Screw threads

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: This invention relates to screw threads. A particular application of the invention is to screw threads for struts used with trench-box panels to shore up the walls of excavations. The criteria for a trench-box strut are as follows:- Firstly, the strut is required to apply force to a trench-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 trench-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 screw-type 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 trench-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 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, see for example DE-B-1027014. However, any machining of a screw thread is a relatively costly operation. One particular size of standard "Acme" thread has a pitch of 12 mm, giving a feed of 12 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 trench-box strut embodying the invention. The illustrated screw-threads 10, 12 are designed and made in accordance with the above-mentioned criteria for a trench-box strut. Referring to the drawing, there is shown a (single-start) male screw-thread 10 of steel, screwed into a female single-start screw-thread 12 of cast nodular graphite iron. The yield stress of the steel is 345 N/mm², whilst the 0.2% proof stress of the nodular graphite iron is 230 N/mm². The steel male screw-thread 10 has machined-flat crown 14, flanks 16, 16 and root 18, somewhat like the well-known "Acme" thread, whilst the cast-iron female thread 12 has a machined-flat crown 20 and un-machined, 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 screw thread 10, that is, the root diameter, having been machined or "turned", is 75.82 to 75.97 mm, whilst its major diameter or crown diameter, also after machining, is 83.82 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 76.00 to 77.00 mm; the major, or root, diameter of the female thread 12 as cast is 85.5 to 86.5 mm; this is left un-machined. The flanks 16, 22 are angled at 14.5° 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 p m of 20 mm. The female screw-thread 12 has a root thickness t rf of 13 mm, a crown thickness t cf of 10.4 mm and (obviously) a pitch of 20 mm. The male screw-thread 10 is 80 mm long. The female screw-thread 12 is longer. 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 un-machined, they are adequate for the purpose required, that is, for use in a trench-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 retain 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.

Claims: A machined male screw-thread (10) screwed into a female screw-thread (12) of cast iron, the female screw-thread (12) being machined on the crown (20) of the thread and having a substantially larger root thickness (t rf ) than the male screw-thread root thickness (t rm ), the flanks (22) and root (24) of the female screw-thread (12) being left un-machined, there being a substantial clearance between the crown (14) of the male screw-thread (10) and the root (24) of the female screw-thread (12). The combination as claimed in claim 1, wherein the female screw-thread (12) is of cast nodular graphite iron. The combination as claimed in claim 1 or 2, wherein the crown (14, 20), flanks (16, 22) and root (18, 24) of each thread (10, 12) are flat in longitudinal axial section. The combination as claimed in claim 1, 2 or 3, wherein the ratio of the female screw-thread root thickness (t rf ) to the male screw-thread root thickness (t rm ) is about 13:7. The combination as claimed in any preceding claim, wherein the female screw-thread (12) has a major diameter in the range of 76 mm to 130 mm. The combination as claimed in any preceding claim, wherein the screw-threads (10,12) are single-start and provide 20 mm feed per revolution. The combination as claimed in any preceding claim, wherein the helix angle is about 5 degrees.

Inventors: COLLINS JOHN, COLLINS, JOHN
Applicants: MABEY HIRE LTD, MABEY HIRE LIMITED
Application year: 1990
Main classification: F16B33
IPC codes: F16B33, E02D17