Bump Test

The Bump Test is a simple method for analyzing the structural modal response of a machine or structure. When impacted, a machine or structure produces a broad frequency band of excitation components. When these frequency components coincide with the structural natural frequencies, then resonant conditions are present which result in a higher than normal vibration level at those frequencies. During the bump test the vibration amplitudes and frequencies are sensed with accelerometers and measured by CoCo-80 and displayed as plots of amplitude versus frequency. The peak amplitudes correspond to the structural flexible modes and the narrowness of the peaks provides insight as to the damaging amplification factors.

Usually, in most cases of machine vibration, it is not so important to determine the exact magnitude of the transfer function; the most important information is in the frequencies of the modes of vibration of the structure, their locations, and to a lesser extent, the damping associated with these modes.

The CoCo can be used to perform an impact test without the need for a special hammer with a force transducer attached. The technique involves simply hitting the structure with a suitable impacting device, such as a wooden mallet, a hard rubber mallet, or in some cases, a sledge hammer.

The bump test can be conducted in Equipment On or Off mode. Equipment Off, a preferred method, means the rotor is not rotating and no other excitation sources exist in the system. A bump impact can cause clear resonance, as shown below:

Bump Test Display


Equipment On is applied when rotor still rotates where the force excitation source exists in the whole vibration system. A special algorithm called negative averaging is applied, as shown below: