July 26, 2016

Feature: A pump for all occasions

Assembly schematic of an Oilgear PVV-250 axial piston pump
Assembly schematic of an Oilgear PVV-250 axial piston pump

Axial piston pumps are used in many industrial applications providing 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

Hydraulic pumps come in many shapes and sizes, but one of the most versatile is the axial piston pump, which can be arranged as a pump or a motor, depending on the application. Coupled with a choice of control mechanisms, the axial piston pump can be applied to a myriad of tasks. The basic design provides an efficient delivery of power, which has led to an increasing number being specified in favour of fixed displacement gear or vane pumps.

At the heart of the axial piston 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, known as 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.

Differences in design

While the essential principles are very similar in all hydraulic axial piston pumps, the finer details, such as the precise construction, materials, tolerances, and control features, are what make each pump design unique and 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 axial piston pumps by the pump manufacturer Oilgear. 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. In fact, because the input shaft bearing does not have to support any of the load induced by the pressure, it will last many times longer. When it finally does require replacement, it can be changed without taking the pump apart, saving time and improving productivity.

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, which allow different application requirements to be met by the most suitable method without needing separate electric motors.

Matching process requirements

Identifying the most appropriate control method is just part of the pump selection process when developing a hydraulic system. In many cases the pump selection will use an existing configuration, which has been developed to suit a number of common applications. 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.

However, certain situations may require a very specific solution, such as a recent development that was created by the Oilgear engineers in Leeds, UK. Having been approached to deliver a hydraulic pump for a steering system on a ship, Oilgear realised that the very specific flowrate required for this application fell between two models of its PVV range of pumps.

It would have been possible to meet the flow requirements by down-rating an existing model, but this would have been a compromise that, in this situation, was not acceptable. As a result, the Oilgear designers set about creating a new model, the PVV-355, which would precisely meet the specification of the client. The creation of this new model has extended the original model range and is now available for general use.

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.

For more information:
This web-exclusive feature was written by Jim Cairney, engineering manager at Oilgear UK. Visit: www.oilgeartowler.co.uk

Assembly schematic of an Oilgear PVV-250 axial piston pump

November 23, 2017
TransCanada’s permit to operate the Keystone crude oil pipeline in South Dakota could be revoked by state regulators, according to Reuters. An initial probe into the recent major spill on the pipeline... [Read More]

November 22, 2017
Emerson has launched a new line of knife gate valves designed specifically for challenging mid-service mining applications. According to Emerson, the new Clarkson SU10R polyurethane knife gate valves... [Read More]

November 21, 2017
Trelleborg Sealing Solutions have launched a unique rotary seal which aims to achieve strong performance and reliability when running at greater speeds and higher temperature. The new Turcon Roto Glyd... [Read More]

November 21, 2017
Hamburg based Implico has launched a new system to allow its customers to connect legacy field technology with the OpenTAS terminal management system. The new TCP-X-Unit is capable of simultaneously converting... [Read More]

November 20, 2017
Based near Wells in Somerset, St Cuthberts Mill specialises in the manufacture of high-quality artists’ papers. With a heritage that can be traced back to the 1700s, the company makes approximately... [Read More]

November 20, 2017
A Denver, Colorado based oil and gas producer is using Coriolis flowmeters for well head allocation. Bonanza Creek is an exploration and production company focused on extracting oil and associated liquids-rich... [Read More]

November 20, 2017
SNC-Lavalin has announced the agreement of a contract with Ecopetrol in Colombia for conceptual, basic and detailed engineering on high and medium complexity projects through to 2020. Founded in 1911,... [Read More]

November 20, 2017
Hubli, India based Weir BDK will supply Indian EPC contractor M/S Tata Projects with £1.4 million (€1.6 million) worth of butterfly valves. The valves will be installed in the Sri Damodaram... [Read More]

November 16, 2017
The ‘most used and recognised offshore pipeline standard in the world’ has been updated to reflect new pipeline research and developments by leading industry players, including quality assurance... [Read More]

November 16, 2017
Producers Midstream will use UOP Russell modular cryogenic equipment to process 200 million standard cubic feet per day of natural gas at its gas processing facility in the Delaware Basin in western Texas,... [Read More]

November 15, 2017
Scientists in the US are developing an analysis tool to more accurately predict how sulphur compounds in crude oil might corrode equipment. The research aims to address a major safety issue for fluid handling... [Read More]

November 15, 2017
Pump solutions specialist Selwood has launched what it dubs a ‘green-friendly' electric solids handling pump. The UK based company has introduced a new Super Silent electric drive addition... [Read More]

November 14, 2017
Flowmeters come in a variety of forms and operate on a range of different principles. Choosing the correct flowmeter for your application is vital for ensuring both accuracy, efficiency and value for money. Coriolis... [Read More]

November 13, 2017
Grundfos, a specialist in the manufacturer of energy-efficient pumps in the world, has announced that its subsidiary Grundfos Pumps India Pvt (Grundfos India) will be launching a new manufacturing... [Read More]

November 13, 2017
US liquid management solutions provider BakerCorp has recently marked the completion of a multi-site dewatering project for a Denver area public-private construction entity. BakerCorp was chosen as the... [Read More]