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2. TYPES OF MAINTENANCE

Three Types of Maintenance

Breakdown maintenance is the oldest type, allows equipment to operate until it fails before it is repaired or replaced. Although this type requires little planning, its drawback make it unacceptable in large plants; all work is done on an emergency basis, which results in inefficient employment of manpower and in excessive downtime.

Preventive maintenance’s goal is to prevent breakdown and so extend plant operating time, by means of scheduled inspections and periodic overhauling of equipment. It achieves this, but does not reduce costs. Recently, its popularity has waned because of skyrocketing costs.

Predictive maintenance borrows strong points from the first two types but adds advanced scientific techniques to maximize operating time and eliminate unnecessary work. Equipment is run to a point just short of failure. Preventive measures are taken only for equipment whose failure could create a serious hazard or whose operation is specially critical.

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2.1 Breakdown Maintenance

The manager of an industrial plant maintenance department often meets resistance from two sides in his efforts to handle his responsibilities. The production department begrudges him the time to work on the machines and equipment, while management begrudges him the money for labor and tools to do the work.

The problem of efficiently handling maintenance is particularly acute in plants having continuous process. Here, the maintenance department may not have a chance to work on a machine until a product change comes up, or until a major breakdown of a piece of equipment shuts down the whole process.

A different of opinion sometimes exists between maintenance and production over the best method of achieving optimum machine performance.

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The production people would like to operate equipment at maximum output for the longest possible time (until a breakdown seems imminent or occurs) and then make quick repairs to get back into production. Operating in this way, repairs cannot be and are not scheduled. Maintenance people, in general, prefer to make repairs and adjustments on a planned and scheduled basis, thereby eliminating (as much as possible) the emergency situations that arise with breakdowns.

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2. 2 PREVENTIVE MAINTENANCE

2.2.1 General

Preventive maintenance is the practice of tending to equipment so it will not breakdown and will operate according to requirements. It entails understanding and maintaining all the physical elements of manufacturing -machine components, equipment, and system- so they consistency perform at a good thing in general.

Although reliable, well functioning equipment is a good thing in general, it is a prerequisite for production system to perform each of its activities to be always in time. A hallmark of JIT production for example is ability to function with little or zero inventory and JIT process simply does not have enough WIP inventory to buffer against chronic equipment problems. Equipment that performs erratically or breaks down will soon bring the entire process to a halt.

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A hallmark of TQM is its ability to hit product or service targets, which means no significant deviation from them. In manufacturing, this situation can only happen if equipment is reliable and well-functioning.

2.2.2 Preventive Maintenance Program

The aim of preventive maintenance is to improve equipment performance. This is done through a number of steps and practices that address and rectify the major causes of equipment.

The previous module has describe five major causes of equipment problems namely: deterioration, equipment ill-suited for the purpose, failure to maintain equipment requirements, failure to maintain correct operating conditions and lack of skills of operator, maintenance crew and setup people.

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In order to address these problems, preventive maintenance emphasize the need to:

. Maintain normal operating conditions

. Maintain equipment requirements

. Keep equipment and facilities clean and organized

. Monitor equipment daily

. Schedule preventive maintenance

. Manage maintenance information

. Use predictive maintenance

1) Maintain normal operating conditions

As all machine have design limitation, preventive maintenance start with knowing the normal operating conditions. Operator should run equipment and monitor conditions to make sure that its requirements are not exceeded. Derating the equipment (running it below normal operating condition) further reduces deterioration and extends the equipment’s useful life. The life of bearing for example, proportional to rotational speed.

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2) Maintain Equipment Requirements

Equipment has physical needs. These needs are, like operating conditions, sometimes listed in manuals which accompany the equipment, sometime however, they must be determined by operational experience. Some examples of equipment requirements are lubrication, bolt tightening, the use of proper tools and fixtures in machine setup etc. Japanese factories indicate that about 80 % of equipment breakdown be reduced just by frequently retightening bolts of equipment.

Machines malfunction usually happen when they are incorrectly adjusted or configured during setup. Reviewing setup procedure, training operators and setup people in proper setup techniques, and using setup checklists are ways to reduce machine problems from setup mistake.

