First, if we are looking for maximum efficiency, we would want to set the relief valve at 1000 PSI since this is all the pressure that is needed to obtain the required force. We find, however, that when the cylinder bottoms out, our pump flow is relieved to tank at 1500 PSI rather than at the initial setting of 1000 PSI. Although all flow across a relief turns energy into heat, a 500 PSI override increases our energy loss by 33%, or a net increase of 3 1/2 hp for this particular example.
Second, if the cylinder encounters a slight increase in load, say 250 pounds, ideally, we would want our system to stall since it was our original intent to limit the output force to 1000 lbs. In actual practice, however, we find that the system will do the work at approximately half speed, while wasting the other 50% of our power input in the form of heat, as oil flows across the relief. When the load is mechanically stalled, our pressure again rises to 1500 PSI creating 500 lbs. more thrust than originally intended.
Friday, December 23, 2016 21:31