid power systems. They convert mechanical energy into hydraulic energy, and vice versa. The performance and efficiency of these components affect the overall operation and energy consumption of the system.
But how can we measure and calculate the efficiency of hydraulic pumps and motors? And what factors influence their efficiency? In this article, we will answer these questions and provide some practical tips to improve the efficiency of your hydraulic system.
What is efficiency?
Efficiency is a ratio of output to input. It indicates how well a component or a system uses the input energy to produce the desired output. The higher the efficiency, the less energy is wasted as heat, noise, or friction.
There are three types of efficiency used to describe hydraulic pumps and motors: volumetric efficiency, mechanical/hydraulic efficiency, and overall efficiency.
Volumetric efficiency
Volumetric efficiency is the ratio of actual flow delivered by a pump or motor at a given pressure to its theoretical flow. Theoretical flow is calculated by multiplying the displacement per revolution by the driven speed. For example, if a pump has a displacement of 100 cc/rev and is driven at 1000 RPM, its theoretical flow is 100 L/min.
Actual flow, on the other hand, has to be measured using a flow meter. This is because the actual flow is always less than the theoretical flow due to internal leakage and slippage in the pump or motor. For example, if the same pump delivers 90 L/min at 207 bar, its volumetric efficiency is 90% (90 / 100 x 100 = 90%).
Volumetric efficiency is an indicator of the condition of a pump or motor. It reflects the amount of internal leakage and wear in the component. As the component wears out, its volumetric efficiency decreases, resulting in lower flow and slower speed.
Mechanical/hydraulic efficiency
Mechanical/hydraulic efficiency is the ratio of theoretical torque required to drive a pump or motor to the actual torque required to drive it. Theoretical torque is calculated by multiplying the displacement per revolution by the pressure. For example, if a pump has a displacement of 100 cc/rev and delivers 207 bar, its theoretical torque is 329 Nm.
Actual torque, however, has to be measured using a dynamometer. This is because the actual torque is always higher than the theoretical torque due to mechanical and fluid friction in the component. For example, if the same pump requires 360 Nm to drive it, its mechanical/hydraulic efficiency is 91% (329 / 360 x 100 = 91%).
Mechanical/hydraulic efficiency is an indicator of the friction losses in a pump or motor. It reflects the amount of energy converted into heat and noise in the component. As the component wears out, its mechanical/hydraulic efficiency decreases, resulting in higher torque and power consumption.
Overall efficiency
Overall efficiency is the product of volumetric efficiency and mechanical/hydraulic efficiency. It is the ratio of power output to power input of a pump or motor. For example, if a pump has a volumetric efficiency of 90% and a mechanical/hydraulic efficiency of 91%, its overall efficiency is 82% (90 x 91 / 100 = 82%).
Overall efficiency is the most important measure of the performance of a pump or motor. It indicates how much of the input power is converted into useful hydraulic power. The higher the overall efficiency, the lower the energy consumption and operating cost of the system.
How to calculate efficiency?
To calculate the efficiency of a hydraulic pump or motor, you need to know the following parameters:
- Displacement per revolution (cc/rev or in3/rev)
- Driven speed (RPM)
- Pressure (bar or psi)
- Flow (L/min or GPM)
- Torque (Nm or lb-ft)
- Power (kW or hp)
You can find the displacement per revolution and the driven speed from the specifications of the pump or motor. You can measure the pressure and the flow using a pressure gauge and a flow meter, respectively. You can measure the torque and the power using a dynamometer, or you can estimate them using the following formulas:
- Torque (Nm) = Power (kW) x 9550 / Speed (RPM)
- Power (kW) = Torque (Nm) x Speed (RPM) / 9550
Once you have all the parameters, you can calculate the efficiency using the following formulas:
- Volumetric efficiency (%) = Flow (L/min) / (Displacement (cc/rev) x Speed (RPM) / 1000) x 100
- Mechanical/hydraulic efficiency (%) = (Displacement (cc/rev) x Pressure (bar) / 20 x ?) / Torque (Nm) x 100
- Overall efficiency (%) = Volumetric efficiency (%) x Mechanical/hydraulic efficiency (%) / 100
For example, let’s say we have a pump with the following parameters:
- Displacement per revolution = 100 cc/rev
- Driven speed = 1000 RPM
- Pressure = 207 bar
- Flow = 90 L/min
- Torque = 360 Nm
- Power = 37.7 kW
We can calculate the efficiency as follows:
- Volumetric efficiency (%) = 90 / (100 x 1000 / 1000) x 100 = 90%
- Mechanical/hydraulic efficiency (%) = (100 x 207 / 20 x ?) / 360 x 100 = 91%
- Overall efficiency (%) = 90 x 91 / 100 = 82%
How to improve efficiency?
The efficiency of a hydraulic pump or motor depends on several factors, such as design, operating conditions, maintenance, and wear. Some of these factors are fixed and cannot be changed, while others can be improved by following some best practices. Here are some tips to improve the efficiency of your hydraulic system:
- Choose the right pump or motor for your application. Select a component that matches the required flow, pressure, speed, and power of your system. Avoid oversizing or undersizing the component, as this will reduce the efficiency and increase the energy consumption.
- Maintain the optimal operating conditions. Adjust the pressure and the speed of the pump or motor according to the load and the demand of the system. Avoid running the component at excessive or insufficient pressure or speed, as this will cause unnecessary losses and wear.
- Keep the fluid clean and cool. Use a high-quality hydraulic fluid that meets the specifications of the pump or motor. Change the fluid and the filters regularly to prevent contamination and degradation. Install a cooler and a reservoir to dissipate the heat and maintain the fluid temperature within the recommended range.
- Check and repair the leaks and the wear. Inspect the pump or motor regularly for any signs of leakage or damage. Replace the worn or damaged parts, such as seals, bearings, valves, gears, pistons, etc. Use proper tools and techniques to avoid damaging the component during installation or removal.
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