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Ron Helton. Kim Jihyun. How useful was this post? Click on a star to rate it! Leave a Reply Cancel reply Your email address will not be published. The history of human reliability may be traced back to the late s when H.
Williams pointed out that human-element reliability must be included in the overall system reliability prediction; otherwise, such a prediction would not be realistic [3]. Many people have contributed to human reliability. The first book on the subject appeared in [4]. A comprehensive list of publications on human reliability is available in Reference 5.
This chapter presents important aspects of mechanical reliability and human reliability. Many different types of failure modes are associated with mechanical parts or items. Some of these failure modes are shown in Figure 6. These are fatigue failure, creep or rupture failure, bending failure, metallurgical failure, instability failure, shear loading failure, material flaw failure, compressive failure, bearing failure, stress concentration failure, ultimate tensile-strength failure, and tensile-yield-strength failure.
Fatigue failure occurs because of repeated loading or unloading or partial unloading of an item or part. Its occurrence can be prevented by selecting appro- priate materials for a specific application. For example, under cycle loading, steel outlasts aluminum. In the case of creep or rupture failure, material stretches i.
Also, creep accelerates with elevated temperatures. Bending failure occurs when one outer surface is in compression and the other outer surface is in tension. An example of the bending failure is the tensile rupture of the outer material. Metallurgical failure is also known as a material failure.
This type of failure is the result of extreme oxidation or operation in a corrosive environment. The occurrence of metallurgical failures is accelerated by environmental conditions such as heat, erosion, nuclear radiation, and corrosive media.
However, this type of failure may also occur because of torsion or by combined loading i. The shear loading failure occurs when shear stress becomes greater than the strength of the material when applying high shear or torsion loads.
Material flaw failure occurs because of factors such as weld defects, poor quality assurance, small cracks and flaws, and fatigue cracks. Compressive failure causes permanent deformation, rupturing, or cracking and is similar to tensile failures except under compressive loads. Bearing failure usually occurs because of a cylindrical surface bearing on either a flat or a concave surface like roller bearings in a race and is similar in nature to compressive failure.
The stress concentration failure occurs under the conditions of uneven stress flow through a mechanical design. Ultimate tensile-strength failure occurs when the ultimate tensile strength is less than the applied stress and leads to a complete failure of the structure at a cross-sectional point.
Tensile-yield-strength failure occurs under tension and, more specifically, when the applied stress is greater than the material yield strength. They are arbitrary multipliers. They can be quite useful to provide satisfactory design if they are established with utmost care, using considerable past experiences and data. Three safety factors are presented below. This safety factor is considered quite good, particularly when the loads are distributed.
The value of this specific safety factor is always greater than unity. More specifically, when its value is less than unity, it simply means that the item under consideration will fail because its strength is less than the applied stress. Generally, for normally distributed stress and strength, this safety factor is a good measure.
However, for large variations in stress or strength, this safety factor becomes meaningless because of the positive failure rate. All in all, for selecting an appropriate value of the safety factor, careful consid- eration must be given to factors such as cost, failure consequence, uncertainty of material strength, load uncertainty, and the degree of uncertainty in relating applied stress to strength [12].
The approach used for determining reliability is known as stress—strength interference theory modeling. Reliability is simply the probability that the failure-governing stress will not exceed the failure- governing strength.
In the published literature Equation 6. By substituting Equation 6. Example 6. Calculate the item reliability. By substituting the specified data values into Equation 6. The mean value of the stress is 6, psi, and the mean and standard deviation of the strength are 25, and 2, psi, respectively. Compute the item reliability. By substituting the given data values into Equation 6. The approach is extremely useful when its associated stress and strength probability distributions cannot be assumed but there is a sufficient amount of empirical data.
Differentiating Equation 6. By inserting Equation 6. Inserting Equation 6. Figure 6. C and D values in Table 6. The total area under the Figure 6. This value is close to but lower than the one obtained using the analytical approach in Example 6. Operator errors are the result of operator mistakes, and the causes of their occurrence include poor environment, complex tasks, lack of proper procedures, operator carelessness, and poor personnel selection and training.
Maintenance errors occur in field environments because of oversights by maintenance personnel. Some examples of maintenance errors are repairing a failed item incorrectly, calibrating equipment incorrectly, and applying the wrong grease at appropriate points on the equipment.
Assembly errors are the result of human mistakes during product assembly. Some of the causes of assembly errors are poor illumination, poor blueprints and other related material, poorly designed work layout, and poor communication of related information.
Some of the causes of their occurrence are failure to ensure the effectiveness of person—machine interactions, failure to implement human needs in the design, and assigning inappropriate func- tions to humans. One typical example of inspection errors is accepting and rejecting out-of-tolerance and in-tolerance components and items, respectively.
Handling errors occur because of improper transportation or storage facilities. There are many causes of human errors. Some of the common ones are poor training or skills of personnel, inadequate work tools, poor motivation of personnel, poorly written product and equipment operating and maintenance procedures, com- plex tasks, poor work layout, poor equipment and product design, and poor job environment i. This curve shows that a moderate level of stress is necessary for increasing the effectiveness of human performance to its maximum.
The moderate level may simply be inter- preted as high enough stress to keep the individual alert. At a very low stress, the task becomes dull and unchallenging; therefore, most people will not perform effectively and their performance will not be at the optimum level.
When the stress passes its moderate level, the effectiveness of the human performance starts to decline. This decline is mainly due to factors such as worry, fear, and other types of psychological stress.
At the highest stress level, human reliability is at its lowest level. Some of these factors are dissatisfaction with the job, possibility of work layoff, inadequate expertise to perform the task, excessive demands of superiors, current job being below ability and experience, tasks being performed under extremely tight time schedules, serious financial problems, low chance of promotion, health problems, difficulties with spouse or children, working with people who have unpredictable temperaments [27].
Some examples of these tasks are scope monitoring, aircraft maneuvering, and missile countdown. Equation 6. More specifically, it can be used to calculate human reliability at time t when time to human error follows any known probability distribution.
The application of Equation 6. By substituting the above values into Equation 6. Each has its advantages and disadvantages. Two of these methods are presented below.
Its application in human reliability work is demonstrated through the following two examples. The job is composed of three independent tasks B, C, and D. If any one of these three tasks is performed incorrectly, job A will not be accomplished successfully. Task B is composed of two subtasks b1 and b2.
For the successful performance of task B, only one of these subtasks needs to be performed correctly. Subtask b1 is composed of three independent steps i, j, and k. All these three steps must be performed correctly for the successful completion of subtask b1.
Develop a fault tree for the top event: job A will not be accomplished correctly by the person. Using the Chapter 8 fault tree symbols, the fault tree for the example is shown in Figure 6. Calculate the probability of occurrence of the top event i. The probability of occurrence of the top event job A will not be accomplished correctly is 0.
The application of this method in human reliability work is demonstrated through the following example: Example 6. The state—space diagram of the person performing the task is shown in Figure 6. Develop probability expressions for the person performing the task successfully and unsuccessfully i.
Also, develop an expression for the mean time to error com- mitted by the person. In Figure 6. The following symbols are associated with Figure 6. Using the Markov method, we write down the following two equations for Figure 6. By rearranging Equation 6. Solving Equation 6. Write an essay on the history of mechanical reliability.
List at least 12 mechanical failure modes. 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.
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