Scheduled Preventive Maintenance





When most people think of preventive maintenance, they visualize scheduled, fixed-interval maintenance that's done every month, every quarter, every season, or at some other predetermined intervals. That timing may be based on days, or on intervals such as miles, gallons, activations, or hours of use. The use of performance intervals is itself a step toward basing preventive tasks on actual need instead of just on a generality.

The two main elements of fixed-interval preventive maintenance are procedure and discipline. Procedure means that the correct tasks are done and the right lubricants applied and consumables replaced at the best interval. Discipline requires that all the tasks are planned and controlled so that everything is done when it should be done. Both of these areas deserve attention. The topic of procedures is covered in detail in the following sections.

Discipline is a major problem in many organizations. This is obvious when one considers the fact that many organizations don't have an established program.

Further, organizations that do claim to have a program often fail to establish a good planning and control procedure to ensure accomplishment. Elements of such a procedure include the following:

1. Listing of all equipment and the intervals at which they must receive preventive maintenance

2. A master schedule for the year that breaks down tasks by month, week, and possibly even to the day 3. Assignment of responsible persons to do the work

4. Inspection by the responsible supervisor to make sure that quality work is done on time

5. Updating of records to show when the work was done and when the next preventive task is due

6. Follow-up as necessary to correct any discrepancies.

Note that there are variations within the general topic of scheduled fixed interval maintenance. Some tasks will be done every Monday whether or not they are necessary. Inspection may be done every Monday and preventive tasks done if a need is indicated. Seasonal maintenance may be directed by environmental changes rather than by strict calendar date. Use meters, such as an automobile odometer, allow quantitative measure of use that can be related to the parameters that will need to be maintained. One must consider the relationship of components to the meter readings; for example, a truck's need for maintenance will vary greatly depending on whether it's used for long hauls or for local deliveries. A truck that's started every few miles and driven in stop-and-go, dusty city conditions will need more frequent mileage maintenance than the same truck used for long trips of continuous driving.

Seasonal equipment such as air conditioners, lawn mowers, salt spreaders, and snow blowers require special maintenance care at the end of each season to clean and refurbish them and store them carefully so that they will not deteriorate and will be ready for the next season. A lawn mower, for example, should have all gasoline drained from the tank and then be run until it stops because it has completely run out of fuel. This ensures that gasoline is completely removed from the lines. Oil should be changed. The spark plug should be removed and cleaned. A tablespoon of engine oil should be poured into the cylinder through the spark plug hole and the cylinder pulled through several strokes to ensure that's well lubricated. The spark plug should be put back in its hole loosely. Grass, dirt, and other residue should be thoroughly cleaned from all parts of the mower.

The blade should be sharpened and checked to see that's in good balance. The mower should be stored in a dry place until it's needed again. Then, when the grass starts growing, all one has to do is fill the tank, tighten the spark plug, and connect the ignition wire. The motor should start on the second try. Careful preparation of equipment for storage will pay a major dividend when the equipment is needed in a hurry.

LUBRICATION

Friction of two materials moving relative to each other causes heat and wear.

Great Britain has calculated that friction-related problems cost their industries over $1 billion per annum. They coined a new term, tribology (derived from the Greek word tribo s, which means ''rubbing''), to refer to new approaches to the old dilemma of friction, wear, and the need for lubrication.

Technology intended to improve wear resistance of metal, plastics, and other surfaces in motion has greatly improved over recent years, but planning, scheduling, and control of the lubricating program is often reminiscent of a plant handyman wandering around with his long-spouted oil can.

Anything that's introduced onto or between moving surfaces to reduce friction is called a lubrican t. Oils and greases are the most commonly used substances, although many other materials may be suitable. Other liquids and even gases are being used as lubricants. Air bearings, for example, are used in gyroscopes and other sensitive devices in which friction must be minimal. The functions of a lubricant are to

1. Separate moving materials from each other to prevent wear, scoring, and seizure

2. Reduce heat

3. Keep out contaminants

4. Protect against corrosion

5. Wash away worn materials.

Good lubrication requires two conditions: sound technical design for lubrication and a management program to ensure that every item of equipment is properly lubricated.

