Speed Sweep Analysis

On rotating equipment, where the dominant vibration is related to the rotational speed, a very powerful method of learning more about the vibration characteristics of the structure is speed sweep analysis. In this procedure the vibration is measured, along with the machine speed, while the machine speed is increased or decreased.

 

This procedure determines at what speed the vibration is the highest. This speed is often a resonant frequency in the structure. The advantage of this method as compared to modal analysis is that the dynamics are measured when the structure is as close as possible to its actual operating condition. The spring stiffness of bearings for instance, can change depending upon thermal expansion. When performing a modal analysis some stiffnesses are slightly different. When modeling the structures with finite element analysis, only handbook approximations are available for some of these components.

 

The types of measurements that can be made are time or speed based spectral maps, order ratio maps, order tracking and order spectrums. Each of these is a different view of the same basic information, which is processed and displayed differently.

 

Much insight about the characteristics of the structure can be gained from these measurements. In particular the resonant characteristics will be much better known. Speeds at which operation should be avoided can also be determined.