Scheduled maintenance policy for minimum cost - A case study

School of Engineering

Scheduled maintenance policy for

minimum cost

- A case study

Växjö, Spring 2011

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Organisation/Organization Författare/Authors

Linnaeus University Mohamad Tabikh

School of Engineering Ammar Khattab

Department of Terotechnology (Systemekonomi)

Dokumenttyp/Type of document Handledare/Tutor Examinator/Examiner

Examensarbete/Degree Project Matias T. Hailemariam Imad Alsyouf

Titel och undertitel/Title and subtitle

Scheduled maintenance policy for minimum cost – A case study

Sammanfattning/Abstract

This report evaluate the maintenance policies that been applied within specific industrial company, Taken into considerations all corrective and preventive maintenance costs ,in addition to optimise best preventive maintenance schedule for minimum cost.

Dynamate Intralog AB was the surveyed company that been encountered high maintenance cost compatible with less productivity, therefore obtaining maintenance schedule policy for minimum cost was the best solution for their problem, then by calculating their corrective and preventive

maintenance cost the optimum time was acquired. Finally, the maintenance schedule approve that organized maintenance based on optimum time enhance the productivity and minimize the company maintenance cost.

Nyckelord/Keywords

Corrective maintenance, Preventive maintenance, Maintenance Schedule, optimum time

Utgivningsår/Year of issue Språk/Language Antal sidor/Number of pages

2011 English 58

Internet/WWW

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Acknowledgements

First of all, we would like to take the opportunity to express our gratitude to

everybody who has helped us with this thesis.

In this context, our special thanks refer to our tutor, Matias T. Hailemariam and our

examiners, Imad Alsyouf, Lars Erikson and Anders Ingwald as they have guided and

helped us with our work.

Moreover, we appreciated the cooperation with Dynamate Intralog AB

presented by Mr. Gaith that helps us during collecting the data

Finally, we are grateful for our families and friends who have supported us

during this period of time, firstly to Bea Cerezo Valera for her constructive

comments, then to Idriss, Nurdan, Akif, Nurdos, Hande and Zehra Aksoy.

Växjö, June 2011

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Table of contents

2.4 Validity and reliability ... 14

2.5 Generalizing ... 15 3. THEORETICAL FRAMEWORK ... 16 3.1 Impact of Maintenance ... 16 3.2 Corrective Maintenance ... 16 3.3 Preventive Maintenance ... 17 3.4 Block policy ... 18 3.5 Age policy ... 18 3.6 Weibull distribution ... 19 3.7 Maintenance optimization ... 22 3.8 Schedule maintenance ... 23

3.9 Life Cycle Cost (LCC) ... 24

4. EMPIRICAL DATA ... 26

4.1 General description ... 26

4.2 The logistics outsourcing ... 26

4.3 The empty boxes process ... 27

4.4 Robot performance and critical components ... 28

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4.6 Corrective maintenance for essential components ... 29

4.7 Preventive maintenance for essential components ... 30

4.7.1 Weekly preventive maintenance ... 30

4.7.2 Monthly preventive maintenance ... 32

4.8 Corrective and preventive maintenance related cost ... 33

4.9 Production list for palletizing robot ... 34

5. ANALYSIS ... 35

5.1 Maintenance cost calculations ... 35

5.2 Distribution for the failure data ... 38

5.3 Optimum time to PM actions ... 39

5.4 Maintenance schedule based on optimum time ... 41

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List of tables, figures and images

List of tables

Table 1. Time Frame ... 11

Table 2. The value of m ... 40

Table 3. The optimum time for each component to perform PM ... 40

Table 4. The optimum time to perform PM ... 41

List of figures

Figure 2. Corrective maintenance cycle ... 17

Figure 3. Example of age policy ... 18

Figure 4. Curve shows the total cost with increasing PM actions to an optimal time ... 22

Figure 5. TPL circulation ... 27

Figure 6. No. of failures for each critical component during 5190 hours ... 38

Figure 7. The time at which the components have failed ... 39

Figure 8. Current PM Time and Optimum time ... 41

List of images

Image 2. The photocell in palletizing robot... 31

Image 3. The brake motor ... 31

Image 4. The gearbox ... 32

Image 5. The air bellows ... 32

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List of abbreviations

CBM Condition based monitoring

CM Corrective maintenance

LCC Life Cycle Cost

MLE Maximum Likelihood Estimation

MTTF Mean time to failure

PM Preventive maintenance

TPL Third-party logistics

tp Time interval between PM actions

Cf/Cp Cost ration

Cf Cost of failure

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1. INTRODUCTION

This chapter will present an overview about the report, which includes: Background, problem discussion, problem formulation, purpose, relevance, limitations and time table.

