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Learn how ». The drum, in which the reaction takes place, has 42 rollers elements , which are subjected to a heavy load and to chemical corrosion.
The components are organized in a ring-type structure. The system failure is defined either as the failure of 2 adjacent elements, or as a failure of any three elements in a set of 6 adjacent elements. The existing servicing policy prescribes replacing only the failed elements at the instant of system failure occurrence. The operational conditions permit the opportunistic replacement of non-failed components at the instant of system failure.
In this paper, we propose a cost-effective policy of preventive maintenance: at the same time the system fails, several of the oldest non-failed components are replaced by new ones. Keywords: preventive maintenance, group replacement, simulation approach 1.
Group replacement is one of the strategies that may The renewal of the broken elements with the be employed for the maintenance of technical systems simultaneous replacement of r oldest elements leads of identical, consecutive components.
The aim of this study is the maintenance costs Gertsbakh In order to ensure opti- : mum efficiency in the latter maintenance approach, we introduce two parameters: a threshold of failures, 1 necessary for implementing the repair; and the number of components to be replaced Dekker et al. Consequently, we propose an ap- The simulation algorithm is written in MA- proach and a solution based on a simulation study.
Problem Description 3. What are the general causes for the occurrence of mechanical failures? Write down at least two distinct definitions of the safety factor. Prove that Equation 6. Discuss factors that increase stress on humans. Assume that the stress and strength associated with an item are described by exponential and normal distributions, respectively.
The mean value of the stress is 7, psi, and the mean and standard deviation of the strength are 20, psi and 1, psi, respectively.
Discuss the human performance effectiveness versus stress curve. Redler, W. Williams, H. Lipson, C. Collins, J. Doyle, R. Phelan, M. Bompass-Smith, J. Juvinall, R. Kececioglu, D. Kenney, G. Rook, L. Kohn, L. Bogner, M. Joos, D. Cooper, J. Meister, D. Beech, H. Hagen, E. Some of the objectives of applying maintainability engineering principles are to reduce projected maintenance time and costs, to determine labor-hours and other related resources required for performing the projected maintenance, and to use maintainability data to estimate equipment availability or unavailability.
When maintainability engineering principles are applied successfully to any product, results such as reduction in product downtime, efficient restoration of the product to its operating state, and maximum operational readiness of the product can be expected [3]. Maintainability refers to measures or steps taken during the product design phase to include features that will increase ease of maintenance and ensure that the product will have minimum downtime and life cycle support costs when used in field environments [3].
In contrast, maintenance refers to measures taken by the product users for keeping it in operational state or repairing it to operational state [1,4]. More simply, maintainability is a design parameter intended to minimize equipment repair time, whereas maintenance is the act of servicing and repairing equipment [5]. The responsibility of the maintenance engineers is to ensure that product or equipment design and development requirements reflect the maintenance needs of users.
Thus, they are concerned with factors such as the environment in which the product will be operated and maintained; product and system mission, operational, and support profiles; and the levels and types of maintenance required. Maintainability Reliability 1 Reduce life cycle maintenance costs Maximize the use of standard parts 2 Reduce the amount, frequency, and Use fewer components for performing complexity of required maintenance tasks multiple functions 3 Reduce mean time to repair MTTR Design for simplicity 4 Determine the extent of preventive Provide adequate safety factors between maintenance to be performed strength and peak stress values 5 Provide for maximum interchange ability Provide fail-safe designs 6 Reduce the amount of supply supports Provide redundancy when required required 7 Reduce or eliminate the need for Minimize stress on components and parts maintenance 8 Consider benefits of modular replacement Use parts and components with proven versus part repair or throwaway design reliability the analysis of maintenance tasks and requirements, the determination of mainte- nance resource needs, the development of maintenance concepts, and maintenance engineering analysis [6].
In contrast, reliability is a design characteristic that leads to durability of the equipment as it performs its assigned function according to a specified condition and time period.
It is accomplished through actions such as choosing optimum engineering principles, testing, controlling processes, and satisfactory component sizing. Some of the important specific general principles of maintainability and reliability are presented in Table 7. They are used to represent repair times of equipment, systems, and parts. After identification of the repair distribution, the corresponding maintainability function may be obtained.
