The Customer: The Center for
System Reliability has developed several optimal spare parts inventories using Sandia's WinR software. The study described here was conducted for a company that makes semiconductor-manufacturing equipment.
The Work: This study was designed to answer the question: "For
a given number of machines, what is the optimal parts inventory that a customer should keep, given that they are willing to spend some specified budget?" The approach also allowed the equipment manufacturer to
estimate for their customers the down time reduction that would result from keeping the optimal parts inventory on site. The study accounted for equipment repair times, shipping times and other down time factors,
spare part replenishment times, cost of parts, and shipping costs.
The above figure shows down time as a percentage of the average down time with no spares on site as a function of optimal inventory cost to support 30 machines.
Key Benefits:
- Minimizes unscheduled down time,
- Reduces inventory cost, and
- Avoids maintaining unnecessary parts in inventory.
Results:
- Optimal inventories were developed for several combinations of the number of machines to be supported and budget constraints.
- Optimal spares kits (recommended spare parts for a single machine) were developed for several budget constraints.
Figure 1. Comparison of Predicted Vs Observed MTBF Figure 2 shows a sequence of 5 predictions corresponding to 5 planned equipment grades. Note that the
predicted MTBF improvements were made several months before all 5 modifications were completed and field failure data collected. 
Figure 2. Predicted MTBF for 5 Planned Upgrades
Key Benefits:
- In both case studies, the customers were able to prioritize equipment modifications.
- Equipment upgrade plans were modified based on study results.
- Reliability predictions gave the customers a realistic estimate of the benefits of equipment upgrades.
- Customers realized savings by avoiding less effective upgrades.
Results:
- The two studies illustrated that predicting the reliability benefits from equipment modifications is feasible.
- Results of the analyses improved communications between equipment customers and suppliers.
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