GB2076079A - Seal assembly - Google Patents

Abstract

A seal assembly for sealing a rotary shaft (1 )to a stator (2) into which the shaft extends to contain a fluid within the stator is described. The assembly comprises a first annular sealing surface (10) provided by a spring loaded first sealing member of relatively hard material which is carried by the stator (2). The first sealing surface (10) surrounds the shaft (1) and faces along the shaft in a direction away from that in which the shaft extends into the stator. A second, annular sealing surface (32) is provided by a second sealing member of relatively soft material which is carried by the shaft (1). The second sealing surface (32) surrounds the shaft (1), and slidably, sealingly engages the first sealing surface (10). The relative hardness of the first and second members is such that the two sealing surfaces (10, 32) can bed-in on initial sliding engagement. The second sealing member is split diametrically, the two halves being connected by bolts 25. A scale 35 may be provided such that the wear of the second sealing member may be indicated. Alternatively the second sealing member may be provided with visable counterable indicia. <IMAGE>

Description

SPECIFICATION Seal assembly The invention relates to a seal assembly for sealing a rotary shaft to a stator, into which the shaft extends, to contain a fluid within the stator.

There have been numerous previous proposals for seals in this field, one of the earliest being to provide a first annular sealing member on the stator with and axially facing sealing surface and a second sealing member on the shaft with a sealing surface facing towards the sealing surface on the stator. Each sealing surface is pre-polished or lapped so that a good seal is formed when the two surfaces engage. In practice, a thin film of work fluid is formed between the two sealing surfaces.

These seals have proved to be reasonably satisfactory at normal shaft rotation speeds, the main disadvantage being that the lapping of the sealing surfaces must be very precisely controlled to obtain high tolerances. Furthermore, during use, the sealing members gradually wear away and it is a characteristic feature of this type of construction that at a certain point the seal will unpredictably and suddenly break down. This of course, is very undesirable and can lead to costly down time of the equipment concerned.

A more recent proposal has been to make the first sealing member on the stator of a relatively softer material than that of the second sealing member and to make the first sealing member with a polygonal cross-section so that an edge of the first sealing member engages the sealing surface of the second sealing member. An example of this construction is illustrated in British Patent Specification No. 1,283,867 where the first sealing member has a substantially square cross-section and the second sealing member has a conical sealing surface. These constructions have proved to be reasonably satisfactory since the sealing surfaces do not have to be finely polished due to the bedding-in action which-will occur on the intial relative sliding motion between the two sealing surfaces.However, since the first sealing member presents and edge towards the second sealing surface, the wear rate of the first sealing member will not be constant. Clearly, as the first sealing member wears away, the first sealing surface will gradually increase in area and it becomes very difficult to forecast when the seal will break down. Also, the softer material of the first sealing member acts as a heat insulator and the only paths by which heat can escape from the seal interface is only very slowly through the hard second sealing member via the shaft to the atmosphere, very slowly directly to the atmosphere, or possibly very slowly into the work fluid adjacent the seal interface. This latter path is of little significance since work fluid can only lie along one edge of the sealing interface. The majority of the heat generated is retained by the insulating first sealing member.This problem becomes acute at high shaft rotation speeds.

One attempt to solve this heat build-up problem is illustrated in British Patent Specification No. 1,554,471. This illustrates a construction of the so called "internal" type where the softer sealing member is mounted instead on the shaft and the hard sealing member on the stator, the soft sealing member being within the stator. This construction does allow the work fluid to conduct heat away from the seal interface more efficiently but suffers from the major disadvantage that the sealing surfaces are not visible or easily accessible, and it is therefore impossible to determine the amount of wear or to replace a worn sealing member without dismantling the entire assembly.

In accordance with the present invention, a seal assembly for sealing a rotary shaft to a stator into which the shaft extends to contain a fluid within the stator comprises a first annular sealing surface provided by a first sealing member of relatively hard material which is carried by the stator, the first sealing surface surrounding the shaft and facing along the shaft in the direction away from that in which the shaft extends into the stator; and a second, annular sealing surface provided by a second sealing member of relatively soft material which is carried by the shaft, the second sealing surface surrounding the shaft, and slidably, sealingly engaging the first sealing surface, the relative hardness of the first and second members being such that the two sealing surfaces can bedin on initial sliding engagement.

