GB2263527A - Fluid seal arrangement - Google Patents

Abstract

Within a hydraulic a bolt tensioning device 10 (Fig. 1) a seal arrangement 22 comprises a carrier ring 23 of acetal copolymer that forms a low-friction sliding fit in the chamber 20 and in each longitudinally extending peripheral wall 24 and 25 has machined a groove 26, 33 with a sectional profile defined in part by a semi-circle whose diameter is inclined to peripheral wall at substantially 60 DEG and the end closest the piston is coincident with the wall to define a feather edge 31, 35 outwardly deflectable under fluid pressure to close any extrusion gap. The groove contains effective O-ring 27, 34 formed from a nitrile cord whose ends are joined within the groove and which projects outwardly to the cylinder boundary wall 151, 152 to form a lipless low pressure seal that also transmits fluid pressure to deflect the feather edge. <IMAGE>

Description

Fluid Seal Arrangement This invention relates to fluid seal arrangements and in particular to such arrangements adapted to seal an extrusion gap, formed between a longitudinally extending peripheral wall of a piston and a longitudinally extending peripherally conforming boundary wall of a cylinder space containing said piston, to fluid at high pressure in a chamber formed within the cylinder space by the piston.

The invention is particularly, but not exclusively, concerned with hydraulic bolt tensioning devices in which the piston and the cylinder space, defined by a cylinder body, are annular and coaxially surround a bolt to be tensioned relative to a machine part, the piston being releasably attached to threads of the bolt and the cylinder body bearing against the machine part. Hydraulic fluid supplied at a high pressure of some 1500 Bar to a chamber in the cylinder space adjacent one end of the piston displaces the piston with respect to the cylinder body and machine part to stretch, and induce a predefined tension in, the bolt. Thereafter, the chamber is vented and the piston retracted into the cylinder space by manual effort or automatically.It will be appreciated that such a device has somewhat conflicting criteria for any sealing arrangement which is required both to withstand and seal against the high fluid pressure necessary to displace the piston against high loading and to provide a relatively low friction between the conforming piston and cylinder boundary walls to facilitate piston retraction. In such configuration the problem is exacerbated by the annular piston having a further peripheral wall defined by the inner annular radius and the cylinder space having a further conforming boundary wall which effectively doubles the frictional resistance.

One example of a sealing arrangement used in such a bolt tensioning device is to be found in British Patent Specification No. 1,590,131 (corresponding USP 4249718).

The seal arrangement comprises essentially a flat annular ring of elastomeric material having chamfered inner and outer annular edges that conform with tapered extensions to the piston (or carrier thereon) at its inner and outer annular faces comprising lineal, or so-called feather, edges. In operation the elastomeric ring provides a low pressure seal to prevent leakage or extrusion of fluid from the chamber by way of the gap formed by the inevitable clearance between cooperating piston and cylinder walls but at high pressures, when the elastomeric material would otherwise also tend to be extruded by way of the gap, the feather edge is resiliently deformed by the pressure to close this gap.It will be appreciated that a feather edge formed of metal, whether part of the piston or a separate attachment, has limited deflection even when exposed to high pressures and thus requires when operational to be a tight, or frictional, fit even in the absence of significant chamber pressure, but also that any such source of friction inhibits easy retraction of the piston.

Furthermore, in the event of accidental overstroking of the piston that causes the fluid to be contained only by an unsupported feather edge, it is possible for the metal thereof to fracture and fly, with ejected fluid, in a dangerous manner. The provision of a chamfered edge to the elastomeric seal to conform with the feather edge results in a lip that may contribute considerable frictional resistance to piston retraction and also be susceptible to damage by the feather edge if trapped or chafed thereby.

Furthermore, such a seal arrangement is relatively complex and expensive to manufacture in small quantities, both in terms of defining the feather edges in the metal of the piston or separate attachment and in terms of the cooperatingly-shaped ring of elastomeric material.

Such a high pressure sealing arrangement is in general applicable to other forms of hydraulic devices, such as rams employing simple rod-like pistons, and preserving the generality of the form of piston and cylinder cavity within which contained, it is an object of the present invention to provide a fluid seal arrangement adapted to fit between a piston and a fluid chamber which is of simple construction and capable of mitigating disadvantages of known arrangements, and a fluid operated device incorporating such a fluid seal arrangement.

