U N I T - 2 M A I N T E N A N C E P O L I C I E S - P R E V E N T I V E

M A I N T E N A N C E

2 mark questions:

1) Define repair cycle. The repeated performance of inspection (I), minor repair (r1) and major or medium repair (r2) in sequence between successive overhauling (O) of equipments is known as repair cycle.

2) Define TPM.

It denotes “Total Productive Maintenance”. Total Productive Maintenance is a system of maintaining and improving the production and quality by keeping all the machines, equipments, processes and employees in top condition to avoid breakdowns and delays in the manufacturing process.

3) List out the objectives of corrective maintenance

Eliminate breakdowns

Eliminate deviations from optimum operating conditions

Eliminate unnecessary repairs. 4) What is predictive maintenance?

Predictive maintenance is a technique that uses regular evaluation of the actual operating conditions of plant equipments, production systems and plant management functions to optimize total plant operation.

5) What are the principle objectives of lubrication? The main objective of lubrication is to reduce the wear and heat between the contacting surfaces in relative motion. Lubrication also helps in

Reduction of oxidation and rust formation

Insulation in transformer application

Transmission of mechanical power in hydraulic power systems

Seal against dust, dirt and water

O I

r1

r2

I r1

I

r2

6) What is meant by downtime scheduling?

Downtime scheduling means to decide when the shutdown time will be done and who will do it without affecting the scheduled activity. The interval of time for which the machines and equipments are switched off is known as the downtime and it is important to keep machines at rest for an interval regularly to prevent failure and excessive heating

7) List out the types of maintenance systems

Preventive maintenance

Corrective maintenance

Predictive maintenance

Condition based maintenance

Reliability centered maintenance 8) Define environmental impact.

The factor or action which significantly affects the quality of the human environment is called as environmental impact. Industrial development, infrastructural projects and automotives cause the major damage to the earth’s biodiversity.

8-mark & 16-mark questions:

1) What are the categories of maintenance? Briefly explain planned maintenance.

Maintenance

Breakdown Maintenance Planned Maintenance

Preventive maintenance

corrective maintenance

predictive maintenance

condition based maintenance

reliability central maintenance

Planned Maintenance: Maintenance that is performed purposely and regularly in order to prevent a machine from deteriorating or breaking down is known as planned maintenance. 5 types of planned maintenance are:

Preventive Maintenance

Corrective Maintenance

Predictive Maintenance

Condition Based Maintenance

Reliability Central Maintenance

i. Preventive maintenance:

It is the set of periodic actions performed to prevent breakdown of machinery. It involves cleaning, lubrication, inspection, retightening, etc. to maintain machinery in good health.

Benefits:

Maintains the equipment in good condition

Detects problems in early stage

Increases the machine life

Reduces scrap/rework

Reduces the unplanned downtime

ii. Corrective maintenance:

Corrective maintenance is defined as a maintenance operation done in order to identify, isolate and rectify a particular fault. It improves equipment and its components so that preventive maintenance can be carried out reliably. Equipment with design weakness must be redesigned to improve reliability or improving maintainability.

The main objectives of corrective maintenance are:

Eliminate breakdown

Eliminate deviations from optimum working conditions

Eliminate unnecessary repair

Optimize all critical plant system

Requirements of corrective maintenance:

Existence of full time trained maintenance planners to identify problems accurately

Trained craftsmen with necessary skills to complete the repair of each problem

Standard maintenance procedure for recurring repair and tasks

Sufficient time to maintain machinery among the production schedule

Thorough verification process to ensure the completion of repair

iii.Predictive Maintenance:

It is the process of maintaining a particular set of machinery based on the prediction that it may fail soon. Machines to be repaired are identified by analyzing their performances and comparing them with standard/optimum performance. If a machine deviates from the optimum performance level, it is selected for repair. It will provide necessary data to schedule preventive and corrective maintenance.

Benefits of predictive maintenance:

Reduces uphold breakdown

Increases the production capacity

Increases the quality

Increases the overall effectiveness

Increases the profit by reducing the maintenance cost

iv. Condition Based Maintenance:

This maintenance is performed after one or more indicators show that equipment is going to fail or that equipment performance is deteriorating. It is the maintenance done when need arises. Following techniques are used to find if a machine needs repair:

Vibration monitoring – study of vibration or noise to find the plant condition

Thermography – study of temperature/infra-red emission to find the plant condition

Tribology – study of condition of bearings, lubrication, supports, etc. by lubricating oil analysis, spectrographic analysis, ferrographic and wear particle analysis.

