Before starting with the concept of maintenance, it is important to define what we mean by machines’ and equipment’s reliability degree and characteristics of maintainability.
A certain machine or manufacturing system can be considered fully reli- able when it is able to perform its own function consistently and in a manner coherent with its established use during the requisite time for a given project.
From a statistical point of view, the reliability degree is the probability that it will work correctly and without failure during the progressive utilization time (t):
r tð Þ ¼ 1 f tð Þ;f(t) being the probability of failure The reliability degree depends on the following factors:
• Intrinsic Functional Reliability, determined by project quality and robustness, components and system quality in assembly. Techniques used for this purpose are:robust design,FMEA analysis, transformation process simulations (during design phase), component and system functional testing techniques (during construction and try out phase).
• Duration Reliability, linked to usage of mechanical and mechatronic parts and fatigue factors. Techniques used to establish and extend the life of components are essentially based on the right choice of material and calculation tests, through CAD-CAE methodologies and the application of standard solutions, tested with positive results.
• Utilization and Maintenance Reliability, determined by correct usage of the
‘‘systems’’ and effectiveness of maintenance processes. It is important to con- sider that the productive operations lead to a risk of failure; in the following sections, we will approach maintenance methodologies that help to improve this factor of reliability.
During the start-up phase, up to the final working speed of a system, the more influential reliability factor is the intrinsic one, while the duration reliability factor is strongly influential on the maintenance cost, if the production process is set, and determines the technical life cycle of machines and equipment.
The total reliability degree is the product of the three factors.
Let us now introduce the concept ofequipment and machine maintainability:
it consists of the easy inspection and substitution of overused parts and the cor- rection of possible failures with simple and reliable operations.
The maintainability degree depends first on the design and development setting of the project and on the application of standard solutions, tested ahead of time in a collaboration between Constructor and User. To establish premium levels of technical reliability and availability (A), it is important to evaluate the possible failure modes, the associated probabilities, and the subsequent effects (FMEA methodology) during the project phase. After this analysis, the right ‘‘project reviews’’ are set, making construction solutions more robust and inspecting activities on critical components easier, assuring quick intervention for the sub- stitution of damaged or used parts during the normal technical life of the equipment.
So far, in logical order, the main criteria that an engineer must consider for the design of easily maintained machines are:
1. Improve detectability of breakdowns by designing easy man–machine inter- faces able to give maximum details about how a particular failure occurred (type, module/component position, instruction on how to access the damaged part…)
2. Improveaccessibilityto the machine: once the position of the affected module/
component has been identified, access must be easy for the maintenance operator; this is possible thanks to two main approaches in design:
• layout optimization (locate critical module/component in easily accessible areas)
• facilitate the tear down of safety repairs by creating fast, user-friendly solu- tions for dismounting and mounting
3. Design by modularity: machines must be designed such that maintenance crews can operate solely on the relevant modules during a failure crisis thus speeding up maintenance operations; once the dysfunctional module has been replaced, it can be removed, analysed and subsequently reconditioned to work again through repair of the damaged component, all done remotely
4. Design by standardization: usage of standard components can improve the maintainability of the machine by:
(a) facilitating the training of the maintenance crew and creating conditions for a quick response (interchangeability of skill trades)
(b) facilitating the management of spare parts (reduction of supply lead times and inventories).
Even for equipment and machines already under production, it is possible to improve maintainability by acting as follows:
• improving the interface between man and machine through monitored instruc- tions for driven intervention and time-based maintenance;
• out of machine ‘‘pre-setting’’ techniques applied for tool changes, to shorten set- up-time;
• improving the removal and re-positioning operations on safety barriers and repairs;
• partner tool adoption to the quick removal and re-positioning of bulky parts under failure, by operating in safety conditions.
For each module of a ‘‘Plant Technical System’’, the failure modes can be different, random and periodic. The techniques for cause and effect analysis—
specifically discussed inChap. 8—allow for selecting the critical phenomena and adopting specific countermeasures. This method is particularly important during the start-up phase: the root causes of the failures are verified, and eventual defi- ciencies in mechanical and electronic control systems are fixed (with particular attention to sensors and automatic system control software). In this way, certain types of failure and their frequency are minimized. In between, the intervention for corrective action is improved, reducing the length of technical breakdowns (maintenance learning curve).
Once the start-up phase is completed, the frequency index Fn, considered physiological for a certain typology of failuren, is estimated on a statistical basis.
These analyses must be performed on a significant and extended temporal scale, such as: working week or month, or a certain progressive working time used for production (ex.: 100 h).
Fn= 100/MTBFnis the average frequency for breakdownnreferring to 100 h of work, whereMTBFn (mean time between failures) are the average working hours between the failure eventsn, without considering inactive time for other causes.
The statistical data Fn is significant for the probability of breakdown and depends on the total reliability of the system.
The second important group of statistical data is the acronymMTTRn(mean time to repair and restart) and corresponds to the average downtime for a machine, expressed in hours, necessary for repairing a breakdown (n) and restarting production. MTTR includes:
• time necessary to detect the breakdown and maintenance intervention management;
• time necessary for repair with machine stopped;
• time necessary to restart production, after necessary test for safety and process quality.
TheMTTRncan also be determined by statistical observation and depends on the ‘‘degree of maintainability’’ and ‘‘maintenance efficiency’’ (intervention speed). To perform correct statistical data collection for MTTRn, abnormal waiting
time, due to a lack of manpower or spare parts (management problems), should be excluded.
