Fixed vs. Variable Displacement Vane Pumps

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 vane 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 vane pumps can be used to power a wider variety of tools, but require more expense and more attention.


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What Is a Variable Displacement Hydraulic Pump?

A variable displacement hydraulic pump is a type of mechanical-electrical motor placed in a fluid channel. Built either to generate electrical power from the natural flow of fluid past it or to apply mechanical force to make stationary fluid flow in a pipe gives the variable displacement hydraulic pump a dual use — adjustable capability. Such pumps have versatile applications because they can be set to capture the maximum amount of hydraulic fluid energy as the velocity of fluid flow in a channel dynamically changes, or they can be switched to applying mechanical energy powered by electricity to increase fluid flow.


Hydraulic motors designed on the variable displacement hydraulic pump axial principle have three common features regardless of their size or the complexity of their controls. These include a central axis, a cylinder of pistons surrounding it, and a circular plate to which the ends of all of the pistons are attached. They are usually built on the axis piston pump or bent axis pump design, which allows the vertical angle at which the pistons are positioned to be adjusted either by angling the axle itself, or the plate that is attached to the ends of the pistons.


The circular array of pistons in a variable displacement hydraulic pump moves in unison, but they displace different amounts of fluid depending on their horizontal positions in the piston shafts in which they move. As the internal retainer plate, or external swash plate, rotates at an off-center vertical angle of 25° to 40° in the pipe, the pistons alternate between pushing fluid through an exhaust pipe, and pulling it into an intake pipe. These outlet and inlet pipes are known as kidney slots and the piston chambers are referred to as cylinder barrel bores.


Another key feature of variable displacement hydraulic pumps is what’s known as the compensator. This is a collection of controls often attached to the external housing of the pump, which adjusts the angle of the swash plate or retainer plate. If the swash plate is set to a perfect vertical angle on the pump as it rotates, the displacement will be zero and fluid flow will stop. Compensators can be simple or complex controls, but their purpose is to change the variable settings on the pump for maximum efficiency.


ANSON is a professional hydraulic pump supplier with rich experiences. We specialize in manufacturing intra vane pump, vane pump cartridge kits, variable displacement hydraulic pump, fixed displacement vane pump, hydraulic power pack pump and more. If you are interested in variable displacement hydraulic pump series, welcome to visit our website and feel free to contact ANSON!


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