Allowable length of hollow shaft under torsion
Allowable length of hollow shaft under torsion
(OP)
Hi everyone,
I am currently reviewing a simple hollow shaft "extension" design which is basically a pipe with a flange at each end under torsion.
Calculations for shear and angle of deflection are readily available, but I have been unable to find a reference for calculating the allowable length. Any references to allowable angular deflection are per unit length or based on an 'allowable deflection' limited by the equipment rather than potential failure of the shaft. The rotational speed of the device is slow, so amount of angular deflection is unlikely to be an issue in operating the device (within reason).
I have scoured the references available and the internet. I have reviewed sources re: thin shell cylinders under pure torsion, but the sources are either far too mathematical to digest and based entirely on theory, or aren't designed for slender use and yield poor results. What I am after is a relatively simple conservative approach - surely this has all been done before and there is a simple method.
Simplified example:
- DN80 SCH40 pipe (OD 88.9mm, ID 77.92mm, thickness 5.49mm)
- Torque of 3000 Nm
I can calculate the deflection, and allowable shear, but how do I know how long I can make the shaft before it fails by 'buckling'?
WillowEng
I am currently reviewing a simple hollow shaft "extension" design which is basically a pipe with a flange at each end under torsion.
Calculations for shear and angle of deflection are readily available, but I have been unable to find a reference for calculating the allowable length. Any references to allowable angular deflection are per unit length or based on an 'allowable deflection' limited by the equipment rather than potential failure of the shaft. The rotational speed of the device is slow, so amount of angular deflection is unlikely to be an issue in operating the device (within reason).
I have scoured the references available and the internet. I have reviewed sources re: thin shell cylinders under pure torsion, but the sources are either far too mathematical to digest and based entirely on theory, or aren't designed for slender use and yield poor results. What I am after is a relatively simple conservative approach - surely this has all been done before and there is a simple method.
Simplified example:
- DN80 SCH40 pipe (OD 88.9mm, ID 77.92mm, thickness 5.49mm)
- Torque of 3000 Nm
I can calculate the deflection, and allowable shear, but how do I know how long I can make the shaft before it fails by 'buckling'?
WillowEng





RE: Allowable length of hollow shaft under torsion
http://naca.central.cranfield.ac.uk/reports/1935/n...
and the Johnny-come-lately's get in on the act
http://shellbuckling.com/papers/classicNASAReports...
I hope they help, I haven't looked through them in detail, looks like the NASA one is a design guide.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Allowable length of hollow shaft under torsion
RE: Allowable length of hollow shaft under torsion
Tmoose, the shaft is able to move axially - I would expect it to fail by buckling along symmetrical curves or by deflecting laterally and failing due to the resulting eccentricity.
WillowEng
RE: Allowable length of hollow shaft under torsion
je suis charlie
RE: Allowable length of hollow shaft under torsion
The risc of buckling reduces drastically when adding stiffening plates (reduction of the shear tension under the critical shear value)
Timoshenko-Greve, Theory of Elastic Stability (1961) documented torsion tests.
RE: Allowable length of hollow shaft under torsion
David Simpson, PE
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist
RE: Allowable length of hollow shaft under torsion
If you can accept the deflections coming from a limit stress (allowable = ultimate / safety factor), then you would have a "first" point to stop.
Deflections (e.g. bending of girders or so) usually find a deflection limit coming from other aspects than instability, say for a bridge girder to have less deflection than 1/300 of the span. So if from that point of view you could derive other limits (from tolerances, force application range limits, surface deflection limits, functionally or operationally permitted deflection) you would have another point to stop. Ideal rotationally symmetric profiles don't warp, point is: ideally.
Third is, base to all elastic stress etc. designs is the assumption that cross sections remain planar and parallel. So if you go beyound the limits of that theory due to slenderness, little thickness of the shell, length of your application: you need other tools & input from people familiar with such kind of quest. Call FEA at some reputable company / university.
So, if in the end you come down to the problem of "warping", that is the instability due to torsional loading, then there is an fullscale apparatus of formulae to be applied, and should so. Dare say: imho
Pls. don't neglect effects of bending from dead weight or other, tolerances from position, shape or fixation or profile or application of forces.
Good luck!
RE: Allowable length of hollow shaft under torsion
If you are looking for some design guidance, have you by chance tried any of the naval architecture references? Some of the larger propeller drive shafts for huge vessels are tubes - as opposed to solid rods - and so there might be some guidance there, based on practical experience.
Dave
Thaidavid
RE: Allowable length of hollow shaft under torsion
Your pipe d/t is similar to what ZDAS does everyday so I don't think you are in the buckling realm.
Thaidavid, good idea - Besides the shafts, I found good data relative to torsion loadings on hulls. Anybody ride a container freighter and heard the containers groan as the ship's hull distorts at sea?