diameter ratio for hollow shaft

[magicme]

hello

In reading various design specs for hollow shafts (that will see torsion and rotating bending), I have come across the design guideline that ID / OD <= 0.65.

In fact I have several Navy specs that say "ID / OD must = .65" .

Going through my books, I cannot find a rationale for this.

Any guidance as to where this comes from would be appreciated.

thanks

magicme



not all that glitters is gold.

 

[roadapple]

This proved to be a hard nut to crack and I, like you, can't find anything about it.

Common sense tells me that the ratio is based on the notion that the outer diameter of a rotation shaft carries a larger portion of the torsional load while it decreases to nothing at the center of the shaft.

So, short of working a spread sheet out to determine the answer my best guess is that the .65 results from some value that states "that 80% of the load" is carried in that id/od range.

roadapple

 

[telecomguy]

I can imagine there's a point where buckling becomes an issue if the ID approaches the OD. Without working the calcs, and not working in a field where I need to be familiar with this type of loading, I can only speculate.

<tg>

 

[magicme]

yes, I have the feeling it has to do with a balance between torsional buckling and some allowable stress level due to steady torsion..... ... I am still hunting..... thanks for the thoughts. I will post the answer here if I ever find it!

I recall years ago when working with press-fit shaft couplngs, the same .65 diameter ratio was used for the coupling hubs.

regards

magicme



not all that glitters is gold.

 

[GregLocock]

Automotive drive shafts typically are only 1-2 mm thick, maybe 3 for an aluminium one. They are typically 75-100 mm OD.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[desertfox]

Hi magicme

I haven't found a definitive source for the 0.65 however
"The Mechanical Engineers Data Handbook" by James Carvill I
have found that Di/Do = k and on page 7 of this book. It lists various k values for hollow shafts and compares the
outer diameters, weight and angular deflection for a particular hollow shaft against that of a solid one ie:-


k 0.5 0.6 0.7
---------------------------
Do/Ds 1.02 1.047 1.095
Wh/Ws 0.783 0.702 0.613
[&Theta;]h/[&Theta;]s 0.979 0.955 0.913

the fomula's given are as follows:-

For a hollow shaft to have the same strength as an equivalent solid shaft:-

Do/Ds = (1/(1-k^4))^0.3

Wh/Ws = 1-k^2/((1-k^4))^0.3


[&Theta;]h/[&Theta;]s = ((1-k^4))^0.3

and finally k= Di/Do as previous

hope this helps

best regards

desertfox

 

[magicme]

thanks desertfox

but I tried ratios of that tabulated data and can't find a reason to pick one value of k over another.

still hunting,

and of course the longer it takes, the more determined I am to find the rationale!

magicme

------------------------------------
"not all that glitters is gold"

 

[desertfox]

Hi magicme

The only reason I can see to pick different values of "k" would be suit your particular critera ie:-

if weight was the main critera then you would perphaps use
a hollow shaft with a k=0.7 as this would only have 61.3%
of the weight of a solid shaft.

If you were restricted for space you might choose a k=0.5
to keep the diameters of a solid and hollow shaft fairly close.

regards

desertfox

 

[dimjim]

Navy specs that say "ID / OD must = .65" .
I find this equation interesting.
If we let the ID equal OD - 2t, then t=.175 OD
ie the wall thickness. I do not know where thin
section vs thick section columns is defined.

If so then the equation

ID / OD <= 0.65.
may make some sense.
Or t >= .175 OD

 

[JStephen]

"Formulas for Stress and Strain" includes equations for buckling of thin-walled cylinders under torsion. But the formula includes length as one of the critical factors, so it wouldn't immediately lead to this kind of diameter ratio, unless length were otherwise specified.