GB2181261A

GB2181261A - Load indicating washer

Info

Publication number: GB2181261A
Application number: GB08623352A
Authority: GB
Inventors: Alastair Aitken Sinclair; John Thomas Bavis
Original assignee: Individual
Current assignee: Individual
Priority date: 1985-09-28
Filing date: 1986-09-29
Publication date: 1987-04-15
Also published as: GB8623352D0; GB2181261B

Abstract

A washer device 3 is provided the elastic deformation of which under load causes a machined gap 6 between two circular projecting flanges 7 to reduce by a suitable degree. Between these machined surfaces and interposed in the gap, a shaped ring 8 can be freely rotated. Upon approaching the pre-determined load for which the device has been previously set the flanges 7 contact and bear upon the shaped ring 8 preventing its free rotation by hand. This serves as an indicator that a bolt 1 has reached its design load and that further tightening is not required. If a subsequent reduction in the clamping force takes place, this can be immediately detected as the ring 8 may again be freely rotated. The ring 8 may be adjusted by a set screw 12. <IMAGE>

Description

SPECIFICATION Load indicating washer This invention relates to a load indicating washer for use with bolts in applications where an accurate valueofbolttension is required and where the value desired is to be checked from time to time during service.

Bolts are conventionally used to connect com ponents together but, in the fields of mechanical and constructional engineering for example, it is often necessary that the clamping force applied to the components is accurately known at the time of assembly and additionally is capable of being checked during the service life of the components.

The tensile force within the bolts, which results from tightening the bolt or its associated nut, represents the clamping force achieved between the components.

A number of methods of measuring orpre- determining the clamping force between components have previously been proposed. As a first example, torque measuring devices which either give a continuous reading ofthetorquevalue being applied during tightening ofthe bolt or which release at a pre-determined value oftorque have been used.

Secondly, specially shaped washers or projecting ribs or lugs designed to deform at a predetermined value of torquf have been used, as has the simple method of measuring the degree of rotation ofthe bolt head or nut. A fourth proposal has been the inclusion of gauge rods within a boltshankto measure directly the extension of the bolt under load.

Each ofthese proposals however suffers from a number of disadvantages. For example inaccuracies can be introduced due to frictional inconsistencies or variability of deformation characteristics and due to poor dimensional tolerances in conventional bolt and nut assemblies. In addition the second and third proposals do not allow for subsequent checking of bolttension after initial tightening.

The use of gauge rods provides a system with high accuracy but suffers from the disadvantage of being both expensive and prone to external damage.

According to the present invention there is provided a load indicating washerforfitmentaround a bolt stem, the washer having first and second portions which are relatively movable and arranged so as to lock together against such relative movement on application of a predetermined applied load to the washer in the intended direction of the bolt axis.

Preferably the first portion is engaged by a nut or by the head ofthe bolt so as to be fixed thereby in use. The first portion may have a pair of axially spa- ced radially-projecting flanges which are deformable on application of load to clamp the second portion between them.

Preferablyalsothesecond portion isannularanda projection on the second portion extends into the space between the projecting flanges of the first portion.

Preferably also the spacing between the projection on the second portion and theflanges on thefirst portion prior to the application of said load is adjust able.

Preferably also means are included associated with the first portion for causing a radially non uniform load applied thereto to produce a substanti ally radiallyuniform deformation oftheflanges thereon.

Embodiments ofthe present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure lisa side view of a nut and bole incorporat ing one embodiment of a load indicating washer of the present invention; Figure2 is a plan view of the load indicating washer of Figure 1; Figure 3 is a part sectional side view of the load indicating washer of Figure 1; Figure 4 is a part sectional side view of a second embodiment of a load indicating washer of the present invention; and Figure 5is a part sectional side view of a third embodiment of a load indicating washer of the present invention.

Referring to the drawings, a conventional threaded bolt stem 1 and nut 2 has a load indicating washer device 3 located between the nut, 2 and a plain washerS which lies on component4 being clamped.

A pair of projecting flanges 7 on the washer are separated by a machined gap 6 and a rotatable ring 8 having an inwardly-tapering inner wall is fitted over the flanges 7,so that the tapering wall projects between the flanges 7. The ring 8 is split at one point on its circumference and is provided with a set screw 12 which can be adjusted to alterthesize of a gap 9 between the ring 8 and the flanges 7. Machined shoulders 10on the washer device 3 are provided with a raised loading point 11 against which the nut 2 and washer5 bear and through which the load in the bolt 1 on tightening is transmitted to the washer device 3.

The loading points 11 are proportioned such that they deform plastically or elastically under load and thus ensure uniform load transmission around the circumference of the shoulders 10.

In use the bolt 1, nut 2, washer device 3, and washer5 are assembled on the component4as shown in Figure 1. Before tightening of the bolt 1 and nut 2 the existence ofthe gap 9 between the flanges 7 and the rings 8 allows the ring to be freely rotated by hand.

