GB2063385A

GB2063385A - Gas seals

Info

Publication number: GB2063385A
Application number: GB7940578A
Authority: GB
Original assignee: Cross Manufacturing Co 1938 Ltd
Current assignee: Cross Manufacturing Co 1938 Ltd
Priority date: 1979-11-23
Filing date: 1979-11-23
Publication date: 1981-06-03
Also published as: DE3043617A1; FR2470313A1; GB2063385B; FR2470313B1; DE3043617C2; US4331337A

Description

1 GB 2 063 385 A 1

SPECIFICATION Gas seals

This invention concerns gas seals and in particular relates to apparatus for and a method of effecting a gas seal between a shaft and a housing 70 which surrounds with clearance the shaft.

There have been designed various forms of seal capable of effecting a seal between a rotatable shaft and a housing which surrounds the shaft, for instance to prevent foreign matter which may be present outside the housing on one side of the seal entering the housing. The provision of an effective seal is particularly important for the case where a bearing for the shaft is provided within the housing, to prevent damage to the bearing, and in such a case, the seal may serve the further purpose of preventing lubricant or other liquids from leaving the housing.

A known form of seal suitable for the above purposes comprises a member which is adapted to be mounted in the housing and fits around the shaft, the member for instance being made of a resilient material such as a natural or synthetic elastomer. Such a seal member is quite satisfactory for effecting a seal to a shaft intended to rotate at low speeds, but as the speed of rotation rises, so does the rate of wear of both the seal member and shaft itself. Consequently, with shafts intended to rotate at very high speeds (for instance, 10,000 r.p.m. or more), it is advantageous to provide a seal member which is retained within the housing in such a way that the member does not contact the shaft but fits very closely there-around with a minimal clearance. For example, the seal member may be elongate axially and provided with a plurality of circular ribs on its inner surface, which ribs fit within but do not touch correspondingly shaped grooves formed in the shaft; such a seal is known as a labyrinth seal.

The sealing effect of a labyrinth seal may be enhanced by providing air or a gas under pressure at an appropriate point to the seal member, so that the air or gas bleeds outwardly through the labyrinth path defined by the shaft and the seal member, thereby reducing the probability of foreign matter entering the labyrinth path.

The above-described form of labyrinth seal suffers from several disadvantages. The seal member must be made in at last two parts in order to be capable of being fitted around the shaft, and 115 this causes problems in manufacture and assembly. Machining of the seal member is critical and must be formed to tight tolerances; machining of the grooves in the shaft may also cause problems. The seal member must be mounted rigidly within the housing to ensure that the member does not contact the shaft at any point, and moreover if clearances are to be maintained at the smallest values possible, then the shaft must run with a very high degree of truth; little if any run-out is permissible. Also the supply of compressed air or gas to the seal leads to further complications and an increase in the amount of equipment necessary.

It is a principal object of this invention to provide apparatus for and methods of effecting a gas seal, which at least reduce the problems discussed above of known forms of seal such as a labyrinth seal.

According to one aspect of this invention, there is provided a sealing member for effecting a seal between a rotatable shaft and a housing which surrounds with clearance the shaft, which sealing member is in the form of a ring adapted loosely to be fitted within the housing so as closely to surround the shaft, the ring having a split which allows circumferential expansion thereof and the inner face of the ring being so formed as to define at least one ramp surface with respect to the shaft whereby when the ring is fitted in the housing to surround the shaft, the clearance between the inner face of the ring and the shaft decreases from a maximum value at one point on the ring to a point spaced angularly from the first point in the direction of normal rotation of the shaft where the ring touches the shaft so that on rotation of the shaft at a sufficiently high speed, gas is carried around by the shaft within said clearance so as to expand the ring circumferentially and lift the ring clear of the shaft.

It will be appreciated that the sealing member of this invention, when fitted in a housing to surround a shaft adapted for rotation in one direction at a relatively high speed (typically in excess of 10,000 r.p.m. ) forms a gas seal utilising the surrounding gas (and usually air) insofar as there is no direct contact between the sealing member and the shaft; instead, a film of gas exists between the shaft and the sealing member, which film is maintained by the shaft rotation and keeps the sealing member lifted off the shaft. There is however no need to provide means to supply gas under pressure to the sealing member, for the gas film is created by the rotation of the shaft itself. Though usually the gas in the region of the seal will be air, it could of course be any other gas or mixture of gases; reference will however hereinafter be made expressly to air, though the term should be interpreted broadly to include all types of gases.

