Hi Ross,

You're right. I just stuck the x4 in somewhere convenient - I only wanted the tool deflection value out at the end, which I guess is what most people might want for design work. If you want the torsion constant out as well then this x4 should go somewhere else in the calculation chain. One option would be to divide the applied torque by 4.

The x4 is an assumption based on reasoning (it's probably in Roark's somewhere but it is quite a read), but I ran it past a colleague and he came to the same conclusion. A bar with a twisting load applied at one end and the other fixed, will be twice as stiff if the load is then applied to the 'centre' of the span (x2). If the free end is then also restrained, with the twisting load still at the centre, then this previously free part of the bar will now also be able to resist the twist, doubling the stiffness again (x2). This is a total of x4. Technically the spreadsheet should be modified to be more correct (in the workings) although the end results are OK. Do you agree with this logic?

If you want to post a modified sheet, or use the equations in your own sheet then go for it. It would be great if there was an special design/calculation area on the forum where people who were asking "what sort of section is best for . . " or "how much deflection might I have . . ." could download a simple spreadsheet and get a rough idea. I think Irving2008's motor calculation was a great example of this.

Barry