Fluid power technology emphasizes the use of efficiencies as key figures of merit for many products across multiple marketplace segments. Such reasoning is sound, especially with the push to reduce energy consumption. However, efficiency is too simplistic a measure, and dare say, tends to be rather abused.
First, efficiency often is implied as “constant throughout the operating range of the machine in question, such as a pump, motor, or cylinder.” Second, the quoted value is usually the peak value that’s achievable by the machine. This is simply not a valid use of known efficiency values.
Industry applies well-known efficiencies to pumps and motors, namely volumetric efficiency (ηV) and mechanical efficiency (Hm). Both are calculated as ratios of ideal to actual flow and torque, or vice versa.
They are somewhat artificial and depend on knowledge of the machine’s displacement, which doesn’t conveniently yield to direct measurement. Instead, it requires some judicious assumptions or total reliance on empirical methods. This doesn’t minimize their value, though, as both of these artifices are employed in pump and motor development.
Overall efficiency (ηO) is the ratio of actual delivered output power to total input power. This “real” efficiency can be determined by means of objective measurements.
It is common knowledge that efficiencies are not constant, but, rather, vary with current operating conditions (e.g., pressure, flow, load torque, speed, etc.) from near zero to values that approach near 100%. As a result, they require interpretation.
Lost Power Method
A better method for assessing losses accrued in rotating machinery is the lost powerconcept. For each machine and each component, suitable mathematical models that use measured input and output power can calculate the amount of power lost (Wlost) to friction and internal leakage. Like efficiency, it’s simple in principle:
Wlost= Win– Wout
Lost power’s main advantage is that actual power levels are retained in any desired system of units. Efficiency is always relative, considering that it’s a dimensionless ratio. The lost power, like efficiency, varies with the operating conditions, such as pressure, speed, and load. However, because actual units of power are retained, the lost-power curve for a machine helps determine the total energy lost during, say, a normal operating shift or any desired time interval.
Two key sets of data are required to make concrete evaluations of total lost energy in a given application. First, a lost-power diagram for the machine can be based on suitable mathematical models, or alternatively, taken directly from measured input and output powers. Second, a histogram typifies the machine’s operating cycle, which requires on-board data acquisition and sampling computers as well as appropriate transducers. The case studied in this article required a rotational speed-sensing device.
Simultaneous use of the lost-power curve and operational histogram can evaluate the relative merits of a machine and its application in terms of lost energy. The methods are simple in concept and don’t require advanced mathematical methods. However, like all empirical processes, it may need lots of data manipulation. That’s why, in this scenario, a computer becomes indispensable.