According to a first aspect of the present invention a fluid seal arrangement adapted to seal an extrusion gap, formed between a longitudinally extending peripheral wall of a piston and a longitudinally extending peripherally conforming boundary wall df a cylinder space containing the piston, to fluid at high pressure in a fluid chamber formed within the cylinder space by the piston, comprises a carrier having a peripheral wall dimensioned to conform o, and form a longitudinally sliding fit within, the cylinder space and having within said peripheral wall, and extending thereabout, a groove, and contained in, and retained between opposing walls of, the groove a resilient low pressure sealing element extending about the periphery of the carrier and projecting outwardly from the groove beyond the peripheral wall to bridge an extrusion gap between the carrier and the boundary wall of the cylinder space and form a low pressure seal therewith, the carrier groove extebdubg longitudinally to undercut the peripheral wall of the carrier and define between the groove and the peripheral at the entrance to the groove a feather edge intermediate the low pressure sealing element and the piston resiliently deformable in response to high pressure in the chamber towards the boundary wall of the cylinder space to prevent extrusion of the low pressure sealing element by way of the extrusion gap.

According to a second aspect of the present invention a fluid operated device includes a piston defined about a longitudinal displacement axis by a longitudinally extending peripheral wall, a cylinder space containing the piston having a longitudinally extending and peripherally conforming boundary wall, a fluid chamber formed within the cylinder space by the piston arranged to receive fluid at high pressure to displace the piston longitudinally and a fluid seal arrangement, adapted to seal an extrusion gap between a peripheral wall of the piston and conforming boundary wall of the cylinder space, as defined in the preceding paragraph.

Preferably, the resilient low pressure sealing element has, at least in the part projecting outwardly of the groove, a sectional elevation profile that forms a segment of a circle prior to insertion into the cylinder space.

It is also preferable that the carrier groove has a sectional profile defined in part by a semi-circle whose diameter is inclined to the peripheral wall and the end of the groove diameter in operation disposed closest to the piston is coincident with the peripheral wall such that the edge of the groove and the peripheral wall intercepted thereby define the feather edge.

It is then convenient that the resilient low pressure sealing element has a sectional elevation profile that is circular prior to insertion into the cylinder space.

When applied to a bolt tensioning arrangement as outlined above in which the piston, cylinder space in which it resides and the fluid chamber formed thereby are of annular form the carrier may comprise an annular ring having both said peripheral and further peripheral walls extending longitudinally adjacent corresponding boundary walls of the cylinder space, a further groove is defined in the further peripheral wall of the ring corresponding to said groove in the peripheral wall and a resilient low pressure sealing element is contained within the further groove, extending about the inner periphery of the annular exclement and projecting from the groove beyond the inner peripheral wall to abut, and form a low pressure seal with, the corresponding longitudinal boundary wall of the cylinder space.

Embodiments of the invention will now be described by way of example with reference to the accompany drawings, in which:- Figure 1 is a sectional elevation through one half of an annular bolt tensioning device incorporating a fluid seal arrangement according to the present invention, Figure 2 is a sectional elevation through the complete form of the seal arrangement of Figure 1 illustrating the parts thereof in greater detail, Figure 3 is a sectional elevation through a hydraulic ram of different form to Figure 1, also incorporating a fluid seal arrangement according to the present invention, and Figure 4 is a sectional elevation through part of yet another form of ram device incorporating a fluid seal arrangement according to the present invention in duplicate.

Referring to Figure 1 this shows in sectional elevation a hydraulic ram in the form of an annular bolt tensioning device 10 extending symmetrically about a longitudinal axis 11 of a bolt to be tensioned (not shown). Such bolt tensioning devices per se are well known in the art and need not be described in great detail. For such description attention is directed to the aforementioned British Patent specification No. 1,590,131. To assist in understanding the present invention it is sufficient to know that the device 10 comprises a cylinder body 12 having therein an annular cylinder space 13 containing an annular piston 14. The piston which is displaceable longitudinally along axis 11, has a longitudinally extending peripheral wall 141 defined by the outer dimension of the annulus, and a longitudinally extending further peripheral wall 142 defined by the inner dimension of the annulus.The cylinder space has a longitudinally extending boundary wall 151 peripherally conforming to the piston peripheral wall 141 and a longitudinally extending further boundary wall 152 peripherally conforming to the piston peripheral wall 142. The piston is a freely sliding fit within the cylinder space, that is, its peripheral walls 141 and 142 are each spaced from the conforming boundary walls 151 and 152 by inevitable clearances, hereinafter called, in reference to the operational problems caused thereby and for uniformity of description extrusion gaps 161 and 162 respectively. Not only is the extrusion gap essential in permitting assembly and operational piston movement but the precise width of the gap is indeterminate as a result of manufacturing tolerances of the component parts.The longitudinal extent of the piston is such that it projects out of the cylinder space and abuts a so-called annular puller 18 which is screw threaded at 19 for releasable engagement with a bolt to be tensioned.

