BULETINUL INSTITUTULUI POLITEHNIC DIN IAŞI

Bul. Inst. Polit. Iaşi, t. LVIII (LXII), f. 3, 2012 31
⎛ 1 ⎞
MTBF = μ= αΓ ⎜1+ ⎟= 2.016×Γ ( 1.24827)
= 2.016× 0.90675 = 1.82months,
⎝ β ⎠
(14)
where the value Γ(1.24827) =0.90675 is obtained form the tabulated values
chart of the Γ function. Then the frequency of failures will be
δ = 1/1.82 = 0.549 failures/month = 6.59 failures/year
f
Calculate the total costs per failures per year TCP f (using Eq. 9)
TCP = 6593 monetary units/year.
f
(15)
(16)
The equivalent annual total cost of 6593 monetary units represents the
value of money that will be needed every year to pay for the problems caused
by failures, during the 10 years of expected useful life;
Calculate the total costs per failures in present value NPV(TCP f ) (using
Eq. 10), for a period T=10 years and discount rate i=9%:
10
( 1+ 0.09)
−1
× ( + )
NPV ( TCP f
) = 6593 = 42631 monetary units.
10
0.09 1 0.09
4. Conclusions
(17)
1. The Weibull distribution failure rate model could contribute to the
enhancement of the specialized wheeled vehicle systems performances by
diminishing the uncertainty within the process of total life cycle cost estimation.
2. The calculus example emphasized the strong points and weak points of
the presented method as follows:
a) the obtained annual equivalent total cost, represent the mean value of
money that will be needed every year to pay for the problems of failures, during
the 10 years of expected useful life, with a discount factor of 9%. The frequency
of failures varies every year throughout the expected cycle of useful life.
b) in order to conduct an accurate assessment of the process of reliability
it is necessary that the data gathering must be based on a detailed and good
quality information about the more important types of failures and, if possible,
for similar operational conditions.
3. The main limitations of this method are:
a) regardless the use of the Weibull distribution, the model quantifies the
impact of the non-reliability annual costs in a constant way over the years of
expected asset useful life;
b) it restricts the reliability analysis to the exclusive use of the Weibull
distribution, excluding other existent statistical distributions, which could also
be used in the calculation of the MTBF and failure frequencies.