GB2033537A

GB2033537A - Piston for pneumatic actuator

Info

Publication number: GB2033537A
Application number: GB7936845A
Authority: GB
Original assignee: Festo Maschinenfabrik Gottlieb Stoll Firma
Current assignee: Festo Maschinenfabrik Gottlieb Stoll Firma
Priority date: 1978-10-31
Filing date: 1979-10-24
Publication date: 1980-05-21
Also published as: GB2033537B; FR2440500A1; FR2440500B1; DE2847274C2; DE2847274A1

Abstract

A piston (12) capable of operating without lubrication is formed of resilient plastics material, e.g. polyurethane, and includes a control section (44), sealing lips (46, 48) and buffers (54, 56), and is a press fit on rod (32) where it is retained by plate (42). The piston may consist of a single plastics material, in which case it may have a skin of higher density, or it may have an outer layer of plastics material with a lower coefficient of friction. <IMAGE>

Description

SPECIFICATION A piston for an operating cylinder This invention relates to a piston for an operating cylinder.

The piston body of known pistons for pneumatic operating cylinders is formed of metal. A number of sealing rings, fitted into circumferential grooves on the piston body, are in resilient contact with the internal wall surface of the cylinder. The manufacturing costs of a piston consisting of several parts are high and as a rule the piston has to be separately lubricated.

It is an object of the present invention to provide a piston which, reduces both the manufacturing costs and the special requirements of lubrication.

According to the present invention there is provided a piston for an operating cylinder, said piston comprising a piston body which is connectable with a piston rod and which is integrally manufactured from a resilient plastics material having a low coefficient of friction, the piston body having a control section and sealing lips, integrally formed onto the ends of the control section and extending circumferentially thereof the control section being radially outermost of the piston body, the diameter of the latter being slightly smaller than the internal diameter of the cylinder.

The present invention also provides an assembly comprising a piston as described in the preceding paragraph and a control and sealing ring which is integrally manufactured from a resilient plastics material having a low coefficient of friction, the ring having a control section and sealing lips integrally formed onto the ends of the control section and extending circumferentially thereof, the control section being radially innermost of the ring.

An embodiment of the present invention will now be described by way of example only, with reference to the single figure of the accompanying drawing, which is an axial section through a pneumatic operating cylinder having an integrally manufactured all-plastics piston.

Referring to the drawing, a pneumatic pistoncylinder assembly comprises cylinder body 10, a piston 12, and a piston 14.

The cylinder body 10 is formed of a cylinder bush 16, a first closed end wall 18 and a second end wall 20 through which the piston rod 14 extends. The two end walls 18 and 20 are fixed to the cylinder bush 16 by means of screws 24, sealing elements 22 being mounted between these end walls and the bush. The piston 12 divides the cylinder 10 into two chambers.

At its end wall 18 the cylinder has a first channel 26 for the pressure medium, a second corresponding channel being located at the end wall 20 but not being visible in the drawing.

The rotationally symmetric piston 12 comprises a disc shaped piston body 28, with a radially inner most mounting section 30 which is press-fitted on to a reduced-diameter end-section 32 of the piston rod 14. This mounting section 30 carries annular, axial projections 34 and 36 on its ends. In the drawing, the dimensions of these projections are exaggerated.

The projections 36,34 support themselves respectively on a radial shoulder 38 of the piston rod 14, which defines the end section 32, and on a radial shoulder 40. The latter is constituted by the inner surface of a holding plate 42 which is fixed to the piston rod 14 by means of countersunk head screws which extend into the front end of the piston rod 14.

The disc shaped body 28 has an external control section 44 which extends practically to the internal wall surface of the cylinder bush 16, leaving only a narrow gap between the two adjacent surfaces.

Sealing lips 46, 48 formed on to the control section 44, each having a cross-section like a truncated cone, taper towards their free ends. In order to maintain the narrow gap between the control section 44 and the bush 16 without having to reduce the radial length of the sealing lips 46,48, the piston body is provided with annular cut-outs 50 and 52 between the sealing lips and the control section.

Axially projecting buffer rings 54, 56, are formed on to the end faces of the piston body 28 at positions which are radially external of the piston rod 14, and radially internal of the sealing lips 46,48. It is by means of these elements that the piston 12, reaching its end positions, makes resilient surface contact with the end walls 18 and 20.

In addition to the passage 58 for the piston rod 14, the end wall 20 has an annular recess 60 through which the piston rod extends and which opens towards the external wall surface of the end wall 20.

The recess 60 accommodates a control and sealing ring 62 for the piston rod 14.

The design of the ring 62 is very similar to that of the piston 12. There are sealing lips 66,68 integrally formed onto a control section 64 which surrounds the piston rod 14, leaving a narrow gap in the radial direction, and the lips are located on either end of the control section. As above, there are annular recesses 70, 72 between the lips and the control section 64. Axial projections 74 and 76 support the sealing ring 62 on the internal surface of a holding plate 78 and on the botton of the recess respectively.

Again, the holding plate 78 is fixed to the end wall 20 by means of counter sunk head screws, but this is not shown in the drawing.

