Field of invention

This invention relates to a locking device for screw threaded members, and more particularly, though not exclusively, to locking means for threadedly jointed bars or tubes of substantially circular cross-section.

Background to the invention

Though the present locking device is not exclusively applicable to circular cross sections, it is to be understood that the locking improves the larger the number of facets in the cross-section of at least one of the members.

As the invention relates particularly to circular cross section members, the invention will be described in relation to such members. It will, however, be appreciated that the invention may equally well be applied to joining members of other cross-sections.

Many methods of joining pieces of round bar, end to end, have been employed, eg one end of each piece of bar may be drilled and tapped longitudinally to accept a threaded insert, such as studding, so that the two ends may be jointed by the insert; or a male thread may be formed on one end of one of the pieces of bar and a female on one end of the other piece of bar, if the finished overall length is not critical. Another method is to "sleeve" the

end of one piece of bar, retaining the sleeve to the bar with a screw, rivet or pin fitted through the sleeve and into the bar, and then inserting an end of the other bar in the other end of the sleeve, where it is retained using a screw, pip-pin, or nut and bolt fitted through the sleeve and the other bar. Specially manufactured clamps with flanges which can be bolted together are also used for joining two bars or tubes at their ends.

All these previous methods have a time factor problem if the bars etc are to be locked together, and if just threaded together they present a security problem since the threaded sections may be easily separated.

It is therefore an object of this invention to provide an improved locking device for screw threaded members which enables them to be easily freed for unscrewing but will not allow them to unscrew of their own accord, thereby substantially overcoming the previous problems.

According to the present invention there is provided a locking device for retaining together two inter-engaging screw threaded members, comprising a coiled spring mounted on one of the members, the spring having inner and outer surfaces, a substantially cylindrical surface formed on the other member for engagement with one of said inner and outer surfaces, the spring being so coiled that upon screwing the members together the coils slip over the cylindrical surface and upon attempted unsolicited unscrewing of the members the coils tighten on the cylindrical surface to retain the members together, and rotation means engageable with an end of the coil spring to rotate said end in a direction to free the spring from the cylindrical surface to permit unscrewing of the

memoers

The invention will now be described, by way of example only, with reference to the accompanying single figure of the drawing, which shows in section a locking device in accordance with the invention.

Referring to the drawing a first bar or tubular member 1 has an annular section 2 into which is fitted a locking device means, generally shown at 3. The device 3 comprises an insert 4 with an internal screw thread 4a. A coil spring 5 of suitable diameter has its left end 5a anchored to the insert 4 and its right end 5b anchored to an external sleeve 6. Preferably the sleeve 6 is of sufficient length to overlap both the spring 5 and a portion of the tubular member 1. To retain the external sleeve 6 adjacent the end of member 1 a shoulder 7 is formed on its inner wall, and the shoulder 7 is trapped between the spring's left end 5a and the right-hand end of the member 1, adjacent to which a snap ring 8, engages the insert 4 to stop the insert being pushed too far into the member 1. Preferably the insert 4 is a push fit in the member 1 and is then retained therein with Loctite (R.T.M) or a similar locking compound. It may equally well be retained by any other known means such as grub screws, rivets or pins. To limit the angle of rotation of the sleeve 6 a stop-pin 9 is mounted on the insert 4 to operate in an arcuate slot 6a formed in the shoulder 7 of the sleeve 6.

A corresponding male threaded section 10 of a second bar member 11 engages with the screw thread 4a of insert 4. The member 11 also has an external cylindrical surface 12 of slightly greater diameter than the inner diameter of

coil spring 5. When the first member 1 and the second member 11 are screwed together via the device 3, the external surface 12 of the member 11 slightly opens the coils of the spring 5 and slips under the coils. Upon inadvertent surface unscrewing of one member relative to the other, the unscrewing movement of the surface 12 drags and tightens the coils of the appropriately handed spring 5 to further clamp end grip upon the surface 12. In order to unscrew the two members from each other the external sleeve 6 must first be turned in such a direction relative to the first member 1 so as to open up the coils of the spring 5, thereby freeing the second member 11 for unscrewing.

It will be apparent to those skilled in the art that the spring may equally well be arranged so that the outside diameter of the spring vice-versa engages an internal cylindrical surface of one of the members. By utilising the locking means in this alternative manner, the outside contour between the joined first and second members can remain smooth and unbroken.

It will also be apparent that the device described may easily be fitted to any two articles which require to be joined in this manner.