Spring failure

[design0058]

I have a compression spring failure issue that I hope someone can help me with. I’ve tried the same spring from 2 different manufacturers. Danly #9-1610-26 and Ready Technologies #9-1610-H. Chrome-vanadium springs. Sometimes the springs break at the first full coil and sometimes they break 2 to 3 coils from the end.
The spring parameters are as follows:
OD = 1”
ID = ½”
Free length = 2.5”
Pre-load = .087”
Deflection = .512” (cannot be changed)
Total deflection = .598” (23.9% of free length)
Spring rate = 691 lbs/inch
Pressure at full compression = 413 lbs.
Application:
Progressive stamping die running at 1,100 strokes per minute. The spring is guided on the inside diameter by a ½” diameter shoulder bolt.
The problems is that we’ve had springs break after 300,000 strokes while others have lasted up to 4.6 million strokes. The goal is for the springs to last 10 million strokes without failure. Is there a way to calculate the life of these springs? Would reducing the percentage of free length to approximately 20% increase the life of these springs? Is so, by how much? Would a longer spring with larger diameter and a lower percentage of free length last longer? Is there a better spring material for this application? Any help will be appreciated.

 

[israelkk]

There is no way to answer your question without exact dimensions of the spring including wire (rectangular) dimensions, treatments that the spring has gone, number of total coils, active coils, etc. However if you look at Danly's catalog you are using the second column deflection and force and this defines for "average life" while the first column deflection defines "long life". My best advice is to call the company and ask them if you want to avoid a custom designed spring. 10 millions cycles is usually means long life.

 

[Alsip]

You primarily need to find out why the spring fractured "prematurely" - this can be done by having a failure analysis done on the broken spring by a competant spring engineer.

There may be tooling marks, scratches, excessive decarb etc. involved in the fracture cause. Can't procedd any further without this information, in my opinion.

Alsip

 

[unclesyd]

I totally agree advice posted above with having your failure analyzed by someone well versed in spring failure.

Two things gave us problems you might want to check.
Our short run spring failures were traced to essentially two things. One was not enough preload and the springs were subject to shock loading. I can't find out how this was resolved, either increasing the preload on the existing sdpring or a long spring preloaded more. The second part of the problem was at times we were changing to a lighter duty spring than what was originally evaluated and specified.

 

[otovar]

one thing to consider is to look into the heat treating of these springs..sometimes the springs are not getting enough heat, since its chrome vandium they should be heated at 750 degrees for 60mins..also this material should be heated the same day they are formed, alot of times they sit around too long after coiling (more then 24hrs) that will effect the properties in this material since they were stretched beyond their limits they should be drawn back right away..just a thought ive seen this happen alot..let meknow if you considered this good luck