Mohd Asyraf Rawi Rev#: 00 01 Apr 2010

LECTURE NOTE / INSTRUCTION NOTE

AZIANASHIMA HEEDAYU BINTI ZAINAL

TYPES OF MAINTENANCE

Maintenance may be classified into four categories:

(some authors prefer three categories- scheduled and preventive maintenances are merged)

Corrective or Breakdown maintenance

Scheduled maintenance

Preventive maintenance

Predictive (Condition-based) maintenance

CORRECTIVE OR BREAKDOWN MAINTENANCE

Corrective or Breakdown maintenance implies that repairs are made after the equipment is failed and can not perform its normal function anymore

Quite justified in small factories where: Down times are non-critical and repair costs are less than

other type of maintenance

Financial justification for scheduling are not felt

DISADVANTAGES OF CORRECTIVE MAINTENANCE

Breakdown generally occurs inappropriate times leading to poor and hurried maintenance

Excessive delay in production & reduces output

Faster plant deterioration

Increases chances of accidents and less safety for both workers and machines

More spoilt materials

Direct loss of profit

Can not be employed for equipments regulated by statutory provisions e.g. cranes, lift and hoists etc

Mohd Asyraf Rawi Rev#: 00 01 Apr 2010

LECTURE NOTE / INSTRUCTION NOTE

AZIANASHIMA HEEDAYU BINTI ZAINAL

Schedule maintenance is the process of periodic

overhauls or service of the equipment.

This may be anything from a lubrication routine to a

time frame for replacement of component parts on a

piece of equipment; the point is that maintenance is set to

a time period, much the same as service to a car is tied to

mileage.

SCHEDULED/PERIODICAL MAINTENANCE

The time may be in hours of operation, number of

shifts of operation, or a given service period in days,

weeks, or months.

Once the schedule has been set, periodic checks should

be run from time to time to ensure that the times set up

are correct, and that the equipment is not being inspected

too frequently or not often enough.

Scheduled maintenance is a stitch-in-time procedure and incorporates

inspection

lubrication

repair and overhaul of equipments

If neglected can result in breakdown.

Generally followed for:

overhauling of machines

changing of heavy equipment oils

cleaning of water and other tanks etc.

Mohd Asyraf Rawi Rev#: 00 01 Apr 2010

LECTURE NOTE / INSTRUCTION NOTE

AZIANASHIMA HEEDAYU BINTI ZAINAL

PREVENTIVE MAINTENANCE (PM)

Principle – “Prevention is better than cure”

Procedure - Stitch-in-time

It locates weak spots of machinery and equipments

provides them periodic/scheduled inspections and minor repairs to reduce the danger of unanticipated breakdowns

WHAT IS PREVENTIVE MAINTENANCE?

Preventive Maintenance (PM) is action taken to

keep an item which is in operating condition by

means of inspection, detection, and prevention of

failures.

WHY HAVE PREVENTIVE MAINTENANCE

Equipment failure usually occurs at the worst possible

time. For example, the production department may be in

the middle of a rush order that is very critical to the

customer.

This is the time the equipment will break down and cause

an interruption in production. Depending on the severity

of the failure, the equipment may be down from a few

hours to a few days.

This delay can prove very costly in the competitive

markerplace that all industry finds itself in today.

How much better it would have been if the maintenance

department had been able to detect a problem in the

equipment, arranged with the production department a

convinient time, and made the necessary repairs before a

failure occured.

A second reason for preventive maintenance is safety.

Proper inspections can detect unsafe conditions in

time to prevent an accident, which might cause

damage to the equipment or injure operating

personnel.

A third reason is reduced repair costs. When a failure

occurs, it usually destroys equipment that is associated

with the detective component. If the detective

component is changed before the failure occurs, the

related equipment will not be damaged; thus, repair costs

will be reduced.

With the price of replacement parts escalating at today’s

rate, this cost savings can be substantial.

CANDIDATES FOR PREVENTIVE MAINTENANCE

Mean Time Between Failure (MTBF)

Frequency of Failure Good candidates have

more normal

distribution with low

variability

ADVANTAGES OF PM

Advantages: Reduces break down and thereby down time

Lass odd-time repair and reduces over time of crews

Greater safety of workers

Lower maintenance and repair costs

Less stand-by equipments and spare parts

Better product quality and fewer reworks and scraps

Increases plant life

Increases chances to get production incentive bonus

Mohd Asyraf Rawi Rev#: 00 01 Apr 2010

LECTURE NOTE / INSTRUCTION NOTE

AZIANASHIMA HEEDAYU BINTI ZAINAL

PREDICTIVE (CONDITION-BASED) MAINTENANCE

In predictive maintenance, machinery conditions are periodically monitored and this enables the maintenance crews to take timely actions, such as machine adjustment, repair or overhaul

It makes use of human sense and other sensitive instruments, such as audio gauge, vibration analyzer, amplitude meter, pressure,

temperature and resistance strain gauges etc.

PREDICTIVE MAINTENANCE

Unusual sounds coming out of a rotating equipment predicts a trouble

An excessively hot electric cable predicts a trouble

Simple hand touch can point out many unusual equipment conditions and thus predicts a trouble

MAINTENANCE COSTS

Maintenance Commitment

Cost

Breakdown Cost

MAINTENANCE COSTS

Maintenance Commitment

Cost

PM Cost

Breakdown Cost

MAINTENANCE COSTS

Maintenance Commitment

Cost

PM Cost

Breakdown Cost

Total Maintenance Cost

MAINTENANCE COSTS

Maintenance Commitment

Cost

PM Cost

Total Maintenance Cost

Breakdown Cost

Optimal

DETERMINING THE SIZE OF REPAIR CREWS

Problem:

A factory has 200 machines and the maintenance engineer supervises the repair crews who repair malfunctioning machines. The maintenance policy is to repair the broken down machine and bring back in production within 2 hours on the average. If average breakdown rate is 3.5 machines/hour and each repair crew can repair 0.25 machine per hour on the average. How many repair crews are required ?

SOLUTION

The formula for average repair rate () is

1

ts = ---------- or = + 1/ ts

-

Where = repair rate

= arrival rate of malfunctioning machines

ts = average time arrivals in the system

Required average repair rate

= 3.5 + 1 / 2 = 4 machines / hour

No. of Crews = machines/hour a crew can repair

= 4 0.25 = 16 repair crews required