A chamber 20 is formed within the cylinder space by the piston, comprising that part of the cylinder space to one end of the piston, and is arranged to receive hydraulic fluid by way of coupling 21 at high pressure in order to displace the piston axially with respect to the cylinder space.

To prevent leakage of the fluid from the chamber 20 by way of extrusion gaps 161 and 162 a fluid seal arrangement 22 is provided.

The fluid seal arrangement 22 is also shown separately and in Figure 2 and in accordance with the present invention comprises a carrier 23 in the form of an annular ring having an axially extending peripheral wall 24 defining the outer boundary of the ring and a further axially extending peripheral wall 25 defining the inner boundary of the axial ring, the dimensions of the carrier ring being such that it conforms to, and forms a longitudinally freely sliding fit within, the cylinder space.

Considering initially the peripheral wall 24 of the carrier, within said peripheral wall and extending thereabout is a groove 26 and contained within, and retained between opposing walls of, the groove and extending about the periphery of the carrier is a resilient low pressure sealing element 27. The sealing element extends or projects outwardly of the groove beyond the peripheral wall 24 to bridge an extrusion gap 281 between the carrier and the longitudinal wall 151 of the cylinder space. Depending on the material selected for the carrier 23 it may have the same nominal raidal dimension as the piston or may be a tighter fit within the cylinder space than the piston 14 such that the extrusion gap 281 is smaller than that 161. Notwithstanding this, the sealing element forms in operation a low pressure seal with the boundary wall 151 to prevent leakage of fluid from the chamber.

The carrier ring 23 is preferably and conveniently formed of a low friction engineering grade thermoplastics material such as Acetal copolymer and groove 26 is machined or otherwise formed therein to extend longitudinally to undercut the peripheral wall of the carrier and define between the groove and the peripheral wall at the entrance to the groove a so-called feather-edge. The groove 26 has a sectional profile defined in part by a semi-circle 29 whose diameter, shown by line 30, is inclined to the peripheral wall 24 at an angle a. For this material the angle a is within the range of 55 to 650, and preferably at substantially 600.

The end of the groove diameter intended to be closest to the piston when the seal arrangement is disposed within the chamber is coincident with the peripheral wall 24 of the carrier such that the edge portion of the groove and the peripheral wall intersected thereby define feather edge 31 between the low pressure sealing element and the piston that extends about the peripheral wall of the carrier and points towards the fluid chamber 20. The other end of the groove diameter 30 demarcates a profile transition from the semi-circle to a tangential continuation thereof shown at 32 which intersects an end face 231 of the carrier ring.

The resilient low pressure sealing element 27 has a profile in sectional elevation that is at least part circular in respect of the part that projects from the groove to make sealing contact with the boundary wall of the cylinder space (and prior to insertion into the chamber and deformation by said boundary wall) and in respect of the part contained within the groove to conform with the semicircular part of the groove 26. For preference the profile is completely circular, and the element is formed from Nitrile cord material made into a continuous O-ring equivalent by bonding together its ends, contained within the groove, with suitable adhesive material, such as that sold under the trade name Loctite Superbond 415.

The further, inner, peripheral wall 25 of the carrier, separated from the longitudinal boundary wall 152 of the cylinder space by extrusion gap 282, has a corresponding groove 33 therein and contains a corresponding resilient low pressure sealing element 34, the groove being inclined at angle a to the longitudinal axis to define a feather edge 35. The sealing element 34 is of course slightly shorter in length than the element 27 but projects outwardly of the groove beyond inner peripheral wall 25 to abut, and form a low pressure seal with, the corresponding longitudinal boundary wall 152 of the cylinder space.

