GB2156469A

GB2156469A - Spring washer

Info

Publication number: GB2156469A
Application number: GB08505135A
Authority: GB
Inventors: John Andrew Brian Hirst
Original assignee: Exotech Ltd
Current assignee: Exotech Ltd
Priority date: 1984-02-29
Filing date: 1985-02-28
Publication date: 1985-10-09
Also published as: GB8405204D0; GB8505135D0

Abstract

A spring washer for use with a stud or bolt 3 and adapted to maintain preload therein comprises a solid substantially cylindrical body member 1 formed with an axial bore 4 to accommodate the shank of the stud or bolt. One or more annular grooves 5, 6 are formed the outer and/or inner cylindrical surfaces 7, 8 of the spring washer to permit the washer to deflect under load without radial expansion or contraction and thus store energy in the washer 20 to maintain the applied preload. The washer may have a single annular groove in its inner or its outer surface, and may be integral with a nut 303 (Fig. 8). The end faces may be dished or chamfered or flat. The side walls of the groove(s) may be parallel. The washer is made of metal or plastics, and may carry load indicator pegs. Alternatively the width of the groove may be tested by a gauge. <IMAGE>

Description

SPECIFICATION Spring washer This invention relates to a spring washer for use with a stud or bolt and adapted to maintain preload therein.

The use of spring washers in conjunction with fastening devices comprising studs or bolts is conventional. The known spring washers, either in the form of a helical spring or of a stack of one or more Belleville washers, are positioned around the shank of the stud or bolt underneath the nut which is then tightened in order to compress the spring washer and induce a desired preload in the shank of the stud or bolt. These known spring washers are satisfactory in use with studs or bolts of smaller diameters up to about 1 2 mm, but attempts to employ them in conjunction with studs or bolts having shanks of larger diameter have typically met with a number of disadvantages.

Bolts used for fixing plant and machinery frequently have a large shank diameter substantially greater than 1 2 mm, diameters of 80 to 1 50 mm being common. It is usual to specify that such bolts must be tightened down to a predetermined torque which is related to a desired preload represented by the tension in which the bolt shaft is placed.

Changes in ambient temperature cause elongation or contraction of the bolt shaft which is necessarily accompanied by an alteration in the preload. In certain locations, the ambient temperature change can be sufficient to relieve the preload in the bolt completely, with the concomitant risk of the bolt failing due to vibration loading, etc.

It would therefore be desirable to use spring washers in conjunction with such fastening devices in order to ensure that adequate preload is maintained at all times. However, to date this has not been possible due to deficiencies of existing spring washers.

A spring washer comprising a stack of Belleville springs initially resists transverse and axial movement of an associated bolt by friction between the individual springs. When the movement becomes sufficient to overcome the friction the springs snap into a new alignment. Therefore there is no continuous compensation for temperature induced movement of the bolt shaft or creep; compensation takes place in discrete steps at unpredictable intervals. Furthermore, compression of the Belleville springs is accompanied by radial spreading, and in a stack of springs this can give rise to alignment problems.

it is the object of the present invention to provide a new and improved spring washer which overcomes the disadvantages of the prior art, which is suitable for use with large diameter studs or bolts, is adapted to maintain a preload in an associated stud or bolt, which does not spread radially under load, and which is self-aligning to accomodate bending of the associated stud or bolt under load in situations where misalignment exists.

The present invention provides a spring washer for use with a stud or bolt and adapted to maintain preload therein, characterised by comprising a substantially cylindrical body member having opposed axial end faces and an axial bore extending between the end faces to accomodate the shank of the stud or bolt, said body member further comprising at least one annular groove.

The present spring washer can be made to give large or small deflections as desired under unit loading, this being achieved by appropriate dimensioning of the at least one groove and of the wall thickness as measured between the outer cylindrical surface of the body member and the inner cylindrical surface defining the axial bore. In particular, the geometry of the spring washer is such that high loads can be carried with low deflection of the spring. This makes the spring washer extremely stable in use, and allows it to exert excellent transverse radial control in all directions.