Maintainability functions for various probability distributions are obtained below [3,6,8—10]. Inserting Equation 7. Calculate the probability of completing a repair in 6 hours.
Using the specified data values in Equation 7. Its probability density function with respect to corrective maintenance times i. By substituting Equation 7. Substituting Equation 7. In order to find, Mg t , by using the tables of the incomplete gamma function, we rewrite Equation 7. In this case, Equation 7. Discuss the need for maintainability. Compare engineering maintainability with engineering maintenance. Compare maintainability engineering with reliability engineering.
Define maintainability function. Write down the maintainability function for an exponential distribution. Assume that the repair times of an engineering system are exponentially distributed with a mean value of 6 hours.
Calculate the probability of accomplishing a repair in 8 hours. Prove that the maintainability function for Weibull distribution is given by Equation 7. Prove that the mean of the gamma distributed repair times is given by Equation 7. Prove that the maintainability function for Erlangian distribution is given by Equation 7. Kumar, U. Downs, W. Smith, D. Blanchard, B. Pearson, K. Some of these approaches have been successfully used in the maintainability area as well. These approaches include fault tree analysis; cause and effect diagram; failure modes, effects, and criticality analysis FMECA ; and total quality management.
An effective engineering design i. This requires careful planning and a systematic effort to bring attention to maintainability design factors such as maintainability allocation, main- tainability evaluation, and maintainability design characteristics.
Many of these factors involve subfactors such as interchangeability, standardization, modularization, accessibility, testing and checkout, human factors, and safety. In every aspect of maintainability design interchangeability, standardization, modularization, and accessibility are important considerations [1,2]. This chapter presents a number of methods for performing various types of maintainability analysis and various aspects of specific maintainability design considerations.
Fault tree analysis FTA starts by defining the undesirable state event of the system or item under consideration and then analyzes the system to determine all possible situations that can result in the occurrence of the undesirable event. Thus, it identifies all possible failure causes at all possible levels associated with a system as well as the relationship between causes. FTA can be used to analyze various types of maintainability-related problems.
FTA uses various types of symbols [3]. Four commonly used symbols in fault tree construction are shown in Figure 8. The circle denotes a basic fault event or the failure of an elementary component. The rectangle denotes a fault event that results from the combination of fault events through the input of a logic gate. The OR gate denotes that an output fault occurs if one or more of the input fault events occur. Finally, the AND gate denotes that an output fault event occurs only if all the input fault events occur.
Needless to say, FTA can be used to analyze various maintainability-related problems. The following example demonstrates its application to a maintainability- related problem: Example 8. A: Skilled manpower is unavailable. B: Equipment is too damaged to repair. D: There are no spare parts. Furthermore, either of the following two factors can result in the unavailability of spare parts: E: Parts are no longer available in the market.
F: Parts are out of stock. In addition, the unavailability of skilled manpower can be caused by either of the following two factors: G: Poor planning. H: Labor shortage. Develop a fault tree for this undesired event: the equipment will not be repaired by a given point in time. Calculate the probability of the occurrence of the undesired event if the probabilities of occurrence of factors B, C, E, F, G, and H are 0. For this example, the fault tree shown in Figure 8.
By substituting the given data values into Equation 5. Figure 8. As per Figure 8. In the published literature, this method is also known as a fishbone diagram because it resembles the skeleton of a fish, or as an Ishikawa diagram, after its originator, K. Ishikawa of Japan [4]. A cause and effect diagram uses a graphic fishbone for depicting the cause and effect relationships between an undesired event and its associated contributing causes.
The right side i. A well-developed cause and effect diagram can be an effective tool to identify possible maintainability-related problems [2].
When FMEA evaluates the failure criticality i. It has proven to be quite useful to organizations in pursuit of improving the maintainability of their products. The term total quality management was coined by Nancy Warren, an American behavioral scientist, in [7]. Many organizations have experienced various difficulties in implementing TQM. Some of those difficulties are failure of top management to devote adequate time to the effort, failure of senior management to delegate decision-making authority to lower organizational levels, insufficient allocation of resources for training and developing manpower, and management insisting on implementing processes in a way employees find unacceptable [9].
