Dynamic loading of springs falls into three main categories, shock, resonance of the spring itself, and resonance of the spring/mass system.
Shock loading:
Shock loading occurs when a load is applied with sufficient speed such that the first coils of the spring take up more of the load than would be calculated for a static situation. This is due to the inertia of the spring coils. This situation can lead to much higher stresses than calculated for the static design. Even worse, shock loading can lead to coil clash, which can result in premature failure.
There are calculations available to predict if a spring system may be subject to shock loading, if the speed of force application is know. If shock loading is suspected, fatigue testing should be done, but in the actual application instead of a conventional cycle test machine.
Spring resonance:
A spring will resonate if the operating speed is the same as the natural frequency of the spring or if it is the same as a harmonic of the natural frequency. Most computerized spring design programs will calculate the natural frequency of a spring design. A good rule of thumb is to make sure that the operating frequency of the spring is no more than one-thirteenth of the natural frequency of the spring. This is a number that has been established over the years as safe from exciting harmonics of the spring.
It is important to avoid resonance because it can cause greatly elevated stresses and even coil clash resulting in premature failures. Contact Rockford Spring Company if you have a design that may be a problem for resonance. We can employ variable pitch, extra closed coils, etc. to help with designing around resonance.
Resonance of the spring/mass system:
When the spring is required to carry a mass attached to its moving end, the combined system is subject to resonance at a much lower cycle rate than the spring alone. Do not simply use the natural frequency of the spring from the design program for this type of spring/mass system. Calculations are available for these situations as well. It is important to identify these situations and bring them to our attention so that we can help you to deal with them.