In order to ensure that the sealing member performs properly in use, the formation of the ramp surface is critical; a typical maximum value for the clearance is of the order of 0.005 inches (about 0. 125 mm) though advantageous results have been obtained with maximum values in the range of 0.002 to 0.003 inches (from about 0.05 to about 0.075 mm). It will of course be appreciated that the maximum value as well as the angular extent of the ramp surface - and hence the "angle" of taper of the clearance depends upon a variety of factors including the diameter of the shaft, the axial extent of the sealing member, the surface finish of the shaft and sealing member, the temperature of the air at the installation location and so on. In the case in which only one ramp surface is provided, it is preferred for that ramp surface to have an angular extent of slightly less than 27r radians, though 2 GB 2 063 385 A 2 typically three or four such ramp surfaces may be provided - in which cases, each ramp surface should have a length of slightly less than 2. 7r/3 or,r/2 radians, respectively. Whatever the number of ramp surfaces choser, the split in the ring should be disposed at an angular position on the sealing member other than in a region where there is a reducing clearance - i.e. not to cut through a ramp surface.

The inner surface of the sealing member may be formed tc, define -,lij ramp surface in various ways. For example, the inner surface of the rnem.ber rnay be machined to provide the ramp su rface. Another pissibb!ity for the case cr,' a metal sealing member is to arrange for the member to be rolied from a metal strip, and to adjust the rolls suitably to provide the or each ramp surface during the rolling step. Yet another possibility, in the case where there;s but one ramp surface, is for the two adjacent end portions of the sealing nember at the split therein to be stepped but in the opposite sense so that one end portion may overlie the other end portion. By appropriate adjustment of the radial thicknesses of the stepped end portions, the ramp surface may be created; the thicknesses should be adjusted so the inner surface of the sealing member is dis--ont;nuous at the split therein, the inner surface at the radially outer end portion which overlies the radially inner end portion being spaced outwardly with respect to the inner surface at the radially inner end portion by an amount equal to the required maximum value of the clearance.

For an arrangement with stepped end portions, in order to accommodate the circumferential expansion of the sealing member, the two end portions must be capable of sliding one with respect to the other. In order to allow free sliding movement, a roller may be accommodated between the two end portions, whereby one end 105 portion may roll over the other. The roller may be free or may be fitted wit h a cage to maintain the roller in the required position. Another possibility is for the roller to be accommodated within a V-shaped groove in one of the end portions, which groove extends parallel to the axis of the sealing member. Whatever the arrangement employed, the thicknesses of the end portions must be adjusted appropriately, to ensure the required clearance is maintained.

Apart from the possibility of using stepped end portions (as discussed above), the sealing member may have the split formed as a simple radial cut, the two ends of the member abutting but separating as the member expands circumferentially. The split may instead be formed by a cut extending at some angle to a radial plane, the ends again abutting. These configurations, inchuding that with st tepped end portions, may be usedwith sealing members other than those having the ramp s7-,.rF.3ce forn)#3J by adjustment of the thickness of everapping end portions.

4 will ta that the sealing member of this invention should be accommodated loosely within the housing in order that the sealing 130 member may fit closely around the shaft. Not only does this obviate the need to machine with great accj;acy that part of the housing which receives the sealing member, but also allows for a certain amount of run-out on the shaft. Because then the sealing member is not held tightly in the housing, it is preferred for there to be means provided to prevent rotation of the sealing member, when installed in the housing. For example, a peg may be provided for mounting in the housing so as to project therefrom into a suitably-formed recess in the sealing member, so as to prevent rotation thereof despite rotation of the shaft. However, it would be possible for some applications for the sealing member not to be restrained against rota'L'Sc.ti,. Then, the sealing member may rotate with the shaft when the latter rotates slowly, but as the shaft speed increases, an increasing differential between the shaft speed and the sealing member speed will occur until an air film is formed, to lift the sealing member completely clear of the shaft. At this point, the friction between the ring and the shaft will be much lower than that between the ring and housing, so that the ring will tend to stop rotating altogether, ensuring the air film is maintained.