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3) Keep Equipment and Facilities Clean and Organized

Cleanliness and organization are essential to preventive maintenance. Noise, oil leaks, wobbling fixtures, cracks, discolorations, and other sources of problems that might otherwise be concealed by dirty, show up readily on a clean machine. Calibrations and settings on clean machines are readily visible, making it easier to see when they need resetting.

Dirty and disorganization are themselves major causes of equipment problems. Grime gets into working parts and causes scratches and friction, which slows down or jams machinery. Clutter (messy) in the shops leads to confusion, difficulty in locating repair parts and tools and mistakes in setting up, operating, and repairing machines. Clutter impedes efficient response to breakdown and increase MTTR (mean time to repair). Responsibility for keeping machines and workplaces clean and organized is assigned to machine operators.

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4) Monitor Equipment Daily

Periodic cleaning of equipment is not enough. Equipment must be monitored daily or, ideally, in real time so that early signs of problems are promptly detected and fixed. Operational monitoring for subtle signs of problems, such as increased heating, vibration, jamming, defects and so on, provides advance warning of emerging problems. Often, quick detection of an emergent problem allow a simple solution like tightening a bolt, adding oil or inserting a new part.

The most practical way to monitor and maintain equipment in real time is to have the operators do it. Giving operators responsibility for basic machine upkeep helps ensure that problems will be detected early, minor problems will be fixed quickly and major problems will be avoided.

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5) Schedule Preventive Maintenance

Even machines that are properly operated and maintained need periodic attention from the experts. Instead of waiting until machine break down, preventive maintenance. Preventive maintenance programs allow for periodic, scheduled downtime during which experts can inspect, replace parts in, and overhaul equipment. As a rule, the trade-off is that relatively little scheduled downtime for preventive maintenance affords big benefit in terms of fewer random, unscheduled machine breakdowns. With scheduled preventive maintenance, everything is stopped at the same time, and production schedules are not disrupted

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Ways of Scheduling Preventive Maintenance

Scheduled time for preventive maintenance can be based on the following:

. Clock or calendar time intervals, e.g. run a two-shift operation then

devote the third shift to preventive maintenance; performs

preventive maintenance tasks monthly, quarterly, annually etc.

. Cycles of usage, e.g. schedule preventive maintenance for every

1.000 hrs of machine operation or every 50 setups

. Periodic inspection e.g. schedule preventive maintenance

whenever a periodic inspection indicates impeding or possible

malfunction or failure

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6) Manage Maintenance Information

Preventive maintenance requires keeping good records about the performance and breakdown history of equipment and the costs of operations and repairs.. Effective preventive maintenance requires a good system for tracking equipment performance and breakdowns, repairs, and related costs. The tracking system should be part of a large computerized maintenance management system that processes work orders for repair maintain preventive maintenance procedures and schedules, releases work orders for all scheduled preventive and repair maintenance, and prepares summary reports.

Information retained by CMMS (computerized maintenance management system) about machine performance, breakdowns, and costs is used to compile statistics on equipment available, MTTR, efficiency and quality and to assess current maintenance activities. The statistics are useful for determining maintenance requirements and schedules for components and equipment and for improving maintenance procedures.

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7) Use Predictive Maintenance

For the case where it is difficult or impossible to determine the useful life a component or piece of equipment, a common practice is to employ predictive maintenance to give warning about potential failures. Rather than perform periodic scheduled maintenance tasks, predictive maintenance performs periodic, scheduled inspections, the results of which are used to determine specific preventive maintenance tasks and replacement of components.

The periodic inspections might be done by workers, though usually they require high-level skill and must be done by the maintenance staffs. Predictive maintenance is some time classified as a function separate from preventive maintenance, though it is always performed in conjunction with preventive maintenance. Predictive maintenance often involves monitoring the vibration, speed, temperature, sound and other physical phenomena of machinery.

