GB2229785A

GB2229785A - Asymmetric screw threaded devices

Info

Publication number: GB2229785A
Application number: GB8907318A
Authority: GB
Inventors: Ryotaro Matsushima
Original assignee: MIKURIYA YK
Current assignee: MIKURIYA YK
Priority date: 1989-03-31
Filing date: 1989-03-31
Publication date: 1990-10-03
Also published as: GB8907318D0

Abstract

To prevent loosening of screws due to bending of screw threads 15, 17 at the time of tightening so producing only line contact between the threads, the screw threads 15, 17 having the same nominal diameter but having mutually different bending strengths of their screw threads 15, 17 because of different dimensions or materials of the screw threads 15, 17, the width of the gap between the flanks of the screw threads 15, 17 is made to increase progressively towards the member of lower bending strength, by cutting away part of a thread flank (e.g. 15b). As shown, the nut 16 and bolt 14 are of the same material, and the bending strength of the bolt thread 15 is less than that of the nut thread 17 because the base of the bolt thread 15 has a smaller axial extent. Instead of the cutting away of the bolt thread 15 at 15b, the nut thread flank 17a may be cut away. <IMAGE>

Description

ASYMMETRIC SCREW DEVICE This invention relates to a screw device comprising male and female screw threaded members.

Various screw devices for tightening purposes are known, and the combination of a bolt (B) and a nut (N) such as shown in Fig. 7 of the accompanying drawings has been widely employed. In combinations of screw members having mutually the same nominal diameter, the flanks 3a, 3b of a screw thread 3 of a male or externally threaded screw member 1 and the flanks 4a, 4b of a screw thread 4 of a female or internally threaded screw member 2 are formed symmetrically so that their opposed surfaces are parallel with each other. The nut (N) is driven towards the head 5 of the bolt and is fastened at the position where it comes into contact with a member 6 to be gripped, as shown in Fig. 8.

As the flank 3a of the male screw thread 3 facing in the opposite direction (L1) to the tightening direction (T1) and the flank 4a of the female screw thread 4 facing in the opposite direction (L2) to the tightening direction (T2) comes into close contact with each other, mutual stress develops and keeps the assembly tight.

In such conventional screw devices, the thread flanks facing in the opposite direction to the tightening direction are in close contact when tightening is effected, as described above. In ordinary screw devices, the bottom land area per pitch is different between the screw threads of the male and female screw members, and the length of or the bottom side of the screw thread 4 of the female screw member 2 is greater than the length of 1 or the bottom side of the screw thread 3 of the male screw member 1, as shown in Fig. 9. Therefore, the bottom area of the screw thread 4 is greater than that of the screw thread 3 and the total volume of the former per pitch becomes greater, so that the bending strength of the screw thread of the female screw member 2 becomes greater than that of the screw thread of the male screw member 1.

For this reason, the screw thread 3 of the male screw member 1, whose bottom area is smaller and whose screw thread has the lower bending strength, is gradually bent in the tightening direction (T1) of the male screw member 1 by the tightening force, as shown in Fig. 10. For this reason, the flank 3a near the bottom 7 of the male screw member 1 and the flank 4a near the top 4c of the female screw member 2 come into a line contact state, so that the contact area becomes very small and the screw device is likely to work loose.

On the other hand, if the materials of the male and female screw members are different and the bending strength of the female screw member 2 is lower than that of the male screw member 1, the screw thread 4 of the female screw member 2 is bent in the tightening direction of the female screw member 2, as shown in Fig. 11. Accordingly, the flank 3a of the male screw member 1 near the top 3c and the flank 4a of the female screw member 2 near the bottom 8 come into a line contact state, whereby the contact area becomes extremely small and loosening is likely to occur.

The phenomenon described above is notable in ordinary screw devices because their male and female screw members 1, 2 are made of the same material and the difference of bending strengths occurs due to the difference in the bottom areas of the screw threads. Hence occurs the technical problem of preventing loosening of the screw members due to bending of one or other of the screw threads at the time of fastening.

According to the present invention there is provided an asymmetric screw device comprising a pair of male and female screw threaded members having the same nominal diameter but having mutually different bending strengths at the screw threads thereof, wherein the width of the gap between the flanks of said screw threads of said male and female members is formed so as to increase progressively towards the member of lower bending strength.

