Background

Technical field

The present invention relates to a method according to the introductory portion of the accompanying claim 1.

The invention further relates to a device according to the introductory portion of the accompanying claim 12.

The invention, in addition, relates to a locking element according to the accompanying claim 23. Prior art

Bolt and nut and bolt in a threaded hole are known methods for creating joints.The bolt thread always has a rise, which makes such a joint to tend to come loose if a friction force in the thread and against a support is below a loosening torque created by tensile force in the bolt and the thread rise. Common methods meant to secure bolt joints, ie to prevent them from coming loose, are to increase the friction against the support and/or in the thread. The friction against the support is often increased by ribbing of the contact surface of the bolt head and the nut, respectively. Increase of the thread friction is done either by designing the threads so that one part comprises a surface, which is not smooth against the bolt thread and thereby increases the friction by a cutting function. Alternatively the thread on the bolt or the nut is provided with glue or a nylon bushing meant to give increased friction.

A substantial drawback of increased thread friction is that torsional stress in the bolt is increased, which means that maximum allowed tensile stress in the bolt decreases. In addition to this an unwanted turning of the bolt appears, which increases the risk for turning loose when the joint is exposed to dynamic loads. The application of glue on the thread gives an unwanted uncertainty due to spread in friction result caused by different used materials, surface treatments and different glue qualities.

In order to use the maximum allowed tensile stress in the bolt and hereby the maximum tensile force of the bolt, the thread friction must be minimized.This can only be achieved by applying a lubricant in the thread. With a low thread friction, turning loose is risked if the bolt and/or the nut are not secured against the support. Increased friction against the support is achieved by providing to the surface some kind of teeth arrangement, which gives a grip in the loosening direction. Such a surface may give a sufficient safety against loosening. At strong dynamic loads small movements however appear between the surfaces and due to the rise of the thread and hereby often a successive turning occurs and thereby loosening of the bolt.

A known method meant to prevent loose turning is to use a pair of locking elements in the form of locking washers having a number of cam or wedge portions on opposing sides, where the rise of the respective cam portions exceeds the one of the thread, often by about 50%, whereby the pair of washers is arranged between a bolt head or a nut and a support. In the loosening direction, ie in the circumferential direction of the bolt, sliding must occur between opposing cam portions of the washers since the washer surfaces against bolt head /nut are provided with ribs, teeth, which increase the friction against the goods in the loosening direction. A grip of the teeth takes place during the tightening.

An increased rise of the cam portions above the thread rise leads to a wedge action during loosening, which gives an increased bolt stress. This stress increase leads to deformation of the cam portions of the pair of washers as the tensile force in the bolt increases and as the contact surface between the washers decrease during loosening. The cam portions then are deformed plastically, ie permanently. This makes reuse of the washers after loosening of the joint difficult or impossible.

Thus, there is a need for a solution to the problem described above and for eliminating the drawbacks described above, which are associated with the prior art. The present invention has hereby as its purpose to solve among other things these problems.

Summary of the invention

This and further purposes are achieved by a method according to the accompanying claim 1, a device according to the accompanying claim 12 and a locking element according to the accompanying claim 23.

Further advantages are achieved by what is specified in the respective dependent claims. Brief description of the drawings

A better understanding of the present invention may be achieved by reading the accompanying description in association with the accompanying drawings, in which like details are designated with like designations and in which - fig 1 schematically shows one embodiment of a bolt joint with a screw or bolt, shown in a side view, with a head and a threaded portion and a dashed marked pair of locking washers under the head and a dashed shown nut with a pair of dashed marked locking washers, two goods portions being arranged to be joined by means of the bolt joint;

-fig. 2 a) to c) schematically shows a general design of a locking washer of a kind actual here seen against its axial direction and from above and below, respectively;

- fig. 3 schematically shows a pair of locking washers seen a) against its axial direction and b) from above in a), respectively, said washers representing prior art and hereby having on mutually opposing sides a pattern of cam portions with rise considerably exceeding the rise of the bolt shown in Fig. 1, which bolt is intended to be locked by means of the locking washers in a bolt joint;

