Machine Diagnostics

It shouldn't come as a surprise that periodic forces in rotating machines appear to be related to the rotating speed. Most of these forces produce excitation at frequencies proportional to the operating frequency of a machine. Machine structure transmits the forces to the foundation. Depending on mass-elastic properties of the machine components the machine structure produces response to the excitation forces. This response is what we measure as vibration. In other words, when operating, stationary machine components move periodically (vibrate) under internal periodic forces. This movement, i.e. vibration, can be measured and analysed.

The excitation forces in rotating machines can be related to the machine function, such as blade pass excitation in turbo machines or to mechanical sources that are present in every machine. For example, there is always residual imbalance in the rotor, which produces force at rotating frequency. If there is coupling misalignment, it will produce forces at 2x, 3x, or other integral multiples of the rotating speed, depending on the design.

When components start wearing out or become defective due to contamination or abuse, the forcing frequencies change. These changes can be picked up by vibration analysis software or a vibration analyst. Often the source of the problem or at least a component that has developed a fault can be determined just by analyzing the vibration data. Every machine component has its vibration signature, which changes when a fault develops.

This is why dynamic vibration analysis is so important. Since the vibration spectrum shows vibration levels vs. frequency, it is the first tool that is often used.