This construction enables service fluid easily and efficiently to cool the first sealing surface. The removal of heat from the seal interface can be improved still further by the provision of a cooling chamber immediately behind the first sealing surface and into which service fluid may flow to absorb heat conducted through part of the first member from the first sealing surface.

The first sealing surface may be formed directly on a casing in which the shaft is received, or, alternatively, it may be formed on a separate stator part which is non-rotatably fixed to the casing. The second sealing member is preferably provided on a rotor mounted non-rotatably to the shaft.

In order to maintain an effective seal between the two sealing surfaces, the sealing surfaces must be urged towards one another. Where the first sealing surface is provided on a separate stator part, this stator part may be axially movable relative to the shaft under the service fluid pressure or under spring action. However, it is convenient if a spring is provided to urge the stator part towards the rotor so that the seal is maintained even when the machined is not in use.

Alternatively, the shaft may be spring'loaded so that it is urged into the stator.

One of the most important advantages of this invention is that the amount of wear of the second sealing member is normally visible. However, this is not absolutely essential and the amount of wear could be indicated by, in the example where a separate stator part is provided, the amount of movement of that stator part.

Preferably, the second sealing member is annular and has a substantially constant, radial thickness as hereinafter defined, over the majority of its axial length. By "substantially constant radial thickness"l mean that neither the inner or outer surface of the second sealing member extends at an angle of more than 1 50 to the shaft axis. With this construction, the wear rate of the second sealing member is substantially constant and this allows the user accurately to determine the lifetime of the seal. It is particularly convenient if the unused second sealing member is tapered to form a leading edge which on initial contact with the first sealing surface forms a line contact. This enables the sealing surfaces automatically to bedin without prior lapping of the second sealing surface being necessary.

Preferably, where the second sealing member is provided on a rotor, the rotor is made symmetrical about a plane perpendicular to the shaft axis so that a second sealing member is provided on the other axial end of the rotor. This enables the user, when the second sealing surface has worn down, simply to turn the rotor around to present a fresh second sealing surface to the first sealing surface. This means that breakdown of the second sealing surface is not accompanied by substantial down time of the machine concerned.

The operator merely has to reverse the rotor and can then apply for a replacement part while the seal still has a reasonably long life time. Thus, the operator is in effect given an early warning of when he will need to replace the second sealing member.

To enable the rotor easily to be removed from the shaft, it may be completely split into halves.

Further, where a separate stator part is provided this may also be divided into halves to enable that part to be easily removed from the shaft.

An example of a seal assembly in accordance with the present invention is illustrated in the accompanying drawings, in which: Figure 1 is a part sectional view of the assembly taken on the line 1-1 in Figure 2; Figure 2 is an end view of the assembly taken on the line 2-2 in Figure f; and, Figure 3 is a schematic view of part of one example of a rotor on an enlarged scale.

The seal assembly illustrated in the drawings is shown sealing a shaft 1 to a stator 2 into which the shaft extends. The stator 2, comprises a generally tubular portion 3 and an integral flange portion 4. The tubular portion 3 is mounted in a cylindrical, tubular cavity 5 in a casing 6 of, for example, a pump housing. An annular groove 7 is provided in the tubular portion 3 of the stator 2, ar "O"-ring 8 being housed in the groove 7 to seal the stator 2 to the cylindrical surface of the cavity 5. The flange portion 4 of the stator 2 has an axially facing sealing surface carrying surface 9 which carries an annular first sealing surface 1 0 made of a suitable hard material such as stellite, tungsten carbide, or a ceramic. The first sealing surface 10 is either coated directly on to the carrying surface 9 or may be bonded in some other way to it.An annular cavity 11 is formed in the flange portion 4 of the stator 2 immediately behind the sealing surface 10. The stator 2 is made of a hard material such as stainless steel or "Nyrnsist" which is a flaked graphite cast iron.

