1 What Is Effective Maintenance?
1.1 The Aim of Effective Maintenance
The aim of Effective Maintenance is to raise the company’s productivity by lowering the total cost of its equipment over every stage from design and fabrication through to operation and maintenance (including the initial cost of the equipment itself, maintenance and other running costs, and losses due to equipment deterioration).
The goals of Effective Maintenance can be summarised as follows:
Goals of Effective Maintenance = (Equipment performs its functions whenever required / At minimal cost)= (Maximise goals / Minimise means of attaining goals)
In a nutshell, the aim of Effective Maintenance is to eliminate failures.
1.2 The Different Types of Maintenance
1.2.1 The two principal maintenance activities
Maintenance activities can be broadly divided into the following two types (see Figure 6.1). These two types of activity need to be carried out in tandem.
(1) ‘Sustainment’: Preventing and correcting equipment failures.
(2) ‘Improvement’: Prolonging the working life of the equipment, reducing the time spent on maintenance, and eliminating the need for maintenance.
1.2.2 Measures for ‘sustaining’
(1) Correct operation
(2) Preventive maintenance (routine maintenance, periodic maintenance and predictive maintenance)
1.2.3 Measures for ‘improving’
(1) Corrective maintenance (improving equipment reliability and maintainability) (2) Maintenance prevention (designing out the need for maintenance)
These measures can be incorporated into operators’ duties using a three-pronged approach: (1) preventing deterioration, (2) measuring deterioration and (3) reversing deterioration. Each of the three prongs can be implemented in a different way, and it is not always necessary to place equal emphasis on all three. However, if any of the three is neglected, it will not be possible to achieve the maintenance objective.
1.2.4 Types of Effective Maintenance
The basic approaches used in Effective Maintenance are periodic maintenance, predictive maintenance, breakdown maintenance and corrective maintenance, as described below. Each has its own merits and should be used selectively to suit the particular situation.
(1) Periodic Maintenance
Periodic maintenance means maintaining equipment at regular intervals, before any failures occur. Although it can be expensive to implement, it often turns out to be cheaper than breakdown maintenance, which consists of waiting for failures to happen and then dealing with them, because the total cost of replacing components in advance is usually less than the major losses caused by a serious breakdown.
There are two types of periodic maintenance: TBM (time-based maintenance), and IR (Inspection and Repair, i.e. periodic overhaul). In TBM, the time it takes for the equipment to deteriorate to a certain level is identified, and the equipment is serviced after it has been used for that length of time. In IR, on the other hand, the equipment is completely stripped down, inspected and overhauled at fixed intervals, and any parts found to be below standard are repaired or replaced.
Other duties of specialist maintenance staff include routines designed to prevent forced deterioration, such as periodically checking lubricant levels; replenishing, cleaning and replacing lubricants; cleaning the equipment, and so on.
(2) Predictive maintenance
Predictive maintenance is based on predicting the working life of key components through inspection and diagnostic testing, and finding ways of ensuring that those components are used right up to the end of their working life. It makes maintenance cheaper to implement and keeps failure-related losses low. However, to permit the warning signs of problems to be spotted at an early stage, inspection and diagnostic techniques enabling the deterioration of the equipment to be predicted accurately must be developed.
One strand of predictive maintenance is CBM (condition-based maintenance), in which trends are monitored by collecting and analysing data indicating the degree of deterioration. It requires a higher level of technology than TBM (for instance, an on-line monitoring system) and is more labour-intensive.
(3) Breakdown Maintenance
Breakdown maintenance consists of repairing or replacing units and parts after they have started to underperform or suffer loss of function (i.e., after they have broken down). With some machines, this reactive approach can be more cost-effective than the proactive approach adopted in preventive maintenance. True breakdown maintenance is intentional, whereas unplanned breakdown maintenance (emergency maintenance) is unintentional, and consists of repairing and replacing equipment on a ‘fire-fighting’ basis, with no pursuit of economy.
(4) Corrective Maintenance
The aim of corrective maintenance is to make equipment more reliable, easier to maintain, and safer to operate. Shortcomings in the existing equipment are systematically and proactively addressed by improving materials, configurations of parts, etc., with the aim of reducing deterioration and failures and thereby achieving a higher level of equipment reliability. (Essentially, the term ‘correction’ has two meanings: in the context of breakdown maintenance, it means repairing equipment that has failed, whereas in the context of corrective maintenance, it means improving equipment so that it will not fail in the first place.)
