GB2067713A - Non-metallic sealing ring - Google Patents

Abstract

The invention relates to a piston- in-cylinder vacuum pump, in particular a vacuum pump for motor vehicles which is subject to severe temperature fluctuations, there being sealing means between the piston and the cylinder wall comprising a sealing ring proper 7 of synthetic plastics material backed by a resilient ring 6 which bears against the plastics sealing ring. The invention is characterised in that these two rings are housed in a stepped groove formed in either the piston flank, Fig. 2, or on the adjacent cylinder wall within which the piston reciprocates, Fig. 3, the sealing ring being housed in the wider shallower portion of the groove and the resilient ring in the narrower deeper portion, the sealing ring being of substantially rectangular cross- section and being in part supported against the step. The resilient ring may be of a rubbery material, Figs. 2 and 3 or may be a spring, Figs. 4 and 5. <IMAGE>

Description

SPECIFICATION Piston-in-cylinder vacuum pump The invention relates to a piston-in-cylinder vacuum pump, particularly a vacuum pump for motor vehicles, in which the pump is subjected to severe temperature fluctuations.

In pumps of this kind a variety of different sealing devices have been applied between the piston and the cylinder wall. For example, in the pump of the US Patent Specification 3 212411 a cup-shaped seal is used made of polytetrafluoro ethylene. The seal is drawn cap-like over the piston head, which has several peripheral grooves in its sidewall, V-shaped in cross section. When the pump is heated to a high temperature the grooves, acting as expansion chambers, thrust the sealing material resiliently outwards against the cylinder wall. If the pump is intended for use at low temperatures, for example in a refrigeration plant, it is necessary to provide supporting rings of a resilient elastomer. The rings are inserted into the grooves and thrust the sealing material resiliently outwards against the cylinder wall.

This arrangement has the disadvantage, in the first place, that there is a risk during manufacture of damaging the cap when it is being pulled over the piston head. And the outer edges of the V-section grooves can damage the cap quickly during operation of the pump.

In a pump known from the Swiss Patent Specification 461 203 the piston has a peripheral groove of rectangular cross section in its sidewall.

The groove contains a PTFE (polytetrafluoro ethylene) ring, to act as a seal.

Under the PTFE ring there is a second ring, made of an elastomeric material, which thrusts the PTFE ring against the cylinder wall. The cross section of the PTFE ring has a blunt V at the inside and chambered edges at the outside. Furthermore, the edges have circumferential grooves to increase mobility.

A disadvantage is the high manufacturing cost of the PTFE ring, due to its compiex shape.

In a pump described in the French Patent Specification 1 464 702 the piston consists of a number of individuai discs which form between them what is essentially a series of peripheral grooves, each rectangular in cross section. Each groove contains a closely fitting PTFE torusshaped ring. One disadvantage is that the PTFE rings are costly to manufacture. Secondly, the sealing action of the PTFE ring does not appear likely to be satisfactory at low temperatures, due to the inadequate resilience of this material.

From the German Auslegeschrift 10 48 081 a device is known for sealing a reciprocating piston against a cylinder wall. The piston sidewall has a peripheral groove with a wavy floor and undercut edges. The groove contains a PTFE ring which has a cylindrical inner surface and a wavy outer surface containing two curved recesses.

Reinforced portions of the PTFE ring are thrust home, during manufacture, into depressions in the floor of the groove, so that temperature fluctuations are compensated by clearance adjustments.

A disadvantage is the high manufacturing cost of the complex groove, with its wavy floor and undercut regions. The PTFE ring also has an irregular cross section and is, for this reason, comparatively costly to manufacture.

A pump of the kind mentioned in the preamble of Claim 1 is described in the German Auslegeschrift 24 09 877. In this known pump the sidewall of the piston has several peripheral grooves which are substantially rectangular in cross section. Nevertheless, the floor of each groove has two projecting peripheral ribs. Each groove contains a PTFE ring of rectangular cross section. The ring is thrust resiliently outwards against the cylinder wall, partly due to the fact that it is prestressed and partly by the effect of the two ribs.

A disadvantage is that the rib edges severely stress and can damage the PTFE ring when it is being inserted into the groove during manufacture and also during the movements of the piston, when the pump is in operation. Furthermore, changes in the elasticity of the ring at both high and low temperatures are not fully compensated.

