helical compression spring

by John Logan
(Australia)

My name is John Logan and I am an engineer with the Australian Department of Defence.
I a conducting a review of a particular design of a system with a spring element in which we are having a few concerns - I wonder if you might be able to offer some advice.
We have a helical compression spring, about 480mm free length and 7mm OD. (I am working from home and don't have the specs in front of me.) The piano wire spring operates in a stainless steel tube (not sure of the tube ID but it is not the 10% recommended!) The ID of the spring also runs over a rod for part of its length. The system operates at about 10-13 Hz.
Our problem is excessive and unpredictable/variable energy loss with the system. The energy loss can change unpredictably with time/cycles. We have measured the force displacement over the operating length (about 120mm)and it produces very erratic curves with a significant area between the forward and return curves. We have used a glass tube to visually see what is going on (just staticallY). As expected, there is a wave generated along the length of the spring and what appears to be very high side forces between the OD of the spring and ID of the tube where contact is made. There is crunching and binding of the springs, with erratic and uneven release of coils during compression. Probably not a good design overall but what we have to work with.
The ID of the tube is highly polished. Do you think there might be a material incompatibility betwen the very hard spring wire and relatively soft stainless causing a galling? We have used a solid lubricant (Mo2S) with limited success. Also phosphated springs to take Moly - rougher surface. We would like to reduce the frictional energy losses in the system and produce something that is more consistent - any suggestions for enquiry or improvement would be most welcome.
Thanks in anticipation from friends in Australia.