GB2074280A

GB2074280A - Screw Thread Fasteners

Info

Publication number: GB2074280A
Application number: GB8109580A
Authority: GB
Original assignee: Sparling L G M
Current assignee: Sparling L G M
Priority date: 1980-04-02
Filing date: 1981-03-26
Publication date: 1981-10-28

Abstract

In order to spread the load evenly between the threads at the loaded end of a threaded member such as a nut (Fig. 2) or a pipe coupling (Fig. 3), the threads at that end (portion X) taper outwards as at 14 or 16 while remaining of full-form. For the rest of the length (portion Y) the threads are parallel as at 15). A tap (Figure 4 not shown) in which the threads are parallel for most of the length but taper outwards at the trailing and (portion A) is used to form the nut of Fig. 2. <IMAGE>

Description

SPECIFICATION Screw Thread Fasteners The invention relates to screw-threaded fasteners.

Normally in use male threaded members such as bolts, set-screws and studs are loaded in tension whilst the corresponding female threaded components such as nuts and other components formed with threaded holes are loaded in compression. For ease of reference in the following description the male and female threaded members will be referred to as simply as bolts and nuts respectively. The most severely loaded threads are the first threads on both nut and bolt counted from the loaded face of the nut and when failure occurs due to static loading it often occurs across the core of the bolt starting from the root of the thread, one thread inside the nut from the loaded face. Furthermore fatigue failure almost always occurs in this area.Both the theory of Sopwith and assessments based upon experiment by various investigators give the proportion of the total load on the various threads of a six thread nut as 1 st 0.34, 2nd 0.26, 3rd 0.15, 4th 0.11, 5th 0.09,6to 0.05. Similar loads occur on the corresponding threads of the bolt.

Various techniques have been suggested to reduce the load concentration on the first three threads of the nut and re-distribute it among the other threads. These include differential pitch, truncation of the crests of the first three threads of the nut and producing all the threads of the nut on an internal conical surface. All these techniques have disadvantages such as transfer of the greatest load from the first thread to the last thread and making the fastener assembly weaker if the nut is inadvertently screwed in the upside down position upon the bolt.

Due to the tension in the bolt and the compression in the nut a progressively increasing differential pitch between the bolt and the nut would be ideal and should be smallest in the bolt near the first engaged thread and largest in the corresponding position in the nut. This ideal differential pitch is difficult to arrange for mass production hence matching equal pitches in both the nut and bolt are conventional. This match when the bolt and nut are unstrained results in a mismatch of pitches when they are strained and consequently severe bending of the first engaged threads occurs, and fairly severe bending of the second engaged threads also occurs.

According to the invention there is provided a nut in which the threads are full form or substantially full form and in which a first zone adjacent to one end of the nut, intended to be the loaded end, the threads converge towards the other end of the nut and in a second zone adjacent to the other end of the nut the threads are parallel. Thus in the first zone the mean diameter of the threads decreases towards the second zone. Preferably the threads comprise a transitional or third zone between the first and second zones.

The taper in the first zone may be in the range 1 in 10 to 1 in 200 on diameter, and is preferably of the order of 1 in 20 to 1 in 25 on diameter.

In this manner extra clearance is provided to relieve the excess load on the first and second engaged threads. The ideal extra clearance should be proportional to the strain due to tension, which is in turn proportional to the cumulative load; plus a constant times the deflection of the thread due to bending and due to direct shear, which are both in turn proportional to the thread load. When combining all three reasons for increased clearance together with the effects of the recession of the nut and bolt (as described by Sopwith) an overall result indicates that considerable clearance is required for the first engaged thread less for the second and almost trivial amounts for subsequent threads.The ideal bell-mouthed shape upon which full form threads are superimposed will normally be modified to cater for mass production and for possible errors in screwing the nut upon the bolt in the "upside down" position.

The three zones are preferably as follows:- (1) A first zone in which the conical shape attains the maximum value of the taper i.e. in the range 1 in 10 to 1 in 200 on diameter and which does not exceed 40% of nut height (axial length); (2) A second zone exceeding 40% of the nut height and of the same form as existing manufactured nuts, that is to say of constant diameter, and (3) A transitional, or third, zone disposed between the first and second zones, 0 to 40% of nut height and in which the taper progressively decreases from that applying in the first zone to the parallel.

The total height (axial length) of the first and transitional zones may be in the range 20 to 60 of the nut height and is preferably 32.5% of the nut height.

Standard nuts have been modified in accordance with the invention by cutting into the hole from the face intended to be loaded with a tap having full form threads produced upon a conical surface or taper of 1 in 20 on the diameter. Tests with such nuts assembled on standard bolts gave considerable increases in life before fatigue failure, and also permitted increased static loads. A further increase in static load and fatigue life was found to. occur when the nuts were rammed with unfluted taps so that residual compressive stresses were produced in the nuts. The increase in fatigue life was statistically significant and was 120%, with a mean tension equal to 40% of the static failure load together with a superimposed cyclic load equal to 20% of the static failure load.For lighter fatigue duties a 200% increase in life was obtained whilst under static loading a 10% increase in load occurred.

Thus for example a six thread nut consists of parallel full-form threads extending from the free (or non loaded) end until about 2 or 3 threads from the loaded end, (22 threads was found to be the optimum for ISO - My 2 Fine Series). From here full form threads are cut on an opening taper (1 in 20 was found to be a suitable value) and the threads may be with advantage rammed by an unfluted tap if desired.

