Mechanical Systems Reliability, MP3701
Your assignment must be submitted electronically via eLearn (Assignments tab) by the submission time, and a paper version should be submitted to the CEPS office by the same time. All materials should be contained in a Word document. No other means of submission will be accepted.
Any references to materials should be given in standard Harvard form.
The assignment should be your own work and written in your own words. Any plagiarism or copying from others will be dealt with through the university's plagiarism procedures.
10% of students will be chosen for an oral examination to assess if the work submitted is their own work.
Learning Outcome to be assessed:
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2.
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Analyse and predict the statistical behaviour of machine failures.
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Mechanical Systems Reliability - Assignment 2010/11
Widgets plc is an engineering company that manufactures domestic washing machines. You are a consultant that has been asked to review the effectiveness of Widgets' ability to test its washing machines. The Director of Engineering has asked you to examine a test report (attached) from the company's Test Department; and provide her with a critical review of the procedures and analysis that were undertaken, and the conclusion that was made.
Provide a report for the Director that contains:
- A description of the approach that was taken to evaluate the reliability of the machines.
- A critical review of this approach; suggesting ways in which you believe improvements could be made, both in the analysis and the procedures followed. This should include any additional analysis that you think should be undertaken.
- Any revised conclusions.
The whole report should be no more than 3,000 words plus any diagrams, charts etc. You should submit a paper copy and also an electronic version. It will be checked against Turnitin for plagiarism.
Widgets plc.Test report
Subject: The new design of washing machine.
Sponsor: Director of Engineering.
Remit: We have been asked to test 20 of the new design of washing machine, and make recommendations on whether the 5-year guarantee currently offered can be sustained.
Method
20 machines were put on accelerated test in our laboratories. The machines were tested over a period of four weeks, and all failed within this period.
The failure data for 20 of the new design of washing machines is presented below, where the time to failure of each machine has been scaled up to estimate the actual life. Thus, for example, machine 3 actually lasted for 28 days, but because of the duty cycle employed, we can say that this represents 200 months of life in a typical household.
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Machine
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Months to failure
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Machine
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Months to failure
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1
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50.0
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11
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6.2
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2
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4.8
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12
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30.0
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3
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200.0
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13
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40.0
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4
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5.2
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14
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3.8
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5
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25.0
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15
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60.0
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6
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6.7
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16
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70.0
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7
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10.0
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17
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80.0
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8
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90.0
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18
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12.0
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9
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15.0
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19
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140.0
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10
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20.0
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20
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5.1
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Analysis
A two-parameter Weibull plot of the failure data is shown overleaf. We deduce from this that the characteristic life, T, is 34 months and beta is 0.92, suggesting a normal probability density function for the machines. The B10 life is determined as 100 months - hence 90% of the machines will survive for 84 months.
If we offer a 5-year guarantee on the machines, then the graph shows that 25% of machines will fail by 5 years - an average of 5% per year. This is better than our existing machines, so we will be able to make a profit even allowing for the warranty repair costs.
Conclusion
Our conclusion is that this a better design than the existing machines, and we should go ahead to mass production.