Maintainability design factors that are most frequently addressed are presented in Table 8. More specifically, it restricts to a minimum the variety of components that a product will require. There are many goals of standardization. Some of the impor- tant ones are shown in Figure 8. Standardization should be the main goal of design, because the use of nonstandard components may result in increased maintenance and lower reliability.
Nonetheless, past experiences indicate that the lack of standardization is usually due to poor communication among design engineers, users, contractors, subcontractors, and so on [12]. The degree of modularization of a product is dictated by factors such as cost, practicality, and function. More specifically, consider design, modularization, and material problems simultaneously.
This will make it easier to replace components. Simplification should be the constant objective of design, and a good design engineer includes pertinent functions of a system or product into the design itself and makes use of as few components as good design practices will permit.
Accessibility is the relative ease with which an item can be reached for repair, replacement, or service. Poor accessibility is a frequent cause of ineffective maintenance. For example, according to a U. Army document, gaining access to equipment is probably second only to fault isolation as a time-consuming maintenance task [1]. It should be added that an item being readily accessible does not in itself guarantee overall cost-effectiveness and ease of maintenance under consideration.
Interchangeability is made possible through standardization and is an important maintainability design factor. However, when functional interchangeability is not required, there is no need to have physical interchangeability. Identification is concerned with labeling or marking of parts, controls, and test points to facilitate tasks such as repair and replacement.
When parts, controls, and test points are not identified effectively, the performance of maintenance tasks becomes more difficult, takes longer, and increases the chances for making errors. Types of identification include equipment identification and part identification. Addi- tional information on identification is available in References 1 and 2. Write an essay on fault tree analysis. What are the most frequently addressed maintainability design factors?
What are the important goals of standardization? Discuss at least six important guidelines associated with designing modularized products. Ishikawa, K. Bowles, J. Evans Associates, Durham, NC, , pp.
Walton, M. Burati, J. Gevirtz, C. Ankenbrandt, F. Rigby, L. Its tasks range from simply man- aging maintainability personnel to effective execution of technical maintainability tasks. Maintainability management can be examined from different perspectives such as management of maintainability as an engineering discipline, the place of the maintainability function within the organizational structure, and the role maintainability plays at each phase in the life cycle of system and product under development [1].
Maintainability costing can be examined from different perspectives including the cost of performing the maintainability function and the cost of maintaining a product in the field. Obviously, this cost must be reduced to a minimal level to make the equipment cost-effective. An effective maintainabil- ity program incorporates a dialogue between the manufacturer and user throughout the product life cycle, which can be divided into four distinct phases as shown in Figure 9.
The concept development phase is the first phase of the product life cycle. During this phase the product operational needs are translated into a set of operational requirements and high-risk areas are highlighted. The validation phase is the second phase of the product life cycle. The production phase is the third phase of the product life cycle. Thus, essential maintainability- related data can be collected for use in future applications [4]. It is developed either by the product or system manufacturers or the user, depending on factors such as the nature of the project and the philosophy of the decision makers.
Some of the important elements of a maintainability program plan are [2,4]: Objectives: These are basically the descriptions of the overall requirements for the maintainability program and goals of the plan. Policies and procedures: Their main purpose is to assure customers that the group implementing the maintainability program will perform its assigned task in an effective manner.
The directives address items such as data collection and analysis, maintain- ability demonstration methods, participation in design reviews and evaluation, and methods to be employed for maintainability allocation and prediction. Organization: A detailed organizational breakdown of the maintainability group involved in the project is provided along with the overall structure of the enterprise. Maintainability program tasks: Each program task, task schedule, major milestones, expected task output results, projected cost, and task input requirements are described in detail.
Maintainability design criteria: This part of the plan discusses specific maintainability-related design features applicable to the item under consid- eration. In addition, the description may relate to qualitative and quantitative factors concerning areas such as interchangeability, accessibility, parts selection, or packaging. Organizational interfaces: This section describes the lines of communica- tion and the relationships between the maintainability group and the overall organization.
