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What is Preventive Maintenance?
•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.
Reasons for the need of a PM system
•Business loss due to production delays
•Production of a higher quality product
•Need for a more organized, planned environment
Advantages of a PM system
•Reduced production downtime, resulting in fewer machine breakdowns.
•Better conservation of assets and increased life expectancy of assets, thereby eliminating premature replacement of machinery and equipment.
•Timely, routine repairs circumvent fewer large-scale repairs.
•If failure can not be prevented periodic inspections can help reduce its severity.
•Reduced cost of repairs by reducing secondary failures. When parts fail in service, they usually damage other parts.
•Identification of equipment with excessive maintenance costs
•The cost of planned maintenance is always cheaper than emergency maintenance
•Better quality products
•Improved safety and quality conditions.
•Are there any risks of running a PM system?
•If there are risks when would it be a good time to use PM?
Potential errors or damage of a PM system
•Damage to an adjacent equipment during a PM task.
•Reintroducing infant mortality by installing new parts or materials.
•Damage due to an error in reinstalling equipment into its original location.
A real life example of a PM error
A review of the data from fossil-fueled power plants that examined the frequency and duration of forced outages after a planned or forced maintenance outage reinforces our point. The data collected from those plants showed that of 3146 maintenance outages, 1772 of them occurred in less than one week after a maintenance outage. Clearly, this is pretty strong evidence that suggests that in 56% of the cases, unplanned maintenance outages were caused by errors committed during a recent maintenance outage.
When Does Preventive Maintenance Make Sense
•This is a logical choice that considers two conditions
–#1. The component in question has an increasing failure rate.
–#2. The overall cost of the preventive maintenance action must be less than the overall cost of a corrective action.
If both of these conditions are met PM would make sense
The Law of PM
•Higher the value of the plant assets and equipment per square foot of plant, the greater will be the return on a PM program.
E.g. -downtime in an automobile plant assembly line at one time cost $10,000 per minute. Relating this to lost production time an automobile manufacturer reported that the establishment of a PM program in their 16 assembly plants reduced downtime from 300 hours per year to 25 hours per year. With results such as this no well- managed plant can afford not to develop a PM program.
How to determine the optimum age of replacement?
How to determine the optimum age of replacement?
•A more technical approach.
•R(t) = reliability at time t.
•CP= cost of planned replacement.
•CU= cost of unplanned replacement
•CPUT(t) = The optimum replacement time
The key for a successful PM system
•Scheduling –Should be automated to the maximum extent possible
•Execution –Should be done before the actual break down occurs.
•The Bath Tub Model –Three stages of new parts
1.Infant morality stage
2.A fairly long run stage
3.Wear out stage
•Traditional View according to Bath Tub Model –Replace components just before they entered wear out stage
•Traditional view says –
Wear out stage = Increased rate of failure.
•Point to Ponder –
Since, wear out stage represents increased rate of failure, wouldn’t it be plausible to execute the PM program at this stage?
•United Airlines Research
Reliability Centered Maintenance based on research done by United Airlines and the rest of the aircraft industry showed that very few non-structural components exhibit bathtub curve characteristics. Their research showed that only about 11% of all components exhibit wear-out characteristics, but 72% of components do exhibit infant mortality characteristics. These same characteristics have been shown to apply in Department of Defense systems as well as power plant systems.
What really should PM focus on?
•Cleaning Found through testing
•Lubrication and inspection
•And correcting deficiencies
Predetermined parts replacement should be minimal and done only where statistical evidence clearly indicates wear-out characteristics
In the absence of data
•Age exploration programs
Cost shouldn’t be a primary factor in deciding against data collection for a PM
Motivating Preventive Maintenance Workers
•Training programs –most effective
•Establish inspection and preventive maintenance as a recognized, important part of the overall maintenance program.
•Assign competent, responsible people to the preventive maintenance program.
•Follow-up to assure quality performance and to show everyone that management does care.
•Provide training in precision maintenance practices and training in the right techniques and procedures for preventive maintenance on specific equipment.
•Set high standards.
•Publicize reduced costs with improved up-time and revenues, which are the result of effective preventive maintenance
The most commonly applied condition- based maintenance techniques are
vibration analysis, oil analysis, thermography, ultrasonics, electrical effects and penetrants.
Vibration can be defined as the movement of a mass from its point of rest through all positions back to the point of rest, where it is ready to repeat the cycle. The time it takes to do this is its period, and the number of repetitions of this cycle in a given time is its frequency
The severity of vibration is determined by the amplitude -or maximum movement -its peak velocity and peak acceleration. Vibration analysis in condition monitoring, is accomplished by comparing vibration characteristics of current operation to a baseline, measured when the machinery was known to be operating normally. The selection of the specific parameters to be measured depends primarily on the frequency of the vibration.