Both the piston 12 and the control and sealing ring 62 are extruded polyurethane elements, the material for their manufacture being chosen with a view to good dry-running properties combined with the required resilience and mechanical strength.

Similarly, the parts of the cylinder 10 may consist of extruded plastics from which it follows that the complete working cylinder can be manufactured at very favourable costs. In addition to this its weight is extremely low.

Should the piston rod 14 of the pneumatic working cylinder described above cant or be radially stressed, a portion of the control section 44 of the piston 12 will be in contact with the internal wall of the cylinder 10, or part of the control section 64 of the control and sealing ring 62 will make surface contact with the piston rod 14. When this occurs the necessary forces of support are restored without difficulty by moving the piston rod 14 slightly away from the axis of the cylinder 10. The resilient sealing lips 66 and 68 would still provide a perfect seal for the operating chambers of this pneumatic cylinder.

Experts appreciate the fact that the design of the pneumatic cylinder is very simple. All components may be manufactured automatically in large numbers, and their assembly is very simple.

If the piston 12 is not to be a differential piston, the end wall 18 is simply replaced by an end wail 20 together with the holding plate 78 and a control and sealing ring 62, so that the second end section of the piston rod 14 extends through these components in a pressure-tight manner.

The piston may be manufactured of one single plastics material providing a high tri-dimensional resilience and ensuring that the piston surfaces have favourable dry running properties. The plastics material may have a surface skin of increased density.

Alternatively the piston is manufactured of a material of a resilience superior to the above and improved mechanical properties while the material chosen for the surface layers has the required good dry running properties.

When there is a slight axial, elastic displacement of the piston relative to the piston rod, and the piston body is in full surface contact with the radial shoulders of the piston rod, the relative axial displacement between the piston body and the piston rod is accommodated and a condition is created where considerable loads may be transmitted.

The piston has to fulfil simultaneously four functions. It applies the piston pressure through its surfaces, transmitting the resulting loads into the piston rod; it provides a seal between the cylinder chambers and the internal cylinder wall; it controls the movements of the piston rod; and it ensures that sliding friction relative to the internal wall surface of the cylinder is moderate.

Separate lubrication, for example effected by spraying oil into the compressed air, is not required when using the piston according to the invention. In consequence thereof also is eliminated any danger that the environment might be contaminated by using a pressure medium which contains a stress relieved lubricant.

The manufacture of the piston is easily automated, the work being completed in only one operation of extrusion. The service life of extruded plastic pistons is moreover longer than that of metal piston bodies with rubber seals inserted into the latter.

With conventional rubber pistons it is impossible to combine the four above mentioned functions.

This advantage is achieved by using a plastics material of the required characteristics. The fact that the parts are integrally made of only one material not only is very favourable with respect to the manufacturing costs, but also offers the possibility of automatic production, this being important for example when small-size cylinders, disposable cylinders, or cheap cylinders have to be manufactured.

Endurance tests confirmed that the expected service life is considerably longer than that of units comprising conventional pistons of metal and having rubber seals.

Various modifications may be made without departing from the invention.

Claims

1. A piston for an operating cylinder, said piston comprising a piston body which is connectable with a piston rod and which is integrally manufactured from a resilient plastics material having a low coefficient of friction, the piston body having a control section and sealing lips integrally formed onto the ends of the control section and extending circumferentially thereof, the control section being radially outermost of the piston body, the diameter of the latter being slightly smaller than the internal diameter of the cylinder.

2. A piston according to claim 1, formed of polyurethane.

3. A piston according to claim 1 or 2, wherein the plastics material comprises a surface skin of increased density.

4. A piston according to claim 1, wherein a main portion of the piston body formed of the resilient plastics material comprises a surface layer of a second plastics material which has a lowercoeffi- cient of friction.

5. A piston according to any of the preceding claims, wherein the diameter of an axial passage through the piston body is smaller than the diameter of a section of the piston rod which is arranged to carry the piston body.

6. A piston according to any of the preceding claims, wherein the radially innermost edge of the piston body carries axial projections through which the piston body is arranged to support itself on radial shoulders of the piston rod.

7. A piston according to claim 6, wherein the said projections are annular.

8. A piston according to any of the preceding claims, wherein the axial ends of the piston body carry second axial projections through which the piston body is arranged to butt against end wails of the cylinder.

9. A piston according to claim 8, wherein the second axial projections are annular.

10. An assembly comprising a piston according to any of the preceding claims and a control and sealing ring which is integrally manufactured from a resilient plastics material having a low coefficient of friction, the ring having a control section and sealing lips integrally formed onto the ends of the control section and extending circumferentially thereof, the control section being radially innermost of the ring.

11. A piston-cylinder assembly comprising a piston according to any of the preceding claims mounted on a piston rod which extends through at least one end wall of a cylinder.

12. A piston for an operating cylinder substantially as hereinbefore described with reference to the accompanying drawing.

13. Any novel subject matter or combination including novel subject matter herein disclosed, whether or not within the scope of relating to the same invention as any of the preceding claims.