GB2054787A - Slit nut - Google Patents

Abstract

In certain corrosive situations conventional nut and bolt assemblies can become frozen whereby it is impossible to remove the nut from the bolt. The present invention solves this problem by providing a one piece nut having a ramp surface 10 at one end of its axially threaded bore and a longitudinally extending slit 12 extending from the ramp end of the bore at least partially along the axial length of the nut and completely through the periphery of the nut. In use the ramp surface of the nut engages a cooperating surface in an end engaging washer whereby opposing surfaces of the slit in the nut are urged towards one another. The slit may extend wholly across the nut diameter, from the ramp end face to within a short axial distance of the opposite end face (Fig. 5, not shown). <IMAGE>

Description

SPECIFICATION Slit nut The present invention relates to one piece nuts and methods of assembling and removing such nuts from externally threaded members such as bolts or studs.

In certain applications where severe corrosion or high temperatures are involved, conventional one piece nuts may become "frozen" in position on bolt or studs by rust or scale so that they can no longer be removed by rotation. In such instances the bolts/studs and nuts have to be cut or burned off with an oxy-acetylene torch to enable removal.

In one known method of overcoming the aforementioned problem, a two piece nut is provided which co-operates with an angular faced washer. The two pieces of this nut are assembled with a bolt without the necessity of rotating the nut over the whole of the protruding bolt threat. If the exposed threads become corroded the two pieces of split nut can be removed after rotation by approximately a quarter to half of a turn to release them from the reaction imposed by the angular seat. While this type of multi piece nut operates satisfactorily, it does have serious disadvantages in that it cannot be economically produced using conventional high volume nut forming processes. In addition there are three separate loose pieces involved in the assembly which can provide difficulties in handling until such time as the parts have been locked together.

The present invention aims at providing a one piece nut which substantially overcomes the aforementioned disadvantages while maintaining an ability of relative ease of removal from a threaded member which may be heavily corroded.

According to the invention there is provided a one piece nut having external driving surfaces, an axial threaded bore passing through said nut, a ramped surface or surfaces arranged around at least one end of said bore, and a longitudinally extending slit extending from said one end of the bore at least partially along the axially length of said nut and completely through the periphery of said nut, said ramped surface or surfaces being adopted to engage a co-operating surface in a nut engaging component when the nut is threaded on an externally threaded member whereby opposing surfaces of said slit are urged towards one another.

According to a preferred embodiment of the invention, the aforesaid slit extends axially along the complete length of the nut. In addition, the portion of the periphery of the nut diametrically opposite to the slit may have a reduced axial cross-section by arranging at least one axially extending groove along the bore and/or along the external surface of the nut. In accordance with another preferred arrangement, the slit may extend axially along a substantial portion of the nut but not along its complete length.

Furthermore, in this embodiment, a further slit may be provided diametrically opposite to the first mentioned slit, and one or more additional axial slits may be arranged in the nut provided that the nut is maintained in one piece. Preferably the ramped surface comprises a frusto conical surface arranged co-axially with the bore of the nut.

Alternatively, the ramped surface may have a curved profile when viewed in cross-section, for example, as would occur if the ramped surface were formed as a slice from a spherical member.

In accordance with an advantageous embodiment, ramped surfaces are provided at either end of the bore of the nut which ensures that the nut does not require any particular orientation during application to an externally threaded member.

The slit (or slits) passing through the periphery of the nut may be arranged at any location around the bore of the nut. The slit(s) may be positioned either through a driving face or corner of the nut.

The invention will now be described with reference to the accompanying drawings showing several preferred embodiments of nuts in accordance with the invention. In the drawings, Figures 1 to 5 shown either side elevation or longitudinal section views and plan views of various preferred embodiments of the invention.

Figure 6 illustrates a longitudinal section view and plan view of a washer component adapted for engagement with nuts in accordance with the invention. Figure 7 illustrates the interengagement of nuts and washers according to the invention.

Figure 8 is a table showing test results and Figure 9 is a graphic representation of comparative fatigue tests.

Referring to Figure 1 there is shown a nut of substantially conventional configuration having hexagonally arranged external driving surfaces and an axially extending internally threaded bore. The nut is formed with frusto conical surfaces 10, 11 co-axial with the bore and having typically an included angle A of 100--1400, preferably 1 19--1200. An axially extending slit 12 extends along the complete length of the nut and completely through the periphery of the nut.

Typically, for a + inch (20mm) nut, the width of the slit B is preferably at least 1.5 mm in the unstressed state of the nut. For smaller nuts, the minimum width of the slit may be proportionally decreased subject to the capability of inserting a spreading tool, if required. For larger nuts, the width of the slit may be larger. Diametrically opposed to the slit 12 is a groove 13 extending the complete length of the nut inwardly from the internally threaded bore. The groove 1 3 has the effect of reducing the cross-section C of the periphery of the nut, thereby reducing the strength of the nut wall at this location. Typically for a i inch (20 mm) nut, the width of the web C is at least 1 mm and should preferably be 3 mm. For smaller and larger nuts, the width of the web C can be varied proportionally.Accordingly, the nut flexes more easily about the remaining portion of the nut. Figure 2 shows a nut similar to that of Figure 1 but having only one ramped or frusto conical surface at one end of the bore of the nut.

