Maintenance Requirement Analysis, Other Engineering

Ask questioA wire rope used as a crane hoist rope has a mean life of 530 days. The wire rope predominantly fails in a wear-out mode caused by either corrosion, abrasive wear or fatigue. Of these, the dominant mode of failure is bending fatigue causing individual wires within the rope to break. The rope is also susceptible to random failures through damage caused by some form of overload with approximately 10% of failures attributable to overload.
A potential wear-out failure condition can be identified by a detailed inspection. For the fatigue failure the rope manufacturer’s product guide states that this particular size and style of rope should be replaced if there are more than 10 broken wires in any length of 0.6 metres. With 10 broken wires the average remaining life before failure is estimated to be 60 days. The likelihood that an inspector will miss 10 broken wires in any 0.6 metre length is deemed to be 10%.
In addition to a detailed periodic inspection, the rope is subject to a daily inspection by the operator and a detailed rope inspection can be initiated if there is any mal-operation involving the hoist. The daily inspection should identify any obvious rope damage and allow for a planned renewal.
The rope costs $9000 to replace and if it fails in service there is an additional secondary cost of $60,000 due to loss of production during the unscheduled repair. A detailed inspection costs $500 and can detect a potential failure by any of the failure modes. For the detailed periodic inspection assume the failure mode ratio is 0.9.
ENGG960/MECH487 - Maintenance Requirements Analysis
© UOW/P.Gordon/R.Dwight Page 3
1. Failure criticality can be quantified using the failure cost rate. What is the criticality of a wire rope failure measured in $/year? (4 Marks)
2. What is the optimum economic interval for the detailed periodic inspection of the wire rope? (6 marks)
3. If the rope is inspected at the optimum interval what does the failure cost rate ($/Year) reduce to? Assume the 10% of potential failures that occur through overload are detected by daily inspection. (4 Marks)
4. What is the total cost rate ($/year) for the preventive renewal of the rope? (6 Marks)
5. You were given the warning time and probability of successfully detecting a potential failure for the fatigue failure mode only but told to assume a failure mode ratio of 0.9 which included the other wear-out failure modes. What conditions need to apply for this analysis to be valid? (4 Marks)
6. The cost of failure in this example excludes any cost related to the possibility that a rope failure will result in an injury or death. Personnel access to areas where the crane is operating is restricted. For a fatality to occur when the rope failed the crane would have to have a suspended load and someone would have had to access the restricted area and be standing under the crane load presumably without the crane operator noticing them. If you assume the probability of this occurring coincident with a rope failure is 0.01 and you wish to ensure the probability of a failure causing a fatality is not above 0.0001 in the life of the rope, at what interval would you inspect the wire rope? (6 Marks)
7. What would the cost of failure have to be for the optimum economic inspection interval to be the same as that required to achieve a probability of a fatality below 0.0001 over the life of the rope. What is the cost of a fatality that is implied by this cost of failure? (10 Marks)
8. If you estimated the cost to the organisation of a fatality to be $2,000,000 and you inspected the rope at the resulting optimum economic interval what is the implied acceptable probability of failure. Assume, once again, that the probability of a failure resulting in a fatality is 0.01 and there is a production loss of $60,000 whenever there is a failure of the rope. (10 Marks)
9. What issues do you see with the inspection intervals calculated in Q2, 6 and 8. What other factors would you consider in determining an appropriate inspection interval? (10 Marks)n #Minimum 100 words accepted#

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