What is spring surge and how do you prevent it?

Spring surge occurs when the natural frequency of a spring coincides with the operating frequency, causing resonance. This leads to coil clash, high stresses, noise, and early failure. Natural frequency fn = (d/pi.D^2.N) x sqrt(G.g/32.rho). To prevent surge, design fn to be at least 15-20 times the operating frequency. Remedies include changing wire diameter, using variable pitch springs, adding friction dampers, or using nested springs. Critical in valve springs and high-speed machinery.