1.1 Background

The definition of maintenance may combine the technical and administrative actions that intended to retain an item or restore it to a state in which it can perform a required function. Maintenance provides crucial support for heavy and capital-intensive industry by keeping machinery and equipment in a safe operating condition. Therefore, maintenance plays a main role in sustaining long-term profitability and competitiveness for an organization (Aditya and Uday, 2006).

Thus, the market demands on products with higher quality, faster delivery and more reliable characteristics led to the development of manufacturing systems combining automation (robotic machines), integration and flexibility. Accordingly this complex system becoming more vulnerable to various kinds of disturbances, such as components failure and incidental faults, but maintenance strategies consistent with high applicable performance may mitigate it. The assimilation of maintenance management could increase the productive time (availability), reliability, and optimise performance due to many policies that correlated with high maintainability techniques. There are two common maintenance strategies dealing with industrial machinery known as corrective and preventive maintenance, the examples of corrective maintenance (CM) are configuration of mechanical part and control equipment when the robot break down (Fix when it’s break). In other hand, preventive maintenance (PM) can be classified into three categories: time driven, predictive, and equipment driven. Scheduled maintenance derived from periodic, time intervals of the system to replace the part. The concept behind it is that wear parts have a fixed number of cycles to failure which is converted to operating time. By determining the optimum time to maintain or replace the part with minimum total cost (Mobley, 2004).

Furthermore, maintenance strategy1 cannot be set up farther cost effective analysis because the economical view tracking the entire process and organization take into consideration all the direct and hidden cost behind their maintenance strategy. The total cost of the maintenance (CM and PM) depends on the number of components replaced during operating period of the system (Chitra, 2003).

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In order to establish an effective Preventive maintenance (time based), failure data are required and it does not have to be based on Expert opinions anymore. Using statistical methods, scheduled maintenance can be optimized, taking into consideration the three types of maintenance: preventive maintenance, inspection maintenance and predictive maintenance (Didson, 1994).

1.2 Problem discussion

Many organizations have been failed in implementing PM due to absent of clear program that should be followed. In addition, many organizations which have program failed also because of not having activity that ensure the program accomplishment. (Mobley, 2004) In order to establish an effective programme, failure data are required to be analysed to calculate the optimum time (Didson, 1994).

There is a survey within industrial sectors covered different production companies 1997 to highlight the maintenance time and in which trend; the present assessment is shared between planning, prevention and correction. The percentages were distributed according to how much of their maintenance time was spent on those factors. Figure 1 show that about half of the maintenance time is spent on corrective actions and two-fifths on preventive or condition based monitoring (CBM). The optimum figure for CM is considered to not exceed 30-40 percent and most firms probably knows this but are still not changing strategies or techniques (Jonsson, 1997).

Figure 1. Maintenance-related time

Source: Jonsson, 1997, pp.242

1.3 Problem formulation

The problem can be formulated in this thesis as following:

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1.4 Purpose

The goal of this report is to identify the optimum time that a certain PM actions should take a place by analyzing the company maintenance policy towards their robot availability.

1.5 Relevance

In today’s business, a valuable utilization of the machinery (robots) and incorporated maintenance policies2 are a prerequisite for keep on challenging into the market and gain profits. Thus, all the efforts regarding to decrease idle time and minimize delivery delays should be made.

The relevance of this report includes the importance of implementing the right maintenance policy at the right time in order to avoid unnecessary PM action which could affect the machine availability.

1.6 Limitations

Maintenance management is wide subject and it’s difficult to consider all the elements within it, therefore, our focusing will be only on the company applied method. In addition to, visits limitation to company that narrowing our observations.

1.7 Time Frame

The preliminary work of this thesis started on week nine by deciding the thesis topic and case company. Afterwards, the authors of this thesis started searching scientific articles, books, etc. regarding the area of study.

In order to follow up with submission dates, a time plan was developed at the beginning of week ten. The time frame for the accomplishment of this work is shown in the following table.