Frequency and length of breakdowns negatively influence the available pro- duction capacity (APC), according to what was demonstrated in Sect. 3.5. The percentage breakdown severity index, referring to the observed working time (t), derives from the following calculation:
s tð Þ ¼PK
n¼1
FnðMTTRÞn twhere:
t is the progressive working time during the period, expressed in hours n are the several types of breakdown observed
k is the total number of breakdowns observed
MTTRn is the ‘‘mean time to repair and restart’’ for each type of breakdown (n), expressed in hours
Fn is the average frequency for each type of breakdown (n).
The above statistical observation allows for the selection of priorities in countermeasure and improvement activities. For this purpose, a pareto diagram gives evidence of the percentage impact of the several types of failuren, both in terms of frequency and severity. This methodology will be detailed inSect. 8.5.
Referring to the working time diagram presented inSect. 3.5, we remember that the technical reliability degree is given by the ratio:
R ¼ PUT=ðPUT þBUTÞ:
For a specific manufacturing system or stand-alone machine, the trend fors(t), relative to the progressive working time cumulated, is typically shaped on a
‘‘bathtub’’ curve, as shown in Fig.5.1. On the y axle, we find the incidence of technical stoppage due to breakdowns, referring to the working time used for production.
As shown in the diagram, the normal technical life period shows a constant breakdown index during the given time if an effective maintenance process is assured, or slightly decreased if continuous improvement activity on the production
Machine try - out Breakdown maintenance and
learning curve Installed productive capacity obtaining
Technical life extension
Productive Up Time PUT [hours/1000]
Obsolescence limit
Normal life (continuous improvement) s(t)
Try-out
Physiological breakdowns
obsolescence Extraordinary maintenance plan
Fig. 5.1 Equipment’s breakdown severity index in relation to technical life
system is applied, according to the logic that will be detailed inChap. 8. During this period, breakdowns are caused by errors in utilization and by some physiological failures.
It is important to focus on the initial and final phases of the technical life of the system:
• Try-out phase, characterized by several types of failure and high frequencies that decrease exponentially during the given time (electronic and mechatronic component with infancy illness); for this reason, the adoption of breakdown maintenance in this phase is suggested, in order to search for the right coun- termeasures by applying cause and effect analysis, which we will be dealing with inChap. 8.
• Obsolescence phase, during which breakdowns become progressively more severe as a consequence of structural components reaching the fatigue limit, with an exponentially increasing trend; this phase can be delayed by using intensive professional maintenance plans (predictive maintenance and extraor- dinary maintenance activities, with the substitution of critical structural parts…).
• Obsolescence limit, which occurs when the system is no longer capable of guaranteeing the necessary quality level of the final products and/or the conti- nuity of the production process; in many cases, technical obsolescence occurs for technological reasons, with the availability of more modern solutions making the employment of new systems more convenient, or as the result of the advent of more severe legal requirements or, finally, for technological needs of new products.
Before concluding this topic, let us examine the interference effect of technical breakdowns in the so-called ‘‘integrated production systems’’.
Having the same reliability in the single modules of operation in a system, that system’s total reliability will be influenced by layout solutions and by intercon- nections adopted within the single stages of the system. Here, we approach the matter in a simple way, without using complex numerical models that should be based on the availability of statistical data, always hard to collect in a complex manufacturing system.
When the single modules are in parallel, with the possibility of working autonomously, even if in a unique integrated system, the total reliability degree for the system corresponds to the average value of the single reliability of each module:
Rt¼ ðR1þR2þR3þ þRkÞ=k where:
Rt is the reliability of the system Rk is the reliability of the module k K is the number of modules in parallel.
Conversely, if the layout of the system is set on modules that operate in sequence, interconnected one to the other in a synchronous way, the total reli- ability degree is equal to the product of the reliability degree of the single modules:
Rt ¼ R1 R2 R3 Rtk:
Practically, this method of determining the reliability degree is valid if the breakdown distribution can be considered random and if, in case of concurrent events on two or more modules, there is no simultaneous repair intervention. In this situation, as it normally occurs, the failure of a single module causes the breakdown of the whole system; in other words, there is full interference of the technical failures of the single modules for operative continuity of the integrated system.
The above-mentioned interference can be reduced when the modules are interconnected by ‘‘flexible material handling systems’’, having appropriate dynamic inter-operational buffers. Even if this solution is recommended for increasing the total reliability degree, it is counterproductive for the continuous one-piece-flow concept that we will discuss inChap. 6.
To calculate the optimal size of the ‘‘buffers’’, some considerations have to be taken:
(1) MTBF and MTTR for each module of the system should be known;
(2) additional investment needed for construction of the buffers, for all the alternative solutions considered;
(3) the obtainable gain in productivity level of the system by introducing the buffers (higher available productive capacity APC, higher overall equipment efficiency OEE).
The modern CAPE supports allow for the simulation of continuity conditions of productive flows, when the programmed technical stoppages are needed for set-up operations(pu) and when probable failure events (n) can occur, considering the frequency index Fnand the duration MTTRn. In this way, during the project setting phase, alternative solutions for layout composition can be compared (modularity level and deployment of operations), searching for the best solutions for module positioning and for dimension of the buffers. These CAPE systems are also useful for defining the convenient situation of simultaneous tool change with the machine stopped.