GB2111139A - Bellows-type mechanical seals - Google Patents

Abstract

A mechanical seal (14) for a shaft (10) rotating in a housing (12) has a seal element (26) connected to seal carrier (18) by a bellows (242). The seal element (26) abuts a second seal element (30) mounted in the housing (12) and forms a running seal between adjacent faces (28,34) of the two seal elements (26,30) respectively. The bellows (242) is constructed from the deformation of a metal tube and is substantially sinusoidal in profile. The tube may be deformed in a metal rolling process and/or in a hydrostatic pressure deformation process. (Fig. 6). <IMAGE>

Description

SPECIFICATION Bellows-type mechanical seals The present invention relates to mechanical seals and in particular to a mechanical seal of the type used to seal two relatively rotating components in pumps, mixers, compressors valves and the like.

Such seals comprise two seal elements, one disposed on the rotating component, usually a shaft, and arranged to rotate with it and the other disposed on the stationary component, usually a shaft housing, and arranged to remain stationary with that. A running seal is formed between mutually abutting faces of the two seal elements. Normally a thin film of the bulk liquid being sealed by the seal is present in the interface between the seal elements as a lubricant.

In order to keep frictional drag and heat generated between the abutting faces to a minimum it is necessary to restrict the closing force on the abutting faces. This is accomplished by reducing the net area on which the pressure of the bulk liquid acts and this may be achieved by stepping the shaft or housing as appropriate. The hydraulic forces acting on the seal are thus partially balanced. In order to maintain a seal however, when the bulk liquid pressure drops, it is usual to employ springs to bias the seal elements together.

One method of both reducing the net area on which the pressure of the bulk liquid acts and producing the spring bias is to use a bellows-type seal.

Here the bellows replaces the spring and is constructed to provide a spring force on compression. Furthermore, in a bellows construction the point of action of any external pressure coincides with the mean diameter of the bellows. Thus if the mean diameter of the bellows is placed so that it coincides with any point inside the inner and outer diameters of the seal interface the net area on which the pressure of the seal product acts will be reduced. The hydraulic force, F, on the seal is determined by the formula: p(A2 ~ C2) F = (A2 - B2) where A is the outer diameter of the interface; B is the inner diameter of the interface; C is the mean diameter of the bellows; and P is the bulk liquid pressure Thus if 'C' is equal to 'A' there will be no hydraulic force on the seal while if 'C' is equal to 'B' the force will only depend on the fluid pressure.

The bellows therefore provides a simple and effective means of control of the hydraulic balance of a mechanical seal.

Unfortunately however such seals have hitherto been to an extent prohibitively expensive by virtue of the construction of the bellows employed in such seals.

The bellows are constructed by welding together along adjacent edges a plurality of frusto conical rings arranged end to end-so as to give a serrated appearance in section.

This method is expensive and subject to a high failure or scrap rate, but until recently had been the only method of producing bellows suitable for use in mechanical seals of the type referred to above.

It is an object of this invention to provide a seal which is as simple and effective as existing bellows-type seals but which is cheaper to manufacture.

In accordance with this invention there is provided a mechanical seal for sealing two relatively rotating components, the seal comprising a first seal element non-rotatably secured to one component, a second seal element non-rotatably secured to the other component, a running seal being formed between mutually abutting faces of the two seal elements, and a resilient bellows connecting one seal element to its component, said bellows being constructed from a metallic tube which is deformed to form a bellows shape.

Preferably the tube is deformed in a metal rolling process. Alternatively or in addition the tube is deformed by a hydrostatic pressure deformation process.

The invention'is further described hereinafter by way of example only with reference to the accompanying drawings, in which: Figure 1 is a section through a conventional bellows-type mechanical seal; Figure 2 is a section through a bellows-type mechanical seal according to the invention; Figure 3 is a section through the bellows of a conventional seal; Figure 4 is a section through the bellows of a seal as shown in Fig. 2; Figure 5 is a schematic representation of one method of manufacturing a bellows for use in the seal shown in Fig. 2; Figure 6 is a schematic representation of another method of manufacturing a bellows for use in the seal of Fig. 2;; In Figs. 1 and 2 a shaft 10 is rotating in a housing 1 2 which is sealed to the shaft by a mechanical seal generally designated 1 4. The seal 1 4 prevents the bulk liquid product contained within the housing from leaking around the shaft.

