can be considered acceptable, because it avoids delays for emergency calls and contains maintenance costs.
To reduce MTTR, we must act on the following factors:
• increasing technicians’ professional knowledge and improving operators’
skills through training, on-the-job in the case of the latter;
• supplying the right tools for detecting failures’ mode and for speeding up maintenance intervention;
• quick availability of spare parts.
In relation to the last point, the logistic organization of the supply system is particularly strategic. Spare parts can be classified into the following categories:
(a) Spare parts necessary for recurrent ‘‘back-up’’, which should be available at machine side to speed-up interventions; these parts are normally recoverable and repairable for further use as ‘‘back-up’’ parts.
(b) Spare parts necessary for non-recurrent interventions, the storage space of which should be nearby the equipment and machines on the shop floor and managed with a ‘‘minimum stock level’’ criteria, considering utilization fre- quencies and supply lead times.
(c) Standard parts widely employed, the storage space of which should be organized on two levels: the first peripheral level should be small and located near the equipment and machines on the shop floor, while the second level should be centrally located and able to serve more plants, operating to replace the parts withdrawn from the peripheral warehouse on the first level, normally at a weekly rate. In this way it is possible to assure a good service level and contain the relative invested working capital.
To simplify the process of supplying spare parts and to minimize working capital, technical solutions and automatic components must be standardized as much as possible (the same type of machine centre, robot and controlling devices, material handling equipment, etc., should all be used in the same plant).
Modern ICT allow for the automatic checking of stock levels in the warehouses in real time and for the organization of and supply plans for spare parts based on
‘‘minimum safety stock’’ criteria.
To purchase spare parts, it is normal to operate on open agreements, engaging the suppliers in obtaining short delivery lead time and then relying on them to be in charge of supplying the necessary stocks.
5.4 Correlation Between Cost and Maintenance
machining), reinforced polymer transformation processes and body welding and painting.
To plan and control these costs, it is first important to distinguish the part considered ‘‘variable’’ in relation to the activity levels (PWT and PAV) from the part considered ‘‘fixed’’ on a yearly basis, depending on the structural asset of the plant’s technical systems.
On first approximation and in proportion to activity level (PWT and PAV), we can consider the expenses as variable for:
• consumable materials and spare parts used in maintenance, evaluated at the supplied cost;
• fees for time-based maintenance activities planned on cumulated working time and evaluated at contract price (if performed by external providers) or at hourly labour cost of Maintenance Department.
We can consider expenses linked to fixing maintenance garrisons set for assistance to production and emergency calls for each of the planned working shifts as ‘‘variable by step’’, in relation to the planned working time (PWT).
Conversely, we can consider expenses as fixed per year for:
• salaries for managers and technicians dedicated to maintenance;
• other technical and logistic services provided by external suppliers on specific initiatives;
• time-based maintenance activities on calendar normally provided by external suppliers, set at contract price.
In this way, it is easy to plan and control maintenance cost trends, analysing variation causes month by month and year by year, distinguishing:
• variation due to activity volume effect (PAV and PWT);
• variation due to purchasing prices and labour unitary cost;
• variation due to efficiencies.
The evaluation of maintenance cost trends should be done together with the evaluation of technical availability (A)and overall equipment efficiency (OEE) for the plant’s technical systems.
We have to emphasize that, in searching for the best maintenance asset (organizational structure and methodologies for intervention), the following points should be considered:
• incidence of maintenance costs referring to total transformation costs, normally higher when accompanied by a quick response to emergency calls and a high level of failure prevention, with positive effect on technical availability (A);
• incidence of costs due to productivity losses, which depends on statistical data (1-OEE), according to what was laid out inSect. 3.5;
• economic advantage obtainable from increase in equipment productivity, as a consequence of a higher available productive capacity (possibility of reducing working shifts, or increasing the sales plan).
To conclude this section, we will focus on the labour factor of maintenance costs, which we classified as ‘‘indirect labour’’ in Chap. 4; looking at all main- tenance parameters, it is possible to estimate the maintenance labour force required for maintaining a production department by measuring the following data:
• frequency and severity of breakdowns by department
• frequency and duration of preventive activities recorded in the maintenance calendar (TBM and CBM activities).
Referring to the calculation of the Direct Labour Requirement (DLR) seen in Sect. 4.2, the calculation of Maintenance Labour Requirement could be carried out by the following calculations in.
5.4.1 Breakdown Maintenance Activity Volume
BMAV ¼XN
i¼1
1
60MTTRiBi
where:
• MTTRiis the Mean Time To Repair at the production department level mea- sured in minutes;
• Bi is the total number of breakdowns occurring during the working period considered in the department for the typology of failure I;
• Nis the number of different typologies of failure occurring in the department.
5.4.2 Preventive Professional Maintenance Activity Volume
PMAV¼XJ
j¼1
1
60TBMjþXK
k¼1
1 60CBMk
where:
• TBMj is the time required for the Time Based Maintenance activities in the department recorded on the professional maintenance calendar, measured in minutes;
• Jis the number of different Time Based Maintenance activities managed in the production department;
• CBMkis the time required to execute the Condition Based Maintenance activ- ities in the department recorded on the professional maintenance calendar, measured in minutes;
• Kis the number of different Condition Based Maintenance activities managed in the production department.
5.4.3 Individual Activity Achievable
IAA =g IWH (12a) equal to the hours worked on average by each worker, being:
IWH working hours, corresponding to the hours of presence on the job determined by the collective agreement for the working period considered (average data that also considers the organization of shifts);
g maintenance labour efficiency (measured as indicated inSect. 4.1);
a absenteeism index, estimated by statistical data.
TheMaintenance Labour Requirement will be determined by:
MLR¼BMAVþPMAV IAA