Up Time Magazine- April/May 2008 - (Page 51) As I have already said, the decision to hold that first spare is based on the determination that the spare is critical and, therefore, required. However, what about the second spare? The decision to hold a second spare is based on the probability (and consequence) of a failure during the time it takes to restock the first spare. This is, of course, a very limited time. The requirement for a third spare is based on the probability of two failures during the restocking period of the first spare. Let’s call these second, third (and subsequent) spares, meta-spares - literally spares for your spare. The second spare that you hold is a spare for your spare. The third spare is a spare for your spare’s spare. As you can see the function of these spares is simply to be available while your first spare is Spares Availability OK, maintenance and reliability engineers are typically conservative, so let’s say that you really do want to have the item in stock in your store room, sitting there waiting for you. Let’s look at some further issues that you can address. 100% In another example, a company that had steadfastly used sea freight for overseas sourced components (because the freight cost is lower) realized that, for some items, the total cost of ownership was less if they used airfreight. This was because they could then hold fewer meta-spares. They still had ‘ready access’ to the items, they just changed their supply chain. When most engineers say to me ‘you can’t do anything about that spare, it is critical’ what they are really suggesting is that because the spare is classified as critical they cannot hold any fewer than they already do. Yet, as we have just seen, this reasoning cannot be allowed to go unchallenged. If the above arguments don’t convince you to include a review of your critical spares in any spares review, consider the following. Engineers often justify critical spares by saying that the costs of failure so outweigh the cost of holding the spare that the cost of the spare doesn’t matter. While based on sound financial sense, this is largely an emotional argument. Logically, when you reach 100% X Number of Items Held Figure 1 - Spares Availability vs. Number Held restocked. My next point is that, if critical really means ‘available’, there may be a number of actions that you can take that allow you to have availability but with lower costs and fewer meta-spares. These actions might include consignment stocking, identifying effective duplication and reviewing supply chain opportunities. important Our Students Call It CSI for Engineers. We call it “Practical Plant Failure Analysis” It’s a reasonably-priced practical seminar with hundreds of hands-on examples designed for plant people and engineers. You’ll work with gears, bearings, shafts, belts, fasteners, seals, and a selection of corrosion examples. With the hands-on analyses, you’ll learn how and why they fail – and how to diagnose the multiple causes. The public session is three days long and is held in Syracuse,NY in November. Private sessions range from two to four days and can be held at your site. From the Reliability Professionals who “wrote the book on logical and practical plant failure analysis”. For more details contact Dale Gamba at 315-487-4390 or email us at reliable@twcny.rr.com We protect your investments like they were our own. Vibra Metrics • Vibration Sensors & Online Monitoring Products • Field Services, Engineering and Training 195 Clarksville Rd • Princeton Junction, NJ 08550 P:(609) 716-4130 • F:(609) 716-0706 • www.mistrasgroup.com www.uptimemagazine.com 51 http://www.mistrasgroup.com http://www.mistrasgroup.com http://www.uptimemagazine.com
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