GB2204648A - Hydrostatic seal - Google Patents

Abstract

In order to provide a seal around a rotating shaft 1 without undue wear of the seal, a non- contact seal 2 is mounted around the shaft in the bore of a stationary body 4 and is formed with its own bore having a step 14, the seal bore being of smaller diameter on the low pressure side of the step. The seal 2 may be formed in a plurality of portions to facilitate replacement. <IMAGE>

Description

HYDROSTATIC SEAL Field of the Invention When a rotatable shaft passes through a stationary body, it is often desired to provide a seal between the shaft and the body to prevent the escape of fluids through the space between the shaft and the body.

Background of the Invention One example of such a seal is an O-ring which is stretched around the shaft and bears on a cylindrical sealing surface on the stationary body. Such a seal is subject to wear between the O-ring and the sealing surface, and after a time sufficient wear will occur to leave a gap between the ring and the surface to allow escape of fluid.

It has also been proposed to mount the sealing ring stationarily in the body, sealing occurring at the interface between the sealing ring and the shaft, but the same problem of wear occurs. If a small escape of fluid can be accommodated, the sealing ring can be formed with a slightly greater bore than the outside diameter of the shaft, but the problem of controlling the gap between the seal and the shaft to be uniform all around the shaft is difficult to solve.

One attempt to solve the problem of wear is described in "The Hydrostatic Gas Non-Contact Seal, by Koga T and Fur it T in ASLE (American Society of Lubrication Engineers) Transactions, Vol 29, 4, 505-514. In order to stabilise the fluid flow between the shaft and the seal, the face of the seal is tapered towards the shaft at the high pressure side, followed by a cylindrical face of the minimum radius of the tapered portion at the low pressure side.

Summary of the Invention We have found that a better solution is to provide a seal assembly as claimed in claim 1, where the bore of the seal is not tapered but is stepped to have a smaller diameter on the low pressure side compared to the hgh presure side. A stepped bore is easier to manufacture than a semi-tapered bore and has been found to give good performance.

Another problem is that a continuous sealing ring has to be placed in position on the shaft by being passed over the shaft at one end, which presents no problem during the assembly of the shaft in its housing, but when a seal has to be replaced, the consequent dis-assembly of the shaft from its housing and re-assembly takes time. According to a preferred feature of the present invention, this problem is overcome by providing a seal divided into at least tWD portions and means to hold said portions together.

Brief Description of the Drawings An example of the invention will now be described with reference to the accompanying drawing which is a schematic diametral section through a seal assembly.

Detailed Description of the Present Invention A shaft 1 is rotatable about its axis within a machine housing 5.

Secured to the machine housing 5 by means of a bolt 11 and an adaptor 12 is a seal housing 4 formed with a bore within which the shaft 1 rotates. Seals such as O-rings are provided in the adaptor body 12 to prevent leakage at the interfaces with the housing 5 and the housing 4. The bore in the housing 4 is formed with an enlarged portion with narrower portions on either side, to form a recess 13 and a split seal ring 2 is mounted within the recess 13. The narrower portion on the low pressure side of the recess a radial wall 17 and the low pressure side of the seal has a corresponding radial face 16 which engages the wall 17 when pressurised by the fluid to be sealed, thus reducing leakage of fluid therebetween.

The ring 2 is divided into two (it could be more, as convenient) portions to allow the ring to be placed around the shaft 1 without requiring access to one end of the shaft. The ring 2 is provided with an outer circumferential recess to receive a garter spring 3 for holding the divided portions of the ring together. The seal housing 4 may also be split around the shaft.

The bore of the seal ring is in the form of two cylinders with an abrupt (radially faced) step 14 between. There is a larger diameter cylinder 21 on the high pressure side (on the left of the figure) and second smaller diameter cylinder 22 on the low pressure side (on the right of the figure). The cylinder 21 is longer (by a factor of at least 2) than the cylinder 22. The step 14 has a radial dimension of between 0.005 rrrn and 0.05 mm and the gap between the smaller cylinder 22 and the shaft 1 is of the same order. The cross-section of the ring 2 is uniform around the shaft.

The purpose of the increase in gap size between the shaft and the seal ring on the high pressure side is to allow hydrostatic pressure of the fluid into the majority of the gap between the seal ring and the shaft to maintain and control the spacing between the shaft and the seal. The remaining narrow portion of the bore keeps the escape of fluid between the seal ring and the shaft to small and controlled amounts, but because the gap size is properly controlled by the hydrostatic pressure in the upstream portion of the bore, the shaft does not contact the seal ring and so no wear occurs.

Claims (9)

1. A seal assembly comprising a rotatable shaft, a stationary body having a bore for passage of the shaft, a seal mounted in the bore of the stationary body around the shaft, the bore of the seal being formed with a step to have a smaller diameter on the low pressure side of the seal than at the high pressure side.

2. A seal assembly as claimed in claim 1 wherein the bore of the seal is in the form of two cylinders with said step between the two cylinders.

3. An assembly as claimed in claim 2 wherein the length of the cylinder on the low pressure side of the step is less than the length of the cylinder on the high pressure side of the step.

4. An assembly as claimed in claim 3 wherein the length of the cylinder on the low pressure side of the step is less than half the length of the cylinder on the high pressure side of the step.

5. An assembly as claimed in any one of claims 1 to 4 wherein said seal has a radial face on its low pressure side and the stationary body has a co-operating radial face on its low pressure side against which the radial face of the seal is in use urged by the pressure of the fluid.

6. An assembly as claimed in any one of claims 1 to 5 wherein the seal is divided into at least two portions and the assembly comprises means to hold said portions together.

7. An assembly as clamied in claim 5 wherein the holding means comprises a band encircling the portions of the seal and the outer face of the seal is formed with a recess for receiving the band.

8. An assembly as claimed in any one of claims 1 to 7, wherein the seal is an annulus of uniform cross-section.

9. A seal assembly substantially as herein described with reference to and as illustrated in the accompanying drawings.