MY
TORQ UE ISN’T WORKING
this: when we push fluid against a resistance, we charge the fluid with energy, whether it is for storage in an accumulator or for immediate use. That energy is then conveyed by the fluid to the work site. Any energy in excess of what is needed to do the work has to be removed and is lost as heat. As long as we continue to think about flow and pressure as separate entities in our design process, we will have difficulty in finding a solution to what we may think are inherent energy issues when using fluid power. In our pneumatic systems, we waste energy in different ways. We store the energy in a receiver just as with a hydraulic accumulator. If we did the same analysis of the energy stored, we would find the potential energy to be the pressure times the cubic inches of compressed air. What would be the potential energy in a one-gallon receiver charged to 120 psi? Well, a one-gallon receiver
holds 231 cubic inches, so we would have 231 in3 x 120 psi for a potential energy of 27,720 lb-in. If we take our 1-in2 piston, power it with compressed air, and require a torque of 60 in-lb in each direction, we find an energy requirement of 180 lb-in each way, for a total of 360 lb-in. As before, we have more energy available than is needed. When we apply a meter-out flow control, we add a resistive load to the cylinder that matches the difference between the available stored energy and the workload, in this case, 60 pounds. We use up 720 lb-in of energy to do 360 lb-in of work. If we apply a meter-in flow control, we do not reduce the energy used. We simply slow down the rate of energy used. There is a 60-psi pressure drop across the flow control, but when the cylinder comes to the end of each stroke, the pressure equalizes and the cylinder fills to 120 psi. We still used 720 lb-in of energy to do 180 lb-in of work. However, if we replace the meter-in flow control with a pressure regulator, something different happens. Air is compressible. Pressure builds as more air molecules are pushed into a confined space. Limiting pneumatic pressure, by definition, limits the amount of air that can enter the space. A pneumatic pressure regulator reduces the energy that is taken out of storage; it can reduce the energy units used to what is needed for the load.
CIRCLE 103
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www.FluidPowerJournal.com • Tech Directory 2016 • www.IFPS.org
In a hydraulic system, flow controls and pressure-reducing valves extract pressure from each energy unit, preserving the volume necessary for the work. In pneumatic systems, flow controls reduce the rate of flow of the energy units but do not save energy. A pressure regulator reduces the number of energy units that are allowed to pass. In order for us get our torque working properly, we must find a way to transform our energy units in a way that will allow us to use the energy without waste. The next article will begin that discussion.
Dan Helgerson, CFPAI/AJPP, CFPS, CFPECS, CFPSD, CFPMT, CFPCC, is Fluid Power Journal’s technical editor. He can be reached at dan@cfpsos.com or by visiting www.cfpsos.com. Visit www. fluidpowerjournal.com to read archives of this article series, and keep the conversation going by visiting Dan Helgerson’s blog, Watts It All About, linked on our homepage.
CIRCLE 104