By Dan Helgerson, CFPS, CFPAI, CFPJPP, CFPSD, CFPMT, Fluid Power Journal Technical Editor
I am not an environmentalist. I do not believe the earth is my mother. I am convinced that we have been placed here to “Be fruitful, and multiply, and replenish the earth, and subdue it” (Genesis 1:28).
So what does that have to do with fluid power?
We have been given the privilege of using and enjoying the resources provided to us on the earth. But we are to be good stewards of those resources. Fluid power is a major consumer of those resources and a vital component of our ability to do work. It harvests our crops, takes our waste to the landfill, moves the landing gear, entertains us, and protects us, all with a power density and flexibility that is unmatched in any other power-transfer system. However, when it comes to efficiency, fluid power systems are not very high on the list. A study a few years ago revealed that the average efficiency of hydraulic and pneumatic systems is less than 30%. This inefficiency comes at great economic cost in wasted energy.
An article in the June 2019 issue of Fluid Power Journal entitled “Transforming Our Thinking About Energy Units and the Cost of Control” explained that many of our control systems are like “driving with the brakes on.” We have come to accept the parasitic losses built into our circuits as simply the necessary cost of control of fluid power. Restrictive flow controls, restrictive flow dividers, pressure-reducing valves, counterbalance valves, and proportional valves are some of the components used to control power by dissipating excess energy as heat. Variable displacement pressure-compensated pumps constantly dump about 5% of their full displacement at maximum pressure through the case drain, even when there is no flow required. Load sensing pumps operate at least 1.4 MPa (200 psi) above the pressure required at the actuator and continually dump at least 5% of full displacement at that pressure through the case drain. The new Digital Displacement pumps are remarkable in that they supply the right flow at the correct pressure but require intricate electronic controls and are 100% inefficient at minimum displacement. The new floating cup pumps show an overall efficiency of 95%, but an extremely efficient pump in a circuit where all the fluid is going across a relief valve is in a system that is 100% inefficient.
A great deal of work continues in developing more efficient components: cylinders with lower breakaway, pumps and motors with greater volumetric and mechanical efficiency, compressors driven with variable speeds, and variable displacement and load sensing pumps, many requiring more sophisticated electronic controls. These are beneficial, but they often come at a substantial increase in cost that must be weighed against the potential energy savings.
There are products emerging that are targeted to replace the restrictive flow, pressure, and directional controls. But these require thinking differently, which may be the greatest challenge.
To be good stewards of the resources available to us, we need to provide and maintain the most efficient systems possible, and that means rejecting “rules of thumb,” forgetting the “fudge factors,” and doing the math.