Electrical Motor Analysis – study of motor’s performance

Visual inspection – study based on seeing the machinery and predicting faults through experience.

v. Reliability Centered Maintenance (RCM):

It is a systematic and step-by-step instructional tool for selecting applicable and appropriate maintenance operation types. It helps in how to analyze all failure modes in a system and define how to prevent those failures at an early stage. It is similar to Failure Mode and Effect Analysis (FMEA). The processes of RCM are:

Target products/system of maintenance should be clearly identified and necessary data should be collection.

All possible failures and their effect on target products/systems are systematically analyzed.

Preventive or corrective maintenance is done

Applications of RCM:

When designing, selecting and installing new systems in a plant

When setting up preventive maintenance for complex equipment for which we are not clear about how they will work

While teaching the workers about maintenance techniques, RCM is greatly helpful

2. Discuss in detail about preventive maintenance.

It is the set of periodic actions performed to prevent breakdown of machinery. It involves cleaning, lubrication, inspection, retightening, etc. to maintain machinery in good health. The primary goal of preventive maintenance is to prevent the failure of equipment before it actually occurs. 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.

Preventive maintenance may lead to excessive spending of money on lubricating, inspecting and cleaning machines which may increase the total cost of the product significantly.

Real life illustration:

An individual bought an incandescent light bulb. The manufacturing company mentioned that the life span of the bulb is 3 years. Just before the 3 years, the individual decided to replace the bulb with a new one. This is called preventive maintenance.

On the other hand, the individual has the opportunity to observe the bulb operation daily. After two years, the bulb starts flickering. The individual predicts at that time that the bulb is going to fail very soon and decides to change it for a new one. This is called predictive maintenance.

The individual ignores the flickering bulb and only goes out to buy another replacement light bulb when the current one fails. This is called corrective maintenance.

Benefits:

Maintains the equipment in good condition

Detects problems in early stage

Increases the machine life

Reduces scrap/rework

Reduces the unplanned downtime

Inspect Lubricate

Retighten Measure performance

Plan for the next maintenance cycle

3. Explain maintenance scheduling.

Maintenance scheduling is a joint maintenance operation activity in which management agrees to make the resources available at a specific time when the unit can also be made available by operations. Resources include manpower, materials, tools and special equipments.

The work scheduling should be aimed at to have least adverse effect on normal operating schedule while optimizing the use of maintenance resources –especially labor. Scheduling should be viewed as ‘marketing arm of the program on start up of any new maintenance management installation’. Scheduling makes need of positive participation of users of maintenance services and unlike preventive maintenance results in tangible measurable results, often within a week. Work scheduling in the vehicle that facilitates the ability of maintenance to meet the challenge of creating maintenance operation that is both effective and responsive. Also it helps to meet the needs of partners in the operation unit.

The success of any maintenance schedule depends on two basic elements:

1. Work should not be scheduled which is not completely ready for scheduling, regardless of pressure

2. People from other operating units should be involved while creating the schedule.

Communication is the main key to establish successful maintenance scheduling. This involves everyone from planners, schedulers, maintenance supervisor, craftsman, and store room personnel, operations superintendent to operator who is responsible is to keep the equipment secure and ready for maintenance. Any discontinuity in communication will heavily affect the success rate.

Different stake holders of maintenance scheduling and their role:

1. Planner – Should ensure that the work is properly planned with respect to the requirements

2. Scheduler – Should ensure the availability of trades, materials and service during scheduling

3. Maintenance supervisor – Should attend to specific tasks such as who, what, where and when.

4. Craftsman – Executes the assigned task and keep informing the maintenance team the outcome and other deviation

5. Storeroom personnel – Maintain a record of receipt of goods

6. Operations Superintendent – Should be aware of the production schedule and determine the opportune time with maintenance to release the equipment.

7. Operator – responsible for securing the equipment and report back to maintenance personnel if any deviation is observed.

Maintenance Scheduling Principles (Doc Palmer’s 6 points):

Scheduling Principle 1 obligates each job plan to estimate labor hours and craft skill levels.

