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Chapter 4
PROBLEM 4,2
The motor in problem 1 is improved by using sealed bearings and by using a
better quality alloy in the casing and impeller. The improved motor has the
following reliability characteristics:
Electrical failure (insulation, windings, etc.) MTTF -- 20,000 hr
Mechanical failure (impeller, casing, etc.) MTTF = 20,000 hr
Bearing wearout (2 brngs)/~ = 2500
~r = 600 hr, each
Brush wearout (2 brushes) # = 1000
o- = 200 hr, each
(a) What is the reliability of the improved model at 500 hr
(b) If the manufacturer wishes to have a replacement or repair rate of
5 motors per 100 sold, what should be his guarantee period?
PROBLEM 4.3
The following data were obtained from the reliability testing of a group of
special gear boxes:
Hours×104
0-1 1-2 2-3 3-4 4-5 5-6 6~7 7-8
Numbers failed
21 12 6 3
1
1
0
1 total = 45
Find the MTTF by plotting the data on semi-log paper.
PROBLEM 4.4
The following data were obtained from tests on hydraulic valves:
Hours cycles× 103 0-1
1-2 2-3
3-4
4-5
5-6
Numbers failed
11
6
3
2
1
1 total = 24
Find the MTTF by plotting the data on semi-log paper.
PROBLEM 4.5
The main boiler feed pump in a power plant has a MTTF = 200,000 hr.
(a) Find the reliability after one year of continuous operation at
24 hr/day, 7 days/week.
(b) Find the reliability after one year of operation at 40 hr/week.