HYDRAULICS & PNEUMATICS
Select hydraulic pumps for industrial applications 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. A pump is at the heart of any hydraulic system, but how do you decide which type to use for your application? Bosch Rexroth explains.
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pecifying a hydraulic pump for an industrial application can be a daunting task. 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. 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 circuits cover approximately 75% of applications and are the most common type. Hydraulic circuits are configured with return lines open to the atmosphere. Open-loop systems provide flexibility for multi-axis applications and scope for future upgrades. Closed-loop circuits are most commonly used on mobile plant and winches or cranes. They 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. However, there is a wide range of other factors which need to be considered.
Pressure
Speed
A pump does not create pressure; it only creates flow with enough power to overcome pressure induced by the load at the pump outlet. If the outlet is connected straight back to a 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 pumps.
Different types of hydraulic pumps have higher or lower operating speeds. 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.
Flow rate
Power
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.
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.
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.
Hydraulic fluids The fluid must be compatible with the pump,
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HYDRAULICS & PNEUMATICS 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 hightemperature applications where there is a risk of fire. However, they are less viscous and can be chemically aggressive, so care is required when specifying seals and coatings. • Biodegradable fluids: Environmentallysensitive applications such as agricultural machinery and marine equipment often use biodegradable hydraulic fluids to reduce contamination risks. • 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-molecularweight polyglycol to modify the viscosity. Pump data sheets usually state a maximum viscosity for the hydraulic fluid, and it’s important to adhere to this.
Hydraulic pump types The most appropriate type of pump will also vary by application. 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 • External gear pump: External gear pumps can operate at high speeds and are relatively quiet and inexpensive. • Internal gear pump: 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 Vane pumps come in fixed- or variabledisplacement types, both with low operating noise levels. Variable-displacement vane pumps benefit from high repetition accuracy, but they are relatively low-pressure, lowspeed units.
Piston pumps • 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 driveshaft 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. www.boschrexroth.co.uk
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