The intention in the present invention is to provide a piston-in-cylinder vacuum pump constructed to ensure that there is always a good seal between the piston sidewall and the cylinder wall, at all operating temperatures. The changes in the elasticity of the PTFE ring must be compensated over the entire temperature range.

The PTFE ring and the groove which accommodates it must both be simple in cross section in order to minimise manufacturing costs.

According to the present invention there is provided a piston-in-cylinder vacuum pump, in particular a vacuum pump for motor vehicles in which the pump is subject to severe temperature fluctuations, there being sealing means between the piston and the cylinder wall, these sealing means comprising a sealing ring proper of synthetic plastics material backed up by a ring of spring material which bears resiliently against the plastic sealing ring, characterised in that these two rings are housed in stepped grooves formed on either the piston flanks or on the adjacent cylinder wall within which the piston reciprocates, these two grooves being formed of different depths, the sealing ring being housed in the deeper groove and the spring ring being smaller, in volume, than the sealing ring and being housed in the shallower groove, the sealing ring being of substantially rectangular cross-section and being in part supported against a rigid step between the deeper groove and the shallower groove.

Several versions of the invention will now be described in greater detail with the help of the drawing, in which: Figure 1 is a cross section of a piston-incylinder vacuum pump.

Figures 2 to 5 show details of four versions of the invention.

In all the figures corresponding parts have been given the same reference numbers.

Figure 1 shows, as an example, a double-action pump with a circular piston 1 having an upper face 1 a, a lower face 1 b and a sidewall 1 c. The lower portion of the piston 1, adjacent its lower face 1 b, decreases in diameter in two steps. The piston 1 has a central bore 1 d in which is fixed the upper end of a sliding piston rod 2 which can be driven, for example, by an eccentric or a lever (not shown). Under the piston 1 and in contact with its lower face 1 b there is a comparatively thin disc 3 whose diameter is the same as the diameter of the upper portion of the piston, adjacent its upper face 1 a. The piston rod 2 has a threaded upper end 2a and a shoulder 2b, the piston 1 and the disc 3 being fixed together on the upper end of the piston rod 2 by a nut 4, with the aid of the shoulder 2b.

The piston 1 and the disc 3 together form (in the version of the invention shown in Figures 1 and 2) a peripheral annular stepped groove 5 consisting of a shallower groove 5a (Figure 2) and a deeper groove 5b. As shown in Figures 1 and 2, and deeper groove 5b contains an O-ring 6 made of a resilient elastomeric material, the outer diameter of the O-ring being greater than the diameter of the floor of the shallower groove 5a.

Positioned radially outwards of the O-ring 6 the annular groove 5 contains a PTFE (polytetrafluoro ethylene) ring 7, rectangular in cross section, which is supported radially inwards by the floor of the shallower groove 5b and by the radially outer surface of the O-ring 6. The PTFE ring 7 is retained axially at its upper surface by the sidewall of the shallower groove 5a and, at its lower surface, by the disc 3. The PTFE ring 7 is thrust radially outwards by the resilient O-ring 6 so that the PTFE ring 7 makes good contact with the cylinder wall 8, ensuring a good seal between the two working chambers 9 and 10 of the pump.

In the version of Figure 3 the thermal behaviour of the pump is improved by making a stepped annular groove 5' in the cylinder wall 8, instead of in the sidewall 1 c of the piston 1. And here again the annular groove 5' consists of a shallower groove 5a' and a deeper groove 5b'. The deeper groove 5b' contains a resilient O-ring 6. A PTFE ring 7, rectangular in cross section and positioned radially inwards of the O-ring 6, is resiliently supported at its radially outer surface by the O-ring 6 and by the floor of the shallower step 5a', the PTFE ring 7 being retained axially by the two sidewalls of the stepped annular groove 5'. The arrangement has two thermal advantages.In the first place, a better seal is obtained at low temperatures because the PFTE ring 7 shrinks, on cooling, into even closer contact with the wall of the piston 1. Secondly, heat is dissipated more rapidly from the two rings to the mass of the cylinder 8.

A still further advantage of this version of the embodiment of the invention shown in Figure 3, is that the piston, which is a one-piece structure, is less costly to manufacture, compared to the version of Figures 1 and 2. Moreover it can be made in the form of a deep drawn pot of sheetsteel which has little mass and consequently requires only a comparatively weak return spring 11.