Lubrication Program Development

Information for developing lubrication specifications can come from four main sources:

1. Equipment manufacturers

2. Lubricant vendors

3. Other equipment users

4. Individuals' own experience.

As with most other preventive maintenance elements, initial guidance on lubrication should come from manufacturers. They should have extensive experience with their own equipment both in their test laboratories and in customer locations.

They should know which parts wear and are frequently replaced. Therein lies a caution-a manufacturer could in fact make short-term profits by selling large numbers of spare parts to replace worn ones. Over the long term, however, that strategy will backfire and other vendors, whose equipment is less prone to wear and failure, will replace them.

ill. 5.1 Recommended lubricants.

- - -

Table 5.1 Lubrication Codes

Methods of Application Servicing Actions

ALS Automatic lube system CHG Change ALL Air line lubricator CL Clean BO Bottle oilers CK Check DF Drip feed DR Drain GC Grease cups INS Inspect GP Grease packed LUB Lubricate HA Hand applied HO Hand oiling

Servicing Intervals

ML Mechanical lubricator H Hourly MO Mist oiler D Daily OB Oil bath W Weekly OC Oil circulation M Monthly OR Oil reservoir Y Yearly PG Pressure gun NOP When not operating RO Ring oiled OP OK to service when operating SLD Sealed SFC Sight feed cups

Service Responsibility

SS Splash system MAE Maintenance electricians WFC Wick feed oil cups MAM Maintenance mechanics WP Waste packed MAT Maintenance trades OPR Operating personnel OIL Oiler

- - - ill. 5.2 Typical lubrication schedule.

Lubricant suppliers can be a valuable source of information. Most major oil companies will invest considerable time and effort in evaluating their customers' equipment to select the best lubricants and frequency or intervals for change.

ill. 5.1 shows a typical report. Naturally, the vendor hopes that the consumer will purchase his lubricants, but the total result can be beneficial to everyone.

Lubricant vendors perform a valuable service of communicating and applying knowledge gained from many users to their customers' specific problems and opportunities.

Experience gained under similar operating conditions by other users or in your own facilities can be one of the best teachers. Personnel, including operators and mechanics, have a major impact on lubrication programs. Table 5.1 shows typical codes for methods of lubrication, intervals, actions, and responsibility.

ill. 5.2 shows a typical lubrication schedule. Detailing of specific lubricants and intervals will not be done here, since they can be more effectively handled by the sources listed above.

The quality and the quantity of the lubricant applied are the two important conditions of any lube program. Lubrication properties must be carefully selected to meet the operating conditions. The viscosity of the oil (or the base oil, if grease is used) and additives to provide film strength under pressure are especially important for bearing lubrication.

Too little lubricant is usually worse than too much, but excess can cause problems such as overheating and churning. The amount needed can range from a few drops per minute to a complete submersion bath.

A major step in developing the lubrication program is to assign specific responsibility and authority for the lubrication program to a competent maintainability or maintenance engineer. The primary functions and steps involved in developing the program are to:

1. Identify every piece of equipment that requires lubrication

2. Ensure that every major equipment is uniquely identified, preferably with a prominently displayed number.

3. Ensure that equipment records are complete for manufacturer and physical location

4. Determine locations on each piece of equipment that needs to be lubricated

5. Identify lubricant to be used

6. Determine the best method of application

7. Establish the frequency or interval of lubrication

8. Determine if the equipment can be safely lubricated while operating or if it must be shut down

9. Decide who should be responsible for any human involvement

10. Standardize lubrication methods

11. Package the above elements into a lubrication program

12. Establish storage and handling procedures

13. Evaluate new lubricants to take advantage of state of the art

14. Analyze any failures involving lubrication and initiate necessary corrective actions.

Lubrication Program Implementation

An individual supervisor in the maintenance department should be assigned the responsibility for implementation and continued operation of the lubrication program. This person's primary functions are to

1. Establish lubrication service actions and schedules

2. Define the lubrication routes by building, area, and organization

3. Assign responsibilities to specific persons

4. Train lubricators

5. Ensure supplies of proper lubricants through the storeroom

6. Establish feedback that ensures completion of assigned lubrication and follows up on any discrepancies

7. Develop a manual or computerized lubrication scheduling and control system as part of the larger maintenance management program