- fig. 4 schematically shows a pair of locking washers according to the invention for locking of a bolt according to Fig. 1 in a bolt joint, where the washers on mutually opposing sides have a pattern of cam portions and are shown in Fig. 4a and 4b in a way corresponding to as in Fig. 3; - fig.5 schematically shows friction means of one side of a locking washer for application of the present invention, where the locking washer is seen against said side 1 Fig. 5a) and in a diametrically running axial section in Fig. 5b);

- fig. 6 schematically shows an against its axial direction seen bolt with a pattern of cam portions on the bottom side of the head; and - fig. 7 schematically shows a goods portion with an imprinted pattern of cam portions.

Detailed description of preferred embodiments

In Fig. 1, 100 designates an against its axial direction seen bolt of a joint 110 for application of the present invention, where in the in Fig. 1, as an example, shown embodiment, two portions 101, 102 of goods G are intended to be joined by means of the bolt joint 110. The bolt comprises a head 101 and a portion 111 of an outer thread with a certain rise vl intended to cooperate in said joint with a portion 113 of inner thread 114, which may be arranged in a portion 102, which shall be joined by the bolt joint, as shown in Fig. 1, or be arranged in a nut 116, as shown dashed in Fig 1.

The present invention relates to a technique wherein a bolt joint is arranged to be locked against unwanted loosening and hereby comprises at least one locking washer 120, via which the bolt head or said nut is intended to act against goods G, which shall be joined by the bolt joint.

Said at least one locking washer, the main design of which is shown in Fig. 2, is substantially ring- and plate shaped and on its one side 121 provided with a pattern of circumferentially spaced and radially running cam portions 122 having a certain rise, The cam portions ara intended for co-operation with a substantially corresponding pattern of cam portions, e. g. as shown in Fig. 4.

According to the present invention said at least one locking washer is arranged with a rise vl, Fig. 4, of the washer cam portions adapted to the thread of said bolt in such a way that the rise of said cam portions is at most the same, i. e., not more than, the rise vl of the thread of said bolt. Hereby it is made possible to draw said bolt loose without increasing the existing tensile stress in said bolt.

According to preferred embodiments the rise of the cam portions is, in addition, not less than the rise of the thread of said threaded portion of said bolt. Under certain circumstances, however, it may be imagined that the rise of said cam portions is somewhat less, e. g. 1-10% less, than the rise vl of the thread without risking the function of the invention. The function according to the invention is described in more detail in the following.

Said at least one locking washer further comprises a pattern of friction means 123 on the other side 124 arranged to create increased friction between the locking washer and the surface, against which the locking washer is intended to act, for example bolt head, nut or goods G. According to one embodiment said friction means comprise a pattern of radially running teeth 125, Fig. 2. According to another embodiment said friction means comprise at least one ridge portion 126, Fig. 5, running in the circumferential direction of the washer. It is preferred that said ridge portion is running in association to the periphery of the washer to obtain a long torque arm for the prevailing friction force. A combination of the described embodiments may also be imagined. According to a preferred embodiment said at least one locking washer is arranged in a pair with an additional locking washer of the same kind so that the cam portion patterns of the locking washers are mutually opposing and hereby arranged for co-operation, as shown in Fig. 4.

According to another embodiment said at least one locking washer is the only locking washer acting against the bolt head/nut and whereby said substantially corresponding pattern of cam portions as the one of said at least one locking washer is comprised by said bolt head or nut, as shown diagrammatically in Fig. 6. According to among other things such embodiments said substantially corresponding pattern of cam portions as the one of said at least one locking washer is imprinted in said bolt head or nut or is imprinted in said goods G, Fig. 7, in association with tightening of the bolt joint.

According to certain embodiments there are friction means 123 imprinted in said bolt head or said nut by tightening of the bolt joint.

