September 15, 2016
MACHINERY VIBRATION PROBLEMS AND SOLUTIONS, PART 6 – RESONANCE
By Stan Riddle
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”.
- Design and engineering. Resonance factors should be a normal part of engineering design. But when speeds, components, or operating conditions are changed, resonance could occur.
- A loose component’s stiffness changes, since it is less constrained. Look for signs of looseness, and make repairs or adjustments as needed.
- Changing mass or stiffness. Changing the mass or stiffness of a machine changes its natural frequency. These changes can be tested simply by temporarily adding mass (such as weights) or by changing stiffness (such as temporarily adding bracing). Permanent changes to either mass or stiffness should be done with the advice of an engineer.
- While observation won’t change resonance, good observation and troubleshooting skills can sometimes find the problem. A start-up/coast down test can be used. Resonant frequencies can be identified by an increase in vibration, and a phase change as the critical speed is passed. A strobe light can sometimes be used to observe the component which is excited into resonance.
OTHER PROBLEMS SOMETIMES INCORRECTLY DIAGNOSED AS RESONANCE