GB2233061A

GB2233061A - Screw threads

Info

Publication number: GB2233061A
Application number: GB8914083A
Authority: GB
Inventors: John Collins
Original assignee: Mabey Hire Co Ltd
Current assignee: Mabey Hire Co Ltd
Priority date: 1989-06-20
Filing date: 1989-06-20
Publication date: 1991-01-02
Anticipated expiration: 2009-06-20
Also published as: DE69016276T2; EP0404318A2; GB8914083D0; DE69016276D1; US5052871A; EP0404318A3; ATE117775T1; GB2233061B; EP0404318B1

Abstract

A modified "Acme" type of male and female thread combination has a single-start male thread (10) of machined steel and a single-start female thread (12) of cast nodular graphite iron. The crown (20) of the female thread (12) is machined but the flanks (22, 22) and root (24) are left un-machined. There is plenty of clearance left between the crown (14) of the male thread (10) and the root (24) of the female thread (12). This reduces cost and yet the screw threads are adequate for use in a trench-box strut. The feed per revolution is 20 mm.

Description

(73 C -1 1 M&C FOLIO: 230P57516 SCREW THREADS WANGDOC: 0651f This

invention relates to screw threads. A particular application of the invention is to screw threads for struts used with strongbox panels to shore up the walls of excavations.

The criteria for a sLrong-box strut are as follows: Firstly, the strut is required to apply force to a strong-box panel so as to press the panel against a wall of an excavation in order to shore up the wall. The strut is also required to pull the panel away from the wall when it is desired to remove the strong-box from the excavation. The strut is not required to position the panel with any particular accuracy, nor does the adjustment of the strut require fine control of the strut. However, considerable robustness and strength is required of the strut. If the strut is a screwtype strut, which is usual, if not universal, the screw mechanism is required to be manually operable in both directions, that is, both when extending the strut to press the panel against the wall and when retracting the strut to release the strong-box, and is required to maintain its extension under external applied force when in use. Hence, the screw must be easy enough to turn manually in either direction. It is preferred for only 2 a few turns to be required to retract the strut or extend the strut as desired.

It is common practice to machine screw threads to achieve dimensional tolerances which are acceptable for the particular uses required of the screw threads. However, any machining of a screw thread is a relatively costly operation.

one particular size of standard "Acme" thread (size to be stated) has a pitch of 12 mm, giving a feed of 12 1 mm or 24 mm for a single-start or double-start thread accordingly. This is not an ideal pitch for every application.

The invention provides a combination of screw threads as claimed in each of the claims, to which reference is directed.

The invention will be described by way of example with reference to the drawing, which is a longitudinal axial section through interengaged screw threads of a strongbox strut embodying the invention.

The illustrated screw-threads 10, 12 are designed and made in accordance with the above-mentioned criteria for a strong-box strut.

3 Referring to the drawing, there is shown a (single-starL) male screwthread 10 of steel, screwed into a female single-start screw-Lhread 12 of cast nodular graphite iron. The yield stress of the steel is 2 345 NImm ' whilst the 0.2% proof stress of the nodular 2 graphite iron is 230 NImm The steel male screw-thread 10 has machined-flat crown 14, flanks 16, 16 and root 18, somewhat like the wellknown "Acme" thread, whilst the cast- iron female thread 12 has a machined-flat crown 20 and un-machined, 1 as cast, flanks 22, 22 and root 24.

The diametrical measurements of the two screw-threads 10, 12 are as follows:- the minor diameter of the steel male scew thread 10, that is, the root diameter, having been machined or "turned", is 75.93 to 75.97 mm, whilst its major diameter or crown diameter, also after machining, is 83.93 to 83.97 mm; the minor diameter - that is, the crown diameter of the female thread 12 is 74.5 to 75.5 mm as cast; after machining, this diameter becomes 76.000 to 76.046 mm; the major, or root, diameter of the female thread 12- 4 as cast is 84.5 to 85.5 mm; this is left un-machined.

There is thus a maximum "concentric clearance" between the crown 14 of the male screw-thread 10 and the root 24 of the female screw-thread 12 of 0.8 mm.

The flanks 16. 22 are angled at 14.50 in axial longitudinal section (as shown in the drawing).

The male screw-thread 10 has a root thickness t rm of 7.07 mm, a crown thickness t cm of 5 mm and a pitch PM of 20 mm.

The female screw-thread 12 has a root thickness t rf of 13 mm, a crown thickness L Cf Of 10.4 mm and (obviously) a pitch of 20 mm.

The helix angle is about 5 degrees, more exactly, 4.85 degrees, compared with the standard helix angle for an "Acme" thread of 2.996 degrees.

Although the nodular graphite iron is only about half as strong as the steel, the geometry of the screw-threads 10, 12 compensates for this. making the female screw-thread 12 substantially stronger than the male screw-thread 10.

Although the flanks 22, 22 and root 24 of the female thread 12 are unmachined, they are adequate for the purpose required, that is, for use in a strong-box strut having the above-mentioned criteria.

Being single-start and with a pitch of 20 mm, the screw-threads 10, 12 provide a feed per revolution of 20 mm. This means that only a few turns are required, either to extend or to retract the strut, and yet it will retatn a given extension reliably under external load.

The invention may be utilised with advantage for female threads (not shown) as above but having different major, or root, diameters in the range of 76 mm to 130 mm.

6

Claims

CLAIMS:-

1. A machined male screw-thread screwed into a female screw-thread of cast iron, the female screw-Lhread being machined on the crown of the thread and having a substantially larger root thickness than the male screw-thread, the flanks and root of the female scew-thread being left un-machined, there being a substantial clearance between the crown of the male screw-thread and the root of the female screwthread.

2. The combination as claimed in claim 1, wherein the female screw-thread is of cast nodular graphite iron.

3. The combination as claimed in claim 1 or 2, wherein the crown, flanks and root of each thread are flat in longitudinal axial section.

4. The combination as claimed in claim 1, 2 or 3, wherein the ratio of the female screwthread root thickness to the male screwthread root thickness is about 13:7.

5. The combination as claimed in any preceding claim, wherein the female screw-thread has a major diameter in the range of 76 mm to 130 mm.

7 6. The combination as claimed in any preceding claim, wherein the screwthreads are single-start and provide 20 mm, feed per revolution.

7. The combination as claimed in any preceding claim, wherein the helix angle is about 5 degrees.

Published 1990 atThe Patent Office, State House, 6671 High Holborn, LondonWC1R4TP. Further copies maybe obtained from The Patent Office Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent, Con. P87