A vibration analysis is conducted on a fan once per month.
Vibration readings beyond a specific level indicate an impending failure. The goal is to
do vibration analysis often enough to catch the machinery in a state of high vibration,
but before failure. This state is referred to as the lapse zone (see Chapter 5 of the
manual for more details). The length of the lapse zone must be estimated. It is not
feasible to run to failure for the purpose of collecting failure data. The length of the
lapse zone is determined by estimating a minimum length, a maximum length, and a most
likely length. For this example, production operators determined that the minimum duration
of high vibration reading before a failure occurs is 3 million feet of production, the
maximum duration is 10 million feet of production, and the most likely duration is 5
million feet of production.
The cost of a vibration analysis is $22, and the cost of a machinery failure is $985. The time to fail data in this case should be the time until increased vibration is first noticed. The file "VBRATION.DAT" contains data for the time until vibration readings began increasing. How often should vibration readings be taken?
The software will display the results in a frame. The first vibration analysis should be conducted after 88.23 million feet of production have been run. The second vibration analysis should be conducted after 92.11 million feet of production have been run, only 3.88 million feet later.
Note that the time between each successive inspection is decreasing. This is because the failure rate is increasing. A shape parameter greater than 1.0 (the shape parameter was 4.503 for this example) indicates "wear-out", which is characterized by an increasing failure rate. The longer the fan runs, the more likely it is to fail. If the shape parameter is equal to 1.0, the time between each successive inspection will be equal. If the shape parameter is less than 1.0, the time between each successive inspection will increase; the longer the item runs the less likely it is to fail.