Spring washers in accordance with the present invention can be designed to suit any particular fastening requirements. The design should ensure that the material of which the spring washer is made, which will normally be metal but which may be a bonded synthetic plastic material, is deformed only elastically during the range of anticipated loads and operating conditions, due allowance being made for anticipated unusual conditions and safety factors, and that its yield point will not be exceeded.

Deflection of the spring washer of the invention is internal only in terms of shear and bending of the material, and is not accompanied by radial expansion or contraction. This substantially solves alignment and vibration problems.

Other features of the invention are described herein and in the subsidiary claims.

By way of example, some preferred embo il- ments of the present invention will now be described in more detail, reference being made to the accompanying drawings in which: Figure 1 is a side elevational view showing one embodiment of the spring washer according to the present invention, installed on a fixing bolt provided with a nut, Figure 2 is an end elevational view of the arrangement shown in Figure 1, Figure 3 is a view similar to Figure 1 but showing the washer in axial cross section, Figure 4 is a side elevational view similar to Figure 1 showing available compensation for creep, Figure 5 is a side elevational view similar to Figure 1 showing compensation for misalign ment, Figure 6 is a side elevational view similar to Figure 1 but showing the spring washer modified to permit of load measurement, Figure 7 is an axial cross sectional view through a further embodiment of spring washer having a single annular groove, Figure 8 is a view similar to Figure 7 but showing the spring washer extended to provide an integral nut, Figure 9 is an enlarged view of the portion of the spring washer within the circle A in Figure 3, and Figure 10 is a view similar to Figure 7 showing a still further embodiment of spring washer.

Referring to Figures 1 to 3 and 9 of the drawings, there is shown a portion of a fixing bolt 3 for securing a machine flange indicated at 23 to a base not shown. A spring washer 20 of the invention is interposed between a nut 24 and the flange 23.

The spring washer 20 comprises a cylindrical or substantially cylindrical solid body member 1 having opposed axial end faces 2 which are parallel planar faces each lying in a radial plane. An axial bore 4 extends through the body member 1 between the end faces 2, to accomodate the shank of the bolt 3 with clearance.

The body member 1 could be of a shape other than cylindrical, although a cylindrical or substantially cylindrical shape is preferred to make it easier to form in the body the one or more annular grooves 5 and 6 now to be described.

The spring washer 20 is formed with three annular grooves comprising, in axial sequence, an annular groove 6 formed in the inner cylindrical surface 8 of the body member which defines the axial bore 4, an external annular groove 5 formed in the outer cylindrical surface 7 of the body member 1, and a further internal annular groove 6 formed in the inner cylindrical surface 8. More, or fewer, than the three grooves 5 and 6 can be provided. However, when there is more than one annular groove, the arrangement is that axially adjacent grooves are formed alternately in the outer and inner cylindrical surfaces respectively of the body member 1.

Each groove 5 or 6 comprises, see the enlarged view of Figure 9, two side walls 1 2 which are either tapered as shown or may be parallel, and a bottom wall 11 which is preferably rounded to avoid sharp corners at the juncture between the side and bottom walls which would lead to concentrations of stress at these points. The groove is preferably symmetrical about a radial mid-plane 10 but this is not essential. The conditions of use in a particular instance might dictate that the midplane of the groove should be inclined to the true radial plane, and/or the groove may be unsymmetrical.

When the nut 24 is tightened down against the spring washer 20, the washer deflects by bending in the region S (Figure 9) under the compressive axial load developed. The energy stored in the spring washer 20 as a result of this deflection is available to maintain the initial preload established in the bolt 3 as a consequence of the tightening torque applied to the nut 24 upon changes in ambient temperature which cause elongation of the shank of the bolt 3. The preload in the bolt 3 can thus be maintained over an axial extension 30, Figure 4, which is adequate to compensate for variations in ambient temperature encountered in use. Further, any bedding in of the fixing comprising the bolt 3 and nut 24 will not cause a significant reduction of preload on the bolt 3 because the same will be compensated by the energy stored in the spring washer 20.

Figure 5 illustrates how the spring washer 20 compensates for misalignment between the design position 32 for the axis of the bolt 3 normal to the face of the flange 23, and an actual position 31 angularly offset therefrom.

Quite substantial angular offsets can be compensated.