Some of the areas of interface are product engineering, design, testing and evaluation, reliability engineering, human factors, and logistic support, as well as suppliers and customers. Technical communications: This section briefly discusses every deliverable item and their associated due dates. Program review, evaluation, and control: This section discusses the meth- ods and techniques to be employed for technical design reviews, program reviews, and feedback and control.
Also, it describes a risk management plan and discusses the evaluation and incorporation of proposed changes and corrective actions to be taken in given situations.
Maintenance concept: This section discusses basic maintenance require- ments of the product under consideration and issues such as organizational responsibilities, qualitative and quantitative objectives for maintenance and maintainability, operational and support concepts, test and support equip- ment criteria, and spare and repair part factors. In addition, it outlines the procedures to be employed for review and control within the framework of those relationships.
References: This section lists all documents related to the maintainability requirements e. The primary objective of design reviews is to determine the progress of the ongoing design effort as well as to ensure the application of correct design practices.
The design review team members assess potential and existing problems in various areas concerned with the product under consideration including maintainability.
Many maintainability-related issues require careful attention during design reviews. There are many ways of increasing maintainability including incorporating discard-at- failure maintenance, increasing self-checking features, increasing the use of auto- matic test equipment, designing in built-in test points, providing easy access for main- tenance, using reduced-maintenance parts, and improving troubleshooting manuals [1].
Nonetheless, many elements of investment cost are related to maintainability. Some of these elements are the costs of repair parts, prime equipment, training, data, system engineering management, new operational facilities, system test and evalu- ation, and support equipment [1]. Major steps involved in life cycle costing with respect to product procurement are shown in Figure 9. Some of the disadvantages of life cycle costing are that it is time consuming, expensive, and it is a trying task to collect required data [4].
They vary in the methods they employ to determine many of the major costs used in the calculation. This section presents three mathematical models to estimate the life cycle cost of an item. These costs are essentially labor effort costs. The OAE is made up of spares inventory, investment and holding, and administrative and operational program man- agement costs. The two main components of the functional operating expense are operational manning and consumables costs.
Army Material Command to estimate the life cycle costs of new equipment or systems [2,4,14—16]. It includes the costs of reliability, maintainability, system engineering, human factors, electrical design, mechanical design, producibility, and logistic support analysis. Manufacturing cost: This includes the costs of manufacturing engineering, fabrication, quality control, tools and test equipment, assembly, tests and inspections, packing and shipping, and materials.
Initial logistic support cost: This includes the costs of test and support equipment, program management, first destination transportation, technical data preparation, initial spare and repair parts, initial inventory, provisioning, and initial training and training equipment.
This includes the costs of maintenance personnel, maintenance facilities, mainte- nance training, spare and repair parts, transportation and handling, technical data, and maintenance of test and support equipment. Example 9. Two manufacturers, X and Y, are bidding to sell the truck. Data for trucks produced by the both manufacturers are presented in Table 9. Determine which of the two trucks is more beneficial to buy with respect to their life cycle costs.
TABLE 9. By substituting the above calculated value and the data given in Table 9. With the specified data values in Equation 9. Using the data given in Equation 9. Inserting the given data values into Equation 9. Define the term maintainability management. Discuss important maintainability organization functions. What is a maintainability program plan?
Discuss at least 10 important elements of a maintainability plan. List at least 12 maintainability-related issues that require careful attention during product design reviews.
Discuss elements of investment cost related to maintainability. Discuss important life cycle costing steps with respect to product procurement. What are the important advantages of the life cycle costing concept? A company is considering procuring an engineering system. Two manu- facturers, A and B, are bidding to sell the system under consideration. Data for systems produced by the both manufacturers are presented in Table 9.
Determine which of the two systems is more beneficial to buy with respect to their life cycle cost. Patton, J. Pecht, M. Coe, C. Strodahl, N. Reiche, H. In the published literature the terms human factors, human engineering, ergonomics, and human factors engineering have appeared inter- changeably.
Human factors are a body of scientific facts concerning human charac- teristics the term includes all psychosocial and biomedical considerations. Although the modern history of human factors may be traced back to Frederick W. Taylor, who carried out various studies to determine the most suitable design of shovels, human factors have only been an important element of maintainability work since World War II [1,2].
During this war the performance of military equipment clearly proved that equipment is only as good as the individuals operating and maintaining it. This means that people play an important role in the overall success of a system.
