By Josh Cosford
There are a lot of components in a hydraulic system that are not required to achieve the result of work, but you wouldn’t get very far without a pump. The hydraulic pump is the heart of any mobile machine, converting mechanical energy to hydraulic energy, and then pumping it out into the system. That a hydraulic pump actually pumps is often where similarities stop, as pump design and construction can vary vastly.
When applying a pump, one first considers the pressure required to meet the design criteria. The choice in pump construction can often be decided by the operating pressure, as a piston pump will be more reliable at 5,000 psi than, say, a vane pump. Once pump design is finalized, you will need to choose the physical displacement of the pump, which is the fluid volume it can pump in a single rotation, normally expressed in cubic centimeters or cubic inches per minute.
Pump construction is often a matter of preference, but some designs are better suited from some applications than others. The three main types of pump are gear, vane and piston. The gear pump family has two siblings; the outside and inside type. The outside gear pump is the popular one in the family, because it is effective, reliable and extremely inexpensive. It uses meshed spur gears, with one driven and one idle, and is as basic as a pump can get. Gear pumps are used in every conceivable application, especially where price is a concern.
Inside gear pumps are the overachieving valedictorian brother to the spur gear pump. They use an inside gear arrangement, with more complicated construction and tighter tolerances. They are highly efficient, very quiet, and unfortunately, can cost ten times as much as their little brother. Regardless, they are the number one choice for some fixed-pump applications, such as the recently popular servomotor driven closed-loop pump systems.
Vane pumps, once one of the most popular choices, use a rotating group with rectangular vanes pushed out via springs, centrifugal force or hydraulic pressure. The offset between the vanes and cam ring allows the pump to create flow, and in variable displacement vane pumps, the offset distance is what dictates volume. Vane pumps are quiet, reliable and easy to repair. They are most popular in industrial applications, especially those running lower pressure.
Piston pumps are used in the majority of high-pressure applications. The lion’s share of the market is the variable-displacement, axial-piston pump, with the remaining piston designs claiming a small slice of the pie. This pump is highly efficient, very reliable and relatively easy to rebuild—even though it is not very inexpensive. They are available with countless control options, such as pressure-compensated, load sensing, remote pressure control, proportional control, etc. Also, the vast majority of closed loop pumps are axial piston type, because of the versatility and robust construction it offers.