Hydraulic Pump Selection

The Hydraulic Pump is the heart of the Hydraulic System,
providing the flow to meet the demands of the actuators at the output end of the system

An early choice based on system demand is that of Fixed or variable displacement.
Both Vane and Piston pumps are available with a range of displacement controls linked to pressure.
As well as flow and speed capability, pressure also is a vital
element within the choice of a particular pump.

Piston Pumps

Piston pumps can have the pistons arranged in a radial or axial fashion. Radial types tend to be specialised
for applications requiring very high power, while axial piston pumps are available in a wide range of displacements
and pressure capabilities that make them suitable for many mobile and industrial tasks.

Axial-piston pumps consist of a set of pistons that are fitted within a cylinder block and driven
by an angled swash plate powered by the input shaft. As the swash plate rotates, the pistons reciprocate
in their respective cylinder block bores to provide the pumping action. Axial-piston pumps are available with the
input shaft and pistons arranged coaxially, or with the input shaft mounted at an angle to the piston bores.
Bent axis pumps tend to be slightly more volumetrically efficient for technical reasons, but they
also tend to be slightly larger for a given capacity and their shape can present packaging difficulties in some applications.
A unique characteristic of a piston-type pump is that the displacement can be changed simply by changing
the angle of the swash plate. Any displacement between zero and maximum is easily achieved
with relatively simple actuators to change the swash plate angle.

Key Details:
• Displacements up to 500cm3/Rev
• Typical Pressure up to 380Bar
• Simple Multiple Assemblies (Tandem Mounting)
• High Overall Efficiency
• Compact Design
• Extensive Range of Control Options
• Requires Extremely Good Contamination Control Systems at All Times
• Good Serviceability

Fixed Displacement Balanced Vane Pump

Used in both Industrial and Mobile applications, this fixed displacement pump can be very
conveniently serviced (or even have its displacement altered) by simply changing the internal
pumping cartridge. When carrying outthis operation it is extremely important to examine
the condition of the drive shaft splines and remove any wear debris from the casing
if the pump before installing the new cartridge. A complete renovation of the pump still requires
removal from the prime mover in order to change the front bearing and the shaft seal.

The vanes are pressed against the cam ring by directing the outlet pressure to the underside of
each vane element. As the outlet pressure raises the vanes press harder to keep the internal leakage
to a minimum. On the suction side of the pump this creates a vane tip force which is unnecessarily
high (and causes wear). More sophisticated versions of this pump limit the vane tip
force by various means.

Key Details:
• Displacements up to 200cm3/Rev
• Typical Pressure up to 280Bar
• Provides Prime Mover (Motor/Engine) with Soft Start
• Simple Multiple Assemblies (Tandem Mounting)
• Low Noise Operation
• Extremely Good Serviceability

Pressure Compensated Vane Pumps

In addition to the Fixed Displacement Balanced Vane Pump, pressure compensated vane pumps
provide a greater level of efficiency by providing pressure only when required.
As the pump pressure rises due to the downstream resistance to flow, it will eventually
generate sufficient force to move the internal compensator spool (known as cracking pressure)
As the pressure rises the pump displacement will reduce until eventually it moves to a de-stroke
position. At this stage it will remain slightly on stroke maintaining its own internal lubrication
and leakage into the pump casing.

Key Details:
• Displacements up to 100cm3/Rev
• Typical Pressure up to 160Bar
• Provides Prime Mover (Motor/Engine) with Soft Start

External Gear Pumps

The external gear pump is the cheapest type of hydraulic pump and is very popular on systems
which are constrained to have a very low build cost. Noise levels on gear pumps are generally
high and efficiency is typically low – both get worse as the pressure increases however,
for low pressure intermittent use, these pumps are perfectly adequate.

Key Details:
• Displacements up to 250cm3/Rev
• Typical Pressure up to 250Bar
• Good Speed Range
• Simple Multiple Assemblies (Tandem Mounting)
• Generally Very Noisy
• Handle Contamination Well
• Poor Serviceability (Generally throw away when worn)
• Compact and Low Weight (Aluminium)
• Low Cost

Internal Gear Pumps

The internal gear pump is more expensive than the external gear pump
but is more robust and significantly quieter.
There is a much smoother output (Pressure Ripple) and they
can run at higher pressures without the excessive noise levels.

Key Details:
• Displacements up to 250cm3/Rev
• Typical Pressure up to 315Bar
• Good Speed Range
• Simple Multiple Assemblies (Tandem Mounting)
• Generally Lower Noise
• Handle Contamination Well
• Poor Serviceability (Generally throw away when worn)
• Compact and Low Weight (Aluminium)