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Square threads: The sides of the flanks of square threads are normal to the axis and hence parallel to each other. The pitch of the threads is often taken as twice that of B.S.W. threads of the same diameter. These are used for power transmission.


V threads: British Standard Whitworth (B.S.W.) thread: It is a symmetrical ‘V’–thread in which the angle between flanks is 55°. These threads are generally used on bolts, nuts and studs et

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For the sake of knowledge. I have used all these threads and many more in many engineering concepts. It is not so easy to qualify the differences all the time, especially if you are talking to people who think they know threads and believe me there are plenty who think they know the Wiki about threads until challenged. It sounds like a minor subject, but can be seriously challenging. A V thread as you call it are mostly used in applications which do not require a lot of linear power. Their application is varied from hydraulics to motor industry and most of these are not machined in industry today. They stamp them. It is probably only the hydraulic fitting industry, which use a lot of internal threads, who machine proper threads in multi spindle Wickmann's. In motor vehicles these threads are used in Engine blocks and heads, where they are used in torquing actions, to stretch the bolt into position. These bolts are then distorted and not re-usable. In applications across the board it matters which type of material you use and for which application. The steel used are most often EN3A or B. In the stretching procedure they will use a steel which has that capacity and I would think it to be EN 8 which is what they produce coal picks from or EN 19 which is a highly stretchable but no wear steel. The EN 23 has more Chrome/Nickel and cannot stretch like EN8. EN 36B is what we use for gears, lots of Nickel for resistance to wear, in all applications, which qualifies it as high wear resistance and very little stretch. All these metals has a further yield when they are heat treated, case hardened or annealed and quenched. I do always refer you to the Machinery handbook about threads. There you will read about things like backlash, relief and helix angle. Most V treads are the same configuration and difference mainly in threads per inch. There is very little diffs between UNC, UNF, BSP, Metric, Etc., as far as that is concerned. This is the most common thread in all applications.

Square threads are not easy to produce and are hardly ever used as a single thread. They are used in items like mechanical vice and most often have double or triple threads. You can apply lots of linear force in their application and they will not slip or break easily. To assist in this arrangement the vice will have a Bronze sister screw which help to reduce friction and are the fuse in case you apply to much pressure. There is also the dynamic of range accomplished. The square thread is usually much coarser than a V-Tread and can screw quicker in meters per minute because of the double or triple application. It is not easy to line up and screw this thread in seconds, so it will not do for a screw application where you need speed in the assembly. You will never spend the sort of money it costs to produce a square thread, on a hydraulic hose fitting. And the assembly time would be prohibitive. Size is another factor. Square thread go as small as perhaps one inch/25.4mm diameter where V-Threads can go to perhaps two millimeters. Apart from a table vice there is not much use for this thread.

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In square thread, the torque is converted into axial force inline with the rod axis. Hence whole force is utilized. 

While in V thread the torque is converted into axial and radial force, just the axial component is useful.

Due to that, square thread is better in power transmition, but it is more difficult to manufacture because it needs a flat cutting tool. V thread is easier to manufacture but generally used for fastening and tighting rather than power transmition.

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