TechArchive

Original article date: June 2000

How has turning a ball-and-socket joint back-to-front led to significant performance improvements in hydraulic pumps and motors?  Tommy Miller explains

Hydraulics is often perceived as an engineering discipline with plenty of rugged, dependable, powerful solutions for problems of motion or high loads, but little innovation. One type of product, however, has just benefited from some lateral thinking combined with up-to-date design and manufacturing methodologies.

Axial piston pumps and motors have been with us for many years and have proved to be particularly suitable for hydraulic transmissions; Linde started using such components in its fork lift trucks in the 1950s. But now the company has come up with a concept that results in pumps and motors that are more compact, have a higher power density, and are cost-effective.

At the heart of the new products is a design of piston and slipper pad that is the reverse of the norm: whereas the ball end of the ball-and-socket joint is usually on the piston and the socket is on the slipper pad, the New Generation Series 02 units have the ball on the slipper pad and the socket on the piston end. This may not sound like a leap forward, but the results speak for themselves.

One of the main advantages of this arrangement is that the swash angle may be increased from the normal 18deg to 21deg. For the same overall size of pump or motor, the swept volume is thus increased. And because fluid power is directly proportional to the displacement per revolution, the power density increases. A secondary benefit of this alternative arrangement is that the piston can now be hollow, which reduces the weight of the rotating components and improves lubrication.

From the outset, one of the objectives for the 02 pumps and motors was to use common parts for both types of product. This has indeed been achieved and the rotating groups – as well as many other components – are common to both. Other elements of the design have also been made modular so that even models of different capacity share common parts. The result of this approach is that Linde can streamline its manufacturing and reduce its stock holding, thereby making cost savings that can be passed on to customers.

Manufacturing technology

Additional investments in dedicated machine tools, automatic manufacturing systems and new machining techniques, together with improved materials specifications, have led to better surface finishes being achieved on the critical components. Running clearances between the dynamic components can therefore be reduced, so there are lower frictional losses, higher volumetric efficiencies and, consequently, an improved overall mechanical efficiency. There are four model ranges that benefit from the new developments: variable pumps for closed circuit operation; variable pumps for open circuit operation; fixed motors for open and closed circuit operation; and variable motors for open and closed circuit operation.

Linde claims that every type provides high performance, high efficiency and low noise, all from a compact, cost-effective package.

And for noise-sensitive environments, there is a special version of the variable pumps for open-circuit operation that feature an accumulator on the high pressure side. This acts as a silencer because it reduces pressure pulsations from 38.7bar to 11.3bar – which gives a useful 2 to 4dB(A) noise reduction.

• Linde Hydraulics

June 2000