Spring failures

Hi,

I have set up a prototype engine test rig using comp. cams Pacalloy automotive valve springs. These springs have been breaking after around 2*10^7 cycles with the classic 45 degree fatigue fracture where the lower coils bind. Just wondering if anyone knows of a compression spring with a spring rate of 300lb./in and a usable travel of 25mm (i.e., overall travel from free length to coil bind of around 40mm). These springs are required to do endurance tests (hoping for 10^8 cycles) under sinusoidal loadings (must have good fatigue properties).

Also if anyone knows about lubricity coatings such as nylon 11 or 12. Also if air springs could do the job and cost?

Thanks,

If you are using Pacalloy valve springs, then you have pretty much done everything you can to maximize fatigue life, as far as materials and processing is concerned. Apparently the stress levels that are produced in your particular spring are quite high. Do you know if there is a dynamic problem (surging) that may be reducing the fatigue life? If so, have you investigated natural frequencies, damping, etc.?

I am not familiar with the use of Nylon 11 or 12 coatings for use in automotive valve trains, so I cannot help you there. Regarding air springs, these systems are frequently used for high rpm racing engines because of limitations with valve springs (fatigue life, surging, etc.). For cost, you really need to work with a competent supplier to understand the design implications, manufacturing possibilities, etc. Del West in Valencia, CA is highly regarded in this area.