Fatigue Life Square Wire Torsion Spring

[xr4titis]

I'm using the Machinery's Handbook to design a torsion spring with square cross sectional wire. The Machinery's Handbook only gives fatigue stress curves for round wire springs. Am I correct in assuming that square wire springs have a higher allowable fatigue stress?

Please don't hate me for being stupid...

 

[israelkk]

No, you are incorrect. More than that the ultimate strength of square and rectangular wire with similar dimensions to round wire is lower than the equivalent round wire.

The reason that the machinery handbook doesn't give this information is that square and rectangular wire use is very limited compared to round wire.

As far as I know the fatigue curves in machinery handbooks (27Th edition) are for compression spring and not for torsion springs.

 

[xr4titis]

Hmmm that makes it more confusing... The torsion spring formula page leads me to believe otherwise, at least in bending. The bending stress is lower in the square wire when using the same 'd' Can you clarify?

You say the tensile is lower with a square wire, despite the cross sectional area being larger. Does that have something to do with forming?

There are fatigue curves for torsion springs for 4 materials in the Machinery's Handbook

 

[israelkk]

Note, that the stresses in a compression spring are shear stresses where in a torsion spring the stresses are tensile (bending) stresses. The lower ultimate tensile strength of the square/rectangular wire are due to forming (% of reduction of area during the drawing process of the wire).

You asked about the alowable stresses (not the calculated stresses) the alowable stresses are % fraction of the ultimate tensile strength of the wire which is detemined by the wire diameter/size and the material type.

What edition of Machiney handbook are you using and where (page number) did you find the fatigue curves for for torsion springs?

 

[TVP]

xr4titis,

You are confusing stress and strength. The bending stress on a torsion spring is lower for a square/rectangular section wire than it is for a round wire section for a given d and deflection. The strength of the material is lower for square/rectangular section wire than for round wire due to the manufacturing process prior to spring coiling.

 

[xr4titis]

Bingo. Thanks for clarifying! I guess I can't use these Design Stress curves for round wire torsion springs on pg 317 of the 27th Ed of the Machinery's Handbook accurately then.

 

[TVP]

The strength of a rectangular or square section will be ~ 10-15% lower than that of round wire. You should be able to adjust the curves based on this and obtain a reasonable estimate.