Abstract: The success in the management of a truck fleet depends upon many factors. An important one is how good the preventive maintenance is executed. For that reason it should be a way or a method which can be implemented to measure or to evaluate the execution of the preventive maintenance. The objective of this work is to use the cost of repairs of the fleet as a base to evaluate the efficiency of the execution of the preventive maintenance, and to find a relationship between the level of executed preventive maintenance and the cost of repairs. In this paper, a new parameter based on cost analysis is introduced. This new parameter, namely the maintenance grade (MG) is used as a measure or indicator of the preformed maintenance level in different working places. A cost study employing the MG was carried out using real data obtained from different branches of a goods transportation company. The fleet of the chosen company is composed of trucks of different makes and having different operating lives. The relationship between the MG for the fleet of tucks and the costs of the truck's maintenance and repairs was studied. Results obtained provide insight into the execution level of preventive maintenance and its relationship to repairs cost.

1. Introduction

Running a big fleet of trucks efficiently, as in the case of goods transportation companies, depends in the first place on the level of executed maintenance (EM) applied to the trucks. The executed maintenance can be divided into two main types. The first type is known as the scheduled maintenance and referred to as preventive maintenance (PM), where the second type is referred to as repairs. The preventive maintenance (PM) is a group of different preventive activities that are performed at predetermined intervals, based on either time, distance traveled, engine working hours, or quantity of fuel consumed. These intervals, of course, vary with the types and makes of trucks and their operating conditions. Basically the truck manufacturer recommends these intervals and the schedule which should be followed. This is denoted as the planned preventive maintenance (PPM). Examples of the PPM activities are oils change, oils topping, greasing, filters change, checking and adjustment of different parts and systems of the trucks.

The other type of maintenance (i.e. the repairs) can be divided into two kinds. The first kind is the scheduled maintenance in which truck aggregates or some of their parts are to be replaced. Examples of such repair activities are servicing of truck systems (braking, cooling, steering,....etc.), and the overhauling of truck main components (engine, gearbox,....etc.). The second kind of repairs is the unscheduled maintenance due to components failure or road accidents. Examples of such repair activities are body, aggregates and chassis heavy repairs.

The negligence in performing the preventive maintenance, either in its execution efficiency or its execution rate, will lead to an increase in the repairs activities, especially unscheduled maintenance which results from breakdowns of truck components. The unscheduled maintenance is considered the most costly and should be minimized, as it will never be eliminated.

The objective of this investigation is to obtain the relationship between the level of the executed preventive maintenance (EPM) for trucks and their cost of repairs. From this relation the efficiency of the executed preventive maintenance can be judged. In carrying this investigation a variety of trucks of different makes and different operating lives are considered.

2. Infield Data

A goods transportation company had been selected to be the source of data for this study. The chosen company had to fulfill the following requirements:

- It has a big fleet composed of different truck makes having different operating lives.

- The fleet is distributed over several company branches (different working places).

- The company suffered no shortage in spare parts or labor force during the period of the study.

- The company has good and reliable records and documentation system.

3. Approach

In order to judge the efficiency of the executed preventive maintenance in general, there are different parameters that could be employed. These parameters are:

- Rate of trucks brake downs.

- Cost of repairs.

- Rate of working mileage.

- The financial return from running the trucks.

All the above parameters depend to some extent on the working conditions and the transportation company management policy. Therefor the above parameters can not be used accurately to compare two different working places. Hence, a new parameter has been proposed in this study as a quantitative measuring scale, which is to be used as a measure or an indicator for the level of performing the executed preventive maintenance. This new parameter which is referred to as the maintenance grade (MG) is basically based on the cost analysis and is used throughout the study as comparative means for judging the level of EPM. The MG is intended to be used as a measure to compare the efficiency of performing the maintenance work in different working places; considering that the level of labor skills and the operation conditions are the same in each working places.

The MG is defined as the ratio of the cost of EPM to the cost of PPM. Thus it can be put in the form:

Cost of EPM

MG = ------------------, (1)

Cost of PPM

where the costs of both the EPM and PPM are calculated for the same distanced covered by the truck. The planned preventive maintenance is the maintenance schedule recommended by the truck manufacturer. In this study both costs do not include labor or overhead costs as well as the loss due to truck breakdown time.

4. Data Collection and Processing

To reach the objective of this study, which is mainly to obtain the relationship between the level of performing EPM and the cost of the spare parts used in repairs, the infield information is essential. The type of information or data needed from the selected transportation company is the cost of EPM, the cost of PPM and the cost of repairs for the fleet of trucks at each company branch. The data was collected from the documentation of the company and tabulated in specially designed data sheets. The data sheets were designed to contain the information of each "sample group" in each branch. The sample group is defined here as the group of trucks, which has the same make and has the same operating life and located in one branch. The data was collected for a "sample period" corresponding to the fiscal year (June 1990 to May 1991). The following information was collected in these data sheets (for each sample group in the sample period):

- Operation data: (the distance covered by the trucks and

their financial return).

