Yuo-Tern Tsai

Optimizing preventive maintenance for mechanical components using genetic algorithms

Abstract This paper presents periodic preventive maintenance (PM) of a system with deteriorated components. Two activities, simple preventive maintenance and preventive replacement, are simultaneously considered to arrange the PM schedule of a system. A simple PM is to recover the degraded component to some level of the original condition according to an improvement factor which is determined by a quantitative assessment process. A preventive replacement is to restore the aged component by a new one. The degraded behavior of components is modeled by a dynamic reliability equation, and the effect of PM activities to reliability and failure rate of components is formulated based on age reduction model. While scheduling the PM policy, the PM components within a system are first identified. The maintenance cost and the extended life of the system under any activities-combination, which represents what kind of activities taken for these chosen components, are analyzed for evaluating the unit-cost life of the system. The optimal activities-combination at each PM stage is decided by using genetic algorithm in maximizing the system unit-cost life. Repeatedly, the PM scheduling is progressed to the next stage until the systems unit-cost life is less than its discarded life. Appropriately a mechatronic system is used as an example to demonstrate the proposed algorithm.

A study of availability-centered preventive maintenance for multi-component systems

Abstract This paper studies preventive maintenance (PM) in simultaneously considering three actions, mechanical service, repair and replacement for a multi-components system based on availability. Mechanical service denotes the activities including lubricating, cleaning, checking and adjusting, etc. which is set to alleviate strength degradation. Repair is defined on that not only slow down the degraded velocity but also restore the degraded strength partly. Replacement is settled to recover a component to its original condition. According to the definitions, the degradation of components is analyzed from its failure mechanisms and the improvements of various actions to it in reliability were measured by using two improved factors. Following the proposed model of reliability, the mean-up and mean-down times of each component are also investigated and the replacement intervals of components are determined based on availability maximization. Here, the minimum one among the intervals is chosen as the PM interval of system for programming the periodical PM policy. The selection of action for the components on every PM stage is decided by maximizing system benefit in maintenance. Repeatedly, the scheduling is progressed step by step and is terminated until the system extended life reaching to its expected life. The complete schedule provides the information, the actions adopted for the components, the availability and the total cost of system on each stage. Validly, a multi-components system is used as an example to describe the proposed algorithm.

A study of modularity operation of systems based on maintenance consideration

A module is a set of some disassembly and/or non-disassembly components or parts. It usually is used not only in supporting or carrying out the same function, but also in decreasing the complexity of a system in maintenance. Traditionally, the module form of a system is created according to either the function requirements or the manufacturing considerations. It is determined mainly depending on the individual condition of systems in designing, and has no concrete and scientific approach to progress system modularity. To overcome the faults in traditional modularity, the present paper presents a method of modularity based on the consideration of system maintenance policy for constructing the system modules. First, the correlations in designing the functions/components within a system are analysed according to four input/output parameters--geometry constraint, mechanical strength, energy flow and signal flow--to build the hierarchy structure of system in combination. Second, the parameters of reliability and cost of these functions/components are then investigated to describe the degradation in usage and to calculate the maintenance cost of each possible module. Third, the aged-maintenance model is used to evaluate the ideal maintenance intervals and the total maintenance costs of these modules so that the optimal module type of the system can be established. Finally, a hydraulic system used in squeezing machinery is adopted to depict the process of system modularity. Ideally, the result in modularity may feedback to designers to modify the design and to ensure the achievement in designing for maintainability.

The preliminary investigation of system reliability and maintainability to develop availability sound designs

Due to the increasing complexity of modern engineering systems, availability is more and more emphasized when systems are used. In order to develop availability sound products, this paper presents a supporting method to evaluate the reliability and maintainability of the feasible solution in the early design phase. The evaluation is implemented based on some scaling factors that are determined according to the differences of the involved engineering activities between the feasible solution and an existing system. The engineering activities are constructed from three aspects: the complexity of product itself, the maturity of product in production and the related factors of product in use. Once the scaling factors are decided, the mean time between failure and the mean time to repair of a design are then estimated according to the data in failure and in maintenance of the known system. No sooner than the mean time between failure and the mean time to repair of a design are established, its availability in future use can also be calculated. Subsequently, the information is considered with the total cost estimation of the system for selecting the optimal one among the feasible solutions. Plausibly, the availability sound designs would be more easily achieved as well as the specified availability goal being further obtained by the design trade-off analysis.

Diagnosis of oxide films in cast aluminum alloys

Aluminum (Al) alloy castings often contain many defects, such as micro-shrinkage, gas pores, shrinkage pores, hot tearing, and oxide film entrapment. When oxide films are entrapped in Al alloy castings, they are very difficult to identify simply by optical observation. In this study we used an auto-scanning eddy current testing method to detect oxide films entrapped in Al alloy castings. The detection signal and the measurement data were confirmed by scanning electron microscope (SEM) observation and partly backed-up by the ultrasonic-vibration method.

