Explain different types of maintenance giving suitable example. Suggest an organisation structure for the maintenance department of a medium- sized company.
Answer. The different types of maintenance are listed below:
1. Planned Maintenance improves uptime and quality of output and reduces repair maintenance costs through the continuous quality improvement of equipment operation.
Planned Maintenance provides guidelines for a total system of activities in which all employees work to improve the quality of product output, increase production uptime, reduce costs of operations and reduce the amount and complexity of machinery required. It includes scheduled and unscheduled maintenance programs with strategies for responding to machinery and equipment failures. Planned maintenance includes two main activities:
a) Preventive maintenance is a schedule of planned maintenance actions aimed at the prevention of breakdowns and failures. The primary goal of preventive maintenance is to prevent the failure of equipment before it actually occurs. It is designed to preserve and enhance equipment reliability by replacing worn components before they actually fail. Preventive maintenance activities include equipment checks, partial or complete overhauls at specified periods, oil changes, lubrication and so on. In addition, workers can record equipment deterioration so they know to replace or repair worn parts before they cause system failure. Recent technological advances in tools for inspection and diagnosis have enabled even more accurate and effective equipment maintenance. The ideal preventive maintenance program would prevent all equipment failure before it occurs.
b) Corrective Maintenance: Corrective maintenance consists of the action(s) taken to restore a failed system to operational status. This usually involves replacing or repairing the component that is responsible for the failure of the overall system. Corrective maintenance is performed at unpredictable intervals because a component's failure time is not known a priori. The objective of corrective maintenance is to restore the system to satisfactory operation within the shortest possible time. Corrective maintenance is typically carried out in three steps:
• Diagnosis of the problem. The maintenance technician must take time to locate the failed parts or otherwise satisfactorily assess the cause of the system failure.
• Repair and/or replacement of faulty component(s). Once the cause of system failure has been determined, action must be taken to address the cause, usually by replacing or repairing the components that caused the system to fail.
• Verification of the repair action. Once the components in question have been repaired or replaced, the maintenance technician must verify that the system is again successfully operating.
2. Routine Maintenance: Routine maintenance is such activities as cleaning, dusting, lubricating, checkup of important parts such as battery. These activities are to be performed on a daily or weekly basis. Some of these form part of scheduled maintenance. Routine maintenance, normally does not involve any replacement of parts.
3. Scheduled Maintenance: Scheduled maintenance is a maintenance activity undertaken on equipments as per a plan of action, which gives the sequence in which various jobs would be attended. The schedule gives the calendar day and time at which a particular job is undertaken.
A maintenance schedule is prepared for a week. As the job content of maintenance activity is variable, the schedule for the next day is usually firmed up at the end of each day. The scheduled maintenance activities may be preventive or break down in nature. The maintenance schedule is prepared based on certain rules such as:
• First come first served
• Emergency priority job first
• Shortest competition time job first.
• Longest competition time job first
4. Predictive Maintenance: Predictive maintenance allows plant management to control the machinery and maintenance programs rather than vice versa. In a plant using predictive maintenance, the overall machinery condition at any time is known, and much more accurate planning is possible.
Predictive maintenance utilizes many different disciplines, by far the most important of which is periodic vibration analysis. It has been shown many times over that of all the non-destructive testing that can be done on a machine, the vibration signature provides the most information about its inner workings.
Certain machines, which would affect plant operations adversely if they were to fail, can be subjected to continuous vibration monitoring, in which an alarm is sounded if the vibration level exceeds a predetermined value. In this way, rapidly progressing faults are prevented from causing catastrophic failures. Most modern turbine-driven equipment is monitored in this way.
Oil analysis and wear particle analysis are important parts of modern predictive programs, especially in critical or very expensive equipment. Thermography is the measurement of surface temperature by infrared detection, and is very useful in detecting problems in electrical switchgear and other areas where access is difficult.
Motor current signature analysis is another technique that is very useful in detecting cracked or broken rotor bars while the motor is in operation, and electrical surge testing of motor stators is used for detecting incipient electrical insulation failure.
Example: Toyota Motor Corp. is no different than any other company. Like the rest, the automaker wants maximum uptime at the least cost possible. But it differs from many manufacturers in that it has an aggressive program aimed at cutting costs by tens of percentage points. And intelligent predictive maintenance is an important element of its strategy for doing so.
Toyota management proved the value of investing in maintenance on a machining line in Japan a few years ago. A team of engineers, technicians and operators there was able to cut downtime due to maintenance problems in half by applying the fundamentals of predictive maintenance. The machining line had been running at about 82 percent of the time that it was supposed to operate. About 12 percent of the downtime was due to maintenance problems.
The Maintenance Department can be organized by trade classifications. This allows the two supervisors to provide technical assistance based on their specialized qualifications to their staff as well as develop and manage capital improvement projects in their areas of expertise. The individual technicians are either assigned regional area responsibilities or specific tasks such as emergency response or planned maintenance.
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