The extrusion gaps 281 and 282 between the carrier body and the walls of the cylinder space may, by use of material like the aforementioned Acetal copolymer with its inherent lubricity, be so small that there is continuous frictional engagement between the peripheral walls 24, 25 of the carrier and the boundary walls 151 and 152 of the cylinder space without this impeding piston displacement significantly in retracting the piston into the cylinder space in the absence of fluid pressure in chamber 20, the only significant frictional engagement being due to the permanently engaging elastomeric sealing elements 27 and 34.Furthermore, the ability to use a sealing element of circular profile, that is, without any directional 'lip' caused by a chamfered edge or the like ensures that its ability to slide with respect to the walls of the cylinder space is minimal for piston movement in both directions.

When chamber 20 is supplied with hydraulic fluid at high pressure the pressure acts on each resilient sealing element to force it into its supporting groove, distortion of the unsupported part of the seal increases the direct sealing effect it has with the cylinder space wall but the supported part transmits the pressure into the carrier and results in deflection of the relatively thin feather edge portions 31 and 35 outwardly of the relevant carrier wall towards the adjacent cylinder space boundary wall, preventing the sealing element from being extruded through the original extrusion gap at very high pressure.

When chamber 20 is subsequently vented the feather edges 31 and 35 deflected within the elastic limits of the carrier material return to their original dispositions.

It will be appreciated that the use of a low friction carrier material may, by enabling a reduced extrusion gap between the carrier and cylinder space boundary walls with a retracted piston, mean that the feather edge deflection under pressure is reduced for any given extrusion gap between and cylinder space boundary and/or, if the gap varies longitudinally with piston displacement due to distortion of the boundary walls under increasing exposure to fluid pressure, the feather edge is better able to accommodate it within its range of elastic deformation.

The seal arrangement according to the invention may be employed with other hydraulic ram configurations as will be appreciated from brief descriptions of examples of such configurations.

Referring to Figure 3 this shows a ram 40 comprising a piston 41 of substantially circular cross section, displaceable along an axis 42, and a longitudinally extending peripherally conforming cylindrical boundary wall 43 of a cylinder space 44, containing the piston, within cylinder body 45. A fluid chamber 46 is defined at one end of the cylinder space and supplied by way of coupling 47 with hydraulic fluid at high pressure to displace the piston longitudinally. Leakage of fluid from the chamber 46 by way of an extrusion gap 48 between the piston and cylinder space boundary wall 43 is prevented by a fluid seal arrangement 50.

The fluid seal arrangement 50 is similar to the arrangement 22 of Figure 2 differing in that the carrier 51 comprises a disc of material from which the inner annular wall 25, groove 33 and resilient sealing element 34 of 22 are absent; that is, it comprises a single peripheral wall 52 having a single peripheral groove 53 containing a single resilient sealing element 54. Operation is analogous to that described above and does not require repetition.

The carrier 51 need not be formed as a solid disc and may instead comprise a ring, similar to ring 22 but without further groove 33 in the inner peripheral wall, as indicated by broken lines 55, saving on material costs and/or permitting location on a piston spigot or for a piston 'tail' or connecting rod, or attachment screws (not shown) to pass through the seal carrier. In such configuration a face seal 56 may be provided between the face of the carrier and the piston and against which it bears against under pressure to ensure no fluid leakage.

Such a face seal would not, of course, be necessary with a disc having no such through-apertures.

Figure 4 shows in sectional elevation part of a different ram configuration 60 in which a piston 61 is disposed within a cylinder space 62 defined by cylinder body 63, the longitudinally extending wall 64 of the piston and boundary wall 65 of the cylinder space being broken by radially extending shoulders 66 and 67 respectively which define with the longitudinally extending walls a fluid chamber 68 supplied by coupling 69.

