HYDRAULICS & PNEUMATICS FEATURE Throughout the industrial workplace, hydraulic pumps provide a great deal of
motive power, efficiently translating the rotary motion of a prime mover into hydraulic oil flow that can be easily routed and used for a vast range of tasks. Jim Cairney, engineering manager at Oilgear in the UK, examines the principles and applications of one of the most common hydraulic pumps - the axial piston pump
A VERSATILE PUMP SOLUTION O
ne of the most versatile hydraulic pumps is the axial piston type.
Depending on the application, this can be arranged as a pump or a motor and, coupled with a choice of control mechanisms, it can be applied to a myriad of tasks. With its basic design providing an efficient delivery of power, an increasing number are being specified in favour of fixed displacement gear or vane pumps. At the heart of the pump is a group of
finely machined pistons that are fitted inside a round cylinder barrel which rotates. In a fixed displacement pump the cylinder barrel is inclined at an angle to the drive shaft so that as the cylinder rotates one revolution, the pistons complete a full stroke. The total displacement of the pump is determined by the size and number of pistons as well as the length of the stroke, which is determined by the angle of inclination. In a variable displacement pump,
the pistons are attached to an angled plate – the swash plate – so that as the assembly rotates the pistons reciprocate, parallel to the driveshaft. By changing the angle of the swash plate, the displacement is adjusted. In both cases the oil flow is controlled by the valve plate which directs low pressure oil to the pistons on the suction stroke and the pressurised oil to the discharge port. As a hydraulic pump, these components are used to provide the flow which, in turn, can actuate hydraulic cylinders, valves, motors and brakes. However, the principle action of the pump can also be reversed, with the flow being fed into the component which, in turn, will rotate the driveshaft. The pump has then become a hydraulic motor.
APPLICATION USE While the essential principles are very similar in all hydraulic axial piston pumps, details such as the precise construction, materials, tolerances and control features can influence the suitability of the component for a particular application. One example would be the use of a hydrodynamic bearing, which is a feature
of all Oilgear’s axial piston pumps. This allows the rotating barrel to ride on a thin film of oil, without the need for conventional bearings, which limit the operational cycle of many other pumps. As the input shaft bearing in the pumps does not have to support any of the load induced by the pressure, it will last longer and, when it does require replacement, it can be changed without taking the pump apart.
PUMP SELECTION Flow rate, maximum operating pressure and control options are key factors in selecting the suitability and efficiency of a hydraulic pump in an application. Usually the pump can be paired with a choice of control features to deliver the required combination. Pressure compensation control uses a cylinder connected to the swashplate to adjust the displacement of the pump as the pressure in the output line approaches a set value. In some cases it may be necessary to combine pressure compensator control with load sensing or with a horsepower limiter depending on the application. Hydraulic systems that employ more than one pump can also have different control types on each pump, allowing different application requirements to be met by the most suitable method without needing separate electric motors. When developing a hydraulic system, identifying the most appropriate control method is just part of the pump selection process. Maximum flow rates and operating pressures are used to determine a specific model while the control characteristics are selected to give the best operation while keeping power input within available limits. Certain situations, however,
may require a very specific solution. In one recent example, Oilgear was approached to deliver a hydraulic pump for a steering system on a ship, and realised that the very specific flowrate required for this
DESIGN SOLUTIONS | JUNE 2016 25
application fell between two models of its PVV range of pumps. While it would have been possible
to meet the flow requirements by down-rating an existing model, this would have been a compromise that, in this situation, was not acceptable. So, the Oilgear designers set about creating a new model – the PVV-355 – to specifically meet the requirements. This is now available for general use.
FACING THE CHALLENGE While the majority of hydraulic applications involve pressurised mineral oil, there is a growing requirement for systems that operate with other fluids, especially low viscosity fluids with high water content and fire resistant fluids. Designing pumps for these applications raises a number of issues, with lubrication, or rather the lack of it, being a key issue. Without the natural lubricating properties of oil, issues such as bearing design, piston wear and pump sealing have to be reassessed. Each industry faces its own challenges
in terms of hydraulic operation, but all have the common goals of improved performance and minimal downtime. These can be achieved by combining design expertise and experience to identify the most appropriate design combination of pump and control system with the application in question, even if that means a new design.
Oilgear
http://oilgear.com
Exploded view of Oilgear’s PVV-250 axial pump assembly
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