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2.2.3 Objectives of Maintenance Program

The major benefits or objectives of a preventive maintenance program include:

. Less production downtime, with related savings and customer bene-

fits because of fewer breakdowns. . Less overtime pay on ordinary adjustments and repairs than for

emergency and breakdown repairs . Fewer large-scale repairs and fewer repetitive repairs, hence less

crowding of maintenance manpower and facilities. . Lower repair costs for simple repairs made before breakdowns,

because less manpower, fewer skills and fewer parts are usually needed for planned shutdowns than for breakdowns

. Few products rejects and less spoilage, because of properly adjusted equipment

. Postponement or elimination of cash outlays for premature replacement because of better conservation of assets.

. Less standby equipment needed, thus reducing capital investment.

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. Decline of maintenance costs-labor and materials-on assets items in the program. Identification of items with high maintenance costs, leading to investigation and correction of causes such a misapplication,

operator abuse, and obsolescence. Shift from inefficient breakdown maintenance to less costly

scheduled maintenance and better work control. Better spare-parts control, leading to a minimum inventory. Better industrial relations because production workers do not suffer involuntary layoffs or loss of incentive bonus due to breakdowns. Greater safety for workers, and improved protection for plant, leading to lower compensation and insurance costs. Improved costs control resulting from the data obtained during the operation of the preventive maintenance program. Lower unit cost of manufacture, as a result of the above savings inthe cost of operations.

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2.2.4 Scheduled PM and Failure Pattern

Preventive maintenance often involves replacing components that are functioning perfectly. This is because the component is believed to be approaching the end of its estimated useful life. Useful life is the point at which the probability of the component failing greatly increases.

To extend the useful life of equipment approaching the wear-out zone, equipment can be run lower-then-normal speed (derating) or exposed to less-than normal workload or stess level. One way of deciding when to perform PM including replacement or renovation of components, is to analyze the trade-off costs between letting equipment breakdown and performing PM on equipment.

Asume the cost of PM on a machine is Cp and the cost of repairing a broken down is Cb. To get the expected cost of letting machines breakdown, first determine the expected time between breakdowns E(n):

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E(n) =∑ n Pn, where n = time period when a breakdown occur Pn = probability of a breakdown in time period n

Number of machine break down in period n P(n) = ------ = --------------------------------------------------------- Total number of breakdown in all periods

The expected number of breakdown is 1/E(n), so the expected cost of letting N identical machine breakdown is:

N Cb = ------ which is the cost of non-performing PM E(n)

To determine the cost of performing PM involves determining two costs namely the cost of performing routine preventive maintenance and the cost of repairing machines that break down regardless of PM. The cost of performing routine preventive maintenance on N identical machine is N(Cp).

nb

nb

BC

BC

BC

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To get the cost of repairing machines that break down is determined as follow:

Let Bn = expected number of breaks down when PM is performed every n periods:

The cost of repair maintenance is . Thus, the total expected cost per period of performing PM every n periods, is the sum of the total cost of PM per period and the cost of repair maintenance per period :

When , then PM every n periods is less costly than letting equipment break down.

1122111 .......... PBPBPBPPNB nnnnn

nC

n

CB

n

NCC bnpn

)( Bn CB

Bn CC

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Example:Suppose a group of 25 identical machines is maintained for 5 months, and without PM the number of break down in each month is as follow:

This indicates that the longest time a machine runs before breaking down is 5 months, and that for example 4 machines run 2 months before they broke down. The cost of repairing a machine after breakdown is $ 500 (Cb), whereas the cost of PM on a machine is $ 100 (Cp). Preventive maintenance does not eliminate break downs: The monthly cost of break down without PM can be calculated as follow:

E(n) =∑ = 1(0.12) + 2(0.16) + 3(0.12) + 4(0.20) + 5(0.40) = 3.60 months

The monthly expected cost of not performing PM is:

NCb 25 machines ($ 500 per machine) = -------- = -------------------------------------------- = $ 3.472,22 E(n) 3.

