Hydraulic systems continue to offer advantages to designers of industrial machinery. Power density is a major benefit, and hydraulics also offer control accuracy, simplicity, safety, reliability and cost-effectiveness. At the heart of any hydraulic system is a pump but how do you decide which type to use for your application? This article provides practical guidance on pump selection.
Specifying a hydraulic pump for an industrial application can be a daunting task. Moreover, you cannot consider the pump in isolation, as you need to take into account the operating cycle, system components, what will power the pump, the type of hydraulic fluid and maintenance issues.
What to consider
Factors to think about include the type of hydraulic circuit (open- or closed-loop), power, flow rate, pressure, noise, the type of hydraulic fluid, the operating conditions and how the pump will be powered.
Open-loop or closed-loop
There are two main types of hydraulic circuit:
- Open-loop: This covers approximately 75 per cent of applications and is the most common type for industrial hydraulic systems. Hydraulic circuits are configured with return lines open to atmosphere. Open-loop systems provide flexibility for multi-axis applications and scope for future upgrades.
- Closed-loop: Most commonly used on mobile plant and winches/cranes, closed-loop systems have return lines piped directly back to the pump inlet. This avoids the need for control valves and provides accurate, compact control. Closed-loop systems are best suited to rotary type actuators but can also be used with cylinders for applications such as steering.
Flow rate
Start by determining the flow rate required for the application, then factor-in the inevitable loss of efficiency due to component wear and leakage in system components. This gives a required flow rate for the pump.
Pressure
A pump does not create pressure, it only creates flow. It generates flow with enough power to overcome pressure induced by the load at the pump outlet. If the outlet is connected straight back to tank there will be no pressure; if connected to a cylinder, it will generate the pressure required to lift the load. The maximum operating pressure varies between different types of pump. For example, the nominal pressure for a vane pump might be 100 bar whereas a radial piston pump could be rated at 700 bar.
Power
Hydraulic power is defined as flow multiplied by pressure. It is best to calculate the power required by the hydraulic system. Beware of guesstimating the pump’s power by looking at a pump on a similar application, as that pump may have been over-specified.
Pump performance charts show the relationships between power and flow rate across a range of different pressures.
Speed
Different types of hydraulic pump have higher or lower operating speeds. For example, the maximum speed of an external gear pump might be 4000 RPM but a bent-axis piston pump might only operate up to 3000 RPM. Running a pump at a lower speed than its optimum rated speed usually results in reduced efficiency, so care needs to be taken to ensure the pump’s speed and flow rate match the application’s requirements. Note that the efficiency of the driving unit, whether an electric motor or internal combustion engine, will also depend on the speed.
Maintenance
A pump’s purchase cost is only one element of its total cost of ownership (TCO). Maintenance is important to avoid performance degradation as well as preventing premature failure, unplanned downtime and spiralling TCO. Furthermore, correct maintenance ensures worn parts are replaced before the pump sustains more extensive damage. However, some types of pump are more expensive to maintain. It is therefore important to consider the utilisation, ease of maintenance (due to accessibility) and system design life.
Hydraulic fluids
The fluid must be compatible with the pump, so the optimum fluid should be selected at the same time as deciding which type of pump to use. Options include:
- Conventional hydraulic fluid: Most pumps work well with these fluids based on mineral oil, which have good lubricity and a high boiling point.
- Phosphate ester: These synthetic fluids benefit from high thermal stability, good lubricity and antiwear properties. Phosphate ester fluids are typically used in high-temperature applications where there is a risk of fire. They are less viscous, however, and can be chemically aggressive, so care is required when specifying seals and coatings. Hydraulic systems with phosphate ester fluid can cost more to maintain.
- Biodegradable fluids: Environmentally-sensitive applications such as agricultural machinery and marine equipment often use biodegradable hydraulic fluids to reduce contamination risks. These fluids may be based on vegetable oils and they typically have high lubricity and are inherently anticorrosive. However, they can oxidize quickly and degrade if contaminated with water.