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11 w.15 w.16 w.17 w.18 w.19 w.20 w.21 Introduction Methodology Theory Empirical findings Analysis Results Conclusions

Review and hand-in

Submission of chapters 1-3 Submission of chapters 1-5 Submission of final thesis

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2. METHODOLOGY

In this chapter different methods had been selected by the authors in order to reveal the process of the research. The chapter contains the research strategy, the research design, data collection, validity and reliability, as well as generalizing.

2.1 Research strategy

Ghauri and Grønhaug (2005) classify two different approaches of research by distinguishing between inductive and deductive approach. These two ways are used by the researchers in order to identify if their statements are either true or false. If the statement is established as true, it can be used as the basis for theories. Once the statement is identified as true or false, it is time to draw the conclusion.

The deductive approach is when the researcher deduces a hypothesis based on what is known in a specific field, and by collecting data the researcher will reach his or her findings. Some researchers prefer the inductive approach which is the opposite from deductive approach. In the inductive approach “theory is the outcome of research” (Bryman and Bell, 2007, p.14).

According to Saunders et al. (2007), there is another approach which is called abductive. This approach is the combination of deductive and inductive approaches. The abductive approach consists on elaborating the theoretical framework in order to explain a specific case, and later testing this theory on other cases.

An abduction research approach which combines deduction and induction approaches will be used in this thesis. The deduction approach will be used to collect different theories, such as life based maintenance, related to the research question in order to obtain conclusions. The induction approach will be used to verify the conclusions in the case study of this thesis making new schedule maintenance after applying optimum time selection methodology.

2.2 Research design

Research design may be classified as exploratory, descriptive and causal. The

exploratory design is used to clarify the research problem by gathering and providing

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As the purpose of this thesis is to determine the exact optimum period to perform PM actions, the causal method will be used. The authors will confront the cause-and-effect problems by determining which policy is the most suitable for the robot.

Moreover, Bryman & Bell (2007) assert that a research design provides a structure for the collection and analysis of data and there are five different types: experimental design, cross-sectional or social survey design, longitudinal design, case study design and comparative design.

Case study design “is concerned with the complexity and particular nature of the case

in question”. A case study can be: a single organization, a single location, a person, or a single event (Bryman & Bell, 2007, p.62).

In this thesis a case study design will be applied because the data will be collected from an industrial company located in Sweden. For this reason this design is the most suitable for the thesis in order to obtain appropriate data and develops analytic and problem solving skills. Furthermore, this kind of research requires a tangible approach whilst connected to failures and statistical distributions, although it allows for exploration of solutions for complex issues and applying new knowledge. The case study here enables us to observe the current situation and monitoring all the factors surrounded by it.

2.3 Data collection

There are two methods for collecting data, qualitative and quantitative. Qualitative method is the collection and analysis of data by words; whereas quantitative method is through statistics and mathematics i.e. researchers employ measurement (Bryman and Bell, 2007). In addition, Creswell (2009) states that the combination of quantitative and qualitative approaches leads to mixed methods research. Employing both methods provides an extensive understanding of research problems.

This thesis is a result of both qualitative and quantitative research, as the authors investigate and evaluate maintenance policy in the industrial sector using data which is verbally coded as well as statistics in order to provide cost effective solutions to the case study. Therefore both methods are suitable in order to create a deeper understanding about our topic.

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2.3.1 Semi-structured interviews

Bryman and Bell (2007, p.213) state that a semi-structured interview “refers to a context in which the interviewer has a series of questions that are in the general form of an interview schedule but is able to vary the sequence of questions”. Interviewers have the opportunity to ask further questions depending on the interviewees’ answers. When a researcher is collecting data by conducting semi-structure interviews, he or she has to control the situation, ask the most important questions and be able to adapt to the situation (Ghauri and Grønhaug, 2005).

The authors of this paper will interview several workers in the company with the purpose of obtaining expert information about maintenance actions. The interviews will consist of ten questions of different types such as open and closed questions. See appendix (1). Thus, we will reduce the duration of the interview as well as making it easier for interviewees to answer. The questions will be accurate in order to reach our purpose. These questions mostly been formulated based on recent observations and meeting with production and logistics manager, in addition to recent data collected from last year of robot effectiveness such as availability, productivity and reliability (failure data).

2.3.2 Secondary analysis

According to Bryman and Bell (2007), secondary analysis involves the analysis of data by researches who have not participated in the compilation of the data. Other researchers such as companies or other kinds of organizations have collected them for their own purposes. Secondary analysis entails the analysis of quantitative as well as qualitative data.