Vibration analysis techniques can be used to monitor the performance of mechanical equipment that rotates, reciprocates or has other dynamic actions. Examples include gearboxes, roller bearings, motors, pumps, fans, turbines, belt or chain drives, compressors, generators, conveyors, reciprocating engines and indexing machines
Ferrography and magnetic chip detection examine the iron-based wear particles in lubrication oils to determine the type and extent of wear, and can help determine the specific component that is wearing.
Spectrometric oil analysis measures the presence and amounts of contaminants in the oil through atomic emission or absorption spectrometry.
It is useful for determining not only iron, but also other metallic and not metallic elements, which can be related to the composition of the various machine components, like bearings, bushings, piston rings, etc. It is useful when wear particles are initially being generated in the early stages of failure, as they are small.
Chromatography measures the changes in lubricant properties, including viscosity, flash point, pH, water content and insoluble, through selective absorption and analysis.
The most common uses for thermography, which measures the surface temperature through the measurement of infra-red radiation, are for determining poor electrical connections and hot spots, furnace and kiln refractory wear and critical boiler and turbine component overheating. An infra-red camera shows surface temperature variations, calibrated to provide the absolute temperature or temperature gradients through black and white or color variations.
There are several techniques for ultrasonic testing, but they all are used to determine faults or anomalies in welds, coatings, piping, tubes, structures, shafts, etc. Cracks, gaps, buildups, erosion, corrosion and inclusions are discovered by transmitting ultrasonic pulses or waves through the material and assessing the resultant signature to determine the location and severity of the discontinuity. This technique is also used to measure flow rates.
Electrical Effects Monitoring
There are several tests for corrosion using a simple electric circuit monitored by varying degrees of sophisticated instrumentation. The Corrator uses the electro-chemical polarization method in a vessel with corrosive liquid. The Corrometer uses the electrical resistance across a probe inserted in the active environment eg. refinery process equipment.
Electrostatic and liquid dye penetrants are used to detect cracks and discontinuities on surfaces, caused in manufacturing, by wear, fatigue, maintenance and overhaul procedures, corrosion or general weathering. The penetrant is applied and allowed to penetrate into the anomalies. The surface is cleaned and the penetrant revealed through direct visual, fluorescent or electrostatic techniques.
Maintenance work carried out with forethought control and record. It can be applied to any of maintenance provided that
(a) The maintenance policy has been considered carefully.
(b) The maintenance policy is planned in advance.
(c) The work is controlled and directed to conform to the prearranged plan.
(d) Historical and statistical record are complied and maintained to assess the results and provide guide for future policy.
BENEFITS OF PLANNED MAINTENANCE
1.Greater Plant Availability
3.High Level of Output
5.Servicing and Adjustment is not overlooked
6.Improved Budget Control
7.Improved Stocks and Spares Control
8.Provision of Information for Realistic Forecasts
9.Focusing Attn. on Frequently Recurring Jobs.
ELEMENTS OF PLANNED MAINTENANCE (PM)
2.List of all Facilities and their Importance (What)
3. Identification (Coding)
4. Facility Register
Trends in Maintenance
•Production machinery is becoming more and more complex and maintenance personnel must keep pace
•Special training programs to maintain worker skill level
•Subcontracting service companies
•Production workers maintain own equipment
•Computer assistance in maintenance
Computer Assistance in Maintenance
•Scheduling maintenance projects
•Maintenance cost reports by production department, cost category, and other classifications
•Inventory status reports for maintenance parts and supplies
•Parts failure data
•Operations analysis studies
Maintenance Issues in Service Organizations
•Maintenance issues are not limited to manufacturing
•Transportation firms (airlines, trucking companies, package delivery services, railroads) must keep their vehicles in top operating condition
•Highway departments must maintain roadways
•Office personnel are reliant on copiers, printers, computers, and fax machines working properly
•As services become increasingly automated, service firms face more and more maintenance issues
Wrap-Up: World-Class Practice
•Empower workers so they “own” their machines
•Implement JIT to help reduce inventories and cycle time
•Invest in factory and service automation projects
•Utilize automated process sensing and control systems
•Use computers in maintenance management
•PM is not difficult to achieve.
•It is an essential part for cost reduction
•There is an element of risk involved in PM
•Careful planning and execution will reduce the risk of PM
•The biggest benefits of a PM program occur through painting, lubrication, cleaning and adjusting, and minor component replacement to extend the life of equipment and facilities.