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dataPAC User’s Guide
Chapter 6 - Frequency Response Function (FRF)
While the force waveforms for the steel and plastic tips appear similar, notice the
significantly lower frequency range that this impact excited. Using a guideline for
acceptable energy of approximately 20%, you can see that this setup (hammer/tip/impact
force) offers a range to about 100,000 CPM. Reviewing the force spectrum taken with the
steel tip would show an acceptable range of approximately 150,000 CPM.
Given the data shown in the graphs above, you might think it would be best to collect all
data with a steel tip to always get the maximum frequency range. However, a lightly
damped structure can cause double hits. So you may want to use a softer tip for some
applications.
Another characteristic of a softer tip is the ability to concentrate more energy into the lower
frequencies. Compare the graphs of the force spectrum taken with the steel and plastic tips.
Notice that the graph with the plastic tip starts on a slightly higher scale; this characteristic
becomes even more pronounced as you use softer tips. Below is a diagram of an impact
taken with a soft rubber tip.
This soft rubber tip produces a much wider pulse due to the compression of the rubber upon
impact. There is a significant reduction in the amount of acceptable frequency content in the
force spectrum shown below.
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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com