According to preferred embodiments said cam portions are arranged with a helix configuration seen against its and the washer main extension plane, said helix configuration serves the purpose of promoting that a good contact is maintained between opposing cam portions in spite of mutual rotation between the cam portions.

The method as well as the function of the device should to a substantial portion have been made clear by the description given above.

Irrespective of how the two mutually opposing patterns of cam portions are provided, i. e. by means of a pair of locking washers or by means of a locking washer with a pattern of cam portions and a pattern of cam portions of any other component comprised by the bolt joint, the rise of the cam portions is adapted to the rise of the bolt thread in such a way that the rise of the cam portions is at most the same as, i. e. not more than, the bolt thread rise and preferably not less either. A bolt joint made in this way provides an optimum functionality from a locking as well as a loosening point of view.

From a locking point of view full safety is obtained due to the fact that no decrease in the tensile stress in the bolt takes place at turning of the bolt in the loosening direction in the tightened bolt joint. Such a decrease is the main problem of a bolt not locked and makes the bolt tend to come loose. At the same time the friction between the mutually opposing cam portions is enough to prevent sliding between the cam portions and hereby prevent the bolt from coming loose. In many cases the friction between the cam portions should be enough to prevent sliding between the cam portions even if the rise of the cam portions is slightly less, e. g. 1-10% less, than the rise of the bolt thread.

From a loosening point of view a bolt joint according to the present invention offers the advantage that it is easier to turn loose since turning loose takes place without an increase of the existing tensile stress in the bolt when it is turned in the loose turning direction due to the fact that the rise of the mutually opposing cam portions is, at most, the same as the rise of the bolt thread. Sliding between the cam portions at loose turning takes place and the tensile stress in the bolt is relieved after turning over one cam portion length. The fact that the tensile stress in the bolt does not increase at loose turning means that the cam portion tips are eformed to a substantially less extent than at prior art technique, which in turn means that washer from loosened bolt joints may be reused. It is from a loosening point of view advantageous if the cam portion extension in the circumferential direction is small, which means that the tensile stress in the bolt is relieved already after a minor turn and that the tensile stress in the bolt prior to that is spread over a great number of tightly arranged cam portions, which increases the loaded cam surface and hereby decreases the risk for permanent deformation of the cam portion tips even if the tensile stress does not increase during the turning. According to preferred embodiments a suitable such a minor turn, corresponding to a small cam portion extension in the washer circumferential direction, is up to about 10°. Preferred according to the present invention is thus that the rise of said cam portions corresponds to the rise of said bolt thread.

Above the invention has been descried in association with preferred embodiments and examples. Of course further embodiments as well as minor changes and additions may be imagined without departing from the basic inventive idea. According to one embodiment there is, not shown, one or two locking washers pre- mounted on the bolt threaded portion prior to forming of the threaded bolt portion so that the locking washer/- s are held between the head and the threaded portion when this is formed. According to another embodiment for nut there is, not shown, one or two locking washers pre-mounted on a neck of the nut and are retained by a forged flange on said neck.

According to one embodiment a pair of washers is pre-mounted to a unit , e. g. by a viscous glue between the opposing cam portions, to make mounting of the washer pair easier. Preferred is that at least one surface layer of the locking washers according to the invention has a hardness higher than the hardness of the surfaces against which they are mounted. A suitable material of the washers is steel, hardened through and tempered. The hardening furnace used should have a carbonizing atmosphere since the preferred initial material, cold rolled steel, often has a reduced carbon content in the surface layer. After punching/indentation and heat treatment, the washers, according to preferred embodiments, are provided with a rust protecting surface coating of the type zink flakes. Hereby the coating is not applied electrolytically. Washers made of acid resistent steel is an alternative when need of a high corrosion protection is at hand. These washers, also, should be treated for an increased surface hardness in order to secure grip in an opposing material.

Thus, the invention should not be considered limited to the embodiments described above but may be varied within its frame specified by the accompanying claims.