Choosing the right hydraulic pump

There are four types of hydraulic pumps:

• Gear pumps
• Piston pumps
• Rotary vane
• Screw pumps

Gear pumps

Gear pumps have very few moving parts. They consist of two intermeshing gears. These pumps have a constant flow rate. They operate at pressures generally between 50 and 210 bar. Gear pumps operate at the highest speeds of any pumps at up to 3000-6000 rpm.

Advantages: 

• They are inexpensive
• They have fixed-displacement

Disadvantages:

• Their volumetric efficiency is low

There are two types of gear pumps: external-gear pumps and internal-gear pumps.

External-gear pumps:

• In an external-gear pump, only one of the gear wheels, the drive gear, is connected to the drive. The other gear wheel, the driven gear, rotates in the opposite direction, so that the teeth of the rotating gear wheels interlock. 

There are also double external-gear pumps, which combine two gear pumps driven by the same coupling shaft. A double external-gear pump has the advantage of supplying two independent hydraulic circuits, and also provides more flow to one circuit.

Internal-gear pumps: 

• They have an eccentric internal gear wheel and an external gear wheel, also called a crown gear. 
• When the external gear rotates, volume is created between the gear wheel profiles and the external gear teeth attached to the gear housing walls.
• The product is sucked up when the two wheels disengage. 

Advantages:

• Internal-gear pumps are very quiet
• They provide a constant flow

Piston pumps

Hydraulic piston pumps handle very large flows at high pressures. They are operated by pistons that move back and forth continuously. The sealing properties of the pistons are excellent, allowing operation at high pressures with low fluid leakage.

Advantages:

• They offer the best overall volumetric efficiency
• They provide the highest pressures
• They are reliable 
• They have high power density
• They are available in variable and fixed displacement models

Disadvantages:

• They are the most expensive hydraulic pumps

There are two types of piston mounting: radial mounting and axial mounting.

Axial mounting: 

The design of the axial piston pump is based on the swash plate principle or curved shaft design. 

• In the case of a swash plate, the rotating pistons are supported by a swash plate; the angle of which determines the piston stroke.
• In the curved shaft configuration, the displacement volume depends on the swivel angle: the pistons move in the cylinders as the shaft rotates. 

Radial mounting: 

Radial piston pumps are available in two different configurations. 

• With an eccentric cylinder block: the piston turns inside the rigid outer ring. The eccentricity determines the stroke of the pistons. 
• With an eccentric shaft: the rotating eccentric shaft causes radially oscillating piston movements. 

Advantages: 

• Radial piston pumps can be equipped with several different independent outputs.

Rotary-vane pumps

These pumps have rectangular shaped vanes mounted into radial groves on the rotor. This allows the vanes to move radially.

Advantages:

• They generally offer better volumetric efficiency than gear pumps.
• They produce less noise while maintaining high speeds (up to 3,000 rpm).
• There are available in variable and fixed displacement models
• When they are variable displacement, the flow rate can be reduced if necessary and thus energy consumption can be reduced.

Disadvantages:

• They are more expensive than gear pumps (and less expensive than piston pumps).
• They are also fragile because the vanes are subjected to bending stress due to the discharge pressure.

The displacement of a pump is defined by the volume of fluid that the gears, vanes or pistons will pump in one rotation. If a pump has a capacity of 30 cm3, it should treat 30 ml of fluid in one rotation. 

• With a fixed displacement pump, it is impossible to vary the amount of fluid at each rotation. 
• With a variable displacement pump, it is possible to set the amount of fluid pumped at each rotation.

The main advantage of being able to vary the displacement of a pump is to save energy when the circuit does not require the pump’s maximum power.

Piston and vane pumps are available in fixed and variable displacement versions. Gear pumps are only available with fixed displacement.

The characteristics of a hydraulic pump are: 

• The pressure Δp expressed in bar
• The flow rate Qv expressed in l/min: this is the volume of liquid delivered by the pump in a given time
• Displacement expressed in cm3: this is the volume of fluid pumped per rotation
• The speed of rotation N expressed in rpm
• Hydraulic power (Ph) expressed in kW
• Efficiency: this is the useful energy of the pump after losses mainly due to friction. 

Calculation of the power of a hydraulic pump:

                                                  Flow rate (l/min) * Pressure (bar)

Hydraulic power (kW) = —————————————-

                                                                 600

50 bar 

• The operating pressure is rarely less than 50 bar

210 – 300 bar: gear pumps

• The maximum pressure is usually 250 bar
• The most common standard used by gear pumps is 210 bar
• The 300 bar standard is used by some internal-gear pumps

300 – 450 bar: piston pumps 

• They work with high pressures, around 450 bar
• 300 – 350 bar is the standard increasingly being used in open circuit
• 420 bar is the maximum standard especially used for hydrostatic transmission in closed circuits

700 bar

• This standard is used for handling jacks

7,000 bar: hydropneumatic pumps

10,000 bar: pressure multipliers

External-gear pumps

• They are more commonly used in hydraulic applications such as for log splitters or elevators.

Internal-gear pumps 

• They are used in particular in non-moving hydraulics (e.g. machine tools, presses, etc.) and in vehicles that operate in a confined space (electric forklifts, etc.). 

Rotary-vane pumps

• They are used in die-casting and injection machines in industry, as well as in construction equipment.

Piston pumps

• They are used in applications involving high pressures (400 bar to 700 bar), such as presses, plastic processing machines, machine tools and construction equipment. 
• They are used to operate hydraulic cylinders for high loads.
• They are also used in mobile and construction equipment; marine auxiliary power; metal forming and stamping; machine tools; and oilfield equipment.
• Some piston pumps are also used to transport hydrocarbons under high pressure in pipelines.
• They can also be used in water jet cutting machines. In this case, the fluid is water and not oil.

Screw pumps

• They are used to pump viscous liquids such as crude oil under high pressure.