Figure 3 illustrates a nut essentially similar to those of Figures 1 and 2 except that the groove 13 has not been provided. It will be appreciated that one of the conical surfaces is not essential.

Figure 4 illustrates alternative embodiments of the present invention which is essentially the same as Figures 1 and 2 except that an additional groove 14 is provided extending axially along the outer surface of the nut which is parallel to the inner groove 13. Again, one of the conical surfaces 10, 11 can be omitted.

Figure 5 shows a further preferred embodiment wherein a pair of slits 15, 1 6 are provided on opposite sides of the internal threaded bore extending through the complete peripheral wall of the nut. The size of the slits 15, 1 6 are generally as indicated above with reference to Figure 1, however, these slits do not extend along the complete length of the nut terminating a short distance from one axial end of the nut. Preferably, the slit has a length of between 50% and 90% of the axial length of the nut. Figure 5 includes a pair of ramped surfaces 10, 11 at either end of the bore, however, as with other embodiments, the ramp surface 11 at the end opposite open end of the slits 1 5, 16 may be omitted.

As described in the foregoing, the accompanying drawings illustrate a number of possible configurations of the nut in accordance with the present invention. All proposed configurations are in one piece similar to a conventional nut and are capable of being produced by usual high speed methods with a further relatively conventional slitting and grooving operations. The slit nut, with either an angular or conical bearing face may be seated and tightened against the seating face of a washer with a matching profile. The reaction forces generated at this conical or angular face force the two halves of the slit nut inwards and so prevent it opening or "splitting apart when subject to tensile force through the bolt.The behaviour in tightening in respect to the applied torque/induced tension relation is in ever way similar to a standard nut with the same finish and conditions of lubrication.

If the bolt threads are corroded or scaled to prevent easy removal, the slit nut is first rotated of the order of - to 2 a turn to reduce the inward force generated from the annular or conical bearing face. If necessary, the nut may also be spread by hammering a cold chisel or similar tool into the slit. This operation "opens" the nut sufficiently to enable it to be rotated over the corroded bolt thread. This "opening" of the slit also alters the "across flats" dimension of the nut so that an adjustable wrench or a pipe wrench of the Stillson or "footprint" type must be used rather than a conventional open end spanner or ringwrench.

Comparative tests have been carried out comparing ease of removal of nuts according to the present invention with similar performance of conventional nuts. Nuts according to Figure 1 designated X, according to Figure 3 designated Y and according to Figure 5 designated Z were compared with conventional nuts of equivalent dimensions in the follpwing manner. Assemblies were prepared consisting of a 't BSW Australian Standard B 100 bolt, two flat mild steel collars with a tubular steel spacer between, and the slit nuts X, Y and Z with conical washer or a standard conventional nut, were tightened to produce an induced bolt tension of 12000 1 bf.Rusting was then performed in two batches as follows: A first batch was held for a total of 1 5 days in a standard salt spray cabinet with an intermediate delay of 24 days, held in the laboratory, and an additional 2 days drying at the completion of the salt spray period.

A second batch was held for a total of 22 days in the salt spray cabinet with the same 24 days intermediate delay and 2 days drying prior to testing.

The ease of removal of the various slit and conventional nut types was assessed by disassembly of the bolted fixtures while they were held in a bench vice. In all cases a torque in excess of 250 Nm(185 Ibf.ft.-the maximum torque able to be measured) was required to turn the nuts through an initial + turn, determined to the minimum nut rotation to enable release of the reaction forces applied to the nut by the conical washer.

The torque required to remove the nut, having slackened it off by - turn, was then assesed and this was compared with the effort required to turn the nut using a 15" crescent head, Shifting spanner.

FIRST BATCH Trial 1: Australian Standard 8100 nut: it was possible to rotate the nut 1-2 turns using the spanner before seizure; then up to 250 Nm (185 Ibf. ft.) torque was required to remove the nuts any further but this did not result in complete removal of the nut.

Trial 2: Australian Standard B100 Nut: the nut was loose for 1-2 turns but then it seized; requiring up to 45 Nm (33 Ibf. ft.) to remove it completely which equated with a moderately high effort using the shifting spanner.

Trial 3: Type X slit nut: the nut was loose for 211 turn before partial seizing; then required up to 1 5 Nm to remove it completely, that is easy removal using the shifting spanner.

Tiral 4: Type Yslit nut: the nut was loose for 3 turn, then up to 30 nm was required to remove it completely, that is moderately easy using the shifting spanner.