Systems may fail for various reasons including poor attention given to human factors with respect to maintainability during the design phase [3]. This chapter presents various important aspects of human factors directly or indirectly related to maintainability. The knowledge of such behaviors can be quite useful in maintainability work directly or indirectly. The five major senses possessed by humans are sight, taste, smell, touch, and hearing. Humans can sense items such as pressure, vibration, temperature, linear motion, and acceleration shock.
Three of these sensors are discussed below [2,3,6]. In main- tainability work the touch sensor may be used to relieve eyes and ears of part of the load. For example, its application could be the recognition of control knob shapes with or without using other sensors. The use of the touch sensor in technical work is not new; it has been used for many centuries by craft workers for detecting surface irregularities and roughness.
Furthermore, according to Reference 7, the detection accuracy of surface irregular- ities dramatically improves when the worker moves an intermediate piece of paper or thin cloth over the object surface rather than simply using his or her bare fingers.
Sight is stimulated by electromagnetic radiation of certain wavelengths, often known as the visible segment of the electromagnetic spectrum. In daylight, the human eye is very sensitive to greenish-yellow light and it sees differently from different angles. However, with an increase in viewing angle, color perception decreases significantly. In fact, the light colors will appear to be white.
In such situations, the signal to the brain may reverse the color. It helps designers ensure that equipment and products under consideration will accommodate operating and main- tenance personnel of varying weights, sizes, and shapes. In turn, these people will perform their tasks effectively. Adult Population 18—79 years No. Some body-related dimensions of the U. A clear understanding of such devices is essential. Examples of warning devices used in maintenance work are sirens, bells, and buzzers.
In maintainability design, with respect to the use of auditory warning devices, attention should be given to factors such as easy detectability, suitability to get the attention of repair personnel, use of warbling or undulating tones and sound at least 20 dB above threshold level, distinctiveness, noncontinuous and high-pitched tones above 2, Hz, and no requirement for interpretation when maintenance people are performing repetitive tasks [2].
Some of the conditions for using auditory presentation are that the message is simple, the message receiving location is too brightly lit, the maintenance person is moving around continuously, the maintenance person is overburdened with visual stimuli, the message is short, and the message requires immediate action.
Three situations that require the simultaneous use of both visual and auditory signals are shown in Figure This section presents some of the formulas considered useful for maintainability work.
The sound- pressure level SPL , in decibels, is defined by [13—14]. Under normal conditions, P0 is the faintest 1,Hz tone that an average young adult can hear. Thus, letter, marking, and number sizes are based on this viewing distance. Example The recommended numeral height at a viewing distance of 28 inches at low luminance is 0.
Calculate the numeral height for the viewing distance of 56 inches. Substituting the specified data values into Equation Its specified value for important items such as emergency labels is 0. Its recom- mended values for various corresponding viewing conditions and illuminations in parentheses are 0. Using the above specified values in Equation This information could be quite useful with respect to structuring various maintenance tasks.
Write an essay on human factors in maintainability. List at least fifteen typical human behaviors. What are the human sensory capabilities? Discuss at least three such capabilities in detail. List at least five useful pointers for engineering designers concerning the application of body force and strength. List factors to which attention should be given in maintainability design in regard to the use of auditory warning devices. List important conditions for using auditory presentation.
List important conditions for using visual presentation. Define decibel dB. The estimated viewing distance of an instrument panel is 50 inches. Calculate the height of the characters that should be used on the panel if the values of the importance correction factor and the illumination and viewing conditions correction factor are 0.
Write down the formula for estimating the maximum lifting load for a person. Chapanis, A. Woodson, W. Nertney, R. Department of Energy, Washington, DC, Lederman, S. Henney, K. McCormick, E. Adams, J. Dale Huchingson, R. Peters, G. Poulsen, E. Oborne, D. Because of this, there is a definite need for effective asset management and maintenance practices that can positively influence success factors such as price, profitability, quality, reliable delivery, safety, and speed of innovation.
In the future engineering equipment will be even more computerized and complex. Further computerization of equipment will increase the importance of software maintenance significantly, approaching, if not equaling hardware maintenance. In addition, factors such as increased computerization and complexity will result in greater emphasis on maintenance activities with respect to areas such as cost effec- tiveness, quality, safety, and human factors [2].
