Manufacturing with PWW
Achieve world class manufacturing standards with the Plant Wellness Way EAM methodology
Shaft Quality Control Gets Rolling Bearing Reliability
For years a steel mill experienced monthly failure of a spherical roller bearing on a 150mm diameter head pulley shaft. The pulley itself drove a critical mill conveyor, which when failed stopped the whole operation. To prevent failures during planned uptimes, the maintenance crew replaced the bearing every three weeks.
Despite their best efforts, the spherical roller bearing in question would still, unpredictably fail during production. The downtime caused by this component failure was flagged by the management team as unacceptable and an investigation into finding the true root cause was ordered.
As part of this investigation, the steel mill involved the supplier of the bearing. The first question asked by the supplier was to define the tolerance and form values of the shaft under the bearing. As these measurements were not a required part of the maintenance inspection, the values were unknown.
To determine this information, the next scheduled bearing replacement included the recording of these values using the appropriate tools and methods.
The measurements clearly showed that the shaft form itself was not shaped to meet the supplier’s allowed specifications. As a result, there were gaps between the shaft and inner ring surface which allowed the ring to move up and down during operation. The flexing of the inner ring rapidly fatigued the bearing materials causing the bearing to fail.
Now aware of the shaft condition, the steel mill maintenance department completed the right quality assurance checks and replaced the defective shaft with one that met the required tolerance and form for the bearing component.
While this example may have some readers thinking ‘what an obvious solution’, the author has experienced and heard of many similar situations at manufacturing sites across the world. Through no fault of the maintenance crew, the routine checking of shaft shape was not included as part of the SOP, and so, was overlooked.
This example highlights the importance of creating standard operating procedures that include precision maintenance and work quality assurance checks. If a maintenance procedure does not contain all aspects of risk reduction and monitoring, it is easy for a cause of failure to remain unidentified until the problem is considered severe enough to address.
By properly following the Plant Wellness Way methodology will produce operating processes that cover the prevention and monitoring of all potential risks in a cost and time effective manner. To demonstrate how PWW could have saved the steel mill years of breakdowns and untold costs, three of the five core concepts are discussed below.
1. Organizational processes are, most commonly, series processes. And so, the success to the next step is dependent on the success of the previous. i.e. Poor work quality multiplies across a process.
2. Every system is perfectly designed for the results it gets (W. E. Deming).
3. Maintenance is an economic decision. Put another way, failures are very expensive.
A good rule-of-thumb to follow is that the total, companywide cost of failure is approximately 10 times the direct maintenance costs. A Total Defect and Failure (TDAF) Cost Analysis is part of the PWW EAM methodology and quantifies the approximate true cost of failure, including hidden costs and opportunity costs as well as direct maintenance costs.
While the cost to replace a bearing is low, the impact on production reliability and wasted resources for each failure event grew over time. Eventually it was considered too expensive to continue with the ‘status quo’. If the steel mill had conducted a Total Defect and Failure (TDAF) Cost Analysis they would have recognized this and made their investigation into the root cause sooner.
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