The seal 1 4 comprises a sleeve 1 6 mounted on the shaft 1 0. The sleeve 1 6 is sealed to the shaft and rotates with it. Axially slidably but rotatably fixedly mounted on the sleeve 1 6 is a seal carrier 1 8. Between the seal carrier 1 8 and the sleeve 1 6 is disposed an O-ring seal 22.

A resilient bellows 241, or 242 connects the seal carrier 1 8 to a first seal element 26 which has a seal face 28 abutting a second seal element 30.

The seal element 30 is axially slidably but rotatably fixedly mounted in the housing 1 2.

The element 30 traps an O-ring seal 32 against the housing 1 2 and has a seal face 34 abutting the seal face 28 of the first seal element.

The bellows 241, 242 is resilient and acts in the same way as a spring. Since it is compressed during assembly of the seal 14 the following effects will occur: the seal carrier 1 8 will squeeze the O-ring 22 against the sleeve 1 6 thereby sealing the carrier with respect to the sleeve; the seal element 30 will squeeze the O-ring 32 against the housing 1 2 thereby sealing the seal element with respect to the housing; and; the abutting faces 28, 34 of the two seal elements, which form a running seal, will be urged towards one another to maintain the seal, even at low bulk liquid product pressures.

The bellows 241 of Fig. 1 is shown in greater detail in Fig. 3 in which its mode of construction can be seen. The bellows 241 comprises a plurality of flattened annular rings 40, although the rings are firstly concentiallyconvoluted and secondly somewhat frustoconical. The rings 40 are arranged end to end and then, around juxtaposed edges (internally 42 and externally 44), they are welded together.

Although bellows manufactured in this manner are operationally, very satisfactory they are somewhat expensive and, owing to the delicate nature of the procedure, prone to a high scrap rate.

The bellows 242 of Fig. 2 however are constructed by deformation of a metal tube.

Fig. 4 shows such a bellows 242 in detail.

Fig 5 and 6 show alternative methods of construction although both may be employed if desired.

In Fig. 5 a metal tube 50 is deformed by three rollers 52, 54, 56, two of which are mounted on the same shaft. The rollers pinch the tube between themselves and are rotated progressively to deform the tube around its circumference.

In Fig. 6 a metal tube 60 is inserted into a die 62. High hydrostatic pressure 'P' arranged in the tube 60 is sufficient to deform the tube so that it adopts the internal convoluted shape of the die 62. The die 62 is subsequently split to release the so-formed bellows.

By either method the bellows 242 soformed have a more sinusoidal profile than the somewhat sawtoothed profile of the conventionally used bellows 241. This results in an increased pitch of the bellows, i.e. less convolutions per unit length, but this can be accommodated in a seal 14 simply by increasing the overall length of the bellows. Seals 1 4 using these bellows are therefore equally effective, if slightly longer, but are considerably cheaper to produce.

Claims (6)

1. A mechanical seal for sealing two relatively rotating components, the seal comprising a first seal element non-rotatably secured to one component, a second seal element non-rotatably secured to the other component, a running seal being formed between mutually abutting faces of the two seal elements, and a resilient bellows connecting one seal element to its component, said bellows being constructed from a metallic tube which is deformed to form a beliows shape.

2. A seal according to claim 1 in which the tube is deformed in a metal rolling process by rolls disposed inside and outside the tube.

3. A seal according to claim 1 or 2 in which the tube is deformed in a hydrostatic pressure deformation process.

4. A seal according to claim 3 in which the tube is placed inside a die and deformed by hydrostatic pressure in the tube to adopt the internal shape of the die.

5. A seal according to any preceding claim in which the bellows has a substantially sinusoidal profile.

6. A bellows-type mechanical seal substantialy as herein before described with reference to Figs. 2,4,5 and 6 of the accompanying drawings.