The piston, shown in part, and the corresponding cylinder space, may be of generally solid cross section, in the manner of Figure 3, or of annular cross section in the manner of Figure 1. The sealing arrangement, indicated generally at 70 comprises two seal carriers disposed at opposite ends of the fluid chamber 68. A first carrier 71 disposed adjacent the piston shoulder 66 is of the form described above in relation to Figure 4 the carrier being of ring form with a central aperture 72 through which a tail of the piston extends. A face seal 73 is disposed between the carrier ring and the piston shoulder. The (outer) peripheral wall 74 of the carrier 71 has a groove 75 extending around the periphery containing resilient sealing element 76 as defining a feather edge 77 as described above with reference to Figures 1 and 2. A second carrier 78 is disposed adjacent the cylinder body shoulder 67, with a face seal 79 between them. This also comprises an annular ring, the (inner) peripheral wall 80 of which has a groove 81 extending around the periphery containing resilient sealing element 82 and defining a feather edge 83 directed into the chamber 68, also in the same manner as described above.

The precise number, disposition and orientation of carriers will, of course, depend on whether the piston 61 is solid or annular. It will be appreciated that in a ram of the type depicted in Figure 4, wherein both cylinder and piston walls can take one carrier-borne seal, the carriers may be of the form shown inFigure 2 wherein peripheral sealing elements are carried on both carried on both inner and outer peripheral walls instead of having to introduce face seals such as 73 and 79.

It is known for a hydraulic tensioning device per se to be incorporated within the structure of a bolt or a nut of suitable dimensions having relatively displaceable parts analogous to piston and cylinder and it will be understood that a seal arrangement in according with the present invention may likewise be incorporated in such a bolt or nut.

Other variations are possible that are applicable to all of the above described arrangements.

The carrier or carriers may be attached to the piston (and if appropriate any other body) or may be free to 'float' longitudinally within the cylinder space, being forced into operational sealing position by the pressure of fluid introduced into the fluid chamber.

The, or each, carrier or carriers may take a shape other than a parallel flat ring or disc, provided that the face transmitting fluid pressure to the piston or other body conforms to the profile thereof for uniform transmission of force.

The, or each, carrier may be formed of different thermoplastics material to that described above, provided it has suitable mechanical and chemical properties, as long as the feather edge defined therein by an appropriate angle between the groove profile diameter and the carrier peripheral wall is suitably dimensioned for elastic deformation by operational fluid pressure that inhibits extrusion of the resilient sealing element past it. Having defined a sealing arrangement in order to mitigate problems found to exist with known structures having a metal feather edge, it is possible to form a seal arrangement according to the present invention with a carrier of metal although performance advantages are as unlikely as with other materials.

The, or each, elastomeric sealing element may likewise be formed of different material and/or as a complete ring for insertion in the groove and, as indicated above, may have a sectional elevation profile that is non-circular, and even non-uniform, provided it conforms about the periphery of the carrier with that part of the groove which effects transmission of force holding the element into the groove and transmitting force by way of it to deform the feather edge and presents a peripherally uniform surface to the cylinder space boundary wall.

The groove as described above has a sectional elevation profile part of which is a semi-circle having one diameter and at the feather edge, and, as described above, the other diameter end forming the start of a continuation as a straight walled tangent. This continuation may take any profile provided it does not impede retention of the resilient sealing element or deform it so that it does not form an effective seal in contact with the boundary wall of the cylinder space.

As yet a further alternative the groove may have a sectional elevation that is other than part semi-circular provided that the feather edge is defined in such manner with respect to a different groove profile to receive by way of the sealing element appropriate deflection forces.

If the sealing element has a corresponding profile such that forces are transmitted into the carrier homogeneously this should present no problem, but a sealing element of different profile may need the strength of the feather edge modified to accommodate unhomogeneous transmission of pressure derived forces.

In all of the above described ram arrangements the piston, whether annular or solid, and therefore the carrier of the seal arrangement, is implicitly circular in cross section about a longitudinal axis. It will be appreciated that such a piston may, if desired be of non-circular section, such as elliptical, and provided its peripheral wall has no sharp corners, the grooved carrier of the seal arrangement may be formed of corresponding shape and the resilient sealing element of such profile as to present a peripherally uniform surface to the cylinder space boundary disposed in the groove.