As well as being used to improve the existing machinery, the maintenance information obtained through corrective maintenance should be incorporated into the design of next-generation equipment through the activity known as MP design (maintenance prevention design). Creating mechanisms for collating the information on MP sheets or suggestion forms and feeding it into the design process is a crucial part of developing a powerful Effective Maintenance system.
(5) Other maintenance methods
One other maintenance approach is ‘opportunity maintenance’ (or ‘opportunity overhaul’), which consists of using gaps in the production schedule to maintain the equipment, rather than halting it specifically for the purpose.
1.3 Effective Maintenance Indicators
(1) Failure Frequency
‘Failure frequency’ denotes the incidence of failure as a percentage of loading time (the time during which the equipment is supposed to be operating). The ‘frequency’ part of this term was borrowed from the discipline of safety management. This index is expressed by the following formula:
Failure frequency = (Total number of stoppages / Total loading time) x 100%
(Loading time = total operating time + downtime)
(2) MTBF (Mean Time between Failures)
MTBF is the average operating time between one failure and the next, in repairable equipment.
MTBF = (Total operating time / Total number of breakdowns)
(3) MTTF (Mean Time to Failure)
MTTF is the average operating time from startup to failure, in the case of a component that is not repaired.
Like failure frequency, failure severity is also modelled on a term (in this case, ‘accident severity’) first used in safety management. It denotes the proportion of time for which the equipment is ‘down’ due to failure, and is expressed by the following formula
Failure severity = (Total breakdown time / Total loading time) x 100
(5) MTTR (Mean Time to Repair)
MTTR, or the average time required for the breakdown maintenance of a piece of equipment, is expressed by the following formula:
MTTR = (Total breakdown time / Total number of breakdowns)
Availability is the proportion of time for which a repairable system or item of equipment is in a state in which it can fulfil its function, within a given period of time. Mean availability (represented by the letter A) is often calculated using the following formula:
A = (uptime / (uptime+downtime)) = (MTBF / (MTBF+MTTR))
(where ‘uptime’ is the time for which the equipment could have been working if it had not broken down).
Intrinsic Availability (At): indicates how unlikely a machine is to fail, and how quickly it can be repaired if it does fail.
At = MTBF (MTBF + MTTR), where MTBF = mean time between failures, and MTTR = mean time to repair
Achievable Availability (Aa): indicates the length of the intervals between maintenance tasks, and how quickly the maintenance tasks are done.
Aa = MTBM (MTBM + Mean), where MTBM = mean time between maintenance activities (including both preventive maintenance and breakdown maintenance), and Mean = mean maintenance time, i.e. the average time required to perform preventive maintenance and breakdown maintenance tasks)
2. The Basic Approach to Implementing Effective Maintenance
2.1 The Three Main Reasons Why Equipment Fails, and the Five Main Factors that Cause It to Fail
2.1.1 The three main reasons why equipment fails
Why does equipment fail? The answer to this question can be explained in terms of the relationship between its strength and the stresses imposed on it. Let us analyse the reasons more specifically:
(1) Uncorrected deterioration
All equipment deteriorates with time, causing its design strength to dissipate. Eventually, it may become unable to withstand the stresses placed on it during operation, whereupon it fails.
(2) Unchecked stress
Equipment sometimes fails, even though it maintains the specified strength, when forces are applied to it greater than those envisioned at the design stage.
(3) Insufficient strength
Equipment may fail under normal stresses if it there is an inherent weakness in its design.
2.1.2 The five main factors that cause equipment to fail
(1) Deficient basic conditions
Equipment will ultimately fail if it is allowed to deteriorate progressively when the operating department neglects to fulfil its daily maintenance responsibilities properly by sustaining basic conditions (regularly checking the equipment and keeping it clean, well- lubricated and tightened), or when no system exists for the operating department to do so, or if abnormal stresses are overlooked.
(2) Leaving deterioration uncorrected
If equipment has badly deteriorated because of Factor (1), and nothing is done to reverse this deterioration, it will be no surprise if it breaks down at any moment. In such a situation, equipment fails because, lacking the necessary skills and not knowing what to look for, neither the maintenance people nor the operators bother about visible deterioration or attempt to investigate hidden deterioration.