In the future creative thinking and new strategies will definitely be required to realize all potential benefits and turn them into profitability. Some of the important ones are shown in Figure Table Usually, the values of these indexes are calculated periodically to monitor their trends or compare them with established standard values. This section presents some of these indexes [5,11—13].
The approximate average values for this index in the chemical and steel industries are 3. Large variances in the values of this index indicate the need for immediate attention.
However, there is a wide variation among industries. For example, the expenditure for the maintenance activity for chemical and steel industries is around 6. For exam- ple, in around Some of the main reasons for safety problems in maintenance are poor safety standards and tools, poor equipment design, poor training of maintenance personnel, insufficient time to perform required maintenance tasks, poorly written maintenance instructions and procedures, poor management, poor work environments, and inad- equate work tools [15].
One of the important ways to improve maintenance safety is to reduce the require- ment for maintenance as much as possible in products and systems during their design phase. Quality in maintenance is very important because poor quality maintenance can lead to severe consequences. A subsequent investigation identified the cause of the disaster as the failure of the pressure seal in the aft field joint of the right solid rocket motor.
Furthermore, the investigation concluded that a high-quality maintenance program would have successfully tracked and discovered the cause of the disaster. Navy ship [19]. Failure of the service turbine generator root-valve bonnet fastener was identified as the main cause of this tragedy. Thresher, a U. Navy nuclear submarine, was lost at sea because of flooding in its engine room [17,19].
An investigation iden- tified a piping failure in one of the salt water systems as the most likely cause for the disaster. Consequently, many changes were recommended in the submarine design and maintenance processes.
Past experiences indicate that postmaintenance testing PMT is quite useful for increasing the quality of maintenance. Its three main objectives are a to ensure that no new deficiencies have been introduced, b to ensure that the original deficiency has been eradicated properly, and c to ensure that the item in question is ready to carry out its stated mission [20].
Discuss the need for maintenance. Discuss at least five facts and figures concerning engineering maintenance. What are the important objectives of maintenance engineering? List at least three maintenance-related data information sources. In your opinion, what is the most important maintenance index or measure? Discuss two general maintenance indexes. Discuss the importance of safety in engineering maintenance. Discuss the need for quality in maintenance activities.
List at least nine useful guidelines for equipment designers to improve safety in the maintenance activity. What are the important causes of safety problems in engineering maintenance.
Zweekhorst, A. Tesdahl, S. Kumar, V. Niebel, B. Kelly, A. Westerkemp, T. Hartmann, E. Stoneham, D. Hammer, W. Joint Fleet Maintenance Manual, Vol. Navy, Portsmouth, NH, Elsayed, E. Normally, corrective maintenance is an unplanned mainte- nance action that requires urgent attention that must be added, integrated with, or substituted for previously scheduled work.
Corrective maintenance or repair is an important element of overall maintenance activity. Preventive maintenance is the care and servicing by maintenance personnel to keep facilities in a satisfactory operational state by providing for systematic inspection, detection, and correction of incipient failures either before their development into major failures or before their occurrence [2,4]. This chapter presents various important aspects of both corrective maintenance and preventive maintenance.
Salvage: This is concerned with the disposal of nonrepairable materials and utilization of salvaged materials from items that cannot be repaired in the overhaul, repair, or rebuild programs.
Failed part replacement or repair: Replacing or repairing failed parts or components. Return system to service: Checking out and returning the system back to service. Corrective maintenance downtime is made up of three major components as shown in Figure Active repair time is made up of six subcomponents: checkout time, preparation time, fault correction time, fault location time, adjustment and calibration time, and spare item obtainment time [4,7].
Careful attention to acces- sibility during design can help to lower the accessibility time of parts and, consequently, the corrective maintenance time. Improve interchangeability: Effective functional and physical interchange- ability is an important factor in removing and replacing parts or components, thus lowering corrective maintenance time.
Improve fault recognition, location, and isolation: Past experiences indicate that within a corrective maintenance activity, fault recognition, loca- tion, and isolation consume the most time.
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