Environmental Engineering Reference
In-Depth Information
It is now straightforward to show that
R
/1,000 at the various warranty intervals
may be easily calculated and converted to FIT or MTBF as follows:
R
1, 000
¼ð
l
t
Þ
1, 000
j
t
¼
T
R
1, 000
¼
t
ð
FIT
Þ
10
6
ð
6
:
29
Þ
1
10
9
MTBF
ð
1
Þ
FIT
¼
The expression in (6.29) shows that
t
=
T
is the prescribed number of operating
hours for a particular warranty interval. The warranty interval operating hours are
listed in Table 6.7.
Table 6.7 Definition of warranty versus operating time
Warranty interval
12/12
3/36
10/150
Operating time (h)
400
1,200
5,000
Rearranging (6.29) and using the operating hours for 10/150 warranty interval,
it is easy to show the equivalent
R
/1,000 for a given FIT level as
1, 000
10
=
150
¼
5, 000FIT
10
6
R
ð
6
:
30
Þ
According to (6.30), a failure level of 1 FIT equates to 0.005
R
/1,000 at 10/150
warranty interval. In a typical hybrid propulsion system, a major component, a
controller, for example, may have 30
R
/1,000 at 3/36 warranty interval. Table 6.8
summarizes this failure rate along with a comparison of the various reliability
metrics for 1-FIT level (comparison values are in bold for that row).
Table 6.8 Comparison of automotive warranty metrics
FIT
MTBF = 1/
ll
R
/1,000|
12/12
R
/1,000|
3/36
R
/1,000|
10/150
1
10
9
1
10
9
1
*
0.0004
0.0012
0.005
25
10
6
25,000
40,000
10
30
*
125
*The values in bold are comparison values for that row.
A failure level of 1 FIT is very stringent as compared to the failures in
R
/1,000
for 3/36 warranty interval (see Table 6.8). Here, the values differ by 25,000 (= 30/
0.0012), as would be expected because that the failure rates,
l
are in this ratio. So,
an effort to require that safety critical systems achieve integer FIT levels of