The version of Figures 1 and 2, on the other hand, has the advantage that a simple low-friction sleeve 1 3 can be used as a liner for the cylinder wall. A sleeve of this kind made by deep drawing can have a very good surface quality.

The pump described above provides an excellent pressure seal over a very wide range of temperatures. And due to its construction manufacturing costs and assembly costs are low.

The invention can be further developed as illustrated in Figures 4 and 5. In this case the resilient O-ring is replaced by a radially-acting spring 12 made of steel wire, which thrusts the PTFE ring 7 radially outwards into contact with the cylinder wall. An advantage is that the thermal expansion behaviour of the steel wire is more favourable than that of an elastomeric O-ring and allows a better seal to be obtained. The radiallyoutwards acting spring 12 can be round or square in cross section and, if desired, several wires can be used.

In still further development of the invention the PTFE ring 7 can be open-ended, ie cut apart to give ends abutting at 45 against each other, the ring being supported axially by the disc 3. When the ring is thrust radially outwards the PTFE material itself does not need to stretch. The two butt ends of the ring merely move circumferentially away from each other, the 450 butt joint nevertheless retaining a sufficient sealing ability.

Claims (9)

1. A piston-in-cylinder vacuum pump, in particular a vacuum pump for motor vehicles in which the pump is subject to severe temperature fluctuations, there being sealing means between the piston and the cylinder wall, these sealing means comprising a sealing ring proper of synthetic plastics material backed up by a ring of spring material which bears resiliently against the plastic sealing ring, characterised in that these two rings are housed in stepped grooves formed on either the piston flanks or on the adjacent cylinder wall within which the piston reciprocates, these two grooves being formed of different depths, the sealing ring being housed in the deeper groove and the spring ring being smaller, in volume, than the sealing ring and being housed in the shallower groove, the sealing ring being of substantially rectangular cross-section and being in part supported against a rigid step between the deeper groove and the shallower groove.

2. A piston-in-cylinder vacuum pump, according to claim 1, characterised in that the piston (1) decreases in diameter in two steps, so that two peripheral grooves (5a and 5b) are formed of different depths, of which the deeper groove (5b) is adjacent one (1 b) of the two faces of the piston (1 ); the deeper groove (5b) containing a resilient O-ring whose outer diameter is greater than the diameter of the floor of the shallower groove (5a); there being positioned radially outwards of the O-ring (6) an outer ring (7) made of a synthetic plastic material, the outer ring (7) being supported at its radially outer surface by the resilient O-ring (6) and by the floor of the shallower groove (5a);.the two rings being retained axially by sidewalls of the grooves (5a, 5b) and by a disc (3) whose diameter is the same as the greater diameter of the piston (1).

3. A pump according to Claim 1, characterised in that the piston (1) has an uninterrupted sidewall (1 c); the cylinder wall (8) having a stepped peripheral groove (5) forming a shallower groove (5a) and a deeper groove (sub): the deeper groove (5b) containing an o-ring (6); the stepped peripheral groove (5) containing, positioned radially inwards of the O-ring (6), an inner ring (7) made of a synthetic plastic material, the inner ring (7) being supported at its radially outer surface by the O-ring (6) and by the floor of the shallower groove (5a).

4. A pump as claimed in Claim 3, characterised in that the piston is deep-drawn of sheet metal.

5. A pump as claimed in Claim 2, characterised in that the cylinder wall has a deep-drawn liner exhibiting a plain cylindrical wall against which slides the sealing ring carried by the piston.

6. A pump according to Claim 1, characterised in that the diameter of the piston (1) is stepped down adjacent one (1 b) of the two piston faces, so that a peripheral groove is formed; the peripheral groove containing at least one open-ended springring (12) made of steel wire, round or square in cross section; the spring-ring (12) supporting and thrusting resiliently outwards a ring (7), rectangular in cross section and made of a synthetic plastic material; the two rings being retained axially by a disc (3).

7. A pump as claimed in one of the Claims 1 to 6, characterised in that the ring (7) of synthetic plastic material is an open ring with ends abutting at 45 the ring being retained axially by a disc (3).

8. A pump according to any one of the preceding Claims 1 to 7, where the sealing ring is of polytetrafluoro ethylene.

9. A pump according to Claim 1 and as described with reference to any one of the accompanying drawings.