(3) Not observing correct operating conditions
Whenever a machine is installed in a factory, it comes with a set of operating standards specifying conditions such as current, voltage, rotation speed, linear speed and temperature. If these standards are not observed, the equipment may be subjected to unimaginably high levels of stress. The same thing can happen when equipment is remodelled or adapted for a different use, if the production engineers do not think hard enough about the new conditions of use.
(4) Lack of skill
Failures are sometimes induced by unskilled maintenance engineers repairing the equipment in such a way as to increase the stress imposed on it, preventing it from reaching its specified service life. The same applies to operators, who may also, through lack of skill, subject the equipment to abnormal stress and cause it to fail by operating it incorrectly.
(5) Inherent design weaknesses
Insufficient strength can be thought of as a problem with the equipment’s original design – in other words, a design error. Whether such errors are due to carelessness or to lack of knowledge, experience or information, they are the responsibility of the equipment’s designers and fabricators.
Figure 6.2 Stress-Strength Analysis
Why Does Equipment Fail?
2.2 The Five Main Factors that Cause Equipment to Fail
Figure 6.3 How the 3 Reasons and 5 Factors Relate to Each Other
Figure 6.4 The 5 Factors, and the 5 Steps to Eliminate Them
Deficient basic conditions
Establish basic conditions
Not observing correct operating conditions
Observe operating conditions
Leaving deterioration uncorrected
Inherent design weaknesses
Eliminate design weaknesses
Raise operating and maintenance skills
2.3 Maintenance Planning and the 4 Phases
Any attempt to maintain the equipment systematically and cost-effectively will be frustrated if it continues to fail regularly and the intervals between those failures are always changing, because this makes it impossible to put together a dependable maintenance plan. To begin with, breakdowns must be systematically reduced by following the four-phase approach illustrated in Figure 6.5.
Phase 1 (Eliminate forced deterioration)
Phase 1, which focuses on reducing variation in the time interval between failures, corresponds to Steps 1 and 2 of the Autonomous Maintenance programme. It contains the following two main strands:
(1) Correct neglected deterioration
The first thing is to correct any obvious deterioration left untreated because of labour or budget constraints or lack of awareness. We should look for the following sorts of situation and deal with them:
- Used all the time but left untended
- Left loose
- Left detached
- Left out of kilter
(2) Eliminate forced deterioration
The second thing is to eliminate forced deterioration, which is abnormal deterioration caused by stress exceeding the value specified in the equipment’s design. To do this, we:
Sustain basic conditions
(Keep the equipment clean, properly lubricated and securely tightened)
Observe correct operating conditions
- Prevent external sources of disturbances
– Knocks, vibration, noise, …
- Ensure that the conditions are compatible with the specifications of the unit and
– Environmental conditions, proper installation method, etc.
- Ensure that the load matches the equipment’s capacity
Phase 2 (Extend lifetimes through corrective maintenance)
In Phase 2, we try to lengthen the equipment’s intrinsic working life, principally through the following three approaches:
(1) By identifying weaknesses, try to increase the working life of any equipment that soon fails even after all forced deterioration has been eliminated
- Correct design weaknesses – Insufficient strength – Installation problems – Fabrication problems
- Improve equipment’s ability to cope with overloading
If the load cannot be reduced, then the equipment’s weak points must be strengthened.
- Select parts suitable for conditions of use
(2) Eliminate unexpected breakdowns
- Eliminate repair errors -Instil basic repair skills -Improve repair methods, etc.
- Eliminate operating errors -Standardise operating methods -Introduce error-proofing techniques, etc.
(3) Correct visible deterioration
Conduct a comprehensive external inspection of hydraulics, pneumatics, drive systems and electrics, and correct any visible deterioration.
Phase 3 (Monitor and control deterioration)
(1) Estimate the equipment’s working life and periodically restore it to its original
The actions taken in Phases 1 and 2 extend the equipment’s working life and increase the accuracy with which that life can be estimated. This makes periodically correcting deterioration very reliable and cost-effective.