In one form of the invention the flank of the male screw thread facing in the oposite direction to the tightening direction is partly cut away so as to increase the width of the bottom land of said male screw thread in the tightening direction.

Alternatively or in addition the flank of the female screw thread facing in the opposite direction to the tightening direction may be partly cut away so as to reduce the width of the top of said female screw thread in the tightening direction.

In another form of the invention the flank of the female screw thread facing in the opposite direction to the tightening direction is partly cut away so as to increase the width of the bottom land of said female screw thread in the tightening direction.

Alternatively or in addition the flank of said male screw thread facing in the opposite direction to the tightening direction may be partly cut away so as to reduce the width of the top of said male screw thread in the tightening direction.

Some embodiments of the invention will now be described by way of example and with reference to the accompanying drawings in which: Figs. 1 to 6 show some embodiments of the present invention, wherein Fig. 1 is a longitudinal section side view of a bolt and a nut and Figs 2(A), (B) to 6 are longitudinal sectional side views showing the principal portions of screw threads of the different embodiments; and Figs. 7 to 11 show a conventional example, wherein Figs. 7 and 8, and 10 and 11 are longitudinal sectional side views showing the principal portions of screw threads; and Fig. 9 is a longitudinal sectional front view of the screw thread.

Referring to the drawings, Fig. 1 shows a nut 11 fitted to a bolt 10 with the head 12 of the bolt shown in contact with the surface of a member 13 to be gripped. The bending strengths of the screw thread 15 of the male screw member 14 constituted by the bolt 10 and the screw thread 17 of the female screw member 16 constituted by the nut 11 are determined by the bottom land areas of the screw threads if the bolt 10 and the nut 11 are made of the same material, and thus the bending strength of the female screw thread 17 is greater.

Figs. 2(A) and (B) show a case where the male screw member 14 and the female screw member 16 are made of the same material and the bending strength of the screw thread of the male screw member 14 is lower than that of the female screw member 16. As shown in Fig. 2(A), a flank 15b is formed by cutting away part of the flank 15a of the male screw thread 15 facing in the opposite direction L1 to the tightening direction whereby the bottom land 18 of the male screw thread 15 is widened in the tightening direction T1. In other words, while the flanks 17c, 15c of the female and male screw threads 17,15 facing in the tightening directions T2, T1 are formed in such a manner that the gap between them is parallel, the flanks 17a, 15b facing in the opposite directions L2, L1 to the tightening directions are formed in such a manner that the gap therebetween increases progressively towards the male screw member 14 which has a lower bending strength in its screw thread.

In the alternative shown in Fig. 2(B), part of the flank 17a of the female screw thread 17 facing in the opposite direction L2 to the tightening direction is cut away to form the flank 17b whereby to reduce the width of the top 19 of the female screw thread 17 in the tightening direction T2.

As a further alternative, both a portion of the flank 15a near the bottom land 18 and a portion of the flank 17a near the top 19 may be cut away respectively, so as to increase progressively the gap between the flanks towards the male screw member 14.

When the screw device formed in the abovementioned manner is tightened, the portion of male screw member 14 having the lower bending strength, near the top 20 of the male screw thread 15, is at first almost in line contact with the flank 17a or 17b of the screw thread 17 of the female screw member 16, as shown in Fig. 3. Then, however, the screw thread 15 of the male screw member 14 is gradually bent in the tightening direction T1 by the tightening force and tightening is completed when the flank 15a or 15b of the male screw thread 15 is in close contact with the flank 17a or 17b of the female screw thread 17.

On the other hand, Figs. 5(A) and (B) show a case where the bending strength of the screw thread of the male screw member 14 is greater than that of the female screw member 16, due for example to differences of the materials of the male and female screw members. As shown in Fig. 5(A), a flank 17d is formed by cutting away part of the flank 17a of the female screw thread 17 facing in the opposite direction L2 to the tightening direction, whereby the bottom land 21 of the female screw thread 17 is widened in the tightening direction T2.In other words, while the flanks 17c, 15c of both female and male screw threads 17, 15 facing in the tightening directions T2, T1 are formed in such a manner as to have the gap between them parallel, the flanks 17d, 15a facing in the opposite directions L2, L1 to the tightening directions are formed in such a manner that the gap therebetween progressively increases in width towards the female screw member 16 whose screw thread has the lower bending strength. As shown in Fig. 5(B), it is also possible to partly cut away the flank 15a of the male screw thread 15 facing in the opposite direction L1 to the tightening direction so as to reduce the width of the top 20 of the male screw thread 15 towards the tightening direction T1.

