Cautionary Tale: Marketing of Extension Springs - Part II
by Mark Hayes
The cautionary tale in the Winter 2010 edition of Springs promised a continuation of research results that showed how to improve the performance of extension springs. So far, in this cautionary tale, it has been shown that German loops are better than English but only at small index (L8).
Often there is no difference between those two loop types providing that both are made accurately (loop not protruding) and without significant tool marks. It has also been shown that shot peening will improve end loop fatigue life, but by a relatively small amount. Finally, the debate about enlarged or reduced loops was closed in favor of reduced diameter loops.
This tale is intended to illustrate by how much the end loop should be reduced so that the loop is no longer the weakest position in the spring. The research was conducted by IST and its European partners with the Techspring Research Project. IST is always looking for opportunities to undertake fundamental research like this, but it is difficult to obtain funding for this type of R&D activity.
Batches of extension springs were made from carbon steel wire to EN 10270-1 DH quality, an example very similar to ASTM A228. They had reduced diameter end loops and the last body coils before the hook were coned down, as seen in Figure 1.
The spring designs are shown in Table 1.
These springs were fatigue tested at a speed of 200rpm and in each test the position of failure was noted. Failures in the end loop are designated L in Table 2, and those in the body B.
The fatigue results are shown in Table 2.
All the springs from batch 2C broke in the body, thus indicating the reduction in loop diameter that was effective in preventing fatigue failure in the loop — the usual position of fatigue failure in extension springs. These results, together with others generated in the Techspring Project, will enable the production of very much improved Goodman Diagrams for extension springs that take account of end loops performance. These will be incorporated into IST’s CAD software.
Mark Hayes is the senior metallurgist at the Institute of Spring Technology (IST) in Sheffield, England. He manages IST’s spring failure analysis service, and all metallurgical aspects of advice given by the Institute. He also gives the spring training courses that the Institute offers globally.
Readers are encouraged to contact him with comments about this cautionary tale, and with subjects that they would like to be addressed in future tales. Contact Hayes at (011) 44 114 252 7984, fax (011) 44 114 2527997, or e-mail Mark's Email.