-Preventive maintenance data: (amount of oil and grease consumed, in addition to the kind and number of oil, fuel, and air filters).

- Repairs data: (spare parts used for maintaining the trucks, also the spare parts used in overhauling trucks’ aggregates and in repairing the truck road accidents, in

addition to the number of tires and batteries used.

- Unit price data: (the price of each replaced unit and the price of the used quantity of fluids).

The repair cost is calculated by multiplying the number of spare parts used in repairs by the price of each part. The price of spare parts is taken from the price list at the time of the sample period. In this study only the repairs connected to the maintenance activities is considered. Thus, the following repair costs were excluded from the value of repair cost:

- repair cost due to road accidents,

- replacement of tires and batteries, also, the cost of engines and gear boxes overhauls, since their operating lives exceed the sample period.

The EPM cost for each sample group in the sample period is collected from the documentation of the company. The EPM cost is calculated by multiplying the quantities of material and spare parts used by their prices. At the same time the costs of PPM are calculated as shown in Appendix (A). From the resulting information of the EPM and PPM costs, the maintenance grade MG for each sample group is calculated. In order to make the comparison between different sample groups more obvious; the cost rate is employed in drawing all figures. The cost rate is defined as the cost of maintenance or repairs in the sample period divided by the total distance traveled by the trucks of the sample group in the sample period (L.E./1000 km). The repairs cost rate (RCR), the executed preventive maintenance cost rate (EPMCR) as well as the total cost rate (TCR) are drawn in one chart, named the (MG-maintenance cost rate chart). Each point in the chart represents the cost rate of a sample group. This set of curves in the cost rate chart is expected to be similar to the well known chart of maintenance and repairs costs shown in Figure 1.

5. Results

Knowing the trend shape of the cost rate curves and using a suitable curve fitting computer program; the curve connecting the points in the chart is drown. From Figure 2, which represents the MG-maintenance cost rate chart, the following could be drawn:

1-The executed preventive maintenance cost rate (EPMCR) can be represented by a straight line in the form:

y₁ = a x, (2)

where: y₁ is the cost rate of EPM (L.E./1000 km traveled), x is the MG (%), a is a constant, representing a slope which depends on the truck make.

2-The repairs cost rate (RCR) can be represented by an exponential curve with negative power plus a constant

in the form:

y₂ = c e^{-b x} + d, (3)

where: y₂ is the rate of RC (L.E./1000 km traveled), x is the MG (%), c, b, d are positive real constants depending on the truck make and its operating life.

3-The total cost rate (TCR) is the summation of the above two cost rates and its curve can be represented in the form:

y = y₁ + y₂, (4)

y = ax + ce^-bx + d. (5)

4-The curve of the total cost rate has a minimum value that can be obtained by differentiation of equation (5) and equating the result to zero. The optimum value of MG will be equal to:

x = (1/b) ln (bc/a). (6)

6. Discussion

As mentioned before the repairs cost due to road accidents and the cost of overhauling of main truck components as well as the cost of the replacement of tires and batteries have been excluded from the cost of repair. Also, the cost of labor, tool and equipment depreciation or capital cost will not be included in this study as they are outside the scope of the present work.

The data was collected for different sample groups. The MG values were calculated alongside the maintenance and repairs rates. The MG-maintenance cost rate charts are then plotted for different sample groups. In the figures presented here each point in the curve represents a sample group, this sample group was referred to by a different letter and number (R6, KW10,.... etc.). Each letter represents a truck make, while the number denotes its operating life (age). From the figures it can be seen that increasing the MG value, which in turn means increasing the rate of EPM cost, will lead to a decrease in the rate of repairs cost. From the figures the repairs cost rate is different for trucks having the same make but having different operating lives. Figure 3 shows the MG-maintenance cost rate chart for different truck ages. From this figure it can be seen that as the truck age increases the corresponding slope (MG) increases. From these slopes one can conclude that, the greater the operating life of the truck the more sensitive the truck is to the change in MG value. The increase in the repair cost rate for old trucks can be seen clearly. The old truck curve is shifted upwards and to the right. The shift to the right can be due to the EPM operation not being carried out as scheduled for new trucks, but is carried out most of the time when the truck is stopped for the repairs (unscheduled maintenance). The other reason for the increase in the EPM cost for old trucks is also due to the cost of oil topping up the engines and gearboxes (bad gaskets and oil seals). The small values of MG for new trucks are contributed to the poor maintenance operation (long time intervals between executing the PM).

