Poisson Distributions

     
Failure Rate (l)  per 106 Hours
Repair Time (R)  Hours
Total # of Assemblies (n)  
Minimum # Spares (s)  
Confidence (CL)  
Calculate:
Minimum # Spares (s)
Confidence (CL)
   

This calculator calculates the sparing needs to satisfy a set of given conditions. The formula is as follows:
where:
n=number of Line Replacement Units in service
l= failure rate per 106 operating hours
R= repair time in hours
s= minimum number of spares required
CL=confidence level
Example use of the Calculator:
The plan is to sell 2,000 assemblies. The failure rate for thse assemblies has been determined to be 121.7 failure per million hours. The time to repair an assembly is 4 hours. The desire is to provide only 1 spare assembly. The goal is to be 90% confident that 1 assembly will be adequate. Using the calculator as indicated, the resulting confidence is 0.7455. In order to be 90% confident, the number of spares must be increased.
Enter: l = 121.7
  R = 4
  n = 2000
  s = 1
Select:
"Confidence"
Press Calculate
Solves for: CL = .7455
The plan is to sell 2,000 assemblies. The failure rate for these assemblies has been determined to be 121.7 failure per million hours. The time to repair an assembly is 4 hours. The goal is to be 90% confident that the number of spares provided will be adequate. How many spare assemblies should be on hand? Using the calculator as indicated, to be 90% confident the number of spares provided will be adequate, a minimum of 2 spares will be required.
Enter: l = 121.7
  R = 4
  n = 2000
  CL = .90
Select:
"Minimum # Spares"
Press Calculate
Solves for: s = 2