In this thesis will be performed a secondary analysis of a large amount of information that our case study have collected for several years and thereby will reveal relevant knowledge on our thesis.

2.4 Validity and reliability

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This thesis will provide the validity of the data by collecting accurate and relevant information from various articles and books in order to find the most appropriate theories for our thesis. Furthermore, the questions for the interview will be formulated in a careful way to get the best relevant answers. Therefore the ways that been performed for designing the questions taken into considerations the internal, external and content validity. To provide reliability, the authors will interview different operators and group leaders in order to know how the maintenance affects the robot. The questions are related to every part of the process with the aim of achieving a wider perspective.

2.5 Generalizing

Generalization of results involves that the researcher create a representative sample in order to generalize the results to other groups or cases beyond than the one of the research (Bryman and Bell, 2007). The qualitative method as well as quantitative can be generalized, some of the reasons may be the situation of the case study or the type of the research (Saunders et al., 2007).

The Generalizability of the results can be guaranteed from the implemented researching approaches and specific methods adapted only in company’s applied same maintenance strategies and have almost similar cost ratios. Hence, these factors may rely on optimum time method in order to perform preventive maintenance schedule. By the way, we cannot generalize the following results statements because of various systems adaptation and different element may involve the optimum time model evaluation.

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3. THEORETICAL FRAMEWORK

This chapter contains theories about maintenance management in order to provide a clear picture of the theories which will be used in the research.

3.1 Impact of Maintenance

Maintenance has been always considered as extra unnecessary actions which mean cost for managers nevertheless; it is an existence cost that could be hidden. Maintenance forms a portion from the total operating plant. Recently Maintenance has been highlighted due to it is impact in industrial field (Mobley, 2004).

Nowadays organizations are not only satisfied with keep machines in good conditions further, maintenance actions could be planned and totally efficient. Maintenance sometimes could be critical aspect in an organization it could turn unnecessary cost into profits. In united state over than $600 billion has been spent on critical plant systems although it has been increased to $800 billion by 10 years (Mobley, 2004).

3.2 Corrective Maintenance

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Figure 2. Corrective maintenance cycle

Source: Langford, 2007, pp.60

3.3 Preventive Maintenance

Preventive maintenance is a very wide concept there is no specific accurate definition, but it can be generalized as all actions that should be taken on the machine in order to minimize the occurrence of unexpected downtime (Duffuaa et al., 1998).

Mean time to failure (MTTF) or the bathtub curve shows that there is a high probability of occurrence of failure in the first phase which is the installation phase. After a period of time the machine goes into the Normal phase where it supposes to work as it designed. Then the cost of maintain the machine goes higher and higher with time because of wear out. In preventive maintenance machine repaired according to schedules based on MTTF statistically (Mobley, 2004).

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comprehensive preventive maintenance starting with replacing a component and ending with minor adjustment of a minor component. Depending on the type of the machine and the type of the component, the production system, and many other factors play a role in implementing preventive maintenance (Mobley, 2004).

3.4 Block policy

Maintenance policies are a set of protocols that followed in order to reduce the number of unexpected stoppages. Performing PM actions at certain point of time regardless of component’s condition defined as a Block policy. Block policy aims to maintain the component so that the failure could be avoided as much as possible that may lead to a catastrophic failure (Thomas.H.Savit, 1993).

A fixed time intervals is identified for PM actions to take a place, these actions are scheduled and is being implemented periodically. This policy wins normally when the cost of preventive maintenance is lower comparing with other PM cost polices.

Moving towered from fixed intervals that are scheduled to a dynamic scheduling for those intervals based on operating hours, defined as age policy (Sherwin, 2010).

3.5 Age policy

This policy is defined as perform a preventive maintenance at tp (time interval between

PM actions) hours of continuous operating. tp could be finite or infinite, in case of

infinite tp no preventive maintenance scheduled. On the other hand, when failure

occurs before tp PM take a place and rescheduling for the next PM actions must be

done.

Figure 3. Example of age policy

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Figure above shows two operating cycle for a component, first cycle component has been operating till tp and scheduled PM took a place. Next cycle component has been

fail prior tp, that case PM actions has been rescheduled for the next cycle. The

objective of this model is that tp is being optimum and PM actions (replacement, break

down maintenance) are performed with minimum cost per unit time (Duffuaa et al., 1998).