According to another aspect of this invention, there is provided an assembly of a shaft mounted for normal rotation in one sense only, a housing surrounding with clearance the shaft and a sealing member as described above for effecting a seal between the shaft and the housing, the housing defining a recess in which the sealing member is located to surround the shaft, the recess being shaped to allow the sealing member to move in a radial plane of the shaft whereby the sealing member may fit closely around the shaft and may effect an air seal therewith when the shaft rotates at least at a minimum predetermined speed.

It is preferred for the recess in which the sealing member fits to be of very slightly greater axial dimensions than the sealing member itself, whereby the sealing member is effectively constrained against axial movement but nevertheless may slide freely in a radial plane. In order to accommodate a shaft which has a relatively large run-out, as well as a possible lack of concentricity between the recess in the housing and the shaft, it is preferred for the sealing member to be capable of moving in a radial direction by at least 0.0625 inches (approximately 1.5875 mm) from a mean position.

Preferably, means are provided within the housing to prevent rotation of the sealing member, such as a peg engageable with a recess in the sealing member.

According to a further aspect of this invention, there is provided a method of effecting a seal between a shaft rotating within a housing surrounding with clearance the shaft, in which method a sealing member as described above and fitted within the housing fits closely around the shaft, and an air film is generated between the sealing member and the shaft by air being carried round with the shaft on rotation thereof in the 3 GB 2 063 385 A 3 direction of decreasing clearance between the ramp surface of the sealing member and the shaft, which film expands the sealing member circumferentially to lift the member clear of the shaft.

It will be appreciated that the sealing method of this invention does not require the provision of any auxiliary equipment such as a source of compressed air and, provided that the sealing member is properly configured and the shaft rotates with at least a minimum speed in the direction of decreasing clearance between the ramp surface of the sealing member and the shaft, the seal is essentially frictionless whilst being self- sustaining. Moreover, the minimum clearance between the sealing member and the shaft, even when the ring is clear of the shaft, is only a very small amount and typically a few microns - and consequently the probability of foreign matter passing through the seal is very small.

By way of example only, certain specific embodiments of sealing arrangement employing sealing members of this invention will now be described, reference being made to the accompanying drawings, in which: Figure 1 is a diagrammatic view of a shaft, housing therefor and sealing member assembled together in accordance with this invention; Figure 2 is an end view on an enlarged scale of the assembly in Figure 1; Figure 3 is a detail view of a modification of the sealing member shown in Figure 2; Figure 4 is an end view of a second embodiment of sealing member of this invention; and Figure 5 is an end view of a third embodiment of sealing member of this invention.

Referring initially to Figure 1, there is shown a shaft 10 which is mounted for rotation in bearings (not shown) in the direction of arrow A. The shaft 105 is disposed partially within a housing 11 which housing has an inwardly-directed flange 12 defining a circular orifice 13 which surrounds with clearance the shaft 10. Clamped to the flange 12, for instance by means of rivets 14, is a support 110 member 15 which defines a generally rectangular recess 16 facing and extending circumferentially around the shaft 10. A sealing member 17 is mounted in the recess 16, which member 17 effects an air seal between the housing 11 and the 115 shaft 10, when the haft rotates at least at a predetermined minimum speed in the direction of arrow A.

The sealing member 17 is shown in greater detail on an enlarged scale on Figure 2. The member is in the form of a ring for instance of a resilient metal such as bronze, the ring being split at 18 and the two end portions 19 and 20 being stepped oppositely so as to be capable of overlapping one another, as shown. In this way, the member 17 may expand circumferentially, by relative sliding movement of the two end portions 19 and 20. The radial thickness of the radially inner end portion 19 and the radial thickness of the radially outer end portion 20 should be 130 selected so that the radius of the inner surface 21 of the end portion 19 is less than the radius of the inner surface 22 of the end portion 20 by a fixed predetermined amount, of typically 0.0025 to 0.0030 inches (0.0635 mm to 0.076 mm).

When the sealing member is assembled in the housing as described with reference to Figure 1, the inner surface 21 of the end portion 19 will bear on the shaft 1 0,whereas the inner surface 22 of the end portion 20 will be clear of the shaft by a fixed amount which is predetermined by appropriate selection of the radial thicknesses of the end portions 19 and 20 in the overlapping region. Because the sealing member 17 is essentially circular and the end portion 20 is spaced from the shaft 1 Owhereas t-he end portion 20 is spaced from the shaft 10 whereas the end portion 19 touches the shaft, the inner surface of the member, from surface 22 to surface 2 1, will form a ramp with respect to the shaft, the clearance between the inner surface of the member 17 and the shaft decreasing from the maximum value down to zero in the direction A of normal rotation of the shaft.

