Pneumatic cylinders & Hydraulics
Pneumatic cylinders are commonly used where linear motion is required, but there are many applications where they are unsuitable. Their reliance on moving parts and seals makes pneumatic cylinders liable to leakage and wear, and they are not usually suitable for use in harsh environments.
A novel type of actuator that has a number of advantages over pneumatic cylinders is now being marketed by the West Group. The ‘air muscle’ is a lightweight single-acting pneumatic linear actuator with a high power-to-weight ratio. Although it is operated by compressed air, the air muscle has little else in common with pneumatic cylinders; for example, applying pressure causes the device to contract instead of extend.
Continuous development
Air muscles have been in continuous development for advanced robotics and animatronics work for several years. In these applications the attractive characteristics of the air muscles are their ability to provide slow, smooth, controlled movement as well as highly dynamic movements with immediate response. Manufactured by Shadow Robotics, the air muscles are now available through the West Group for use in industrial applications as a powerful lightweight actuator. The basic principle used in the air muscle is similar to that of a biological muscle. Consisting of a hermetically sealed rubber tube that is wrapped in a tough plastic woven sheath, the actuator is expanded radially by air filling the tube. The specially-shaped weave deforms in such a manner as to make the tube shorten axially as it expands radially – creating an axial pulling force.
Depending on the size, air muscles weigh as little as 10g, but they can have a power-to-weight ratio as high as 400:1, vastly out-performing both pneumatic cylinders and DC motors that can only attain a ratio of about 16:1. This characteristic makes them particularly useful for applications where weight is critical.
Air muscles behave in a very different manner to pneumatic cylinders; as the air muscle contracts under constant pressure, the pulling force produced between the end points decreases. The maximum possible force at a given pressure is obtained when the Air Muscle is extended as far as possible; if it is not taut, then the full potential will not be yielded. Furthermore, the relationship between pressure and force is linear at constant extensions, allowing the movement to be controlled by regulating the applied pressure.
Because of their simplicity, air muscles are cheaper to buy and install than other actuators and pneumatic cylinders – prices start at around (UK pounds)6. Being flexible, they can be operated when twisted axially or bent around corners, and they do not need to be aligned precisely.
Being sealed, the air muscles are suitable for difficult or dangerous situations, including cleanrooms and explosive atmospheres. And, having no sliding or rotating parts, they are also well-suited to situations where there are fine, abrasive particles such as on woodworking and grinding machines. One recent application employed a series of air muscles to uniformly tension fibreglass hull mouldings during boat construction. It must be remembered, however, that the air muscle is not able to guide the load directly, so this must be catered for in the application’s design. Neither is it capable of taking compressive loads.
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.