To find out the effect of truck makes on the repair cost, two sets of data were used to draw Figures 4. Presented in Figure 4 is the MG-maintenance cost rate for two truck groups having the same operating life. From these curves one can see that in general, increasing the maintenance level decreases the repairs cost. The slopes of the maintenance curves are a little different due to the difference in the volume of oil used and the number and cost of filters involved in the maintenance operation. Though the maintenance cost rate is nearly the same for the two makes, there is a big difference in the repair cost rate. This difference is mainly due to the difference in the price of the spare parts.

When the MG-maintenance cost rate charts were examined thoroughly two cases were observed in some of the sample group data, which are considered here as abnormal ones and deserved close study. In the first case although the EPM cost is greater than the PPM cost, which lead to an increase in the MG value, no decrease is observed in the repair cost. This indicates that there was overdoing of the maintenance operation, yet it had little or no effect on maintaining the trucks. After infield investigation it was found that, the obvious reason was due to abnormal increase in the cost of the oil used. The increase in oil consumption is attributed to the following causes:

-the change of engine oil does not follow the manufacturer recommendation but takes place at shorter intervals,

- a great amount of oil is used during the period of running-in the engines and gearboxes after conducting the overhauling,

- bad gaskets and oil seals lead to repeatedly topping-up

of the engine oil in the period between two successive engine oil changes,

- the bad habit of the garage technicians of topping up the engine oil over the level indicated by the engine dip stick.

In the second case, the EPM cost is found to be much less than the PPM cost (unusual low value of MG) and is accompanied by high repair cost. That is thought to be due to abnormal circumstances in maintenance operations practice. This case is investigated closely and the results can be summarized as follows:

- shortage occurred in some of the high priced filters, due to bad company administration policy.

- the maintenance operation is not performed correctly (as in the case of greasing which lead to abnormal replacement of universal joints and intermediate supports of the propeller shafts).

7. Conclusions

Several results from the analysis of the data were obtained. Several conclusions can be drawn based upon these results. The main conclusions that were drawn from the obtained results are as follows:

1- Evaluation of different truck makes and the level of performing their preventive maintenance can be done based on the cost-analysis.

2- The defined parameter, MG, can be used successfully as a measure of the level of maintenance executed in the different working places.

3- The MG-maintenance cost rate chart could be used to evaluate the effectiveness of executing the PPM operations in the different branches of the transportation companies. At the same time, it could be used to show the effect of the operating life of each truck make on the total cost rate at a later stage, when making the decision of buying new trucks.

4- Increasing the maintenance level MG leads to a decrease in the repair cost rate. There is an optimum

MG value which corresponds to the minimum total cost rate. These optimum values depend on truck make and age and also depend on maintenance and working conditions.

5- The old trucks (with higher operating lives) are very sensitive to negligence of performing the preventive maintenance than the new ones.

6- The MG-maintenance cost rate can be used to reschedule and modify the PPM with respect to:

-the truck operational life; so the executive interval of PPM should be shortened as the truck operational life increases.

-the truck make; taking into consideration the working and execution maintenance conditions.

8. References

1-Salem, A., Feasibility Study of Preventive Maintenance Implementation",

M.Sc. Thesis, Ain Shams University, Cairo, Egypt, 1992.

9. Acknowledgment

The authors of this paper wish to express their gratitude to the personnel of the General Nile Direct Transport Company for helping in providing the necessary data and documentation.

APPENDIX (A)

Calculation of the PPM Cost

The PPM is scheduled according to the manufacturer’s recommendation, to be carried out every certain distance traveled by the truck. The PPM consists of three main types of maintenance, namely maintenance A, B and C, which are carried out every 5,000, 10,000 and 40,000 km respectively. For each type of maintenance, A, B, and C there are several different operations which are to be carried out.

To calculate the cost of the PPM for each sample group in the sample period, some steps were followed. First, the total distance traveled (TDT) by the trucks in each sample group were collected from the company documentation. Second, the number of each type of scheduled maintenance were calculated according to the following:

Number of maintenance C (NC)

TDT

NC = ---------, (7)

40,000

Number of maintenance B (NB)

TDT

NB = -------- - NC, (8)

10,000

Number of maintenance A (NA)

TDT

NA = --------- - (NC + NE). (9)

5,000

The cost of each operation executed in each maintenance type was obtained from the price list during the sample period. The cost of all operations in each type of maintenance was summed up for each sample group. Finally, the total PPM cost was obtained by multiplying the cost of each maintenance type by its number, then added them together.

APPENDIX (B)