As shown in Figure 2, the circum ferential length of the radially outer stepped end portion 20 is less than the circum.herentiaily length of the radially inner stepped end portion 19, so that a recess 23 is formed between the free end of the outer end portion 20 and the remainder of the sealing member 17. A peg 24 is mounted in the housing 11 and projects into the recess 23, so as to prevent rotation of the sealing member 17.

In use and on rotation of the shaft 10 in the direction of arrow A with at least a minimum speed, air in the vicinity of the seal will be carried round by the shaft and owing to the reducing clearance between the ring inner surface of the sealing member 17 and the shaft 10, a film ot air will be fornied between the member and the shaft, thereby lifting the member completely clear of the shaft. Typically, this film will be of the order of 2.5 microns thick, and though the circumferential expansion required for such a clearance is minimal, nevertheless such expansion may occur by relative sliding movement between the two stepped end portions 19 and 20. Moreover, any run-out present when the shaft rotates can be accommodated by movement of the member 17 in the radial plane, by the member sliding in that plane within the support member 15.

Particularly in the case of a seal to be effected between a stationary housing and a shaft which rotates at very high speeds - and typically not less than 10,000 r.p.m. - it is advantageous to make the sealing member from a material other than a metal, such as a ceramic which may, within relatively narrow limits, deform resiliently in the required manner. Another possibility is to coat a metallic ring with a vjear-r3sistarit coating, such as a chromium plating, in order to minimise wear during the period when the sha-ft is running up to speed and before the air sew is established -for wear rapidly could diminish the formation of the air film between the ring and the shaft.

4 GB 2 063 385 A 4 Figure 3 shows a detailed view of a modified form of sealing member for use in a seal configured generally as shown in Figure 2. In this modified form, there is provided a roller 25 between the two stepped end portions 19 and 20 of the sealing member 17, which roller 25 is located in a groove 26 extending parallel to the axis of the ring in the inwardly-directed face of the stepped radially-outer end portion 20. The provision of such a roller 25 allows friction between the two end portions 19 and 20 to be reduced to a minimal value, thereby assisting free relative sliding movement between the two end portions as the ring expands circumferentially on formation of the air film, lifting the ring clear of the shaft.

Figure 4 shows an alternative form of sealing member for use in the arrangement depicted in Figure 1. In this arrangement, the sealing member 27 is split at 28 in a radial plane, the two radial end faces of the member simply abutting when the member is relaxed. Once fitted closely to surround around a shaft 10, the two end faces may move apart as required, leaving a gap therebetween. By a machining operation, the inner surface of the ring is provided with four spaced but similar ramp surfaces 29, each of approximately 7r/2 radians extent. Each ramp portion defines in conjunction with the shaft a clearance which decreases from a maximum value of typically 0.0025 inches (0.0635 mm) down to zero. Thus, with the member 27 shown in the relaxed position in Figure 4, the ring touches the shaft at both ends thereof, to either side of the split 28, as well as at three intermediate points; but on rotation of the shaft in the direction of arrow A, the member is lifted of an air film by means of the four ramp surfaces 29, air being carried round by rotation of the shaft within the clearances defined by the ramp surfaces.

In a similar manner to that described above, 105 with reference to Figure 2, the member 27 is prevented from rotating by virtue of a peg 30 fixed within the housing and located in a recess 31 provided in the member 27.

Figure 5 shows a further possible configuration 110 of sealing member of this invention, which member 32 is generally similar to that shown in Figure 4, but has four ramp surfaces 33 formed in a different way. In the embodiment of Figure 4, the ramp surfaces are formed by machining, whereas in the embodiment of Figure 5, the ramp surfaces are formed by a rolling operation, during the manufacture by rolling of the member from a strip of metal. This is achieved by appropriate adjustment of the spacing of the rolls forming the strip into the ring-shaped sealing member 32. The free ends of the member 32 abut in a radial plane, as in the embodiment of Figure 4, and the sealing member forms an air seal in the same manner as has been described above, with air being carried round on rotation of the shaft 10 in the direction of arrow A, to form an air film between the shaft and the sealing member 32.