A grease service inlet 1 2 is provided in the carrier 6, while a flush inlet 13, closed by a removable, screw-threaded plug 14, is provided in the flange protion 4 of the stator 2. The inlet 1 3 communicates with the annular cavity 11. The internal diameter of the tubular portion 3 of the stator 2 is slightly larger than the diameter of the shaft 1 so that service fluid may pass under pressure behween the tubular portion 3 and the shaft 1 into the cavity 11 to the first sealing surface 10.

The stator 2 is urged to the left, as seen in Figure 1, by means of three compression springs 1 5 which are mounted between an annular stop plate 1 6 mounted on the casing 6 and a respective nut 1 7 screw-threaded onto a corresponding bolt 1 8. Each bolt 1 8 is screwed into an aperture of a respective bracket 1 9 which, in turn, is mounted to the flange portion 4 of the stator 2 by a respective screw 20 received in a corresponding bore 21.

The stator 2 is non-rotatably mounted to the casing 6 by means of a peg on the stop plate 1 6 (not shown) which is received in an aperture in the stator. The stator 2, however, may move axially along the shaft 1 urged by the springs 1 5.

A rotor 22 is non-rotatably mounted to the shaft 1 adajacent to the stator 2. As is apparent from Figure 2, the rotor 22 is symmetrical about a vertical plane perpendicular to the shaft axis and so parts which are duplicated will be described 'with reference to those adjacent to the stator 2 in Figure 1 , the corresponding symmetrical parts having the same reference numerals but with the addition of a prime.

The rotor 22 has two sections 23, 24 which are semi-circular to enable the rotor to be mounted about the shaft 1, whereupon the two sections are secured together by screws 25 which extend into bores 26 in the section 23. The rotor 22 is maintained non-rotatable on the shaft 1 by means of four grub screws 27 received in respective screw-threaded bores 28.

Turning now to the right-hand part of the rotor 22, as shown in Figure 1, an internal, annular groove 29 houses an "O"-ring seal 30 which seals the rotor 22 to the shaft 1. An integral, annular spigot portion 31 of the rotor 22 has an axially facing annular second sealing surface 32 which abuts the first sealing surface 10 on the stator 2.

The spigot portion 31 has a substantially constant radial thickness along the majority of its length but tapers to a leading edge 33 which forms initially a line contact with the first sealing surface 1 0. The rotor 22 is made of a relatively soft material such as "Railko" or "Ferobestos" which are made from an asbestos reinforced resin, Tufnel which is fabric filled with PTFE, a grade of carbon or a grade of PTFE commonly used for fluid sealing.

When the shaft 1 rotates, in used, the rotor 22 also rotates with the shaft. Since the stator 2 is urged towards the rotor 22 by the compression springs 1 5, the first and second sealing surfaces 1 0, 32 abut and initial rotation causes the sealing surfaces automatically to bed-in due to the initial line contact between the edge 33 and the first sealing surface 1 0. Thus, although lapping of the first sealing surface 10 is desirable, no prior lapping of the second sealing surface 32 is necessary. During use, since the material of the spigot 31 is softer than that of the first sealing surface 10, the spigot will begin to wear away and the stator 2 will move towards the rotor 22.

Service fluid supplied under pressure passes axially out of the flange portion 4 of the stator 2 into the spigot 31 of the rotor 22 and lubricates the abutting sealing surfaces 10, 32. Cooling flush and service fluid enters the annular cavity 11 immediately behind the first sealing surface 10 to absorb heat conducted through the relatively thin, good heat conducting part of the flange portion 4.

The service fluid, and flush supplied through inlet 13 may subsequently flow out through the inlet 13. In some applications, for example with pumps, the service fluid may be recycled. The fluid may be water or a water based fluid.

In this example, the abuttment between the first and second sealing surfaces 10, 32 is visible and thus the degree of wear of the second sealing surface can be readily observed. However, in order to give and accurate indication of the amount of wear that has occurred, a suitable scale 35 mounted to the stop member 1 6 is provided. This may have a scale in, for example millimetres.