Trial 5: Type Z slit nut: the nut was loose for1 turn, then up to 20 Nm was required to remove it completely, that is easy removal using the shifting spanner.

SECOND BATCH Trial 1: Type X slit nut: A quarter of a turn was required to loosen the nut, then the nut required at least 45 Nm to rotate it any further. The testing was stopped and an engineer's cold chisel was driven into the slit using a medium blow of a 1 T Ib hammer, the tip of the chisel held along the length of the slit with the nut resting on a steel bench.

This resulted in the nut being spread open quite widely and it could then be easily removed using the 15" crescent head shifting spanner.

Trial 2: Type Yslit nut: A one sixth of a turn was required to loosen the nut, then the test was stopped at 45 Nm torque. A medium blow with a 12 Ib hammer and cold chisel, positioned in the slit, failed to spread the nut sufficiently for easy removal. A second heavier blow spread the nut so that a 30 Nm torque was required to remove the nut.

Trial 3: Type Z slit nut: A one third turn was required to loosen the nut, then the test stopped at 45 Nm torque. A light blow with a hammer and chisel spread the nut wide open and permitted easy removal using the shifting spanner.

Five slit nuts as illustrated in Figure 2 were tested forfatigue strength and compareaiiS resuE~ obtained with conventional nuts of equivalent size.

The nuts were made from standard hot forged r" BSW nuts which were subsequently machined to form the cone and slit. The nut had a hardness of 76 HRB (equivalent to 30 tonf/in2 tensile strength). The washers were turned from bright rod with a hardness of 92 HRB (equivalent to 40 tonf/in2 tensile strength). The web C was 1.5 mm in width, the slit gap B was 3 mm and the include angle on the nut A was 1 190. The corresponding angle on the washer was 1140 whereby the compressive load acting on the nut cone was concentrated in a narrow annulas at the outer edge. Examination of the slit, prior and subsequent to testing, indicates that during tightening the slit closes up, the closure being in the order of 0.01 6 in. It was noticed that the slit had closed more at the cone face than at the free face (see Figure 8).

The nuts were assembled with a i"BSW 'R- Grade (45 tonf. min tensile strength) hexagon head bolts. These bolts were hot forged and thread rolled prior to heat treatment. The thread form was good and the under head radius was of generous proportions to minimize premature underhead fatigue failure. The nut/washer/bolt assemblies were pretensioned to about 3.5 tonf tensile load and were then mounted in an Amsler 10 tonf Vibrophore fatigue testing machine. The nut/washer/bolt assembly was further tensioned in the fatigue testing machine up to an operating mean load of 8 tonf. The testing machine was operated at a frequency of about 1 60 Hz with a fluctuating load component of + 1.68 tonf (5 tonf/in2).

During fatigue testing, failure was indicated by automatic machine cut off due to shedding of load. At this point a crack was visible in the bolt at the first thread root inside the nut. To cause complete fracture, it was necessary to run the machine for a further 5,000--20,000 cycles. No 'sign of cracking appeared on the nuts. The results of the fatigue tests are shown on Figure 9 superimposed on the results of previous tests on 50 tonf/in2 minimum tensile strength aircraft bolts fitted with standard nuts. It will be observed the slit nuts in accordance with the present invention have a fatigue life within the range of existing standard nut fatigue lives.

Claims (10)

1. A one piece nut having external driving surfaces, an axial threaded bore passing through said nut, a ramped surface or surfaces arranged around at least one end of said bore, and b longitudinally extending slit extending from said one end of the bore at least partially along the axial length of said nut and completely through the periphery of said nut, said ramped surface or surfaces being adapted to engage a co-operating surface in a nut engaging component when the nut is threaded on an externally threaded member whereby opposing surfaces of said slit are urged towards one another.

2. A nut according to claim 1 wherein said slit extends along the complete axial length of said nut.

3. A nut according to claim 1 or claim 2 wherein a said ramped surface or surfaces is arranged around both axial ends of said bore.

4. A nut according to any one of claims 1 to 3 wherein said ramped surface or surfaces comprise a frusto conical surface co-axial with the axis of said bore.

5. A nut according to claim 4 wherein the included angle of said frusto conical surface is greater than the corresponding included angle of a frusto conical co-operating surface on said nut engaging component.

6. A nut according to claim 2 or any one of claims 3 to 5 when appended to claim 2, wherein the peripheral width of said nut diametrically opposite to said slit is reduced.

7. A nut according to claim 6 wherein a longitudinally extending groove is provided along said bore to achieve said reduction in peripheral width.

8. A nut according to claim 6 or claim 7 wherein a longitudinally extending groove is provided along the outer surface of said nut to achieve said reduction in peripheral width.

9. A nut substantially as described herein with reference to the accompanying drawings.

10. A nut and a nut engaging component substantially as described herein with reference to the accompanying drawings.