Allowable torsional deflection in shafts 1

[CudjoeCodger]

Allowable torsional deflection in shafting. I am looking for a less conservative specification for allowable torsional deflection in shafting.
I am aware of the two references in Machinery Handbook. One allows .08 degrees per foot of length. The other allows 1 degree in a length = to 20 shaft diameters.
In this particular case the shaft speed is 1 rpm, the torque is 37,000#" and the service is seldom and short. That is perhaps 1/2 hour per day. Of course I want a safe design but shaft size drives up the cost of bearings, couplings, shafting and labor, and there are multiple units.
I do not know if the slow speed or very light service allows me to use a higher angle of twist in this application.
I am not looking for calculations or formulas, but rather some reference, authority or spec that I can offer as a reference in our submital. Thx CudjoeCodger

 

[MikeHalloran]

Corvairs transmitted the torque from a 180HP engine to the transmission through a shaft no fatter than your little finger.

Early Pontiac Tempests transmitted about the same power from front engine to rear transaxle through a shaft that was a little fatter, a lot longer, and curved to boot.

You can allow quite a bit more twist than the rules of thumb for shafting, but you have to calculate the strain, and select a material that can take it.



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[ccw]

Hi Cudjoecodger,
If windup is not an issue, then design for stress, rather than strain. Use an old power shafting code criteria such as ASME B17c-1927 and see what you come up with. I recall this limits maximum SHEAR stress for mild steel material to 8000 psi SHEAR, and has some knock downs for stress concentrations, rotating, and stationary shafts.

One caveat, be sure that you do not introduce undesirable low frequency stick-slip into your system with a more flexible shaft.

This code is documented in older Spotts and/or Shigley Machine Design texts.