At PCMS Engineering, we have the expertise and resources required to implement cost-effective CBM programmes that help organisations of all sizes move from reactive maintenance to predictive maintenance by using a combination of tools, products and technologies
What is CBM?
Condition based maintenance (CBM) is a maintenance strategy which, simply put, measures operational parameters in assets to determine a change in their condition, performing maintenance only when the need arises.
By using instrumentation to monitor equipment performance in real time, maintenance personnel can identify an adverse change in one or more operational parameters and trigger corrective action to ensure ongoing performance – preventing in-service failure.
Similar to an overall health check at the doctors to detect any underlying issues that might arise with age, or regularly maintaining your car to keep it running, CBM uses various process parameters (e.g. pressure, temperature, vibration, flow, noise, visual, thickness etc.) and material samples (e.g. oil) to monitor conditions in machines.
Engineers can then measure equipment health, performance, reliability and integrity to understand how machines perform, as well as predicting and preventing failures before they happen.
How does CBM work?
The techniques used to monitor the performance of machinery or a component whilst in operation is known as condition monitoring. Condition monitoring techniques – such as vibration analysis, ultrasonics, thermography and oil analysis – can be used solely or in combination and are vital in making sure maintenance is only performed when absolutely necessary.
Generally, condition monitoring techniques are used on a variety of assets such as pumps, electric motors, internal combustion engines, gearboxes, fans, electrical control panels, compressed air and hydraulic systems etc.
What are the benefits of CBM to my organisation?
The main benefits of applying an effective condition based maintenance programme are that repairs can be scheduled during non-peak times, machine productivity and service life are enhanced, and repair costs due to a loss of production time are eliminated.
How do I identify my most critical assets?
The key to having a cost-effective and balanced condition based maintenance programme is to assess which machines are most valuable by performing an asset criticality audit. This addresses the impact of temporary or permanent loss that a key asset would have on your business. The characteristics that make an asset valuable aren’t always obvious. It’s important to rank assets based on the significance a failed asset has on your business.
For example, a small motor gearbox with a replacements cost of a few hundred pounds, which can be sourced quickly, is not critical equipment that needs constant monitoring.
In contrast, a motor gearbox that powers a major conveyor belt with a replacements cost of thousands of pounds, which could take weeks to source and halts your production line until it’s repaired or replaced, is a critical piece of equipment that needs to be monitored around the clock.
How do I implement a CBM programme?
The first phase to implement a condition based maintenance programme is to identify your critical assets by conducting a CBM survey. Our reliability experts will discuss your maintenance requirements, identify critical assets and pinpoint maintenance resources to where they are needed most.
If you are looking to increase productivity, lowers costs and improve the bottom line throughout your entire organisation, then contact us to discuss how we can implement an efficient and effective CBM strategy.
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Due to the problems encountered implementing the RCM technique in small and medium sized process and manufacturing industries, PCMS Engineering developed an alternative technique known as a Review of Equipment Maintenance (REM), which concentrates on improving the availability of existing equipment.
This was in response to the trend towards larger, more integrated production facilities for which the cost of downtime is a major factor in the cost effectiveness of the facility as a whole.
This methodology aims to develop an optimised maintenance plan through a review of the existing maintenance tasks already carried out on the equipment. These maintenance tasks will have generated a large volume of maintenance information on the plant’s historical performance.
The quality and location of this information will vary considerably from plant to plant, but will usually be available from maintenance and production records, or by interviewing plant personnel. For the purpose of an REM study, information on breakdowns is often more useful than information on Fixed Time Maintenance.
The information gained from different maintenance strategies can be summerised as follows:
- OFM: Learning by damage and disgrace;
- CBM: Learning by measurement and knowledge.
- The learning effect from FTM is virtually non-existent.
The primary steps undertaken as part of an REM review are shown above. It can be seen that this is a similar process to RCM, but with an emphasis on the maintenance audit as the main source of failure information.
Ideally, REM is not a process which should be carried out once for a particular plant but should be a continuous process which reflects changes to the operating characteristics of the plant such as product mix, changes to the production cycle or changes to the equipment itself.
During the development of the Boeing 747 a Maintenance Steering Group was formed to look at the maintenance requirements of the aircraft. This steering group carried out a survey on the failure characteristics of aircraft components, which revealed that 94% of failures were not time dependent.
Up until this time, all aircraft maintenance had been based on flying hours, therefore a new method of maintaining aircraft had to be devised. The method chosen was a systematic analysis of the components which made up the aircraft based on a Failure Mode Effect Analysis (FMEA) technique originally developed by the United States Military. The basis of an RCM analysis follows the structure shown in the diagram below.
Moubray took this process and adapted it to suit industrial applications, using it to select the correct mix of maintenance strategies to maximise maintenance effectiveness. He called his process RCM II. RCM as a process has two primary objectives:
- To determine the maintenance requirement of each item of plant and equipment in its operating context.
