GB2024340A - Shaft seals - Google Patents

Abstract

A temperature responsive expansion device comprises a piston and a cylinder containing a substance having a high coefficient of thermal expansion in which the piston comprises a cylindrical shaft 1 which is urged outwardly of the cylinder when the substance expands, the shaft passing through a central bore of a bush 2, located in the cylinder towards one end thereof, and the bush carrying a gland seal engaging about the shaft to provide sealing yet to permit axial movement of the shaft in the bush, characterised in that the gland seal comprises two rings, one 11 of p.t.f.e. surrounding the shaft, and in turn surrounded by the second ring 12 which is of nitrile rubber, the two rings being located in a cylindrical cavity 14 formed in the bush, and the rings being held within the cavity by a washer 15 inserted behind the rings and in turn secured in the cavity by a part 16 of the bush at the open end of the cavity being inwardly deformed behind the washer. <IMAGE>

Description

SPECIFICATION Adjustable thermal actuator This invention relates to temperature responsive devices of the kind utilizing the volumetric expansion of substances or compositions having high coefficients of thermal expansion. Typical of such substances are waxes or jellies, and in particular a mixture of paraffin wax and petroleum jelly. The volumetric expansion and contraction of such substances occurring when there is a change in the ambient temperature is employed to operate mechanisms such as bell cranks or linkages, say for controlling ventilators, windows, fanlights, shutters, valves and so forth.

Temperature responsive actuating devices which utilize the volumetric expansion and contraction of a substance have been proposed which essentially include a straight tubular cylinder containing the substance and immersed in said substance, a piston which is constituted by a cylindrical shaft and extends into the cylinder through a close fitting guide means incorporated a gland sealing means.

The ends of the cylinder are closed by bushes which are sealed against the cylinder walls, and the piston shaft passes through at least one of these bushes.

The present invention is particularly directed to the provision of a gland sealing means for location in a central bore of such a bush and which sealing means surrounds the piston shaft and seals thereagainstwhilst axial sliding movement of the shaft within the seal takes place.

In such an environment the gland seal has to provide sealing to prevent passage of the thermally expansive substance which, as stated above, is commonly a mixture of paraffin wax and petroleum jelly. Sealing problems arise because the piston shaft has to be allowed axial movement and because the thermally expansive substance, while not being very lacking in viscosity at normal room temperature and pressures, may generate very high pressures of up to or even more than 5,000 p.s.i. on being heated and expanding. Under these circumstances, it may behave in a manner somewhat resembling a pressure fluid of low viscosity.

The present invention provides a temperature responsive expansion device comprising a piston and a cylinder containing a substance having a high coefficient of thermal expansion, the piston being constituted by a cylindrical shaft which is urged outwardly of the cylinder when the substance expands, the shaft passing through a central bore of a bush located in the cylinder towards one end thereof, the bush carrying a gland seal engaging about the shaft to provide sealing yet to permit axial movement of the shaft in the bush, and wherein the gland seal comprises two rings, one of p.t.f.e. surrounding the shaft, and in turn surrounded by the second ring which is of nitrile rubber, the two rings being located in a cylindrical cavity formed in the bush, the seals being held within the cavity by a washer inserted behind the seals and in turn secured in the cavity by a part of the bush at the open end of the cavity being inwardly deformed behind the washer.

Preferably this gland seal seal is characterised in that, when the washer is in position, the nitrile seal is resiliently precompressed. This precompression may be obtained by making the rubber ring seal of larger outside diameter than that of the cavity within which it is fitted.

The p.t.f.e. ring acts to support sliding movement of the shaft and also to prevent extrusion of the nitrile ring.

The invention will be described with reference to the embodiments shown by way of example in the accompanying drawings, in which: Figure 1 is a sectional view of a thermally responsive piston and cylinder assembly; while Figure 2 is a detail view of one embodiment of gland seal arrangement; and Figures 3(a) and 3(b) are detail views of a second embodiment of a gland seal arrangement showing the components immediately before final assembly in Figure 3(a) and after assembly in Figure 3(b).