When the spigot 31 has substantially completely worn away, which, as explained earlier, will occur at a fairly constant rate, the operator merely has to dismantle the rotor by unscrewing the bolts 25 and then reverse the rotor 22 to bring the second sealing surface 32' adjacent to the first sealing surface 10. As may be appreciated, this operation can be carried out very quickly and, having made the reversal, the operator can then order a new rotor well before the old rotor has to be discarded.

As well as observing the contact between the sealing surface 10, 32, in use, the operator can also lever the stator 2 away from the rotor 22 to observe the condition of the sealing surfaces.

Instead of a scale 35, where the spigot 31 is visible, a number of circumferential grooves 36 may be machined into the outer surfaces of the spigots 31,31 as shown in Fiugre 3. Thus, merely by counting the number of visible grooves 36 the operator immediately has an indication of the amqunt of wear that has occurred.

As a further possibility, in the situations where the amount of wear cannot be observed regularly, electronic sensors may be provided on the rotor and stator to sense their relative sepraration. The sensors may be linked to some convential warning mechanism which, for example, could stop operation of the apparatus when too much wear has occurred.

Claims (1)

1. CLAIMS:

   1. A seal assembly for sealing a rotary shaft to a stator into which the shaft extends to contain a fluid within the stator the assembly comprising a first annular sealing surface provided by a first sealing member of relatively hard material which is carried by the stator, the first sealing surface surrounding the shaft and facing along the shaft in a direction away from that in which the shaft extends into the stator; and a second, annular sealing surface provided by a second sealing member of relatively soft material which is carried by the shaft, the second sealing surface surrounding the shaft, and slidably, sealingly engaging the first sealing surface, the relative hardness of the first and second members being such that the two sealing surfaces can bed-in on initial sliding engagement.

   2. An assembly according to claim 1, wherein a cooling chamber is provided immediately behind the first sealing surface and into which service fluid may flow to absorb heat conducted through part of the first member from the first sealing surface.

   3. An assembly according to claim 1 or claim 2, wherein a seperate stator part is provided, the stator part being divided into halves to enable it to be removed from the shaft.

   4. An assembly according to any of the preceding claims, wherein the second sealing member is annular and has a substantially constant, radial thickness, as hereinbefore defined, over the majority of its axial length.

   5. An assembly according to any of the preceding claims, wherein the unused second sealing member is tapered to form a leading edge which on initial contact with the first sealing surface forms a line contact.

   6. An assembly according to any of the preceding claims, wherein the rotor is symmetrical about a plane perpendicular to the shaft axis.

   7. An assembly according to any of the preceding claims, wherein the rotor is split into halves to enable it to be removed from the shaft.

   8. An assembly according to any of the preceding claims, wherein the radially outer surface of the second sealing member is provided with a plurality of machine grooves to enable the amount of wear of the second sealing member to be monitored.

   9. An assembly according to any of the preceding claims, wherein a scale is provided adjacent to the postion of engagement between the first and second sealing surfaces to enable the amount of wear of the second sealing member to be monitored.

   10. An assembly according to claim 1, substantially as described with reference to the example shown in the accompanying drawings.

   New claims or amendments to claims filed on 28th August, 1981.

   Superseded claims 1 New or amended claims:

   1. A seal assembly for sealing a rotary shaft to a stator into which the shaft extends to contain a fluid within the stator, the assembly comprising a first annular sealing surface provided by a first sealing member of relatively hard material which is carried by the stator, the first sealing surface surrounding the shafw and facing along the shaft in a direction away from that in which the shaft extends into the stator; and a second, annular sealing surface provided by a second sealing member of relatively soft material which is carried by the shaft, the second sealing surface surrounding the shaft, and slideably, sealingly engaging the first sealing surface, the relative hardness of the first and second members being such that the two sealing surfaces can bed-in on initial sliding engagement, and the wear rate of the second sealing member, in use, being substantially constant to enable the amount of wear to be monitored.