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Hydraulic Pumps: Fixed vs. Variable Displacement

A hydraulic drive system uses a pressurized fluid to deliver force to distant machinery. Each system has several common components; the most universal is the hydraulic pump. The pump’s purpose is to pressurize the hydraulic fluid so that it will travel down the line and perform work on the other side. In an ‘open loop’ system, the fluid is drawn from a reserve tank, and deposited into the same tank after it has done its work. In a ‘closed loop’ system, the fluid is brought directly back to the hydraulic pump after passing through a hydraulic filter.

Fixed Displacement Pumps
A fixed-displacement pump has a set flow rate — every stroke of the motor moves the same amount of fluid. Fixed-displacement pumps are

  • Simple
  • Relatively inexpensive
  • Easier to maintain

The simplest type of fixed-displacement pump is the gear pump, in which the hydraulic fluid is pushed by rotating gears. In some models, the gears are sequential; in the quieter and more efficient version, the gears are interlocking. Another common variation is the screw pump, which uses the classic Archimedes screw, which looks much like a drill bit, to move the fluid. They have the advantage of providing a high rate of flow at relatively low pressures.

Variable Displacement Pumps
In a variable-displacement pump, the flow rate and outlet pressure can be changed as the pump operates. This results in pumps that are

  • More complex
  • More expensive
  • Capable of doing a wider variety of jobs

The most common type of variable-displacement pump is the rotary vane pump, which is a variation of the gear pump in which the ‘gear’ is offset and the ‘cogs’ aren’t fixed, but rather extend and retract as the gear turns, allowing the pump to increase the pressure of the fluid by compacting it as it pushes the fluid through. The top-tier pumps, however, are bent-axis piston-and-cylinder pumps, much like the ones that are used in an internal combustion engine.

Simple, fixed-displacement pumps are perfect for single jobs that need to be repeated indefinitely over long periods of time; variable-displacement pumps can be used to power a wider variety of tools, but require more expense and more attention.