PRINCIPLES
Industries are operated in an era of global competition. They are in a fight to survive and to produce profit among competitors in global front. There is an increased awareness in use of modern and sophisticated tools for engineering analysis of the industry. Although impressive progress has been made in maintaining equipment in the field in an effective manner, maintenance of equipment is still a challenge due to various factors such as size, cost, complexity, and competition. Due to various factors, it was established in the previous century that “maintenance” must be an integral part of the production strategy for the overall success of an organization.
The planning in engineering system is classified into three major approaches.
(a) Long range planning
(b) Short range planning and
(c) Planning for immediate activity
(a) Long range planning:
Long range planning refers to the planning spread over span of five to ten years. In this type of planning, the goals are set, strategies are developed and operational programmes are devised for the period of planning.
(b) Short range planning:
Short range planning is prepared by concerned departments for a relatively shorter period of time, say 1 to 2 years.
(c) Immediate activity planning:
Immediate activity planning is a routine and it is done at the working level when required.
RELIABILITY
Reliability is defined as the probability that a component/system when operating under given condition will perform its intended functions adequately for a specified period of time. It refers to the possibility that equipment will not fail during its operation.
The objectives of reliability engineering are as follows.
(i) To apply engineering knowledge and specialist techniques to prevent or to reduce the possibility or frequency of failures.
(ii) To identify and correct the causes of failures which occur despite the efforts to prevent them.
(iii) To determine ways of coping with failures that occur if their causes have not been corrected.
(iv) To apply methods for estimating the likely reliability of new designs and analysing reliability data.
Reliability can be generally of two types.
(a). Inherent reliability:
It is associated with the quality of the material and design of machine parts.
(b). Achievable reliability:
It depends on other factors such as maintenance and operation of the equipment.
The four important factors required in the determination of reliability are as follows.
(i) Reliability expressed as probability:
Probability is the ratio of number of times we can expect an event to occur to the total number of trails undertaken. A reliability factor can be expressed as probability. A reliability factor is equal to one means that the device performs satisfactorily for the prescribed duration under the given environmental condition. A reliability factor of zero means, all cases the equipment would fail to meet the required performance level.
(ii) Adequate performance requirements:
It defines the role expected of a device or system. A system may perform satisfactorily even though one or more components may not function properly. In reliability analysis, there is a need to define the magnitude of satisfactory or adequate performance of the system.
(iii) Duration of adequate performance:
The duration of adequate performance is used to state the time up to which the desired performance of the system is achieved under the given operating conditions.
(iv) Environmental or operating conditions:
Environmental condition indicate the prevailing conditions at which the system is under operation.
RELIABILITY IN MAINTENANCE ENGINEERING
(i) The concept of reliability has found the increased use in industrial engineering maintenance and management.
(ii) The reliability of any engineering system is based on the reliability of product, equipment and various components of the system.
(iii) The concept of reliability has been incorporated to maintenance management to ensure the robustness of the system.
(iv) Higher reliable equipment ensures the uninterrupted availability to achieve the desired objective of the industry.
Need for Reliability in Maintenance Engineering
Introduction of reliability into maintenance system is to increase life and operational availability of equipment by implementing design and manufacturing features. Strict quality control measures that minimizes the failure.
The main advantage of imposing reliability requirements are increased productivity and reductions in forced outages equipment due to planned maintenance activity.
The application of reliability techniques to any equipment helps to
(i) Identify the flaws in system design
(ii) Compare several possible system configurations
(iii) Minimize downtime
(iv) Maximize operational readiness
(v) Reduce operating costs and
(vi) Develop maintenance policies
Benefit of availability of reliability information or advantages of reliability:
(i) Increased productivity.
(ii) Reduction in forced outage of equipment due to planned maintenance activity.
(iii) Minimized downtime.
(iv) Minimized operational readiness.
(v) Reduced operating cost.
EQUIPMENT LIFE CYCLE AND RELIABILITY
(i) Reliability engineering is engineering that emphasizes dependability in the lifecycle management of a product.
(ii) The failure pattern of equipment over its whole lifecycle can be represented as bath-tub curve as shown in Fig.
Fig. Equipment Life Cycle
(iii) This curve represents the pattern of failure for many products, especially complex products such as Cars and washing machines.
(iv) The bath-tub curve is divided into three regions such as infant mortality, useful life and wear out.
Phase I
The failure pattern inherent in a new product because of manufacturing or design defects. Therefore, the failure rate at the beginning of infant mortality stage is high and then it decreases with time after early failures are removed by burn-in or other stress screening methods. It is referred to infant mortality period of equipment.
Phase II
Shows the useful life period of an equipment where the failures rates are normally moderate as the equipment gets set to the working environment. This period is usually given the most consideration during design stage and it is the most significant period for reliability prediction and evaluation activities.
Phase III
The failures are occurring due to wear out failures which are caused due to aging of the equipment. The equipment life cycle is the essential requirement for the prediction of system reliability.
RELIABILITY MODELS
A device or system is described as a collection of parts or components. The system operates successfully if all its components operate successfully but it may also operate if a subset of components has failed.
A reliability model represents a clear picture of functional interdependencies of each sub system of a system.
The various models used for reliability assessment are as follows.
Series Reliability Model
In this model, the components are arranged in series and the success of the system depends on the success of all of its components.
Parallel Reliability Model
In this model, the system can be partially operative even if some components are in the state of failure.
Series-Parallel Reliability Model
The reliability structure of complex system is decomposed into simpler structure through successive application of the conditional probability theorem.
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