A piston pump is capable of operating with large flows at high hydraulic system pressures.
Applications commonly using a piston pump include: marine auxiliary power, machine tools, mobile and construction equipment, metal forming and stamping and oil field equipment.
As the name suggests, a piston pump operates through pistons that move back and forth in the cylinders that are connected to the hydraulic pump. Some of the main features of a piston pump include:
- Small and compact;
- High power density;
- Highly efficient and reliable;
- Can reach high speeds and torque;
- High operating pressures.
A piston pump aslo has excellent sealing capabilities.
With large volumetric levels, a hydraulic piston pump is able to function, thanks to low oil leakage. Some plungers require valves at the suction and pressure ports, whilst others require them with the input and output channels. If there are valves at the end of the piston pumps, they are more likely to be able to work at higher pressures. This is due to the sealing properties.
Axial Piston Pump
The axial piston pump’s design is characterised by two principles: the swash plate or bent axis design and the system parameters being considered. System parameters include the decision on whether or not to use the pump in an open or closed circuit.
The return line in a closed loop circuit is under constant pressure. This must be considered when designing an axial piston pump that will be used in a closed loop circuits. It is also very important that a variable displacement volume pump is installed and operates alongside the axial piston pump in the systems. Axial piston pumps can interchange between a pump and a motor in some fixed displacement configurations.
The displacement volume of a bent axis pump is determined by the swivel angle: when the shaft rotates, the pistons in the cylinder bore move. The turning pistons are sustained by the swash plate in the swash plate design. The piston stroke is decided by the angle of the swash plate.
Common features include:
- Displacement: 5 to 1,000 cc;
- Maximum pressure: up to 450 bar;
- Speed: between 1,500 to 11,000 rpm.
Radial Piston Pump
These pumps can usually be found in applications needing higher pressures (if pressures reach above 400 bar, up to 700 bar), including: presses, machines for processing plastic and machine tools. They are the only pumps that are able to work continuously under high pressure for long periods of time.
Common features include:
- Displacement volume: 0.5 to 100 cc;
- Maximum pressure: up to 700 bar (size dependent);
- Range of speeds: 1,000 to 3,000 rpm (size dependent).
A vane pump can be classified as a “positive displacement” pump.
Working by rectangular shaped vanes moving back and forth inside slots and forcing the fluid through the system, a vane pump can also be known as a sliding vane pump.
A simplistic vane pump includes a circular rotor turning inside a large circular cavity. The centres of the two circles may oppose, which can result in eccentricity. Vanes have the capability of moving in and out of the rotor, and seal on all of the edges so that vane chambers can finish the pumping work. They may also experience a volume increase in the intake margin of the pump. Through the increase in volume, the vane pump is loaded with fluid that has been forced through by the inlet. Pressure in the inlet is caused by the pressure from the hydraulic system that has been forced around the pump. The volume in the vane chambers is lowered where oil is discharged, and this, along with the movement of the vane, results in oil being forced out of the pump with each rotation.
Normally, the mechanism of the vane pump contains a cylinder-shaped rotor that turns within an asymmetrical casing or cavity. The edges of the rotor have some rectangular slots and as the rotor turns, the centrifugal force enables the vanes’ outward movement, meaning the exterior is constantly in contact with the interior of the asymmetrical casing or cavity. The asymmetrical shape of the case may cause the vanes to move in and out of the slots as the rotor turns, which may lead to vane tensioning.
When the vanes travel further than the suction port of the pump, a vacuum is generated and this is how the oil is drawn into the pumping chamber. The oil is then forced out of the discharge port of the pump, once it has travelled through the ports. Direction of the oil flow may alter, dependent on the rotation of the pump, which is the case for many rotary pumps.
Many applications use a vane pump where positive displacement is a requirement.
With low viscosity oils, like water and petrol, a vane pump is known for efficient operation. Vane pumps do not work so well with higher viscosity fluids. This is due to the higher viscosity causing issues for the vanes rotation, preventing them from moving easily in the slots. The majority of vane pumps are found in fuel loading terminals, fuel transport vehicles, solvents, alcohol and even soft drinks and syrups and can also function with oils with differing temperatures and pressures.
A vane pump typically operates under the following specification:
- Flow rate ranges between 20 to 9500 lpm;
- Total head (pressure) ranges between 1 to 14 Bar;
- Horsepower ranges between 1 to 300 hp.