Claims

1. A sealing member for effecting a seal between a housing which surrounds with clearance a shaft mounted for rotation normally in one sense only, which sealing member is in the form of a ring adapted loosely to be fitted within the housing so as closely to surround the shaft, the ring having a split which allows circumferential expansion thereof and the inner face of the ring being so formed as to define at least one ramp surface with respect to the shaft whereby when the ring is fitted in the housing to surround the shaft, the clearance between the inner face of the ring and the shaft decreases from_ a maximum value at a first point on the ring to a second point spaced angularly from the first point in the direction of normal rotation of the shaft at which second point the ring touches the shaft so that on rotation of the shaft at a sufficiently high speed, gas is carried round by the shaft within said clearance so as to expand the ring circumferentially and lift the second point of the ring clear of the shaft.

2. A sealing member as claimed in claim 1, wherein the ring is pre-formed so that the maximum value of the clearance at said first point is no greater than substantially 0.005 inches (about 0. 125 mm).

3. A sealing member as claimed in claim 2, wherein the ring is pre-formed so that the maximum value of the clearance at said first point is in the range of from 0.002 to 0.003 inches (from about 0.05 to about 0.075 mm).

4. A sealing member as claimed in any of the preceding claims, wherein the split allowing circumferential expansion of the ring is provided at a circumferential position on the ring not coincident with the extent of the ramp surface.

5. A sealing member as claimed in any of the preceding claims, wherein the ring is provided with not more than four distinct ramp surfaces, each of which has an angular extent of less than 27r/n radians, where n is the number of distinct ramp surfaces.

6. A sealing member as claimed in any of the preceding claims, wherein there is only one ramp surface and the two adjacent end portions of the sealing member at the split therein are stepped respectively in opposite senses so that one end portion overlies the other end portion.

7. A sealing member as claimed in claim 6, wherein the radial thicknesses of the stepped end portions are controlled to predetermined values whereby the ramp surface is created, the inner surface of the sealing member thereby being discontinuous at the split.

8. A sealing member as claimed in claim 7, wherein there is a roller accommodated between the two end portions, whereby one end portion may move with respect to the other on circumferential expansion of the ring.

9. A sealing member as claimed in claim 8, wherein one end portion is provided with a V-shaped groove in which the roller is W GB 2 063 385 A 5 accommodated, the groove extending parallel to the axis of the sealing member.

10. A sealing member substantially as hereinbefore described and as illustrated in the accompanying drawings.

11. A sealing assembly of a shaft mounted for normal rotation in one sense only in a housing surrounding with clearance the shaft, and a sealing member as claimed in any of the preceding claims for effecting a seal between the shaft and the housing, the housing defining a recess in which the sealing member is located to surround the shaft which recess is shaped to allow the sealing member to move in a radial plane of the shaft whereby the sealing member may fit closely around the shaft and may effect an air seal therewith when the shaft rotates at least at a minimum predetermined speed.

12. A sealing assembly as claimed in claim 11, wherein the recess in which the sealing member fits is only of a sufficient axial dimension to allow the sealing member slidably to fit therein in the radial direction, the sealing member being effectively constrained against axial movement.

13. A sealing assembly as claimed in claim 12, wherein the recess in the housing allows the sealing member to move in a radial direction by at least 0.0625 inches (approximately 1.5875 mm) from a mean position. 30

14. A sealing assembly as claimed in any of claims 11 to 13, wherein means are provided within the housing to prevent rotation of the sealing member.

15. A sealing assembly substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

16. A method of effecting a seal between a shaft rotating in one sense within a housing surrounding with clearance the shaft, in which method a sealing member as claimed in any of claims 1 to 10 and fitted within a recess in the housing fits closely around the shaft, and an air film is generated between the sealing member and the shaft by air being carried round with the shaft on rotation thereof in the direction of decreasing clearance between the ramp surface of the sealing member and the shaft, which film expands the sealing member circumferentially to lift the member clear of the shaft.

17. A method of effecting a seal between a shaft and a housing as claimed in claim 16 and substantially as hereinbefore described with reference to the accompanying drawings.

Printed for Her Majesty's Stationery Office by the Courier Press, Leamington Spa, 1981. Published by the Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.