- To ensure that these requirements are fulfilled as cheaply and as effectively as possible.
The application of RCM has proved to be time consuming and has been applied with little success outside High Intensity Process Systems (HIPS) industries, with notable success mainly in the Nuclear, Oil, Aircraft Manufacturing and Electricity Generation industries.
The 4 maintenance strategies are not mutually exclusive. For any given piece of equipment appropriate maintenance strategies should be sought to counteract each identified mode of failure. The strategies are then combined together for the equipment to create what is known as the “Asset Maintenance Plan”.
The selection of a maintenance strategy for any given mode of failure should take into account the failure characteristic, the frequency of failure (mean time to failure) and the lead time to failure.
The “Maintenance Plan” for a site or organisation is produced by co-ordinating the individual unit maintenance plans in order to make the best use of available resources and time. In effect, the unit maintenance plans identify what is to be done, and the site maintenance plan identifies when and how it is to be performed.
There are a number of methodologies which can be used to produce a maintenance plan, including Reliability Centered Maintenance and Review of Equipment Maintenance. Many sites are still operating using a maintenance plan which was generated when the plant was first installed, based on the plant manufacturer’s servicing information.
These plans often do not reflect the current maintenance requirements of the plant as the equipment, production cycle, operating conditions and/or product mix may have changed radically since the plant was commissioned.
Maintenance which is carried out at regular intervals of either time, output, cycles of operation etc. is known as Fixed Time Maintenance. To minimise disruption Fixed Time Maintenance is, where possible, carried out off-line i.e. when production has stopped.
Fixed Time Maintenance will only be effective where the failure characteristic is time dependent with the failure occurring within the life of the equipment, and where the total cost of the repair is substantially less than the cost of allowing the equipment to breakdown.
Unfortunately, time dependent failures are a relatively uncommon characteristic in a complex plant, resulting in FTM having a negligible or occasionally detrimental effect on improving equipment performance.
The more predictable the time to failure, the more effective Fixed Time Maintenance is. This does not necessarily mean that FTM is the optimum strategy to use for time dependent failures, as Condition Based Maintenance can sometimes be more economic if it can be applied.
In British industry, the maintenance plan has often evolved rather than being consciously set-up resulting in an over reliance on breakdown and fixed time maintenance (planned maintenance). Historically plant equipment has been robust and less automated than modern equipment making it relatively easy to work on. These plants have relied on large and expensive maintenance departments to enable rapid attention to breakdowns. Single-line single-product plants with no standbys have allowed profitability to be increased through the economies of scale. On the downside, this has resulted in ever increasing breakdown costs. In this changing environment, historical maintenance plans cannot provide the required level of plant performance.
The goal of any well run maintenance organisation is to have the lowest cost of the sum of two quantities, i.e:
- Maintenance labour and material
- Production loss reduction resulting from an inadequate maintenance programme (which includes lack of ability to produce, and value added material that is lost as a result of a breakdown).
Maintenance itself can result in excessive downtime and costs. This results from the requirement to take the machinery off-line to carry out (possibly unnecessary and invasive) maintenance. The danger of infant mortality after it has been put back on line again and also the cost of the maintenance action itself contributes to costs. Achieving the lowest cost is an optimisation technique shown graphically in the diagram below.
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Once a maintenance plan has been determined, it is the task of the maintenance department to ensure that it is carried out. This function is known as Maintenance Work Control, and covers all aspects of day to day maintenance management activities. This is a separate function from activities designed to improve the maintenance plan, such as REM or RCM analysis. The figure below shows how these two aspects of the maintenance department function relate to each other.
One of the primary challenges in maintenance management is balancing these two aspects of maintenance, namely long term improvement in the reliability of the assets employed and managing the work force to ensure that maintenance tasks are carried out. Many companies expend much effort on the day to day management of maintenance, with the result that potential reliability improvements suffer. This is often extenuated by the erroneous view held by many that maintenance costs companies money. This view was confirmed by a survey of maintenance managers, 67% of whom responded that their senior management regarded maintenance as an overhead rather than an investment.
The maintenance improvement cycle is often ignored or under resourced. Many companies concentrate effort on the maintenance work control cycle to the detriment of plant improvement activities. This can be due to a number of reasons, including budget or labour constraints or too much “fire fighting”. Clearly any time saved on day to day management by maintenance engineers could be more effectively utilised in planning and implementing reliability improvement policies, which in turn will lead to both higher plant availability and lower operating costs. However, lack of time is the most common reason quoted for not undertaking plant improvement studies.
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Operate To Failure involves no advanced planning other than ensuring that enough resources are available to carry out appropriate repairs following plant failure. This is usually the least desirable strategy to employ, as the planning and control becomes very difficult to predict and the equipment can breakdown at any time.