It will be appreciated that because deflection of the spring washer of the present invention under compressive axial loading is taken up internally by shear and bending forces in the material, the washer 20 does not spread radially when compressed. This is a substantial improvement in controlling alignments.

The installation of the washer 20 to a preset load, and the inspection of the fixing at intervals thereafter to establish whether or not the intended preload is being maintained, can be assisted by providing on the visible outer face of the washer 20 indicia the axial spacing of which is proportional to the axial load on the washer. Figure 6 shows the spring washer 20 as described above, modified by the addition of indicia in the form of two pegs 33 set into the outer cylindrical face 7 of the washer the axial distance 34 between the pegs 33 or indicia can be measured in any suitable manner to obtain an indication of the axial compression of the washer and hence the load.

Alternatively a simple gauge of the go/notgo variety can be employed in conjunction with the annular groove 5.

Figure 7 illustrates a further embodiment of spring washer 20 comprising a single external annular groove 5. At least one of the axial end faces 202 is inclined axially inwardly, i.e.

dished, to allow the washer to deflect under load by bending of the wall region S adjacent the bottom of the groove.

Figure 8 shows a further embodiment wherein the washer 20 of Figure 7 is extended axially at 30 and the extension portion 30 is internally threaded to constitute an integral nut member 303 cooperable with a stud or bolt. In this embodiment, effectively one of the end faces 202 of the washer is eliminated. The remaining end face should be an inwardly inclined face 202.

The washer 20 in either of the forms illustrated in Figures 7 and 8 may, of course, have a single internal groove 5 instead of the illustrated internal groove 6. An example is shown in Figure 10 when the single annular groove is internal, the direction of inclination of the at least one inclined axial end face 202 must be axially outwardly i.e. domed.

In addition, the integral nut extension portion 30 of Figure 8 may be employed with a spring washer 20 having a plurality of annular grooves 5 and 6. In this event, the remaining end face 2 can be a planar radial face. The spring washer of the present invention may be made for use with studs and bolts of any diameter, although it is especially useful in the larger diameter sizes of over 1 2 mm, as indicated above. By appropriate choice of material and the design of the configuration, spacing, depth and number of the grooves substantially any loading requirement may be met. The invention thus has wide application throughout industry.

Claims

1. A spring washer (20) for use with a stud or bolt (3) and adapted to maintain preload therein, characterised by comprising a substantially cylindrical body member (1) having opposed axial end faces (2, 202) and an axial bore (4) extending between the end faces to accomodate the shank of the stud or bolt (3), said body member (1) further comprising at least one annular groove (5, 6).

2. A spring washer according to Claim 1, wherein at least one of the axial end faces (202) is inclined axially inwardly and comprising a single said annular groove (5) formed in the outer cylindrical surface (7) of the body member (1).

3. A spring washer according to Claim 1, wherein at least one of the axial end faces (202) is inclined axially outwardly and comprising a single said annular groove (6) formed in the inner cylindrical surface (8) defining the axial bore (4).

4. A spring washer according to Claim 1, comprising a plurality of said annular grooves (5, 6) wherein axially adjacent grooves are formed alternately in the outer and inner cylindrical surfaces (7, 8) respectively of the body member (1).

5. A spring washer according to Claim 4, wherein the axial end faces (2) are planar parallel radial faces.

6. A spring washer according to any one of Claims 1 to 5, wherein the at least one annular groove (5, 6) is symmetrical about a radial mid-plane (10).

7. A spring washer according to any one of Claims 1 to 5, wherein the at least one annular groove (5, 6) has a rounded bottom wall (11) and parallel side walls (12).

8. A spring washer according to any one of Claims 1 to 5, wherein the at least -)ne annular groove (5, 6) has a roundest bottom wall (11) and tapered side walls (1.").

9. A spring washer according to dny one of Claims 1 to 8, wherein the body member (1) is extended axially (30) and the extension portion is internally threaded to constitute an integral nut member (303) cooperable with a stud or bolt (3).

10. A spring washer according to any one of Claims 1 to 9, wherein the body member (1) is provided with indicia (33) for enabling measurement of the axial compression of the washer.