4
sequence of equipment out of revenue-producing activity. When the plant has to be shut down
because of an emergency maintenance, costs are even higher, given the fact that most in-process
products are lost. Emergency shutdowns also put a great strain on personnel and can lead to
unsafe methods of operation and danger to the environment, but not least, is the difficulty of
hiring specialized maintenance personnel in case of an emergency.
Maintenance planning involves planning of the periods in which preventive maintenance is to be
performed, but also determines the size of the maintenance crew, when and how many materials
should be purchased, and the scheduling of the different maintenance jobs. Each of these
functions will be elaborated on next.
Preventive maintenance
Preventive or planned maintenance is less costly than corrective maintenance. Preventive
maintenance includes preplanned and scheduled adjustments, major overhauls, inspections and
lubrications, to maintain equipment and facilities in such condition that breakdowns and the need
for emergency repair are minimized. Corrective or breakdown maintenance on the other hand,
involve the activities to repair, replace or otherwise restore equipment or facilities to operational
status after a failure has occurred.
In order to set up an effective preventive maintenance plan, management should know the length
of time the unit has been in operation and the length of time needed to repair the unit, subdivided
into different phases of the maintenance job. Numerous quantitative models that determine when
preventive maintenance should take place can be found in the literature (see e.g. Gertsbakh
(1977) and Lyonnet (1991)).
Determining the size of the maintenance crew
The problem of determining the size of the maintenance crew can be found in many articles in
the literature. Most authors use queuing theory or simulation to analyze the problem. The
complexity of the models depends on whether single or multi-skilled craftsmen are present (for a
summary of the literature on sizing of maintenance crews we refer to Mabini (1991).
Scheduling maintenance jobs
The operational problem of scheduling different maintenance jobs consists of assigning certain
jobs to particular operators for completion at a specified time, including sequencing and
dispatching. While sequencing refers to determining the succession of jobs assigned to
individual workmen, dispatching is concerned with putting the "sequenced" jobs into effect.
PERT scheduling and related techniques may be used to address this problem (see e.g. French
(1982), Selen and Heuts (1990)).
Planning maintenance material
Planning of maintenance material implies a good inventory and ordering strategy. This topic has
been addressed extensively in the literature and will not be elaborated on. A summary of
methodologies is given by Silver and Peterson (1985).The planning of maintenance material also
involves replacement of equipment, which is an investment decision. Replacement problems
have also been discussed extensively in the literature. For a summary, we refer to Mabini (1991).
Most replacement models are based on a Markovian deterioration process, that is, the
deterioration of a system to a future state depends only on the immediately preceding state.
Computer aided maintenance
Because of the vast quantity of technical and economical data involved in maintenance
management, computer support is very desirable. Installing a computer-aided system brings these
economies (e.g. see Pintelon (1990):