Alternatively, both a portion of the flank 17a near the bottom land 21 and a portion of the flank 15a near the top 20 may be partly cut away, so as to progressively increase the width of the gap between the flanks, towards the female screw member 16.

When the screw device formed in the abovementioned manner is tightened, a condition is reached where the portion of the female screw member 16, whose screw thread 17 has the lower bending strength, near to the top 19 of the screw thread, is approximately in line contact with the flank 15a or 15d of the screw thread 15 of the male screw member 14. The screw thread 17 of the female screw member 16 is then gradually bent in the tightening direction T2 by the tightening force, and as described already with reference to Fig. 4 the flank 15a or 15d of the male screw thread 15 comes into close contact with the flank 17a or 17d of the female screw thread 17 when tightening is completed.

As described with reference to Figs. 2(A), (B) or 5(A), (B), if the gap between the flanks of the screw threads of the male and female screw members is formed in such a manner as to increase in width towards the member having the lower bending strength, the flanks come into close contact as a result of bending of the screw threads at the time of tightening, and the frictional area thus becomes greater so that tendency for loosening of the screw thread members can be reduced.

Incidentally, the size of the gap between the flanks is determined in accordance with the material of the male and female screw members, the difference of their bending strengths due to the shapes of the screw threads, or with the tightening torque. Furthermore, the configuration of the flanks of the screw threads is not necessarily limited to a straight line in that flanks having a curved, or stepped, or other shape may exhibit the same effect.

As described in detail with reference to the embodiments given above, in the combined screw members having mutually different bending strengths of their screw threads, the gap between the flanks of the screw threads facing in the opposite direction to the tightening direction is set so as to increase progressively towards the member of lower bending strength. Accordingly, when the screw thread is bent in the tightening direction by the tightening force, the flanks come into close contact so as to increase the frictional area. Accordingly the tendency for loosening of the threaded members can be reduced.

It is to be clearly understood that there are no particular features of the foregoing specification, or of any claims appended hereto, which are at present regarded as being essential to the performance of the present invention, and that any one or more of such features or combinations thereof may therefore be included in, added to, omitted from or deleted from any such claims if and when amended during the prosecution of this application or in the filing or prosecution of any division application based thereon. Furthermore the manner in which any of such features of the specification or claims are described or defined may be amended, broadened or otherwise modified in any manner which falls within the knowledge of a person skilled in the relevant art, for example so as to encompass, either implicitly or explicitly, equivalents or generalisations thereof.

Claims

CLAIMS:

1. An asymmetric screw device comprising a pair of male and female screw threaded members having the same nominal diameter but having mutually different bending strengths at the screw threads thereof, wherein the width of the gap between the flanks of said screw threads of said male and female members is formed so as to increase progressively towards the member of lower bending strength.

2. An asymmetric screw device according to claim 1, wherein the flank of the male screw thread facing in the oposite direction to the tightening direction is partly cut away so as to increase the width of the bottom land of said male screw thread in the tightening direction.

3. An asymmetric screw device according to claim 1 or 2, wherein,the flank of the female screw thread facing in the opposite direction to the tightening direction is partly cut away so as to reduce the width of the top of said female screw thread in the tightening direction.

4. An asymmetric screw device according to claim 1, wherein the flank of the female screw thread facing in the opposite direction to the tightening direction is partly cut away so as to increase the width of the bottom land of said female screw thread in the tightening direction.

5. An asymmetric screw device according to claim 1 or 4, wherein the flank of said male screw thread facing in the opposite direction to the tightening direction is partly cut away so as to reduce the width of the top of said male screw thread in the tightening direction.

6. Asymmetric screw devices substantially as hereinbefore described with reference to Figures 1 to 6 of the accompanying drawings.