The cost of PM actions and break down maintenance are associated with the Total expected cost per cycle which is:

Where R(tp) is the probability that the component will survives till tp (Duffuaa el at.,

1998).

3.6 Weibull distribution

In probability and statistics Weibull distribution is the most commonly used model in modern reliability engineering. Weibull used in statistical analysis due its flexibility and ability to deal with small sample size in order to evaluate lifetime of a system, component. It was named after Waloddi Weibull. Weibull and lognormal are called the lifetime distribution because they tends better in representing the measurement of product life. Weibull distribution is mostly applicable in manufacturing industries and can be applied in a variety of forms of parameters (Murthy et al., 2004).

Exponential function is a special case from Weibull distribution. It is widely used to represent the lifetime for a set of data and also a modelling trend with decreasing or increasing failure rate the probability density function f(t) for Weibull distribution is give as:

f(t) = β/ηβ(t-ɣ)β-1exp[-(t-ɣ/η)β] And

R(t) = exp[-(t-ɣ/η)β] Where: γ = Location parameter

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The Weibull distribution model gives an insight of how the distribution will look like. By using the three parameters mentioned above as an input. Obtaining a graph from those inputs could be possible which will show the probability that a certain component will fail at a certain point in time. The two parameters are obtained by setting γ=0 which give:

f(t) = β/ηβ(t)β-1exp-(t/η)β

Weibull distribution one-parameter occurs when the value of shape parameter β is being determined or assumed. The one-parameter Weibull p.d.f is obtained by setting γ = 0 and assumed β = k = constant.

f(t) = k/ηβ(t)k – 1exp-(t/η)k

The first parameter that is used is called the Location parameter denoted by γ (Gamma). This determines where the graph will start and in most cases it always assumed to be zero showing that it is the minimum time to failure. When gamma parameter is set to be zero, this is called the two-parameter Weibull distribution. The shape parameter is the second parameter usually denoted by β. This parameter determines the shape of the graph and known as the Weibull slope. The β parameter is the function of the hazard rates (Murthy et al., 2004).

β=1, constant hazard rate and Fit to exponential function. β<1, Decreasing hazard rate.

β>1, Increasing hazard rate.

β=3.5, fit to normal distribution.

With some values of (β), the equation will reduce to other distribution model. For example if β=1, the p.d.f of the three-parameter, Weibull will reduce to the two-parameter exponential distribution with the probability density function f(t) given as:

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Parameters estimation methods

According to Dodson (1994), there are four most commonly used methods to estimate Weibull parameters:

- Maximum Likelihood estimation

MLE refers to Maximum Likelihood Estimation, one of the most widely statistics method to estimate Weibulls’ parameters. Based on maximize the value that maximize the probability of a data. Let X1,X2,…..Xn be independent random variables which are

the representations for the probability density function f(x,Ɵ).

The Likelihood function is being maximized by a natural logarithm in order to simplify the calculations.

- Moment estimation

This method is used in estimation parameters by matching the moment of the sample to the moment defined by distribution. In case of Weibull two parameters, first and second moment for a sample data would be variance and mean which equal to:

And

- Probability and Hazard plotting

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3.7 Maintenance optimization

Figure 4. Curve shows the total cost with increasing PM actions to an optimal time

Source: Levitt , 2003, pp.12

Figure above shows the minimum overall cost with increasing PM costs to an optimum time. Notice that cost becomes stabilized with time since PM actions going no more effective.

L is the time interval which the total cost is the minimum.

Implementing a maintenance policy does not mean avoiding extra cost all the time. Defining the right time interval in order to PM actions take a place is exactly what all organizations looking for. With time PM costs increased and become ineffective then, it would preferable to have breakdown instead of running ineffective PM actions. This caused by age of the system wearing out (Levitt, 2003).

The area denoted by X is all organizations interest in which the total cost during this time interval would be the minimum. As mentioned before, determining this time interval is the biggest challenge. Dodson (1994) asserts that minimizing the total cost per unit time in order to find the optimum time according to the following equation:

Where: Cp is the cost of preventive maintenance.

Cr is the cost of failure.

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Instead of minimizing the previous equation every time by usual numerical routine, Dodson (1994) has developed a table that can easily used and applicable by following assumptions:

- Time to failure follows Weibull distribution.

- Preventive maintenance is performed at time = T with cost equal to Cp. - If a failure occur before time = T cost of failure is incurred.

- The last and the most important condition is when the component is being maintained it is return into its’ initial state “As good as new”.