All of the other four maintenance strategies, to some extent, allow for repair or overhaul to be scheduled so that production loss is minimised. Operate To Failure by its nature will result in breakdowns occurring when the equipment is in use.
The correct strategy to employ on non-critical equipment, or where suitable standby equipment is available.
If the maintenance cost or downtime cost of equipment is high, then the Design Out Maintenance strategy can often be effective. This strategy differs from all the others in that it is a one-off activity, as opposed to a repetitive activity designed to prevent failure. Design Out Maintenance aims to redesign those parts of the equipment which consume high levels of maintenance effort or spares cost or which have unacceptably high failure rates.
The high maintenance costs may have been caused by a number of factors, including:
- Poor maintenance
- Operation of equipment outside of its original design specification
- A poor initial design
The Design Out Maintenance strategy can only be implemented effectively if high maintenance cost items can be identified and the reasons for the high cost understood. It is often the best strategy to take when breakdowns are too frequent or repair is too costly.
Some examples of where this concept has been applied successfully are disposable razors, cigarette lighters, ball-point pens, disposable syringes, etc.
The prime objective of a maintenance management strategy is to achieve the optimum balance between equipment performance, availability and the cost of maintenance. Reducing the spend on maintenance can have serious consequences to production.
In one pottery company, in an attempt to reduce costs following reduced profits, the maintenance staff was halved. As a direct result, a machine failed causing £90,000 of damage and lost production. Later, with outside help and improved maintenance procedures, they were able to save 15% of their total production costs.
A Ministry of Technology Working Party report in 1971 estimated that maintenance was costing the United Kingdom around £3 billion per year. They considered that by improving maintenance management and paying greater attention to other factors in the equipment life cycle, substantial savings could be made. The life cycle approach to maintenance cost reduction has since been defined as terotechnology.
The British Standards Institution defines terotechnology as:
“A combination of management, financial, engineering, building and other practices applied to physical assets in pursuit of economic life cycle costs. … Its practice is concerned with the specification and design for reliability and maintainability of plant, machinery, equipment, buildings and structures, with their installation, commissioning, operation, maintenance, modification and replacement, and with feedback of information on design, performance and costs.”
During its life cycle, equipment passes through a number of stages, from specification to replacement. The figure above shows these stages and the inter-relation between them. It can be seen that the successful operation of the equipment is dependent on all the stages in the life cycle.
The maintenance function has traditionally only had an influence on the latter stages of the life cycle, from commissioning through to replacement. However, the earlier stages in the life cycle are where the maintainability of the equipment is set and, without major redesign, maintenance can only really be considered as a “fire-fighting” function, responding to unexpected events with no ability to plan accurately in advance. Ideally, the maintenance function should contribute at all stages of the life cycle, allowing for equipment to be specified, designed and manufactured with the future maintenance implications in mind.
Reactive And Preventative Maintenance
Maintenance activities can be split into two broad areas, reactive and preventative. Reactive maintenance, often known as unplanned or corrective maintenance, occurs where the plant is allowed to run until something fails; after which it is repaired. Preventative maintenance, also known as planned maintenance, aims to forecast when failures are likely to occur, and to fix the problem before it occurs, at a time that is convenient to both production and maintenance.
Traditionally, maintenance was mainly reactive in nature; as machines broke down they were fixed. It is only since the 1970′s that preventative maintenance has become more common within UK industry. The Central Policy Review Staff report in 1975 identified poor plant maintenance as a significant cause of low productivity in the British motor industry. The report stated;
“Effective preventative maintenance programmes are essential to maintaining consistent levels of output in car assembly plants. Despite the fact that British manufacturers employ 50 to 70% more plant maintenance personnel than continental competitors, on identical equipment, mechanical breakdowns result in the loss of about twice as many production hours in the UK as on the continent.”
This situation occurred as a result of the UK plants employing an almost exclusive reactive maintenance strategy and dealing with breakdowns as and when they occurred. On the continent, the use of preventative maintenance was commonplace, resulting in more effective maintenance being obtained for lower cost. Also as preventative maintenance actions are by definition deterministic they can be scheduled in advance, usually according to a planned preventive maintenance programme. This allows maintenance resources to be utilised more effectively as resource usage can be levelled by reducing the peaks and troughs commonly caused by a reactive maintenance policy.
In addition to the two broad categories of predictive and reactive maintenance, maintenance can be further sub-divided into four major strategies. These strategies may be employed, individually or in any combination, to maintain any individual asset throughout its lifetime.
The four strategies are as follows:
- Design Out Maintenance (DOM)
- Fixed Time Maintenance (FTM)
- Condition Based Maintenance (CBM)
- On Failure Maintenance (OFM)
It is highly improbable that equipment failure can be eliminated completely through the adoption of other maintenance strategies; there is always the possibility that equipment will fail in an unexpected manner. As a result, a combination of strategies tend to be used by maintenance departments in order to cover all eventualities.
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