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| Shaft diameter for a given torque ? |
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| picclock:
Hi Can anyone advise me, or point me to a solution for the minimum shaft diameter I would need to transmit a torque of 9 Nm or around 80 lb.inch (6-7 ft.lb). I presume the unsupported length comes into it somewhere. which in this case is around 6". This is a kind of intermediate shaft and is supported by bearings at either end. I was planning to use unhardened silver steel. I have found some internet info (http://www.engineeringtoolbox.com/torsion-shafts-d_947.html) which goes through lots of calculations and comes out with an angle of deflection (twist) figure, but what is a 'safe' angle of deflection? Any help much appreciated Best Regards Picclock |
| RotarySMP:
That formula will give you the shear stress. The material data sheet will give you the allowable stress. The usage will determine the factor safety you need between the two. |
| picclock:
@ RotarySMP I came across another site that said that 1 thou of movement over 20x the diameter was a good way to go. So I tried applying it to one of the formula in the example section and it came up with nonsense. I then tried working through the example and it doesn't give the answer they say. From the site : The angular deflection of the shaft can be calculated as θ = L · T / (Ip ·G) = L · T / ( (π · D4 / 32) · G) = 1 (m) · 1000 (Nm) / (π · (0.05 (m))^4 / 32) · 79 (GPa) = 0.021 (radians) You can see that the second line from the bottom has a numerator of 1000 and a denominator of ~4.8x10^-5 giving an answer of over 2x10^7 so how you derive 0.021 radians from this. ( I also wrote to the site editor). I have edited the formula to show that the 0.05 value is to the power of 4 as this did not copy correctly. Hence the reason for trying to seek further advice from the forum. Best Regards picclock |
| David Jupp:
You have your calculation of denominator wrong. Maybe you missed out the 10^9 in GPa ? |
| picclock:
@David Jupp Hi David - The formula is the one posted on the engineeringtoolbox website - no mention of an extra 10^9. If the error is simply that (and in truth the figures are the same) I have it cracked and can sort it myself. Thanks for having a look at it for me. Best Regards picclock |
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