Scheduling Principle 2 encourages not interrupting jobs already in progress through proper prioritization of work.

Scheduling Principle 3 commits crew leaders to forecast labor hours for craft skills available for the next week.

Scheduling Principle 4 combines all of the forecast crew labor hours with the estimated labor hours of the planned jobs, generally in order of job priority.

Scheduling Principle 5 has crew leaders schedule and assign daily work (even though the planning department allocates the week’s goal of work).

Scheduling Principle 6 establishes the importance of measuring schedule success. Measuring this outcome helps management insure that planning and scheduling does take place.

Maintenance Scheduling principles

Importance of schedules & job priorities

Crew leader handles: i. daily schedule

ii. Assigns names to tasks iii. Co-ordination of resources & clearences

iv. Emergencies

4. Explain repair cycle.

Generally, the maintenance scheduling embraces the following activities:

i. Inspection ii. Repair iii. Overhauling

Here, the term repair does not reflect the actual meaning but only the times duration consumed to perform the corrective action. Based on the time the repair may be minor one like adjustment of fasteners, belt tension, etc. or major one like reconditioning the bed surfaces, guide ways, cleaning of bearings, etc.

To create maintenance scheduling program, the various maintenance activities may be classified into four categories which are as follows:

1. Inspection (I) 2. Minor repair (r1) 3. Major or medium repair (r2) 4. Overhauling (O)

The repeated performance of inspection (I), minor repair (r1) and major or medium repair (r2) in sequence between successive overhauling (O) of equipments is known as repair cycle.

It is clear from the above figure that first an inspection activity is scheduled and followed by minor/major repair. Then an inspection takes place followed by a minor repair. Again a second inspection takes place followed by a major repair. Likes this it goes and complete one repair cycle. This can be represented as O1-I1-r1-I2-r1-I3-r2.

From the above it is understood that the repair cycle is mainly time dependent between activities. An index number generally known as “Repair Complexity Number” is used to denote the complexity of repairing equipments. More the number more will be activities involved and in turn, more staffing is required to complete the cycle.

O I

r1

r2 I

r1

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r2

5. Explain the principle and methods of lubrication.

Principle of lubrication:

Lubrication works on the principle of viscous reduction of frictional wear. The lubricant reduces friction between the contacting surfaces, extracts heat from the machinery and dissipates it to the atmosphere and also protects the machinery from corrosion. Different types of lubrication are chosen according to the factor (R) which is mathematically defined as,

For Boundary lubrication, R =1 For Fluid Film lubrication, 5 ≤ R ≤100 For Mixed lubrication, 1≤ R ≤ 5

Methods of Lubrication:

a. Hydrostatic Lubrication b. Hydrodynamic or Fluid Film Lubrication c. Boundary Lubrication d. Elasto-hydrodynamic Lubrication (EHD) e. Extreme pressure (EP) lubrication

I. Hydrostatic Lubrication:

Hydrostatic lubrication is when an external pressure is applied to the lubricant in the bearing, to maintain the fluid lubricant film where it would otherwise be squeezed out. A thin film of lubricant is created between the journal and the bearing by the pump pressure. It is also called as externally pressurized bearing.

II. Hydrodynamic lubrication:

Hydrodynamic lubrication is the term given when a shaft rotating in a bearing is supported by a layer or wedge of oil so that the shaft is not in contact with the bearing material. It is also called fluid-film, thick-film, or flooded lubrication.

Characteristics:

A thick film of lubricant is interposed between the surfaces of bodies in relative motion.

There has to be pressure buildup in the film due to relative motion of the surfaces.

Fluid friction is substituted for sliding friction.

Coefficient of friction is decreased.

Prevalent in journal and thrust bearings

It is classified into two types namely,

(i) Journal bearing: Journal or plain bearings consist of a shaft or journal which rotates freely in a supporting metal sleeve or shell. There are no rolling elements in these bearings. Their design and construction may be relatively simple, but the theory and operation of these bearings can be complex. This article concentrates on oil- and grease-lubricated full fluid film journal bearings; but first a brief discussion of pins and bushings, dry and semi lubricated journal bearings, and tilting-pad bearings.

(ii) Thrust bearing:

A thrust bearing is a particular type of rotary rolling-element bearing. Like other bearings they permit rotation between parts, but they are designed to support a predominately axial load.

