IWA tolerances for spring rates
Tolerances for Spring Rates of Compression Springs
In standard compression spring applications, spring forces at the low and high ends of the scale normally determine the switching functions intended for a compression spring.
Practicable force tolerances as defined in DIN 2095, the established standard in many EU states and generally throughout the world, are stipulated for such applications.
In some cases, however, the decisive factor is specific force development after an adjustable switching point. This clearly relates to the increase or decrease in force per unit of spring travel, i.e. the spring rate.
Examples that spring to mind include spring balances, mechanical letter or diet scales
as well as suspension systems in automotive engineering applications.
Unfortunately, a very wide range of tolerances has been specified for spring rates to date.
In the United Kingdom BS 1726 is the standard, in America the M3 formula is used while in Japan JIS B 2707 is the decisive standard. All these standards apply to all types of compression spring and give rise to large discrepancies in specified tolerances.
In Germany, DIN 2096 Part 1 is the authoritative standard that applies only to helical springs on motor vehicle axles.
All the above formulae for calculating spring rate tolerance are outlined in Annex 1.
By considering the specified tolerances and comparing them to test results that were determined for ESF based on real springs in America, Denmark, Germany, France and Japan, we arrive at Annex 2.
The only effective way to derive correlations was to sort and compare all results corresponding to the number of active coils of the individual springs.
Trends indicate that the British formula for calculating spring rate tolerances as defined in
BS 1726 results in ever greater spring rate tolerances for springs with increasing number of active coils while the Japanese formula for calculating spring rate tolerances as stipulated by JIS B 2707 leads to precisely the opposite results, i.e. ever closer spring rate tolerances as the number of active coils increases.
In the same way as JIS B 2707, the German tolerance formula in accordance with DIN 2096 Part 1 for spring rates of vehicle suspension springs also leads to ever closer tolerances as the number of active coils increases.
The American M3 method indicates a trend similar to the Japanese JIS B 2707, however, it exhibits extreme values both for low as well as for high numbers of active coils.
With the aim of finding alternatives for an IWA document currently being prepared (i.e. tentative standard to ISO) for spring rate tolerances, the Japanese (end of March 2006) and the Americans (mid-April 2006) each submitted a revised proposal.
The Japanese proposed to split the spring rate tolerance formula of DIN 2095 into two parts
to make one part of it the future formula for calculating spring rate tolerance.
See Annex 3.
The Americans also put forward a completely new derivation for the previous M3 method. See Annex 4, 1-4
Therefore, on the occasion of the last International ESF Standardisation Meeting held on
26 April 2006 in Düsseldorf all associations of the ESF, the Japanese Spring Manufacturers Association (JSMA) and the Spring Manufacturers Institute of America (SMI) were requested to come up with an alternative that meets all requirements for the rational tolerance standardisation for spring rates of compression springs by the next international meeting of spring specialists to be held in the Black Forrest, Germany in October 2006.
Horst Dieter Dannert
Dipl.-Ing., Dipl. Wirtsch.-Ing.
General Secretary – ESF
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