Background When the two threaded members are interengaged, it is usual practice that the retention which effected by such threaded engagement is effected as a result of the frictional interengagement between the opposed axial faces of the member and the interengaged faces of the threads of each of the members. When it becomes necessary to disengage the members the frictional forces which have been generated as a result of the interengagement and which may have increased over time present a significant difficulty in attempting to effect disengagement. As a result, it becomes necessary to utilise large and/or powerful tools in order to effect disengagement. As an alternative is has been known to locate annular elements between two threaded members which can be removed to enable two components to be separated. However such removal requires the use of oxy-acetylene or like cutting equipment or the use of mechanical cutting equipment which involve the use of saws and/or abrasive wheels. The use of cutting equipment which requires the application of heat can have damaging effects upon tempering and/or hardness characteristics of the interconnected components while the use of mechanical cutting equipment can result in physical damage to at least the abutting faces of the interconnected components.

It is an object of this invention to provide a washer which can be utilised between a pair of threaded members which can facilitate the disengagement when required without detracting from the positive interengagement in use.

Disclosure of the Invention This invention relates to a washer adapted to be received between the opposed axial faces of a pair of threaded components to be clampingly retained between said axial faces, said washer being formed of a brittle hard material, said washer 5 being formed with at least one zone of weakness which is adapted to be able to break on an impact force being applied thereto.

The invention will be more fully understood in the light of the following description of one specific embodiment.

Brief Description of the Drawings 10 The description is made with reference to the accompanying drawings of which : Figure 1 is a schematic exploded view illustrating the interengagement of a drive sub to the piston case of a down hole hammer utilising a washer according to the embodiment; and Figure 2 is a plan view of a washer according to the embodiment.

15 Detailed description of Specific Embodiment The embodiment relates to a washer which can be used between a drive sub and piston case of a downhole hammer. It is usual practice when locating a drive sub on a piston case to load the threaded interengagement between the drive sub and piston case to a level sufficient to prevent disengagement of the 20 drive sub from the piston case in use. This loading is accommodated by the frictional interengagement between the opposed axial faces of the drive sub and the piston case as well as the interengaged faces of threads provided on the drive sub and piston case. When it becomes necessary to disengage the drive sub from the piston case, the frictional forces which have been generated from 25 engaging the drive sub to the piston case must be overcome. In addition, it is usual that those frictional forces will have increased in use. The frictional forces

present significant difficulty in disengaging the drive sub from the piston case and necessitate the use of large and/or powerful tools to effect the disengagement.

The present embodiment comprises a washer 19 which is to be received over the threaded shank 15 of the drive sub prior to the threaded shank 15 being engaged with the threaded opening 17 of the piston case 13, whereby the washer will be clampingly engaged between the opposed axial faces of the drive sub 11 and the piston case 13. The washer is formed of a hard brittle metal such that it can accommodate the clamping forces which are applied thereon by the opposed faces of the drive sub 11 and the piston case 11. In addition, the outer perimeter of the washer is formed with a plurality of indentations 21 at angular spaced intervals around the perimeter of the washer 19. The indentations are each formed with a radial face 23.

In use the washer is located in position on the threaded shank of a drive sub prior to the engagement of the shank with the piston case. On engagement of drive sub 11 with the piston casing 13 the washer 19 is clampingly received between the opposed axial faces of the drive sub and the piston case in order that the drive sub is adequately securely fixed to the piston case.

When it becomes necessary to disengage the drive sub from the piston case, an impact force is applied to the radial face 23 of at least one of the indentations 21 by means of a suitable impact tool (eg : a cold chisel and hammer) in order that the washer is broken at the impacted indentation at least. On further impact forces being applied to washer, the broken pieces of the washer can be extracted from between the opposed faces of the drive sub 11 and the piston case 13 such that the washer is fu! ly removed between those opposed faces.

The removal of the washer significantly reduces the frictional engagement which are applied between the opposed axial faces of the drive sub and piston case and relieves the interengaged faces of the thread provided on the threaded shank 15 and threaded opening 17. The reduction of the frictional forces then enables the drive sub to be extracted from the piston case without the need for any heavy and/or powerful tools. In addition the use of the embodiment does not

require the application cf heat to remove the washer which may affect the tempering and/or hardness characteristics of the interconnected components.

Furthermore the removal of the washer does not require the use of abrasive and/or cutting equipment which may damage the abutting faces of the interconnected components.

While the embodiment has been described as being used with two threaded components utilising a tapered thread it should be appreciated that the invention has equal applicability to parallel threaded components. In addition, while the embodiment has being described in relation to a mounting of a drive sub to the piston case of a down hole hammer, it can have equal application to the threaded mounting between the top sub and piston case of a down hole hammer and between any two components which are threadably interengaged.

Throughout the specification, unless the context requires otherwise, the word "comprise"or variations such as"comprises"or"comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

It should be appreciated that the scope of the present invention need not be limited to the particular scope of the embodiment as described above.