Maintenance may be classified as:
(a) Breakdown or corrective maintenance
(b) Preventive maintenance
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(c) Scheduled maintenance
(d) Predictive maintenance.
(a) Breakdown or Corrective Maintenance:
It refers to the repair work undertaken after the failure of machine or equipment e.g., an electric motor will not start, a belt is broken, etc. The replacement of the torn belt is a case of breakdown Maintenance.
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Under such conditions, production department call on the maintenance department to rectify the defect. The Maintenance department checks into the difficulty and makes the necessary repairs. After removing the fault, maintenance engineer do not attend the equipment again until another failure or break down occurs.
The causes for breakdown or the need for breakdown maintenance include:
(i) Failure of lubrication.
(ii) Indifference towards minor fault.
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(iii) Indifference towards equipment vibrations, unusual sounds coming out of rotating machinery, equipment getting too much heated up etc.
(iv) Failure to replace worn out parts.
(v) Neglected cooling system.
(vi) External factors (such as too low or too high line voltage, wrong fuel etc).
This type of breakdown is suitable for small factories which are indifferent to the benefits of scheduling, do not feel a financial justification for scheduling techniques and get seldom (temporary or permanent) demand in excess of normal operating capacity. It is also suitable for factories which use those types of equipments who are down time and repair costs are less this way than with any other type of maintenance.
Merits:
1. Breakdown type of maintenance involves little administrative work, few records and a comparative small staff.
2. There is no planned interference with production programmes.
Demerits:
1. Reduction in output
2. Spoilage of material
3. Direct loss of profit
4. Deterioration of plant
5. Increased chances of accidents
6. Excessive delays in production.
(b) Preventive Maintenance:
Preventive maintenance is a system of scheduled, planned or preventive maintenance with the objective to minimise the problem of breakdown maintenance. It is done before the failure arises or prior to the equipment actually breakdown. It is a safety measure designed to minimise the possibility of unanticipated breakdowns and interruptions in production.
Preventive maintenance includes:
1. Usual case of equipment, such as lubricating, cleaning, adjusting, resulting the equipments or parts while the equipment is either idle or running.
2. Periodic inspection or overhaul of equipment and scheduled replacement of parts.
3. Contingent work done at regular intervals when the equipment is down.
4. Conditioned monitoring, which is a new technique where high cost critical equipments are monitored continuously or at frequent periodic intervals to observe the values of various parameters reflecting the condition of the equipments and the components within the parameters being monitored, may be temperature, vibration, noise, lubricant, corrosion, leaks and others.
Preventive maintenance consists of following steps (Fig. 25.2):
The main problem in preventive maintenance is to decide accurately when equipment should receive attention.
Objectives:
The reasons and objectives of doing preventive maintenance are:
1. To maintain the operational accuracy of the plant equipments.
2. To maintain the optimum productive efficiency of the plant, equipment and machinery.
3. To minimise the possibility of unanticipated production interruption or major breakdown by locating or uncovering any condition which may lead to it?
4 To ensure safety of life and limbs of the workmen.
5- To reduce the work content of maintenance job.
6. To maintain the value of equipment and machinery by periodic inspections, repairs, overhauls etc.
7. To make plant, equipment and machinery always available and ready for use.
Benefits:
1. Reduction of total downtime and consequent reduction in production losses.
2. Lesser immediate time repairs and reduced overtime to the maintenance work force.
3. Greater safety for workers.
4. Reduction in the number of rejects and an improvement in product quality.
5. Increased equipment life.
6. Better industrial relations because production worker do not face unvoluntary lay off or loss of incentive bonus because of breakdowns.
Besides these benefits, there are certain costs associated with the preventive maintenance programme, which include:
1. Replacement parts of supplies.
2. Remuneration of preventive maintenance technicians and staff.
3. Scheduled down time of production.
4. Record keeping costs though they are minor.
(c) Scheduled Maintenance:
This includes all work undertaken to keep the production equipment in efficient condition. It may cover periodic inspection, cleaning, lubrication, overhaul, repair, replacement, etc. All those operations are performed either while the equipment is running or during pre-planned shutdowns.
Scheduled maintenance is a stitch in time procedure aimed at avoiding breakdowns as the breakdowns can be dangerous to the life of the workmen and as far as possible should be minimised.
Scheduled or routine maintenance is of two types:
(i) Running maintenance
(ii) Shutdown maintenance.
Scheduled maintenance practice is generally followed for overhauling of machines, cleaning of water and other tanks, whitewashing of building etc.
(d) Predictive Maintenance:
In this technique, on the prediction of any fault, maintenance is being done. It is comparatively a newer maintenance technique.
For example, unusual sounds coming out of rotating equipment predict a coming trouble, or an electric cable excessively hot at one point predicts a trouble.
In predictive maintenance, equipments’ conditions are measured periodically or on a continuous basis and this enable maintenance men to take a timely action such as equipment adjustments, repair or overhaul. Predictive maintenance extends the service life of equipment without fear of failure.