By Gary Alexander, CFPHS
The hydraulic positive displacement pump is the heart of the hydraulic system. While it's true that pumps don’t create pressure, they do create flow. It's also true that resistance to flow creates pressure. This seemingly simple concept must be understood by anyone who designs or troubleshoots hydraulic systems. And although pumps don’t create pressure, they must be able to withstand the pressures generated in the hydraulic system by the load and other resistances.
In mobile hydraulic systems, two types of positive displacement pumps are widely used: gear and piston. The gear pumps are rugged, low cost and come in a wide range of displacement sizes and pressure capabilities, but are only available as fixed displacement units. Piston pumps cover an even wider range of sizes and pressure capabilities, are very volumetrically efficient, and have variable displacement capabilities with a variety of control options. However, piston pumps are significantly more expensive than gear pumps and are more sensitive to damage and failure due to both contamination and cavitation.
Gear pumps are typically rated for about 3,000 psi operating pressure, though some can operate as high as 4,500 psi. Piston pumps typically operate up to 5,000 psi and some are rated well beyond that, with some in the 20,000-30,000 psi range. While some piston pumps are large enough to pump several hundred gallons per minute, most gear pumps max out at around 50 gpm or less.
One of the most useful features, however, of piston pumps is their ability to be variable displacement. This means that controllers such as pressure compensators, flow compensators or load sense controls can reduce pump flow when there is little or less than full demand for flow. The result is that the hydraulic system can be made to operate much more efficiently because less flow is wasted while dumping over a relief valve creating heat and wasted horsepower.