TechArchive

Original article date: January 1998

Many of problems of rodless cylinders are well known to users — leakage poor stiffness under load inadequate load capacity and service life. But could an oval piston from Vickers change all that?

Outwardly the design of rodless pneumatic cylinders has changed little since the product was first introduced nearly 20 years ago. Improvements to the internal design of the cylinders have been undertaken over time but these improvements have tended to be “tweaks” rather than complete re-designs. As a result many of the fundamental problems with the original designs of rodless cylinders are still with us today: leakage poor stiffness under load inadequate load capacity and service life.

The new SRL 2 series of rodless cylinder from Vickers-CKD claims to have addressed all these problems with an innovative design based upon an oval piston. Because of its shape the oval piston offers fundamental design benefits which are analogous to those of a roller element bearing when compared to a ball bearing. The roller offers a greater and more stable load bearing area whereas the ball element – which is analogous to the piston of a conventional round bore rodless cylinder – has the disadvantage of single point contact not only for load bearing but for sealing also.

The greater moment receiving area which the wide oval piston provides gives added strength to resist radial axial and vertical moment loads. As a result according to laboratory tests the CKD cylinders are able to offer up to three times the load capacity size for size and typically six times the life of conventional non-guided rodless cylinders.

The new design which also uses a new type of piston packing material enables speeds up to 2m/s to be achieved and a very low level of leakage leading to more accurate speed control and considerable energy savings.

Because loads are not always put equally on the centre of the table or carriage external guides often have to be employed with conventional round bore rodless cylinders. The guides are required by the narrow yoke used on the latter type of cylinder. The radial moments on the yoke limit the load it can carry while any off-centre loading results in abrasion of the yoke leading to wear and premature failure of the cylinder.

In contrast the oval piston design of Vicker’s new CKD units with its wider yoke avoids concentrated loads on the piston. It allows a freer location of the load and ensures stable operation in any mounting mode. The benefits are higher reliability and longer service life plus the ability to specify a smaller bore cylinder.

In the tests a 16mm bore CKD unit was compared with a standard unguided rodless cylinder of the same bore size. An overhanging load was suspended 50mm from the centre line of both cylinders to test their radial moment capacity. The tests revealed that the CKD cylinder was able to operate reliably with an overhung load of 20N (2kg) whereas the standard round bore rodless cylinder could only sustain a load of 6N (0.6kg) – less than a third.

Another advantage of the oval piston is its length. With a longer moment receiving area for resistance against cross moment loads the capacity of the CKD cylinder is twice that of an equivalent round bore rodless cylinder.

The higher cross moment characteristics mean that in many cases external guide units are not required leading to savings on both cost and installation space. Additional benefits are greater stability for high speed operations and no pitching and rolling actions of the type which impair service life on conventional designs.

The problems of leakage encountered with many conventional rodless designs are also minimised with an oval piston. A wide flat surface mates neatly with a specially designed composite belt to ensure positive sealing between the cylinder tube opening and the piston itself. The urethane sealing belt is reinforced with Kevlar aramid fibre a material which has higher specific strength and specific stiffness than stainless steel. This ensures that elongation of the belt is minimal so users are not inconvenienced with disassembly operations to effect re-tightening.

The benefits of this simple and very effective sealing arrangement are that external leakage is extremely small and energy wastage is kept to a minimum. With minimum leakage highly accurate speed control is possible and an enhanced cushioning effect is maintained.

The greater area of the oval piston allows for a larger work table fixing than is the case with conventional rodless cylinder designs. This combined with four strategically placed mounting holes ensures a high degree of stability and flexibility. In addition because the piston design dramatically reduces overall cylinder height the table height is also low resulting in a smaller working envelope overall and easier installation in confined spaces.

As a major aid to installation and reducing space the SRL2 cylinders are designed to allow both connections on one end one on either side of the cylinder or both on one side. To ensure smooth deceleration and shock free stopping at the end stroke positions the SRL2 cylinders are designed with high efficiency adjustable cushions.

External shock absorbers are also available for particularly heavy duty applications. Finally the range is complemented by a full range of mounting and carriage adaptors.

• Ray Marston
• Vickers Pneumatics
• Halesfield 6
• Telford
• Shropshire TF7 4LF
• Tel: 01952 586000
• Fax: 01952 586798

January 1998