Data Analysis Tip 3 – Compare Identical Machines

www.dliengineering.com
Posted 3-14-05

When troubleshooting or using vibration in a Predictive Maintenance program comparing identical machines to each other is often the best method to diagnose a problem – especially if you suspect a problem exists in one or more of the machines. When vibration data is collected on a bearing housing using an accelerometer, one is not only measuring vibration forces, but also the effect of the structure on these forces. There is no vibration severity guide in existence that accommodates for the effect of the structure and this is one reason guides are only “guides”. Not only that, the effect of the structure is different at every frequency.

Some small machines normally vibrate “a lot” and some large machines barely vibrate at all. In either case, they all produce unique patterns. The best comparison one can make therefore is between two identical machines, or between reference data from the same machine from an earlier test date. Always test the machines in the same locations and under the same conditions. (speed and load). “Normalize” the data and compare them. If troubleshooting, note the test conditions and save the data for later use.