disc spring load tolerance

[ozzkoz]

I am designing a disc spring stack (first time) and was wondering how they are made and how the process can be used to achieve the specified load tolerance.

I designed a spring based on nominal dimensions and then checked a few catalogs for tolerance guidelines. When I recalculate the force and stresses using the limits allowed per the dimensional tolerance I get loads @ 75% deflection which are about ±80% from nominal, yet the cat. claims load at 75% should be more like ±15%. Does the manufacturer tweak something when he makes the spring in order to tighten the load tolerance?

As an example, if I had a helical spring the manufacturer would buy the wire (industry tolerance), coil it on a machine and try to keep the OD in tol, then he would tweak the number of coils / free length to get the load @ length / stiffness to be in tolerance. Does a disc spring maker do something similar?

Thanks

btw, every example design of disc springs I have seen never considers the dimensional variability due to tolerance, am I being silly by doing so?

 

[israelkk]

How did you get to to ±80% from nominal? Which dimensional tolerance caused this deviation?

 

[desertfox]

Hi ozzkoz

Which way round have you assembled the stack of springs, have a look at this site:-

http://www.bellevillesprings.com/stacking-disc-springs.html

desertfox

 

[desertfox]

Hi Again

This page also as some tolerances for loads etc.

http://www.bellevillesprings.com/din-2093-disc-spring-tolerances.html

desertfox

 

[ozzkoz]

The calculation is very sensitive to changes in t and somewhat less to h. Not so much to changes in D and d.

Currently my parameters are:

D = .245 - .250
d = .125 - .130
t = .005 - .008
H = .016 +.004, -.002

MAT = 17-7PH

Calculated Load @ .75*.016 = 11.6 lb ± 67%

Do they do something at the time of production to tailor the thickness or height to meet the load?

Thanks

 

[desertfox]

Hi ozzkoz

How many are you using in a stack and which way round have you assembled them?

desertfox

 

[israelkk]

ozzkoz

As you see you answered your question. The accuracy of the strip thickness is crucial. I assume disk spring manufacturers either order a special more accurate strip or sheets or play with "h" . However the ratio h/t affects the the load to deflection curve shape drastically.

Also take into account that the formulations given in the literature are approximations based on Almen and Laszlo article which assumes that the disk stay conical all the time (doesn't become dome shape). The article can be downloaded here:

http://www.shotpeener.com/library/pdf/1935000.pdf

 

[ozzkoz]

Thanks, in regards to the number of springs in the stack I am planning on using 6-8 in series, however the tolerance on the load I speak of is in regards to a single washer.

I guess I was hoping that someone who knows how they make the disc springs could tell me if they meassure the thickness and then set h such that you get the specified h/t ratio or something indicating that there is more to making the springs then taking a sheet, cutting the OD / ID and then forming the height such that all dimensions are within spec because it seems like this would lead to very large swings in load (lot to lot, prob fairly consistant for all springs made from a stock sheet).

Thanks

 

[israelkk]

Do you intend to custom design and manufacture yourself or to give the job to disk spring manufacturer?

 

[desertfox]

Hi ozzkoz

Have a look at this catalogue it as all about manufacture and a section on load tolerances depending on material thickness, the washers are basically punched out of strip and formed and heat treated.
You will get far more from this catalogue then we can tell you:-

http://schnorr.com/docs/Handbook.pdf

desertfox

 

[btrueblood]

I know of one manufacturer of devices using Belleville springs that machined, hand-lapped, tested, re-lapped, etc. to get springs that met their specifications (approx. +/-5% tolerance on load at a specific height). They typically scrapped several dozen in a batch of 100 springs, and felt they were doing pretty well.

 

[unclesyd]

You can purchase precision disk springs from Key Bellevilles and imagine other companies as well. You should be able to checkout their tolerances from the specifications listed.

Quote from the Key site:
Key Bellevilles disc springs have design standards based on DIN 2092 specifications and are manufactured to DIN 2093 specifications.

PRECISION BOLTING SPRINGS
(TO DIN 6796)

http://www.keybellevilles.com/prebolt.html