Managers have cause to ask for cost justifications or projections before they sign off on proposed equipment upgrades (see my Mar. 27, 2020, article). These projections are preferably on the safe side. They should not promise unrealistic returns on the investment. Here’s where some rule-of-thumb calculations can provide value.
EXAMPLE RULES OF THUMB
A realistic and time-tested return-on-the-investment calculation may be surprisingly simple. Let’s just call it another rule of thumb, one that uses an exponential approach. It states that if a fully upgraded machine has a reliability of 1.0, then a missed upgrade will lower the reliability to 90% of 1.0 = 0.9; two missed upgrades to 90% of 0.9 = 0.81; three missed upgrades to 90% of 0.81= 0.73; four missed upgrades to 90% of 0.73, equaling only 0.66, and so forth. We consider this elementary rule of thumb rather optimistic; reliability with four deficiencies is probably less than 50% of what would be achievable with better bearings, better mechanical seals, better couplings or whatever other upgrades are achievable.
Here’s another rule of thumb worth sharing. Again, a reasonable assumption is made: A probable 20% improvement in failure avoidance, or repair-cost reductions, or life extension is thought to result from each upgrade. In that case, an upgrade will move the equipment reliability from 1.0 to 1.2. A second (different) upgrade would capture 1.2^2=1.44. Further upgrades would capture 1.2^3 = 1.73, and 1.2^4 = 2.07. The implementation of four proven upgrade measures would cause the MTBR (mean time between repairs) to be extended two-fold. Yearly repair expenditures would be one half of what they were before (and workers previously laboring on repairs would now spend time on repair-avoidance tasks). Safety would go up, community goodwill would be given a boost, and so would worker morale.
WHY BOTHER?
The value proposition is simple. Using a no-nonsense rule of thumb is clearly better than the other two widely used alternatives: doing nothing or offering a wild guess.TRR
ABOUT THE AUTHOR
Heinz Bloch’s long professional career included assignments as Exxon Chemical’s Regional Machinery Specialist for the United States. A recognized subject-matter-expert on plant equipment and failure avoidance, he is the author of numerous books and articles, and continues to present at technical conferences around the world. Bloch holds B.S. and M.S. degrees in Mechanical Engineering and is an ASME Life Fellow. These days, he’s based near Houston, TX. Email him directly at [email protected].
Tags: reliability, availability, maintenance, RAM, work processes, professional development
