Maintenance Philosophy

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Maintenance Philosophy

• Single spares storage warehouse?
• Assumed to be the case
• Multiple spares storage warehouses?
• If so, each will need to be equally stocked to
  provide the necessary spare when needed.
• If it was determined that only 1 spare was
  mathematically needed to support the system, then
  the quantity will need to be increased to match
  the number of spare storage warehouses.

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Spares Calculation Parameters

• Annual Operating Requirement (AOR) in Hours
• Pipeline Time (average time to receipt of part
  after ordering)
• Target Confidence Factor value (probability of
  having a spare when needed) usually 95
• Total system quantity for parts to be spared

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Confidence Factor Formula

• The confidence factor value is the probability
  that a spare will be available in the event of a
  failure, expressed as a percentage. The higher
  the value, the greater the probability that a
  spare will be available.
• Higher spares quantities yield higher confidence
  factor values.
• Lower pipeline time yields higher confidence
  factor values.
• Higher MTBF values yield higher confidence factor
  values.
• Lower system quantities yield higher confidence
  factor values.

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Spares Modeling
Goal Determine a recommended quantity of spare
parts to achieve a desired confidence
level. Tool Applies the following logic to
determine the recommended Spare Quantity value.
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Spares Modeling
This example shows how variable inputs impact the
confidence factor
Confidence Factors expressed as
percentages. System Qty represents how many of
the spared item are contained in a system.
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Replenishment Methodology Example
For module XYZ12345, expected failures for 60
months is 0.26 The condemn rate indicates the
percentage of failed assemblies that cannot be
repaired after failure (5 for our example) When
this occurs, a Replenishment Spare is needed as
a replacement. Replenishment Spares values are
driven by both the part criticality and the
comfort level of program management
Minimal Sparing (expected failures condemn
rate) 0.0 to 0.99 no spare 1.0 to 1.99 1
spare 2.0 to 2.99 2 spares Etc
Average Sparing (expected failures condemn
rate) 0.0 to 0.49 no spare 0.5 to 1.49 1
spare 1.5 to 2.49 2 spares Etc
Maximize Sparing (expected failures condemn
rate) 0.01 to 1.0 1 spare 1.01 to 1.99 2
spare 2.01 to 2.99 3 spares Etc
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Replenishment Quantity Report
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Spares Cost Report
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Expensive Spares Trade-offs
Here is an example to illustrate the impact of
reducing the spare quantity by 1 of the most
expensive items on the list. In some cases, the
Confidence Factor may still be an acceptable
level considering the money saved. For example,
lets say we have a radar system and we are
looking to spare the most expensive part in the
system. In our example, the system normally
requires 3 spares to support 4 radar systems (for
Annual Operating Requirement of 2000 hours) to
have a Confidence Factor greater than 95.
Consider the following 3 spares yields a 98.23
Confidence Factor 2 spares yields a 92.36
Confidence Factor Is the trade-off acceptable?
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For more information contact Rich
Herman 972-344-6179 rherman_at_raytheon.com