Claims (17)

1. A fluid seal arrangement adapted to seal an extrusion gap, formed between a longitudinally extending peripheral wall of a piston and a longitudinally extending peripherally conforming boundary wall of a cylinder space containing the piston, to fluid at high pressure in a fluid chamber formed within the cylinder space by the piston, comprising 1) a carrier having i) a peripheral wall dimensioned to conform to, and form a longitudinally sliding fit within, the cylinder space and ii) within said peripheral wall, and extending thereabout, a groove, and 2) contained in, and retained between opposing walls of, the groove a resilient low pressure sealing element extending about the periphery of the carrier and projecting outwardly from the groove beyond the peripheral wall to bridge an extrusion gap between the carrier and the boundary wall of the cylinder space and form a low pressure seal therewith, the carrier groove extending longitudinally to undercut the peripheral wall of the carrier and define between the groove and the peripheral wall at the entrance to the groove a feather edge, intermediate the low pressure sealing element and the piston resiliently deformable in response to high pressure in the chamber towards the boundary wall of the cylinder space to prevent extrusion of the low pressure sealing element by way of the extrusion gap.

2. A fluid seal arrangement as claimed in claim 1 in which the resilient low pressure sealing element has at least in the part projecting outwardly of the groove a sectional elevation profile that forms a segment of a circle prior to insertion into the cylinder space.

3. A fluid seal arrangement as claimed in claim 1 or claim 2 in which the carrier groove has a sectional profile defined in part by a semi-circle whose diameter is inclined to the perpipheral wall and the end of the groove diameter in operation disposed closest to the piston is coincident with the peripheral wall such that the edge of the groove and the peripheral wall intercepted thereby define the feather edge.

4. A fluid seal arrangement as claimed in claim 3 in which the other end of the groove diameter defining said part of semi-circular profile demarcates a profile transition from said semi-circle to a tangential continuation thereof.

5. A fluid seal arrangement as claimed in claim 4 in which the longitudinal thickness of the carrier is related to the diameter of said part of semi-circular profile such that the tangential continuation thereof intersects a face of the carrier ring remote from the piston and not said peripheral wall.

6. A fluid seal arrangement as claimed in any one of claims 3 to 5 in which the resilient low pressure sealing element has a sectional elevation profile that is circular prior to insertion into the cylinder space.

7. A fluid seal arrangement as claimed in any one of claims 3 to 6 in which semi-circular part of the groove has a profile diameter inclined to the peripheral wall at an angle in the range 550 to 650.

8. A fluid seal arrangement as claimed in any one of the preceding claims in which the carrier is formed from a low friction engineering grade thermoplastics material.

9. A fluid seal arrangement as claimed in claim 8 in which the carrier ring is formed from acetal copolymer.

10. A fluid seal arrangement as claimed in claim 9 when dependent on claim 7 in which the angle is substantially 600.

11. A fluid seal arrangement as claimed in any one of the preceding claims in which the carrier comprises an annular ring having in addition to said peripheral wall defining the outer boundary of the annular ring a further axially extending peripheral wall defining the inner boundary of the annular ring.

12. A fluid seal arrangement as claimed in claim 11 for use within an annular fluid chamber defined within an annular cylinder space by an annular piston, in which the annular carrier ring has both said peripheral and further peripheral walls extending longitudinally adjacent corresponding boundary walls of the cylinder space, a further groove is defined in the further peripheral wall of the ring corresponding to said groove in the peripheral wall and a resilient low pressure sealing element is contained within the further groove, extending about the inner periphery of the annular exclement and projecting outwardly from the groove beyond the inner peripheral wall to abut correspondingly, and form a low pressure seal with, the corresponding longitudinal boundary wall of the cylinder space.

13. A fluid seal arrangement as claimed in any one of the preceding claims in which the, or each, resilient low pressure sealing element comprises Nitrile.

14. A fluid seal arrangement as claimed in claim 13 in which the element comprises an elongate cord the ends of which are secured to each other to effect a fluid tight sealing ring.

15. A fluid operated device including a piston defined about a longitudinal displacement axis by a longitudinally extending peripheral wall, a cylinder space containing the piston having a longitudinally extending and peripherally conforming boundary wall, a fluid chamber formed within the cylinder space by the piston arranged to receive fluid at high pressure to displace the piston longitudinally and a fluid seal arrangement, adapted to seal an extrusion gap between a peripheral wall of the piston and conforming boundary wall of the cylinder space, as claimed in any one of the preceding claims.

16. A fluid seal arrangement substantially as herein described with reference to, and as shown, and one of the accompanying drawings.

17. A fluid operated device substantially as herein described with reference to, and as shown in, any one of Figures 1, 3 or 4 of the accompanying drawings.