Understanding Hydraulic Pump and Motor Efficiency
Hydraulic pump and motor efficiency can have a significant effect on your hydraulic system. It influences the power demand and therefore running costs of a hydraulic system. It determines the amount of heat generation and dictates the requirement for cooling. It can also effect the operating speed of a hydraulic system.
It is for these reasons that an understanding of pump and motor efficiency can assist greatly in understanding and diagnosing issues with a hydraulic system. The three most considered efficiencies are volumetric efficiency, hydro-mechanical efficiency and total efficiency.
Volumetric efficiency is a measurement of the actual flow from a pump, expressed as a percentage of the theoretical flow. Actual flow can be measured using a flow meter. Theoretical flow is easily calculated for a pump by multiplying the displacement by the drive speed and converting units as required. For instance, a 20cc/rev pump being driven at 1500rpm will have a theoretical flow of 30 litres per minute (in this case the product is divided by 1000 to convert cubic centimetres to litres). The volumetric efficiency of a motor is a measurement of the actual rotation speed for a given flow rate, expressed as a percentage of its theoretical speed.
Reduction in volumetric efficiency over time is considered a reliable indicator of pump and motor wear or degradation. For pumps and motors with case drains, flow from these drain lines can also be used to monitor changes in volumetric efficiency due to degradation as these components often leak into their casings. As the volumetric efficiency of a pump drops, the speed of actuators in the system will be reduced accordingly as they receive less flow. Eventually, a hydraulic system may become inoperable and pumps and motors showing symptoms of degradation as determined using volumetric efficiency will likely require hydraulic repair or replacement to restore proper system functionality.
Hydro-mechanical efficiency is a measurement of the theoretical torque required to drive a pump, expressed as a percentage of the actual torque required. For a motor, hydro-mechanical efficiency is a measurement of the actual torque output, expressed as a percentage of the theoretical torque output. This torque difference is caused largely by internal fluid and mechanical friction within pumps and motors. Theoretical torque can be calculated for a pump or motor by multiplying the displacement (in cc/rev) by the pressure drop (in Bar) and dividing by 20π, the resulting answer in Newton meters (Nm). For instance, the theoretical output torque of a motor with a displacement of 20 cc/rev, a pressure of 200 Bar would be 63.7 Nm.
Actual torque is relatively difficult to measure as it requires a dynamometer or other specialised and calibrated equipment. As a result, hydro-mechanical efficiency is most useful in estimating the actual output torque of a hydraulic motor having already calculated the theoretical torque. Using a high efficiency motor, such as a Char-Lynn geroler motor, will result in a high output torque for a given displacement and pressure.
Finally, total efficiency is a measurement of the theoretical power required to drive a pump (or developed by a motor), expressed as a percentage of the actual power requirement. It considers both losses due to leakage or bypass (volumetric efficiency) as well as losses due to internal friction (hydro-mechanical efficiency). It can be calculated by multiplying these two efficiencies together. Total efficiency is used to calculate the power demand of a hydraulic pump. Power (in kW) is equal to pressure (in Bar) multiplied by flow (in litres per minute) divided by 600 and the total efficiency. For instance, a pump with an output of 30 litres per minute and 200 Bar, with a total efficiency of 80%, would require 12.5kW of input power.
A hydraulic system whose pump has a higher total efficiency will require less power to produce the same pressure and flow. This difference in power will be reflected in lower running costs. However, total efficiency can often have a much larger bearing on the cooling requirements of a hydraulic system. The power lost through total efficiency will take the form of heat energy transferred into the hydraulic fluid, raising its temperature and at times necessitating a heat exchanger to dissipate this energy.
Berendsen Fluid Power has 9 branches Australia-wide which can provide assistance in testing and repairing hydraulic motors and pumps.