Such nuts can be produced in one operation by special taps. These taps must have a normal front-end up to the threads of full form at mid length followed by full threads produced upon a taper (say 1 in 20) of increasing diameter. The flutes must start to run out as the taper begins and after about 2w threads on the taper the flutes must be completely eliminated. At least 3 threads should be produced on the tap beyond the runout of the flutes. Tapers of progressively increasing magnitude may be advantageous as an alternative to the above taper.

It is preferred that the nut be marked in some way to distinguish the end intended to be loaded from the non-loaded end to prevent accidental incorrect assembly. Marking may for example consist of providing the nut with an integral washer at its load bearing end or by directly marking the nut with a distinctive chamfer or even an arrow directed towards the load bearing end.

Although in the above description the modified thread form has been described in relation to a nut it will be appreciated that the invention could be applied to male threaded members e.g. a hollow bolt in compression with a nut in tension e.g. a pressure line or pipe connection. Thus the taper thread is still formed on the compressive member, in this case the bolt.

It will also be appreciated that the threaded member described above need not necessarily be made by tapping and could conceivably be made by other methods for example-by rolling or by siritering.

The invention is diagrammatically illustrated by way of example in the accompanying drawings in which Fig. 1 is cross-section through a nut made in accordance with the invention; Fig. 2 is a scrap cross-section on an enlarged scale of the nut of Fig. 1; Fig. 3 is a side elevation, partly in section, of a second embodiment of the invention as applied to a pressure pipe connection; Fig. 4 is a side view of a tap for use in the manufacture of a nut as shown in Figs. 1 and 2; and Fig. 5 is a scrap view on an enlarged scale of part of the tap of Fig. 4.

Referring to Figs. 1 and 2 of the drawings there is shown a nut I having a screw-threaded bore 2 and an axial end face 3 which is intended to be loaded in use of the nut. As can be seen more clearly in Fig. 2 of the drawings the threaded bore 2 includes a first zone 4 delimited by the dimension X in which the threads taper outwardly as indicated by the dotted line 14 and a second zone 5 delimited by the dimension Y in which the threads are parallel as indicated by the dotted line 15.

In Fig. 3 of the drawings there is shown a pair of pipes 6 and 7 formed with internal axial extending cavities 9 and 10 respectively and connected together at their adjacent ends by means of male and female screw threaded connections respectively. For this purpose the end of the pipe 7 is enlarged at 8 and is formed with an internal screw thread whereas the external surface of the pipe 6 is formed with an enlarged annular end section 1 6 which is formed with an external thread. Since in this embodiment the externally threaded member 16 acts in compression while the internally threaded member 8 acts in tension, it is the member 16 which is formed in accordance with the invention so that its screw thread includes a first zone in which the threads taper outwardly towards the compressed end of the member and a second zone in which the threads are parallel.

In Figs. 4 and 5 of the drawings there is shown a tap 11 for producing a nut in accordance with Figs. 1 and 2 of the drawings which tap 11 consists of a generally conventional front end 17 formed with flutes 12 which extend into, but not through, a threaded zone 13 at the rear end of the threaded portion of the tap. The zone 13 which is delimited by the dimension A comprises six threads which taper outwardly in the direction away from the front end 1 7 of the tap as indicated by the dotted line 18 in Fig. 5 as compared with the dotted line 1 9 which extends parallel to the threads on the remaining portion of the tap.

In the description and claims the word nut is intended to include, in addition to conventionally shaped hexagonal female threaded members, other female threaded members such as machine parts formed with female threaded holes.

Furthermore the word nut is intended to include male threaded components such as bolts where in use they are intended to be loaded in compression.

Claims

1. A nut in which the threads are full form or substantially full form and in which in a first zone adjacent to one end of the nut, intended to be the loaded end, the threads converge towards the other end of the nut and in a second zone adjacent to the other end of the nut the threads are parallel.

2. A nut according to claim 1 wherein the threads comprise a transitional or third, zone intermediate the first and second zones.

3. A nut according to claim 2, wherein the third zone does not exceed 40% of the axial length of the nut and in which the taper progressively decreases from that applying in the first zone to the parallel.

4. A nut according to any preceding claim, wherein the taper in the first zone is in the range 1 to 10 to 1 in 200 on diameter.

5. A nut according to claim 4, wherein the taper is of the order of 1 in 20 on 25 diameter.

6. A nut according to any preceding claim, wherein the first zone does not exceed 40% of the axial length of the nut.

7. A nut according to any preceding claim, wherein the second zone exceeds 40% of the axial length of the nut.

8. A nut according to any preceding claim, wherein the total axial length of the first and transitional zones is in the range 20 to 60% of the nut height.

9. A nut according to claim 8, wherein the total axial length of the first and transitional zones is around 32.5% of the nut height.

10. A nut substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.

11. A tap comprising a threaded frontend, threads of full form at mid length followed by section having full threads produced upon a taper of increasing diameter away from the front end, the tap having axial flutes extending from the front end into the section having full threads produced on a taper, the flutes extending into the section for an axial distance of about 26 threads and the section having at least 3 threads extending beyond the run-out of the flutes.

12. A tap substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.