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Free Length Tolerances

Free Length Tolerances

Free Length Tolerances

How would one go about calculating the free length tolerance on the following spring?

d = .016"
OD = .292"
Lf = 1.2"
Ntotal = 20

Material = Phos. Bronze

Standard equations I have seen come out with tol. bigger than the free length! Are there any equations that practically take into account large spring indices and low spring rates?
I would appreciate some opinions or experience in this area.

Thanks, Jack

RE: Free Length Tolerances

Usually I base tolerances on similar springs, and then consult vendors about their capabilities.  Some of our vendors are more capable of holding tolerances than others.

Due to illness, the part of The Tick will be played by... The Tick.

RE: Free Length Tolerances

Usually a spring is designed for two working forces at two specified spring lengthes, or for designed for a spring constant. Therefore, unless the zero force is essential in the design the free length is left with no tolerance. Anyway, the first ~10% and the last ~10% of the spring deflection are non-linear and ususlly are not used.

RE: Free Length Tolerances

I performed a similar calculation for your design according to DIN 2095 and likewise came up with a meaningless free length tolerance.  For small diameter (both wire & mean) springs that we need tightly toleranced, we usually specify the free length to be + 0.50 mm, but have some drawings with the tolerance specified as + 0.25 mm.  These are for music wire springs, however.  It is best to talk to the spring manufacturer about this, as they should have material-specific & geometry-specific targets that they can typically achieve.  Good luck.

RE: Free Length Tolerances

One more thing, I forgot to mention that these springs have the ends closed and ground.  This will make a difference if your design is not of this type.

RE: Free Length Tolerances

Hi jackpot

Working from BS1726 I have two formula for tolerance on free length thus:-

            (Lo + 10mm)*(c+25mm)/(2000) = tol on free length

this is for group A type springs

where Lo = free length in mm

       c = spring index

For group B type springs use the same formula as above and multiply the answer by 1.2

so for your spring index is 17.25

free length is 1.2" = 1.2 * 25.4 = 30.48mm

using the formula for group A tol on length is 0.855mm

for goup B springs tol on length = 0.855mm * 1.2 = 1.026mm

The group A and B springs are just for different tolerances
classes of springs and obviously from above group A springs are made to tighter tolerances than group b springs.

Hope this helps

regards desertfox

RE: Free Length Tolerances

MIL-STD-29A gives tables for free length tolerances when no load is specified. For all other cases where loads are defined or where rate is defined the free length is given as REF on the drawing.

RE: Free Length Tolerances

All of your comments and years of experience are appreciated, as they expose me to some of the more practicle means of determining free length tolerances.

Thanks, Jackpot

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