In the drawings, 1 represents a piston comprising a plain cylindrical shaft adapted at one end to abut against a push rod 11 adapted to be connected to a load. The piston 1 extends through a bush 2 into a cylinder 3. The cylinder 3 in part confines a space 4 which is fully occupied by a substance having a high coefficient of thermal expansion. This substance is adapted to expand when the ambient temperature increases and to contract when the ambient temperature decreases.

The space 4 within the cylinder 3 is obturated at one end by a bush 5 having an external sealing ring 6 engaging with the wall of the cylinder. The space 4 is closed at the other end of the cylinder by the bush 2 which also has a seal 6 engaging with the cylinder wall.

The piston 1 passes through the bush 2 and through a gland seal, generally designated 10 in Figure 1, which gland seal is housed within a cavity of the bush 2. The gland seal will now be described in greater detail with reference to Figure 2, 3(a) and 3(b).

The gland seal shown in Figure 2 comprises two rings surrounding the piston 1. Firstly there is a ring 11 of p.t.f.e. which directly bears upon the piston 1.

Surrounding the ring 11 is a second ring 12 of nitrile rubber. The two rings are located in a cylindrical cavity 14formed in the bush 2. The rings are held within the cavity 14 by a washer 15 suitably of brass. The washer 15 is inserted behind the rings 11 and 12 and is in turn secured in the cavity by a lip 16 of the bush 2 being inwardly deformed behind the washer.

The ring or seal 12, in the embodiment depicted in Figure 2, has a portion extending in contact with the piston shaft 1. The expansive substances acts against this portion of the seal 12 and a hydrostatic sealing action is obtained preventing escape of the substance along the surface of the shaft 1. The seal 12 may have an outer diameter larger than that of the cavity 14 so that it is under a small degree of precompression when housed in the cavity 14.

In the embodiment shown in Figures 3(a) and 3(b) the seal 12 of nitrile rubber again surrounds a ring 11 of p.t.f.e. which surrounds the shaft 1. However, instead of the seal 12 having a portion extending into contact with the shaft 1,there is provided a separate rubber sealing ring 17 interposed before the brass washer 15, the shaft 1 sliding within bores of both these elements, that is the sealing ring 17 and the washer 15.

The p.t.f.e. ring 11 supports sliding movement of the shaft which is of course also supported by the cylindrical bore of the bush 2 through which the rod 1 passes.

Also, the p.t.f.e. ring 11 serves to prevent extrusion of the nitrile rubber seal 12 into the very small space between the shaft 1 and the bush 2.

Claims (5)

1. Atemperature responsive expansion device comprising a piston and a cylinder containing a substance having a high coefficient of thermal expansion, the piston being constituted by a cylindrical shaft which is urged outwardly of the cylinder when the substance expands, the shaft passing through a central bore of a bush located in the cylinder towards one end thereof, the bush carrying a gland seal engaging about the shaft to provide sealing yet to permit axial movement of the shaft in the bush, and wherein the gland seal comprises two rings, one of p.t.f.e. surrounding the shaft, and in turn surrounded by the second ring which is of nitrile rubber, the two rings being located in a cylindrical cavity formed in the bush, the seals being held within the cavity by a washer inserted behind the seals and in turn secured in the cavity by a part of the bush at the open end of the cavity being inwardly deformed behind the washer.

2. A device as claimed in ciaim 1 wherein a further ring of nitrile rubber is disposed on and about the shaft and is located between the washer and the p.t.f.e. ring surrounded by the first mentioned nitrile ruing.

3. A device according to claim 1 and wherein the nitrile ring has a portion extending into annular abutment with the shaft at a location between the p.t.f.e. ring and the retaining washer.

4. A device according to claim 1 and in which, prior to insertion into the cavity, the nitrile rubber ring has a diameter which is larger than that of the cavity so that the ring is compressed when it is inserted into the cavity.

5. A device responding to thermal changes substantially as hereinbefore described with reference to the accompanying drawings.