Posted by John Swensen on February 11, 2002 at 13:19:01:
In Reply to: Rotary Springs posted by Keith C. on February 11, 2002 at 11:17:18:
The short answer: if your springs are not corroded, uniform in resistance to spatula pressure, and return your valves quickly enough, they are fine.
A longer answer: springs for rotary valves (mousetrap type) are double torsion springs, and should last indefinitely, assuming they do not corrode, do not get too hot (no more than, say, 200 degrees F (100 C)), and are designed so that maximum deflection is well within the elastic limit for the material. Put another way, for steels (including stainless varieties), if you don't bend them too much, you can continue bending them forever without fatiguing them; this does not apply to aluminum, and probably not for bronze.
I recently did the analysis for piston-type springs, and discovered that, with common stainless steel spring wire sizes, forces, and deflections (piston travel), the springs are operating in their "safe" regions, as far as spring life is concerned. I have not, yet performed the analysis for rotary-valve springs, but I suspect they will turn out to be "safe", also.
Springs can go bad if they are abused (say, nicked or stretched out of shape during installation or instrument servicing), corroded, or if the metal has defects (impurities in the wire), or if they are under-designed (usually by using too-few turns of small diameter wire, rather than more turns of larger-diameter wire to create a spring with the desired stiffness). I suspect poor steel quality is reponsible for the sudden failure of some new, Russian, clocksprings I have tried, that showed no evidence of corrosion. As your horn was made in East Germany, the steel (bronze?) quality may have been suspect, but they would have failed years ago, were they defective.
By the way, springs for rotary valves are about the easiest in the world to make (except for, say, needle springs for woodwind instruments); if you can drill holes in three pieces of metal, drill and tap holes for small screws, and file notches, you can wind a spring in about minute (stress relieving will take another half hour to an hour in your home oven, and passivating stainless steel will take another hour (well away from the house)). The website below has excellent instructions for making common springs yourself.