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Bump Test


The bump test is a useful vibration analysis technique to help identify resonance frequencies in a machine's structure. It requires ‘bumping' (i.e. hitting) the machine structure when the machine is not running.

Traditional Bump Test

The traditional method of performing a bump test involves using an instrumented force hammer and an accelerometer. The hammer introduces an excitation force and the accelerometer measures the result.

The procedure often seems to be a trial and error process. You must hit the machine or structure with enough force each time to get the instrument to trigger, but yet not hit it too much and get a signal overload. More often than not, you have a crowd of people watching and judging the bump test. The more people watching, the harder it seems to get a consistent striking force.

If you are tired of receiving messages such as: "signal overload" and "waiting on trigger" or you do not understand parameter setups such as "window type", "trigger type", "% pre trigger " you should consider a revolutionary alternative.

Bump Test using Long Time Waveform

Ensure you use a hammer with enough mass to excite the structure but small enough that you do not cause damage to the machine.

Collect longtime waveform data. Impact the machine every four or five seconds. As you impact the machine, the instrument collects the data. Collection often takes less than a minute. You can store the data in the instrument and transfer this to your computer.

You no longer need to wait for triggers. No more signal overloading and other warnings that cause delay. The time waveform yields fast, accurate results using no guesswork.

Commtest Bump Test

In the Commtest vb7TM, a bump test is a simple three step procedure:

  1. From the Main Menu press .
  2. Measure and use the arrow keys on the left to select Bump Test.
  3. Press .
  4. Use the arrow keys on the left to select a parameter set or press to create a new one.
  5. Set the parameters as required.
  6. Press  to begin sampling and 'bump' the machine with a hammer > Repeat this several times.

Bump test measurements are taken in free run mode and use peak hold averaging. The instrument will take continuous samples and update the peak value for each spectral line whenever a line exceeds its previous value.

Long Time Waveforms Explained

Long time waveforms take a continuous recording over a very long time period (even several hours if needed), making it particularly useful for capturing transients. Once measuring has begun, recordings are taken continuously until your specified time period has elapsed or you manually stop the measurement. The amount of time you can record for depends on the Equivalent Fmax (which determines the sample rate/lines of resolution) and how much available memory is left in the instrument.

Measuring Long Time Waveforms

Duration is the total amount of time available for recording, and is displayed in seconds. You can record for the full length of time available or set a shorter time period. Your chosen time period is shown on top, while the available time is shown in brackets below this value.

The Equivalent Fmax (which determines the maximum frequency displayed onscreen) will affect the available recording time. A high Fmax reduces recording time, while a low Fmax increases the total amount of time available for recording.

While the instrument is recording you can display the onscreen signal as a waveform, or convert it to a spectral display by pressing Display Type & Resolution and selecting your preferred display type.

  1. From the main menu, press  Measure
  2. Use the arrow keys on the left to highlight Long Time Waveform.
  3. Press  .
  4. Use the arrow keys on the left to select a parameter set or press  to create a new one.
  5. Set the parameters as required.
  6. Press  to begin data collection - During data collection, the number at the top of the screen indicates how many recordings have been taken. The default onscreen display during measurement will show spectrum and waveforms simultaneously.

When data collection is complete, the screen displays a waveform 'block' (or spectrum if you selected this option) and a trend of the overall power in the last recording. To view the power in another recording, use the arrows on the right to move the cursor.

The top chart can also display a trend of the machine speed if you used a speed sensor. If required you can toggle between overall power and RPM.

When measuring is finished, the selected measurement location will contain several individual recordings.

When the waveforms are transferred to the software and displayed as spectra, the equivalent Fmax value will be used on the chart. (The actual sampling rate is 2.56 times higher than this Fmax value.)