Months of Operation before Break down 1 2 3 4 5

No. of break down, b 3 4 3 5 10

Probability of break down 0.12 0.16 0.12 0.20 0.40

nnP

BC

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The expected number of breaks downs with PM performed every n months, Bn is: N(

11221121 .....)....... PBPBPBPPP nnnn

36.11)12.0(36.7)16.0(3)4.0(25)(

6.7)12.0(3)28.0(25)(

0.3)12.0(25)(

22213213

1212

11

PBPBPPPNB

BPPNB

PNB

45.30)12.0(9.17)16.0(36.11)12.0(36.7)2.0.(3)00.1(25)()()()()(

90.17)12.0(6.11)16.0(36.7)12.0(3)6.0(25)()()()(

14233241543215

13223143214

PBPBPBPBPPPPPNB

PBPBPBPPPPNB

Month between PM service , n

Expected breakdown in n months, Bn

Expected Breakdown Cost=$500(Bn)

Expected breakdown cost/month =$500(Bn/n)

Expected PM Cost/month = 25 ($100)/n

Expected total Cost, Cn (Break down + PM)

1 3 1.500 1.500 2.500 4.000

2 7.36 3.680 1.840 1.250 3.090

3 11.36 5.680 1.893 833 2.726

4 17.90 8.950 2.238 625 2.863

5 30.45 15.225 3.045 500 3.545

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Assignment:1. PT. Professional Maintenance adalah sebuah perusahaan jasa pemeliharaan fasilitas manufakturing di Sumatera Utara. Saat ini perusahaan pemeliharaan ini sedang menangani fasilitas perusahaan industri manufakturing PT Global Engineering untuk 22 buah mesin yang sedang dioperasikannya. PT Global Engineering sedang menerapkan preventive maintenance system. Perusahaan jasa pemeliharaan menyarankan agar preventive maintenance dilakukan per minggu dengan penawaran biaya $ 500 per siklus untuk seluruh mesin tersebut. Disamping, itu biaya perbaikan mesin setiap kali sebuah mesin mengalami break down ialah sebesar $ 1.225.

Data historis lain yang tersedia yang berkaitan dengan jumlah mesin yang mengalami break down dari minggu-ke minggu adalah sebagai berikut :

Operasi minggu ke 1 2 3 4 5

Jumlah mesin break down 4 5 7 4 2

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Pertanyaan: a. Apakah PT Global Engineering setuju dengan usul PT Pro-

fessional Engineering ? b. Berapa penghematan atau pemborosan apabila preventive

maintenance dilakukan dengan siklus 2, 3, 4, 5, 6 minggu? c. Siklus berapa minggu yang terbaik bagi PT Global Enginee- ring?

2. Misalkan downtime dan repair cost setiap mesin rusak adalah $ 2.000 sedangkan comprehensive preventive maintenance costs adalah $ 800 per unit mesin. Catatan Departemen Pemeliharaan menunjukkan bahwa frekuensi mesin rusak dalam operasi produksi adalah sebagai berikut: Jumlah Minggu antara PM Jumlah mesin rata-rata rusak antara PM

1 0.60

2 1.50

3 2.90

4 5.50

5 8.00

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Pertanyaan: Berikan rekomendasi sdr tentang berapa sering preventive maintenance sebaiknya dilakukan

3. Sebuah mesin mempunyai failure pattern menurut pola A. Umur pakai mesin tersebut dapat diperpanjang 18 bulan jika overhaul dilakukan pada 18 bulan. Diasumsikan, tanpa overhaul MTBF adalah 24 bulan. Misalkan downtime dan repair cost adalah $ 6.000 dan overhaul cost adalah $ 2.000.

Pertanyaan:

a. Jika preventive maintenance dilakukan dengan siklus 18 bulan apakah overhaul tersebut efektif? b. Jika preventive maintenace dilakukan setiap hari dengan total biaya $ 1.200 per tahun akan memperpanjang MTBF menjadi 3 tahun, apakan sistem maintenance dengan siklus ini efektif?

4. Uraikan pengalaman sdr pemeliharaan fasilitas yang diterapkan di perusahaan sdr dan jelaskan bagaimana pengaruhnya terhadap kinerja fasilitas tersebut.