III. Boundary lubrication:

Boundary lubrication exists when the operating condition are such that it is not possible to

establish a full fluid condition, particularly at low relative speeds between the moving or sliding surfaces. The oil film thickness may be reduced to such a degree that metal to metal contact occurs between the moving surfaces. The oil film thickness is so small that oiliness becomes predominant for boundary lubrication.

Characteristics: -Boundary lubrication happens when a shaft starts moving from rest. -The speed is very low. -The load is very high and the viscosity of the lubricant is too low.

Examples for boundary lubrication: Guide and guide shoe in two stroke engine, lubrication of the journal bearing in diesel engines (mainly during starting and stopping of engine). Piston rings and when cylinder liner is at TDC and BDC position when the piston direction changes and if the relative speed is very slow.

IV. Extreme pressure Lubrication:

When the moving or sliding surfaces are under very high pressure and speed, a high local temperature is attained. Under such condition, liquid lubricant fails to stick to the moving parts and may decompose and even vaporize. To meet this extreme pressure condition, special additives are added to the minerals oils. These are called “extreme pressure lubrication.” These additives form on the metal surfaces more durable films capable of withstanding high loads and high temperature. Additives are organic compounds like chlorine (as in chlorinated esters), sulphur (as in sulphurized oils), and phosphorus (as in tricresyl phosphate).

V. Elasto-hydrodynamic (EHD) lubrication:

A “thick” fluid film, low friction and no wear are the defining characteristics of hydrodynamic lubrication, which generally occurs at conformal contacts. A lubricated non-conformal contact will experience elastohydrodynamic lubrication (EHD).

6) a. Compare TQM and TPM

The Total Productive Maintenance (TPM) program closely resembles the popular Total Quality Management (TQM) program. Many of the tools such as employee empowerment, benchmarking, documentation, etc. used in TQM are used to implement and optimize TPM. Following are the similarities between the two:

Total commitment to the program by upper level management is required in both programs

Employees must be empowered to initiate corrective action, and

A long range outlook must be accepted as TPM may take a year or more to implement and is an on-going process. Changes in employee mind-set toward their job responsibilities must take place as well.

The differences between TQM and TPM are summarized below:

Enrollment in local colleges, 2005

Category TQM TPM

Object Quality (output and effects) Equipment (Input and cause)

Means of attaining goals

Systematize the management. It is software oriented

Employees participation and it is hardware oriented

Target Quality for PPM Elimination of losses and wastes

6) b. Discuss the various stages involved in implementation of TPM.

Stages of implementation of TPM:-

Initialization

Introduction of TPM

Implementaiton of TPM

Institutionalization

PREPARATORY STAGE:

STEP 1 - Announcement by Management to all about TPM introduction in the organization:Proper understanding, commitment and active involvement of the top management in needed for this step. Senior management should have awareness programmes, after which announcement is made to all. Publish it in the house magazine and put it in the notice board. Send a letter to all concerned individuals if required.

STEP 2 - Initial education and propaganda for TPM: Training is to be done based on the need. Some need intensive training and some just an awareness. Take people who matters to places where TPM already successfully implemented.

STEP 3 - Setting up TPM and departmental committees: TPM includes improvement, autonomous maintenance, quality maintenance etc., as part of it. When committees are set up it should take care of all those needs.

STEP 4 - Establishing the TPM working system and target: Now each area is benchmarked and fix up a target for achievement.

STEP 5 - A master plan for institutionalizing: Next step is implementation leading to institutionalizing wherein TPM becomes an organizational culture. Achieving PM award is the proof of reaching a satisfactory level.

INTRODUCTION STAGE

This is a ceremony and we should invite all. Suppliers as they should know that we want quality supply from them. Related companies and affiliated companies who can be our customers, sisters concerns etc. Some may learn from us and some can help us and customers will get the communication from us that we care for quality output.

IMPLEMENTATION STAGE

In this stage eight activities are carried which are called eight pillars in the development of TPM activity. Of these four activities are for establishing the system for production efficiency, one for initial control system of new products and equipment, one for improving the efficiency of administration and are for control of safety, sanitation as working environment.

INSTITUTIONALISING STAGE

By all their activities one would has reached maturity stage. Now is the time for applying for PM award. Also think of challenging level to which you can take this movement.