Previously, various devices have been proposed for preventing or resisting the loosening of a nut on a screw, bolt or other threaded member such as a stud. Stiffnuts have certain conveniences over positive locking devices but do have certain limitations and disadvantages.

One known type of stiffnut is a nut having a conventional body but with the addition of a nylon insert which is engaged in an interference fit with the bolt on first application of the nut on a bolt whereby a high resistance to rotation is created to resist removal of the nut under vibration when in use. However, a high application torque is needed and after the nut has been removed after a period of service it has a significantly reduced resistance to rotation if re-used, and it may be necessry to discard the nut. Furthermore, this type of nut is relatively expensive. Another known approach is to use a spring device such as a spring washer but in the event that the nut is sufficiently loosened so as not to be compressing the washer, the spring washer has no effect whatsoever and no further resistance to rotation is provided.

Generally, positive locking devices for nuts are relatively cumbersome to apply and remove, often require precise rotary alignment of locking parts and are accordingly expensive in labour time to fit, have the disadvantage of a multiplicity of parts and can be relatively expensive.

Accordingly, there is a need for new useful alternatives to previously proposed arrangements.

Examples of specifications in the field of nuts, fasteners and locking devices include the following: GB-A-372209 (S.A. DES ATELIERS GAMAIN)

AU-B-53657/65 (294766) (R.K.E. GOTZ)

AU-B-1993/45 (146453) (CLARKE AIRCRAFT PRODUCTS LTD)

AU-A-60943/80 (AJAX NETTLEFOLDS LIMITED) GB-A-207907 (HINTON)

GB-A-570905 (ROWING)

GB-A-371109 (S.A. DES ATELIERS GAMAIN)

CH-A-141119 (HUGLI)

CH-A-153562 (AMEZ-DROZ and KEHRER) AU-A-60943/80 (AJAX) describes a nut with a longitudinal slit with a conical end to engage with a corresponding work piece to prevent the nut slit opening when in use. However, the nut is of conventional dimensions and rotates freely on a bolt. The disclosure is concerned with removal of the nut from a corroded thread by the opening up slit. The disclosure does not address the question of locknuts or stiffnuts.

According to the present invention, there is provided a nut comprising an annular body extending around an axis with a screw-threaded interior passage extending along said axis and having first and second ends, a slit extending through the annular body and extending axially from the first end to the second end so as to provide a longitudinal slit, the nut being of resiliently expandable form and, compared with a conventional nut for application to an intended bolt, the interior passage has a reduced diameter, the resilience of the nut and its dimensions being such that the nut has a high resistance to rotation on the intended bolt.

Any one or more and preferably most of the following features are included in a preferred embodiment of the invention: (a) Preferably one end (and possibly both of the ends) of the nut is provided with a lead-in on the screw-threading to permit easy starting of the nut or bolt with similar ease of rotation as that which

applies to a conventional nut on a bolt; this features can be achieved by appropriate cutting back of the screw-threaded interior passage adjacent the end of the nut thereby leaving the remaining portion of the screw threaded interior passage to provide the gripping region for firm engagement with a bolt.

(b) At least one end of the nut has a ramp-like profile such as a conical shaped or part-spherical shaped surface and adapted for co-operation with a corresponding seat on a washer or work piece. This feature operates when the nut is tightened against the washer or workpiece to counteract forces which might otherwise tend to expand the nut radially outwardly, whereby the intended resistance to rotation of the nut is maintained. Furthermore, by appropriate dimensions being selected the ramp-like profile can be arranged to positively urge the nut inwardly onto the screw-thread lhereby tending to distribute the load over an extended region of the threading of the bolt within the nut body. (c) The longitudinal slit is the order of 5% of the diameter of the interior passage and preferably is formed in a manufacturing process in which initially the slit is approximately 10% of the diameter of the interior passage, cold pressing occurs to close the slit substantially, and the pressure is removed thereby resulting in partial recovery of the nut to leave the slit with the width being about half of that initially provided.

(d) The nut is preferably of a high tensile steel which is heat treated to provide and maintain the necessary resilient qualities. One convenient material is that known as a Grade 8 nut having high carbon, magnesium, chromium and molybdenum contents.

It will be appreciated that nuts according to the present invention can be made generally to substitute for conventional nuts according to various standards. While most usually the nuts will have a conventional hexagonal profile, the invention is not limited to such

a shape as any suitable form of exterior profile could be provided.

At least preferred embodiments of the invention can be advantageous in permitting the following advantages to be achieved:

(i) The nut can be used many times as it has been found that its necessary characteristics can be preserved despite periods of use, substantial vibration during periods of service, removal and reapplication.

Obviously the nut cannot safely be used again if it has been damaged or stressed during removal, e.g. due to an excessively corroded exposed bolt. (ii) In the event that the nut cannot be removed due to corrosion of a bolt thread, after a small degree of slackening of the nut, the nut can be opened up by a chisel hammered into the longitudinal slit thereby premitting the nut to be rotated further and removed from the offending corroded thread. The nut should then be discarded, (iii) If the nut is not fully tightened against a washer or work-piece e.g. due to user error, it appears that a bedding-in effect occurs when the nut is subjected to vibration, whereby the initial torque subsequently needed to commence slackening of the nut is of the order of twice the torque actually applied.

(iv) Even if the nut is not tightened onto a washer or workpiece, it has been found that a high degree of resistance to vibration over an extended period of time can be achieved. (v) Nuts can be manufactured economically and manufacture can be based on conventional nut production techniques; essentially conventional coned nuts are formed and then

subjected to additional processing steps to provide the requisite slit, the pressing step can be effected and the preferred heat treatment applied. Use of the present invention can be advantageous in applying frictional forces over most if not virtually all the portion of the bolt thread within the envelope of the nut body and this is in contrast to known stiff nuts. Furthermore, nuts embodying the present invention may be acceptably effective on bolts which are either slightly oversize or slightly under the nominal design size.