8. Motivate lubrication personnel to check equipment for other problems and to create work requests where feasible

9. Ensure continued operation of the lubrication system.

It is important that a responsible person who recognizes the value of thorough lubrication be placed in charge. As with any activity, interest diminishes over time, equipment is modified without corresponding changes to the lubrication procedures, and state-of-the-art advances in lubricating technology may not be undertaken. A factory may have thousands of lubricating points that require attention. Lubrication is no less important to computer systems, even though they are often perceived as electronic. The computer field engineer must provide proper lubrication to printers, tape drives, and disks that spin at 3,600 rpm. A lot of maintenance time is invested in lubrication. The effect on production uptime can be measured nationally in billions of dollars.

CALIBRATION

Calibration is a special form of preventive maintenance whose objective is to keep measurement and control instruments within specified limits. A ''standard'' must be used to calibrate the equipment. Standards are derived from parameters established by the NBS. Secondary standards that have been manufactured to close tolerances and set against the primary standard are available through many test and calibration laboratories and often in industrial and university tool rooms and research labs. Ohmmeters are examples of equipment that should be calibrated at least once a year and before further use if subjected to sudden shock or stress.

Standards

The purpose of a calibration system is to provide for the prevention of tool inaccuracy through prompt detection of deficiencies and timely application of corrective action. Every organization should prepare a written description of its calibration system. This description should cover the measuring of test equipment and standards, including the following:

1. Establishment of realistic calibration intervals

2. Listing of all measurement standards

3. Establishment of environmental conditions for calibration

4. Ensuring the use of calibration procedures for all equipment and standards

5. Coordinating the calibration system with all users

6. Ensuring that equipment is frequently checked by periodic system or cross-checks to detect damage, inoperative instruments, erratic readings, and other performance-degrading factors that can't be anticipated or provided for by calibration intervals

7. Providing for timely and positive correction action

8. Establishing decals, reject tags, and records for calibration labeling

9. Maintaining formal records to ensure proper controls.

Inspection Intervals

The checking interval may be in terms of time (hourly, weekly, monthly) or based on amount of use (every 5,000 parts, or every lot). For electrical test equipment, the power-on time may be critical factor and can be measured through an electrical elapsed-time indicator.

Adherence to the checking schedule makes or breaks the system. The interval should be based on stability, purpose, and degree of usage. If initial records indicate that the equipment remains within the required accuracy for successive calibrations, then the intervals may be lengthened. On the other hand, if equipment requires frequent adjustment or repair, the intervals should be shortened.

Any equipment that does not have specific calibration intervals should be (1) examined at least every 6 months, and (2) calibrated at intervals of no longer than 1 year. Adjustments or assignment of calibration intervals should be done in such a way that a minimum of 95% of equipment or standards of the same type is within tolerance when submitted for regularly scheduled recalibration. In other words, if more than 5% of a particular type of equipment is out of tolerance at the end of its interval, then the interval should be reduced until less than 5% is defective when checked.

ill. 5.3 . A typical calibration label.

ill. 5.4 . A typical calibration card.

Control Records

A record system should be kept on every instrument, including the following:

1. History of use

2. Accuracy

3. Present location

4. Calibration interval and when due

5. Calibration procedures and necessary controls

6. Actual values of latest calibration

7. History of maintenance and repairs.

ill. 5.3 shows a typical calibration label.

Test equipment and measurement standards should be labeled to indicate the date of last calibration, by whom it was calibrated, and when the next calibration is due. When the size of the equipment limits the application of labels, an identifying code should be applied to reflect the serviceability and due date for next calibration. This provides a visual indication of the calibration serviceability status. Both the headquarters calibration organization and the instrument user should maintain a two-way check on calibration. A simple means of doing this is to have a small form for each instrument with a calendar of weeks or months (depending on the interval required) across the top that can be punched and noted to indicate the calibration due date. An example of this sort of form is shown in ill. 5.4.

If the forms are sorted every month, the cards for each instrument that should be recalled for check or calibration can easily be pulled out.

Prev: Planning and Scheduling
Next: Maintenance Engineering Roles and Responsibilities

Article Index    HOME