- Water glycol: These fluids are fire-resistant but have a lower operating temperature than phosphate ester fluids. Water glycol generally comprises water, ethylene or diethylene glycol, and a high-molecular-weight polyglycol to modify the viscosity. Additives can enhance corrosion resistance, antimicrobial properties, oxidation resistance and antiwear properties.
Pump data sheets usually state a maximum viscosity for the hydraulic fluid and it is important to adhere to this. A viscosity that is too high or low can reduce efficiency and introduce further problems. The environmental conditions and operating temperature due to the pump duty cycle will have an effect on the temperature and hence viscosity of the fluid. Therefore, the designer may need to consider using heating or cooling to maintain the desired operating viscosity.
Hydraulic pump types
Pumps are classified as positive displacement or non-positive displacement. Most hydraulic pumps are positive displacement types as outlined below. With non-positive-displacement pumps, the flow rate varies in response to the pressure exerted on the outlet, which is usually undesirable for hydraulic systems.
Gear pumps
Gear pumps can be subdivided as follows:
- External gear pump: Meshing gears within a close-fitting casing force hydraulic fluid to travel in the voids between the gear teeth and the casing. Where the gear teeth come out of mesh, a volume expands, creating an area of lower pressure that draws in fluid via the inlet port. Conversely, where the teeth come into mesh, the volume between the teeth deceases, pressure rises and the fluid flows out of the outlet port. External gear pumps can operate at high speeds and are relatively quiet and inexpensive.
- Internal gear pump: These have an inner gear with teeth facing outward to mesh with inwards-facing teeth on an outer gear. The two gears are located so they mesh on one side; on the opposite side, a crescent-shaped barrier fills the space between them. Where the gear teeth come out of mesh, an area of lower pressure is created, which draws in fluid via the inlet port. Where the teeth come into mesh, an area of higher pressure is created. Hydraulic fluid is carried in the gaps between the gear teeth. Compared with external gear pumps, internal gear pumps produce smooth flows with little pulsation. However, internal gear pumps are more costly to manufacture.
Vane pumps
In vane pumps, the rotor has radial slots in which vanes slide. The rotor is offset in the bore of the casing cavity so, as it rotates, the vanes move in and out to remain in sliding contact with the cavity wall. Fluid is trapped in the chambers created by pairs of vanes and is transported from the inlet port to the outlet port.
Vane pumps come in fixed- or variable-displacement types, both of which are characterised by low operating noise levels. Variable-displacement vane pumps benefit from high repetition accuracy but they are relatively low-pressure, low-speed units.
Piston pumps
Piston pumps are available in axial and radial types:
- Axial piston pumps: A circular array of pistons operate in a cylinder block, with an angled swashplate controlling the piston strokes. As the cylinder block rotates, the pistons move axially, drawing fluid in when the pistons are at the inlet port and pumping it out when they are at the outlet port. In variable-displacement axial piston pumps, the angle of the swashplate is altered to change the piston’s stroke and displacement – and, consequently, the flow rate.
- Bent-axis piston pumps: These are similar to swashplate axial piston pumps but the axes of the drive shaft and cylinder block are at a fixed angle relative to each other. The pistons are caused to move in and out by a drive flange.
- Radial piston pumps: Radial piston pumps have three or more radial pistons in fixed cylinders. As the drive shaft rotates, a cam causes the pistons to move along their axes. Each cylinder is fitted with inlet and outlet ports and non-return valving. Radial piston pumps benefit from high efficiency, smooth flow, low noise levels, high reliability and can operate at high pressures.
Fixed or variable displacement
As we have seen above, some types of pump are available as fixed-displacement units while others can be fixed or variable-displacement. While variable-displacement pumps tend to cost more to purchase, they enable the flow rate to be varied without altering the pump’s speed. For simple applications where movements are always the same, fixed-displacement pumps are usually preferred, whereas variable-displacement pumps are better for applications where motions are less predictable.