Reliability design optimisation for practical applications based on modelling processes

There has been extensive development of computer-aided design/computer-aided engineering software in support of product design. These computer-aided tools usually focus on the problem of deterministic design. Recently, probabilistic design, namely reliability design, is receiving greater emphasis for handling the greater or lesser variations that exist in loadings and strengths. This study reports on a modelling process meant to perform reliability design optimisation (RDO) that integrates reliability evaluation with the probabilistic design system (PDS) of Advanced Analysis Guides (ANSYS). Geometric models of the designs are constructed and the design variables are formulated in probabilistic form based on the stress–strength interference reliability model for performing RDO. The optimal solutions for the design variables are obtained by goal-driven optimisation utilising the PDS of ANSYS. The sensitivities of the design variables to the stress, deformation and mass are investigated so that the priority for design modifications can be determined accordingly. A horizontal joint robot is used as an example to describe the model-based design processes. The modelling processes, which integrate engineering analysis software with reliability evaluations, are important features in the probabilistic design. Using this approach, optimal design solutions can be rapidly obtained when the constraints and the reliability targets are given. This proposed approach empowers the design engineers to be able to use finite element methods to fulfil RDO.

Fatigue life and reliability evaluation for dental implants based on computer simulation and limited test data

Dental implants have been extensively utilized on edentulous patients for many years. The fatigue life of dental implants is critical for them being approved to use in human body because they are within the area of biomedicine. To perform a preliminary investigation of fatigue life of dental implants, this article reports a method of fatigue life estimation based on the combination of computer simulation and limited test data. The method is developed based on a probabilistic form of fatigue life given according to the properties of material fatigue strength. The procedure is carried out by shifting of the regression line (representing the fatigue–life curve) to the desired value of the probability of occurrence. Computer simulation includes both stress analysis and life estimation which are done using the ANSYS software. This estimation model offers a method for fatigue life evaluation and yields the life distribution in respect to the scatter of the cyclic properties of dental implants. Furthermore, the reliability of lifetime is calculated based on the probabilistic form. The purposes of this study are to predict fatigue life using a small amount of testing data and to provide a risk assessment for dental implants in use.

Optimizing reliability design for mechanical systems using geometric programming

Reliability design of mechanical or structural components progresses primarily based on the information of strength and stress distributions. Reliability design is sometimes hardly carried out when a system is composed of many components and the loads resulting in two independently paired sets of information are ambiguous. In this paper, the paired information is represented directly with performances, such as power, speed and maintainability, and so on, for treating the vague problems. The performance supply (provided by a design) is analogized to the strength as well as the performance requirement (requested by the customer) to the stress. According to the performance distributions, four types of performance-related reliabilities are constructed so the reliability designing of a product can be fulfilled based on strength–stress interference theory. Integrating the reliability theory with an optimization technique, the designed problem is further formulated as a constrained minimization problem for minimizing the cost subjected to the reliability goal. The optimization problem is solved using complementary geometric programming. By this approach, the optimal values of the design variables can be determined rapidly so that the optimization of reliability design of a complex system can be achieved. The related theories and an example are reported in this paper to depict the design process of this approach.

A Study of Structural Stress Analysis of Reducers for Supporting Reliability Design

Reducer is extensively used in many devices for speed reduction of rotational motions. It must possess the properties including high rigidness, large reduction ratio and structure compactness when it is applied in robots. This paper presents an innovative design for reducers based on differential displacements of deceleration gears for satisfying the above properties. The structural stresses including vibration frequencies of the reducer are analyzed using Finite Element Methods (FEM) to observe the designed weaknesses of the related components. The kinematic characteristics of the reducer are simulated by SolidWorks motion analysis to identify the accuracy of the mechanisms. The geometric models are leaded into ANSYS to analyze the structural stresses and the vibrations. The analyzed results are further integrated with probabilistic theories to perform reliability design for the reducer. The studied results can provide useful information including the allowed loading and the reliabilities for the high speed-reduction reducer in using.

A Study of Fatigue-Life Estimation and Reliability Analysis for Dental Implants

Recently, dental implants (DIs) are extensively utilized on edentulous patients. The bio-compatibility & physical properties of DIs are severely specified since it belongs to the products of biomedicine. Generally, DIs must pass a series of tests before they are approved to use in human body. In this paper, a method of probabilistic fatigue-life estimation was proposed to fulfill reliability life prediction of DIs. The probabilistic form of fatigue-life evaluation is developed based on material constants namely fatigue strength coefficient and fatigue strength exponent. The procedure is developed based on the shift of the fatigue-life curve to the desired value of the probability of occurrence. This estimation model yields the life distribution in respect of the scatter of the cyclic properties of DIs.The CAD models of DIs are first constructed to perform computer simulation analysis for establishing the fracture spots. The stress analysis and life estimation were carried out by ANSYS software. The simulation results are further compared with the experimental data obtained by fatigue testing to determine the estimated model of fatigue life. The parameters of the model were determined by linear regression method based on the combination of the simulated and experimental data. The reliabilities of DIs were further investigated to provide an index of life-safety of DI at different cyclic loads. The analyzed results may be useful while programming the fatigue testing of DIs.© 2012 ASME