1 Improve maintainability
Modify the structure of the equipment to make it easier to maintain. For example:
- Use standard components wherever possible
- Make it possible simply to swap failed units in and out
- Simplify disassembly and assembly procedures
- Improve existing jigs and tools, and develop new special-purpose ones
2 Standardise periodic maintenance, and implement the following:
- Periodic checking
- Periodic inspection
- Periodic servicing
- Spare parts standardisation
(2) Use the five senses to spot signs of internal deterioration
If the working life of a unit is hard to estimate, or is inherently variable, then operators must rely on identifying the warning signs of failure. Based on past experience, they should answer questions such as:
- Did the failure give off any warning signs before it happened?
- Is it the type of failure that could actually produce warning signs?
- What kinds of warning sign could be used to identify the impending failure?
- Why weren’t the warning signs recognised before the failure occurred?
- What must be done to make sure that the warning signs are recognised in the future?
- What knowledge and skills does the operator need in order to recognise the warning signs?
Phase 4 (Carry out predictive maintenance)
(1) Use diagnostic techniques to predict onset of failure
Some typical failure modes, and the diagnostic methods used to measure them, are listed below:
- Fracture – magnetic scanning, X-rays
- Abnormal noise
- Abnormal temperature – infra-red thermography
- Abnormal vibration – vibration measurement
- Deterioration in materials – AE (acoustic emission)
- Deterioration in lubricants – SOAP (spectrometric oil analysis programme)
- Electrical anomaly – insulation measurement
(2) Estimate and extend working life through engineering analysis of catastrophic breakdowns
- Analysis of fracture surfaces – Stress concentration
- Analysis of material fatigue – Repeated load – Alternating load
- Analysis of gear tooth surfaces
Figure 6.5 The 4 Phases to Zero Breakdowns
3. Types of Maintenance, and How They Should Be Allocated
3.1 Types of equipment maintenance
Up until this point, the main thrust of our discussion has been the thinking behind equipment maintenance. This section splits the maintenance task into three types and explains what each type consists of and who should be responsible for it.
The three types of activity are sometimes referred to as the ‘three elements of maintenance’. They consist of (1) activities to prevent deterioration, (2) activities to monitor deterioration, and (3) activities to reverse deterioration (see Fig. 6.6)
(1) Activities to prevent deterioration
This consists of protecting equipment against forced deterioration by looking after it on a day-to-day basis. Specifically, it means operating the equipment correctly and carrying out routine daily maintenance (cleaning, lubricating, retightening, etc.).
(2) Activities to monitor deterioration
A machine will eventually stop working properly, even if only subject to natural deterioration, unless action is taken before a certain limit is reached. This is what makes it necessary to monitor how far the deterioration has progressed, by measuring it periodically. Specifically, ‘monitoring deterioration’ means checking the equipment daily or at longer intervals, overhauling it, and testing it, using the required measuring technology and diagnostic techniques.
(3) Activities to reverse deterioration
The decision as to how to reverse a particular type of deterioration will be based on
information obtained from (2) above. It might consist of periodically replacing or overhauling a part, or monitoring its condition, and it requires a relatively high level of technical expertise and skill, plus a lot of time.
3.2 The Roles of the Production and Maintenance Departments
The production and maintenance departments have their own respective roles to play in sustaining and improving OEE (Overall Equipment Effectiveness) by maintaining equipment reliably and economically (see Figure 6.7).
First, the equipment’s operators must:
• Carefully sustain basic equipment conditions (by periodically cleaning, lubricating and re-tightening).
• Maintain the correct operating conditions (mainly by performing external checks and following correct operating practice).
• Identify the areas that tend to deteriorate most rapidly (chiefly by visual inspection when restoring the equipment), and use their five senses to spot early warning signs of trouble.
• Continually improve their inspection skills as well as their operating skills (for working the equipment, performing changeovers, etc.).
These are the four principal activities the operators should engage in, under the umbrella of Autonomous Maintenance.
Meanwhile, the maintenance department must:
• Provide technical assistance to support the production department’s Autonomous Maintenance programme.
• Scrupulously eliminate deterioration through regular checking, inspection and strip-down overhaul.
• Identify design weaknesses, specify precise operating conditions, and improve the equipment effectively and reliably.
• Improve their skills for checking, inspection, strip-down overhaul and other maintenance tasks.
It is essential for the production and maintenance departments to work closely together if the maintenance programme is to go ahead smoothly. This means that the production department must take responsibility for some of the maintenance function in addition to its operating duties, while the maintenance department must assume
responsibility for production in order to ensure that productive maintenance takes place.
