In this article we will discuss about the maintainability of equipment in an industry.
Maintainability Criteria of an Equipment:
The main criteria of maintainability of an equipment is that it must have high availability, mean time to repair the fault should be minimum, it should be possible to increase mean time between failure by undertaking preventive maintenance at regular intervals.
Maintainability criteria has to be defined by the user and dependent on the degree of complexity of equipment/system and the required safety, availability and reliability, the maintainability requirements have to be suitably incorporated in the design of system.
Achievement of operational requirements including maintainability requirements in design within specified constraints and costs is a responsibility of the designer. Various maintainability studies should be performed during the design stage to satisfy maintainability criteria and requirements.
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Maintenance concept, which is concerned with maintenance support planning, is also an important aspect in operational environment of system/equipment. It provides for the practical basis for design, layout and packaging of the equipment and its test devices, etc.
For an optimum design and to achieve the lowest life cycle costs, it is essential that designer develops in parallel the maintenance concept and the maintainability requirements. Maintainability is a part of design process.
Maintainability analysis is carried out at:
(a) Preliminary stage and
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(b) Detailed design stage.
Various analysis required during preliminary stage include:
(i) Maintainability Requirements:
These are defined clearly both qualitatively and quantitatively in terms of specific functional, performance, reliability, maintainability and safety requirements and then interpreted in terms of design tasks.
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(ii) Maintenance Concept:
It provides for the practical basis for design, layout and packaging of the equipment and its test equipment, the proposed methods of repair, necessity of spare parts, etc.
(iii) Maintainability Feasibility Estimation:
It is concerned with the identification and estimating duration of the dominant corrective and preventive maintenance tasks.
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(iv) Maintainability Allocation:
It relates to the process of translating system maintainability requirements to lower functional levels. Attempt should be made to improve the maintainability characteristic by better diagnosis, easy access of parts to be replaced, concept of modularisation facilities, etc. All identified potential maintainability problems are ranked according to their impact of system availability or other primary operational requirements.
Various activities performed during detailed design phase are:
(i) Maintainability predictions (assessment of meeting the allocated maintainability requirements and identification of areas for modification/change;
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(ii) Preparation of guidelines and checklists to prevent recurrence of frequently observed maintenance problems;
(iii) Design trade off studies;
(iv) Other maintainability analyses (like address diagnostic requirements, diagnostic effectiveness level, location of repair, ease of maintenance, maintenance task analysis, skill analysis, etc.)
Maintainability Study:
There are several methods to perform maintainability studies.
Some of these are:
(i) Functional Level Maintainability Diagram:
This method consists of breaking down an equipment into various levels beginning with the highest level and continuing down to levels at which faults can be localised and sub-items replaced, repaired or adjusted.
The functional analysis provides a description of major item functions and defines the equipment arrangement concept. The diagram indicates, the replaceable and repairable items and also indicates the fault localisation, isolation and test points (Refer Fig. 11.94).
(ii) Failure Mode and Effect Analysis (FMEA):
It identifies the way of occurrence and the causes of all possible item failures. The analysis provides an input to maintainability predictions by identification of failure modes, their frequency and the subsequent maintenance action required.
It facilitates maintainability allocation process. It also serves as a basis for design and location of condition monitoring and fault sensing devices and development of automatic test and diagnostic procedures to minimise corrective maintenance downtime.
(iii) Maintainability Allocation:
This is the process of determining target maintainability values for various units of the equipment, considering the relative complexity of units involved and the overall maintainability requirement for the item.
Maintainability Prediction:
It is the process of analytically assessing the maintainability features and calculating the quantitative maintainability characteristics of an item using a defined maintainability model including stated maintenance support conditions.
Maintenance tree prediction method is frequently used to evaluate the maintainability characteristics of a sub item, taking into account the maintenance policy and reviewed reliability data. This method is also useful for trade-offs at design stage. Maintenance trees are also a most useful way of representing a sequence of elementary tasks.
A maintenance tree is a graphic pattern of a complete maintenance operation (preventive or corrective) giving qualitative and quantitative information on how to carry out maintenance tasks.
The elementary tasks in a maintenance tree are:
(i) Diagnosis phase (failure confirmation and localisation),
(ii) Restoration phase (isolation, disassembly, exchange, reassembly), and
(iii) Check-out phase.
Maintenance Support Resources:
For cost effective maintainability of any equipment/ system, the following maintenance support resources must be adequately provided:
i. Personnel and training
ii. Technical manuals
iii. Test and support equipment
iv. Provision of spare parts.
The appointment of properly qualified and adequately trained personnel is the basic requirement for safe and reliable operation of any equipment. These persons must be well versed with maintenance requirements of the equipment, their design, and support equipment.
A technical manual should contain information and procedures required by operators and maintenance personnel to carry out their job correctly, efficiently, and in cost effective manner.
It must contain:
(i) Description,
(ii) Theory and principle of working,
(iii) Procedures of operation, testing, fault diagnostics and troubleshooting, repair, service, and preventive maintenance,
(iv) Reference data. It must cover those tasks which are difficult to perform, frequent faults, influence mission success.
Support equipment is the equipment required for maintenance, servicing and inspection of a system but essential for its operation. Support equipment includes hand tools (wrenches, hammers, screw drivers, soldering guns and pliers, etc.), special hand tools, common support equipment (for removal, assembly/disassembly of equipment like cranes, slings, force lift trucks, etc.), mechanical and electronic measuring and monitoring equipment (pressure gauges, multi-meter and oscilloscope, etc.), special fixture to allow measurement of mechanical parameters or special electronic analyser/tester, etc., industrial fixtures and equipment for overhaul, inspection, and repair of major systems.
Where availability of system is important and manual testing is not possible due to complexity of the system, built-in test features must be incorporated. Automatic test equipment, (usually computer driven) is used to provide for automatic test equipment diagnosis and the localisation of fault.
The availability of all spare parts, consumables, special supplies and related inventories is essential to carry out corrective and preventive maintenance of the system. Spare parts are required to repair the equipment faults, for preventive maintenance to fill the logistics pipeline to compensate for repair lines and supplier lead times.
All spare parts must be provided a unique identification number to clearly indicate the equipment/assembly/subassembly they belong to. These should be adequately packaged to allow for their transportation and storage without damage caused by either handling or environmental exposure.
Maintainability is the probability that, when maintenance action is initiated under stated conditions, a failed system will be restored to operable condition within a specified time. System maintainability can be improved by providing accessible test points, built-in test equipment, built- in diagnostic aids, training the operating personnel, and providing spare parts and equipment for incorporating repairs.
Important Points Should be Taken Care for Maintainability:
The following is the check list to ensure high maintainability:
i. Use modular or unit packaging.
ii. Use standardised parts.
iii. Use self-lubricating principles.
iv. Use sealed and lubricated components.
v. Provide built-in testing and calibration feature.
vi. Employ self-adjusting mechanisms.
vii. Use gear-driven accessories in preference to belt and pulleys.
viii. Simplify operator and maintenance functions and use simple design.
ix. Design for quick and positive recognition of marginal performance/malfunction.
x. Design for quick position recognition of the replaceable defective components.
xi. Design to minimise the types and number of tools and test equipment required for maintenance.
xii. Design for optimum accessibility of all components/ system/equipment.
xiii. Design for maximum safety and protection of personnel.