Yet another feature which embodiments of the invention can provide is a nut which does not function in a manner which damages or deforms the thread on either the nut or the bolt.

Since nuts embodying the present* invention can be designed to be substantially of conventional shape and dimension but having a slightly inwardly compressed normal position, ordinarily conventional spanners may be used with the nuts.

Preferred embodiments of the invention are used with a workpiece or washer having a ramp-like profile e.g. domed or conical corresponding to the end of the nut; the surface of the washer or work piece may be provided with a serrated finish or a saw-tooth profile with the serrations or teeth angled so that when the nut engages the surface of the washer or workpiece it has much less resistance to motion in the direction in which the nut is tightened up as opposed to the direction in which the nut is loosened.

The invention also extends to the combination of the nut and a suitable washer and the washer for some applications advantageously may be formed as a spring washer, the spring washer having a ramp-like surface for co-operation with the end of the nut and being adapted to be displaced from an initial position until a portion of the radially innermost part engages the

workpiece .

Examples of the invention will now be given with reference to the accompanying drawings, of which:

Figure 1 is an elevation of a nut embodying the invention;

Figure 2 is an inverted plan of the nut of Figure

1;

Figure 3 is a cross-sectional view through a first embodiment of washer for use with a nut of Figure 1; and Figure 4 is a cross-sectional view through an alternative embodiment of washer to that Figure 3 and prior to engagement with a nut.

Referring first to Figures 1 and 2, the nut is of substantially conventional shape and dimension and has a hexagonal body 1, an axial screw-threaded passage 2 extending between a plane upper end 3 and a lower end 4 having" a frusto-conical surface 5 having an angle X to the end plane typically in the range 3G to 40°. A longitudinal slit 6 extends radially through one- side of the nut.

As indicated in an exaggerated manner in Figure 2, the screw-threaded passage 2 has a lead-in 2A formed by setting back the end portion of the threading so as to turn freely on a bolt upon initially engagement. For a typical nut of 20 mm size (i.e. dimensioned to fit a bolt of overall diameter 20 mm) , the width of the slit is about 1 mm in the final product, the nut being of a high tensile steel such as a Grade 8 steel which is believed typically to include 0.4% carbon, 0.8% magnesium, 0.9% chromium and 0.2% molybdenum. The nut is formed basically by conventional nut-making technology including the domed end but without the slit 6, and with the threaded bore being of conventional dimensions and configuration. However, in this embodiment the lower end 4 has the screw-thread formed to be of slightly larger diameter at the end of the nut to provide an easy lead-in onto a bolt after the nut has been pressed down to a reduced diamter as

described below.

The slit 6 is formed by any convenient cutting technique and transverse pressure applied to close the slit from an initial width of about 2 mm so that the radial faces substantially come together. This is conducted as a cold pressing operation and it has been found that after pressure has been removed the nut only partially recovers and the slit has a final width of about 1 mm. Heat treatment is then effected typically by a hardening operation from 850°C with an oil quench followed by a tempering operation to about 450°C followed by an air cool or oil cooling technique. The resultant nut has a degree of resilience suitable for the present purposes. Referring now to Figure 3 a solid washer 7 is illustrated and has a frusto-conical bore 8 of corresponding angle X to the conical face 5 on the nut. Thus, when the nut and washer are used on a workpiece and applied over a bolt and the nut is screwed down, the conical surfaces interact to restrict expansion of the nut under load.

Referring now to Figure 4, a spring washer 10 is illustrated, the washer being of spring steel and having a cylindrical skirt 11 and a resilient frusto-conical funnel portion 12 having initially a smaller angle Y to the transverse plane than the angle X. The radially interior part of the funnel terminates in an axially extending throat portion 13 with a flat transverse annular end face 14 which normally is set back from the plane 15 of the end face of the skirt 11. When a nut 1 co-operates with the washer 10, deformation of the washer occurs pressing the funnel portion 13 downwardly until the annular face 14 rests on the workpiece. Deformation of the washer tends to cause the angle of the cone surface to alter to adopt the position shown in dotted lines and to be equal to that of the conical surface 5 of the nut 1.

The outer annular skirt 11 may have a thickened

base portion for engagement on the work piece for example as shown in the embodiment of Figure 4 where feet 16 are provided.

In either of the embodiments of Figures 3 and 4, the conical face may be provided with serrations or saw tooth projections preferentially orientated to resist rotation in the direction in which a nut is turned to loosen it and a similar profiling may be provided to the surfaces of the washer adapted to engage the work piece.

Compared with a conventional nut, which is free spinning on its bolt, the present nut can have a clamping effect around the bolt thereby spreading the load over a considerable length of screw-threading on the bolt. By contrast when a conventional nut is under tension, dilation tends to occur thereby tending to increase the clearance between the threads on the nut and bolt.

Test have been made on nuts of -approximately 20 mm size. All nuts were Grade 8 high tensile steel hardened from 850°C and oil tempered to varying temperatures. The following table gives the results of torque required to rotate the nut shown in Figures 1 and 2 when fully wound onto a bolt and free to be rotated:

Slit Width Hardness Temperinσ Temp Torque

0.95 mm 38-39 RC 450°C 27.2 Nm

1.05 mm 38-39 RC 450°C 18.1 Nm

1.30 mm 35-36 RC 500°C 12.3 Nm

By contrast a corresponding conventional stiff nut having a nylon insert was tested and upon first removal of the nut the torque required was 12.7 Nm and on a second removal 9.1 Nm.