نشریه مهندسی و مدیریت کیفیت
سال یازدهم شماره 2 (تابستان 1400)

There are many factors and variables that affect the reliability of organizations equipment that neglecting them may cause irreparable damage to organizations. Despite the importance of high reliability of equipment on the profitability of organizations, so far no research has considered the factors affecting it simultaneously and together. Because the relationship between these factors has a lot of dynamics and feedback, system dynamics is a good tool for analyzing the equipment reliability. The purpose of this study is to create and develop a new way to evaluate and improve the reliability of equipment of one of the most important industries in the world using the system dynamics approach in a 5-year horizon. In this regard, first, the key variables affecting the improvement of reliability, identification and their relationships in the form of accumulation and flow diagrams are completed and simulated in vensim software. Then, after creating a flow-accumulation diagram, suitable scenarios for improving performance are discussed. The validity of the model was also assessed by three tests of reference behavior reconstruction, limit behavior and sensitivity. Simulation results indicates that by implementing policies to improve staff training, allocation of resources to preventive maintenance, etc., the reliability of equipment is significantly increased and this will increase the sales and profits of the organization and managers should pay more attention to these variables.

Precision machining of complex industrial parts requires special machines, one of the best of which is the CNC milling machine. Malfunction of machines can cause the production line to stop, while repairs of these machines when they break down without prior possibility and preparation of spare parts with Paying attention to the need to import them is time consuming and costly. In this article, the failure of the device on an annual basis is extracted from the failures recorded in the repair and control sheets of the device, which have been prepared and maintained by the personnel. Then, using Markov chain, to estimate the reliability, maintenance and repair and availability of four-axis CNC milling machine in 2011 was used. The results of implementation in 2011-2018, were compared with the indicators of 2011. The evaluation results show that the availability of the device has increased from 96.3% in 2011 to 99.28% in 2018.

The economic design of control charts depends on the process shock model distribution and due to difficulties from both theoretical and practical aspects. This paper pursues to develop the economic design of X ̅ control chart for monitoring continuous flow processes under Bur XII shock model. The Burr XII distribution is flexible and its hazard rate can take many forms like as fixed, increasing, decreasing, and single mode or even U-shaped. Continuous flow processes are used in batch processes such as those meet to filtering, chemical processing and etc. A sensitivity analysis is executed and numeric examples are given to illustrate the effects of changing the parameters of the shock model distribution on the optimum values of the economic design models as discussed in this paper. In addition, we use uniform and non-uniform sampling schema and comparison between them. This comparison show that for non-constant hazard rate, non-uniform sampling scheme is better.

In this study, the design of the Response Surface Method (RSM) of the Box-Behnken model (BBD) was used by MiniTab-19 software to determine the optimal conditions for the removal of AB-74 dye by nanozeolite in dimensions smaller than 100 nm. Effective parameters such as pH, temperature, dye concentration, stirrer speed and adsorbent dose were investigated in an ANOVA statistical analysis system to study the dye adsorption process. The results showed that with decreasing pH and temperature and concentration, the dye had the highest dye absorption. To remove anionic contaminants and increase the negative ion exchange capacity by nanozeolite, the nanozeolite surface was modified using tween-60 surfactant. BET and DLS tests were performed to ensure the level of correction and study of structure features. Experimental results showed that by adding the tween-60 surfactant with a concentration of 15% to the nanozeolite due to surface modification, the amount of dye adsorption increased. The kinetic model and isotherm model of the adsorption process were also studied and the results showed that the reaction kinetics follows the pseudo-second-order model and adsorption process for nanozeolite and modified nanozeolite follows the Freundlich model.

Reliability of a degrading load sharing k out-of-n:F system is studied in which the total load on the system is shared among all the functioning components. The performance of the system is evaluated based on degradation of its components, and the system lifetime is determined accordingly. It is assumed that degradation of the functioning components follows a Gamma process, such that the shape parameter is a time dependent function and the scale parameter is constant. The effect of load sharing is modeled by considering a power law function as the shape parameter of the Gamma process. A recursive formula is obtained to calculate the reliability function, and its sensitivity is discussed with respect to the model parameters. The estimation problem of the load sharing parameter is also investigated based on degradation data measured at some pre-specified inspection times. Since the obtained formulas are complex, the reliability function is numerically computed. Further, the performance of the proposed estimator is studied using a simulation algorithm.

Hybrid censorship is a combination of the type I and II censoring schemes, which is divided into two types of type I and type II hybrid censoring schemes based on the criteria for ending the experiment. One of the issues in the censoring is choosing the best censoring scheme. Different criteria can be considered to determine the optimal censoring scheme that each of them may lead to a different design. One of the most important criteria is the cost of testing. In this paper, considering the cost of testing as an optimization criterion in type I hybrid censoring, the optimal censoring scheme is determined, when the lifetime of data is Burr type XII distribution. Numerical calculations as well as an example are presented to evaluate the results of the paper. Finally, a summary of the results of the article is given.