Tightening of the bolt head or nut 2 imposes a compressive force into the washer device 3 which is equal in valueto the tensile force in the bolt shank. A compressivestress is induced into an annularpart 13 of the device which as a result deforms elastically. In addition, the eccentricity of load application to the annulus 13 via the loading points 11 induces a bending stress causing the projecting flanges 7 to distort elastically in such a way that the small movements involved are exaggerated at the outer end of the flanges 7. The clearance 9 between the machined flange ends and the outer shaped ring 8 reduces as a result. When sufficient load has been applied, this clearance reduces to zero and hand rotation of the ring 8 becomes no longer possible.The load at which this condition occurs is the desired pre-set load.

This serves as an indicatorthatthe bolt has reached its design load and thatfurthertightening is not required. If, during service, the bolt orthe assembly connected by it relaxes causing a reduction in the desired clamping force, this can be immediately detected at any time by checking if the shaped ring may be freely rotated. If free rotation is found to exist, the bolt may be re-tightened until hand rotation is again prevented thus preserving the design clamping criteria required.

The washer device 3 has a number of important features. The shaped ring 8 can be adjusted in diameterto allow the device to be pre-setforany particular load within the elastic range of bolt size forwhich it is intended. During manufacture of the washer device, the ring 8 can be set by pre-loading the device and adjusting the set screw 12 to obtain the condition where the ring 8 is just prevented from rotation at that load. The basic device may thus be adjusted for a wide range of applications. In addition this method of presetting the device ensures that the accuracy is unaffected by manufacturing tolerances.

The shape, relative position and dimensional relationship of the machined features ofthewasherdevice and its ring 8 may be varied to ensure satisfactory performance with different diameters and types of bolts. The principle of operation of the load indicating washer device nevertheless remains unchanged.

Modified embodiments of the washer device 3 are illustrated in Figures 4 and 5. In the device of Figure 4 the raised loading point 11 is replaced by an annular spring steel insert 14which engages a groove 15 in the shoulder 10 of the washer device 3. The insert 14 serves to ensure that in the event of an uneven load being transmitted from the nut2 and washer Sthe accuracy of the movement ofthe flanges 7 is unaffected. This is achieved byvirtue of the insert 14 deforming the groove 15 under load so that differences in the spacing around the circumference ofthe device 3 between it and the nut 2 or washer 5 are compensa- ted for.

Asimilar effect is provided by the device of Figure 5. In this embodiment the raised loading point 11 is replaced by a soft washer 16 which has point projections 17 which deform elastically under load againstthe shoulders 10 of the washer device 3.

Modifications and improvements may be made without departing from the scope of the invention.

Claims

1. A load indicating washerforfitmentaround a bolt stem, the washer having first and second portions which are relatively movable and arranged so as to locktogether against such relative movement on application of a predetermined applied load to the washer in the intended direction of the bolt axis.

2. A load indicating washer as claimed in Claim 1, wherein the first and second portions are relatively rotatable about an axis which passes through a central aperture for receiving the bolt stem.

3. A load indicating washer as claimed in Claim 1 or 2, wherein the first portion is deformable to en gagethesecond portion on application of said load to the washer.

4. A load indicating washer as claimed in Claim 1, 2 or 3, wherein the first portion has a radiallyprojecting flange which is arranged to deflect on application of said load to engagethe second portion

5. A load indicating washer as claimed in Claim 4, wherein the first portion has a pair of said flanges arranged on opposite sides ofthe second portion so as to clamp the second portion between them on application of said load.

6. A load indicating washer as claimed in Claim 5, wherein the second portion is in theform ofanan- nulus extending around and projecting between the flanges of the first portion.

7. A load indicating washer as claimed in Claim 6, wherein the annulus has an inwardly-tapering wall projecting between said flanges and engageable by the flanges so as to be clamped on application of said load.

8. A load indicating washer as claimed in any one of the preceding Claims, wherein the spacing between the first and second portions is adjustable.

9. A load indicating washer as claimed in any one ofthe preceding Claims, including means for causing a radially non-uniform load applied thereto to produce a substantially radially-uniform bias of the first portion in the axial direction.

10. A load indicating washer as claimed in Claim 9, wherein said means comprises a projecting member which engages and enters a deformable recess in the first portion on application of said load, the projecting member and recess being of noncomplementary shape.

11. A load indicating washer substantially as hereinbefore described with reference to and as shown in Figures 1,2 and 3 of the accompanying drawings.

12. Aload indicatingwashersubstantiallyas her- einbefore described with reference to and as shown in Figure 4 of the accompanying drawings.

13. A load indicating washer substantially as hereinbefore described with reference to and as shown in Figure 5 of the accompanying drawings.