The roles of the operating and maintenance departments
(1) The operating department
The day-to-day management of the equipment is what determines its safety, quality and productivity, and the operating department must undertake this as one of its essential duties (managing the equipment on a daily basis must be treated as part of the operating department’s responsibility; it should not be thought of as the operating
department doing some of the maintenance department’s work for them).
For this to happen, operators must learn how to check, clean and lubricate their equipment and perform other essential routine management tasks, while developing their ability to recognize abnormalities.
(2) The maintenance department
As the maintenance specialists, the maintenance department should perform tasks such as repairing equipment when it breaks down, periodically overhauling it to reverse deterioration, monitoring its performance, and performing corrective maintenance to increase its reliability and maintainability. To accomplish this, it must cultivate higher and higher levels of the necessary maintenance expertise and skills.
The maintenance department must also provide the necessary training to support the operating department in acquiring the maintenance knowledge and skills it needs to manage its equipment from day to day.
The real job of the maintenance department, however, is to sustain and progressively raise its level of technical expertise while increasing the efficiency of the maintenance work. The maintenance department should focus its energies on these tasks in the conventional way.
Scope of Autonomous Maintenance performed by the operating department
1 Routine management tasks for keeping the equipment working properly
• Cleaning, checking, lubricating, re-tightening, replacing consumable parts, etc.
• Detecting and correcting abnormalities, and improving the equipment, through the medium of these tasks
• Taking action to reduce the level of minor stops
• Creating and continually updating standards associated with the above work
2 Work related to operating conditions and quality conditions
• Robot teaching, controlling welding conditions, grinding and replacing welding tips
• Maintaining and controlling the precision of gauges, clamps, etc.
• Keeping painting lines clean (this leads directly to better quality)
• Controlling the conditions under which guns and nozzles used for paint spraying, adhesive application and so on are operated
• Controlling die-moulding conditions
Figure 6.7 How the Responsibility for Maintenance Should be Allocated (Example)
3.3 Management Issues
Failures become chronic as a result either of management problems or of technical problems. This section discusses the former. Figure 6.8 summarises the weaknesses and vicious circles that can occur within the management structure, based on the experiences of a range of different companies.
One of the most serious problems affecting equipment management in many companies is the ‘I make, you fix’ mentality of the production department, which leads them to think that maintenance is none of their responsibility. A second is the lack of specialised training given to the maintenance department, making them unable to keep up
with technological progress and keeping their morale low. A third is the heavy reliance of equipment design departments on outsourcing, introducing time and budget pressures that inevitably result in equipment riddled with problems from the start.
Two principal reasons for this poor attitude to productive maintenance among managers can be identified.
First, many of them are still trapped in the ‘throw-away’ attitude to equipment that developed during the period of rapid economic growth. This is demonstrated by the fact that there are still plenty of companies whose priority when investing in new equipment is to minimise initial costs, and plenty of others whose first move when profits start to decline is to cut their maintenance spend by 20 or 30%. Even if no money were spent on maintaining it, equipment might last long enough to keep production going for a few years. However, if it were not properly maintained during this time, it would probably grind to a halt in the fourth or fifth year in such a state as to be virtually irreparable. In other words, the very equipment that ought to have been continuing to generate profits
for the business would have dragged it down into the mire.
Second, many of these managers do not understand the full scale of the losses (the causes of higher costs) that arise if equipment maintenance is not up to scratch. Substandard equipment maintenance not only causes losses due to unexpected breakdowns, but also has harmful effects going well beyond this, such as increased changeover times, more frequent idling and minor stops, slower speeds (i.e. longer cycle times), poorer quality, and lower yields, as well as the energy losses and labour losses that derive from these. Experience has shown that the combined total of these losses often reduces the equipment’s effectiveness by 30 – 50%.
It is absolutely essential for management to clearly identify and fully understand the problems in the management system and the precise structure of the vicious circles it generates, before taking any action to reduce the number of equipment failures. Nothing will be achieved if the management aspect of equipment failure is neglected, and any
efforts made to improve the technical aspect will simply go to waste.
Figure 6.8 Management Issues
Chapter 6. Planned (Effective) Maintenance. Part 2