Resonance is when a force causes an object to “resonate”, due to excitation of its natural frequency. Think of a machine coasting down to a stop. There are certain speeds in coasting down that excite natural frequencies of the machine components. This is often observed in the coast down of a bench grinder. As the grinder slows down, it can vibrate more at certain speeds, then get better as it continues to coast down. Every machine and component of a machine has a resonant frequency. Engineers normally try to design around resonances, so the machine is not excited into resonance, but sometimes it happens. Resonance can cause damaging levels of vibration, if not corrected.
Let’s take as an example a simple belt-driven fan. The motor has a running speed. The fan usually operates at a different speed. The belts also have a rotational speed, slower than either the motor or fan. The number of blades on the fan can generate vibration due to aerodynamic forces (# of blades x fan rpm). So there could be some vibration at all of these frequencies. If any component natural frequencies are at or near any of these speeds, its force could cause the component to resonate, or “ring”.
OTHER PROBLEMS SOMETIMES INCORRECTLY DIAGNOSED AS RESONANCE