The optimum time between preventive maintenance actions can be easily calculated by the following formula:

Where: m is the cost ration Cf/Cp.

is the shape parameter.

is the location parameter.

3.8 Schedule maintenance

When preventive maintenance is being mentioned, a number of fixed PM actions done every month, quarter or season are coming to our minds. Actually PM actions are based on two main aspects procedure and discipline.

Procedure is that the right actions are being carefully taken at the right time. Discipline is that all actions are planned and under control. Discipline is the check aspect for PM actions; hence it could not be overlooked. Failing of implementing a scheduled maintenance for some organization mainly comes from discipline aspect, which is being ignored. Scheduled maintenance is one step forward of improving PM actions to be in accurate need of it (Mobley, 2004).

According to Mobley (2004), there are six main elements for accurate procedure:

 Listing of components plus the intervals that they should received a preventive maintenance.

 A schedule for a year that breaks down by month.

 Person responsibility to do the work.

 Updating the records for the actions that had done, and when next action due to.

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Some PM actions could take a place every interval weather it is necessary or not, hence, unnecessary stoppage may occurs. Since scheduled maintenance aim to reduce unplanned stoppages, scheduled maintenance should be based on real actual need of PM actions.

3.9 Life Cycle Cost (LCC)

Life Cycle Cost is a method to calculate the total cost of a structure during its lifetime. A system will not be considered as an economical when the maintenance costs are high. From an economic perspective, the purpose of cost optimization is decreasing the total life cycle cost (Sarma and Adeli, 2002). LCC analysis is applied with the aim of selecting the most cost effective approach from different alternatives in order to reach the lowest long-term cost (Negrea et al., 2007).

According to Negrea et al. (2007, p.217), the LCC includes the following steps: 1. Define the problem which requires LC

2. Alternatives and acquisition/sustaining costs 3. Prepare cost breakdown structure

4. Select analytical cost model

5. Collect cost estimates as well as cost models 6. Make cost profiles for each year of study 7. Make break, even charts for alternatives 8. Pareto charts of vital few cost contributors 9. Analysis of high costs

10. Study risks of high cost items

11. Select preferred course of action using LCC

Furthermore, Liu et al. (2010) state that the value of LCC can be obtained by calculating the equation below.

CLCC = CI + CO + CM + CF + CD

Where CI is the investment costs, CO is the operation costs, CM is the maintenance costs, CF is the failure costs and CD is the disposal costs. But only analyzing the LCC one by one is not enough since the impact of the system will be ignored. It is necessary to include the cost of externalities (Cexter), which results in the following equation.

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LCC can be discounted using the present value method at a certain interest rate (i) after n years, in the present time. The following equation shows the present value of LCC:

PC LCC = F C LCC / (1 + i)n

Where F C LCC is the future value of LCC and is equal to CLCC calculated above, and PC LCC is

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4. EMPIRICAL DATA

In this chapter, a description of case company, the production process is presented. Moreover, data gathered from the interviews and observations are included through this part.

4.1 General description

Dynamate Intralog AB is a Swedish company established in 2000, the company is located in Oskarshamn city and it includes 60 employees in total that distributed into managers, technicians and workshop workers. The company main roles are categorized into three functions: providing warehousing, transportation and production support services.

Dynamate Intralog AB considered Daughter Company of heavy industrial firm known as Scania that placed in Södertalje; Scania objectives are to deliver optimised heavy trucks, buses, and engines. Thus, obtaining logistical services from Dynamate Intralog AB are critical issues among Scania’s production system because of intensive workload within manufacturing and assembling lines.

4.2 The logistics outsourcing

Dynamate Intralog AB is a third-party logistics (TPL) provider that transports on-demand the containers that required in Scania’s production line, and those containers contain boxes filled up with truck elements such as electronic items, steering wheels, doors, and so on.

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Figure 5. TPL circulation

Proceeding from the figure, Dynamate highlighted a core value through TPL circulation, for that reason Dynamate was our interest of study and we are going to present the empty boxes process in details after it arrives to company and then to robots.

4.3 The empty boxes process

The container arrived to Dynamate Intralog AB from Scania filled by empty boxes (30), the workers unloading following container and transported through conveyor belt towards the workstation whilst fork-lifts waiting to carry those empty boxes to palletizing robot. Then the wooden boxes have been checked by taken lids away to be ensured that boxes don’t contain any forgotten items by Scania inspectors. The next step is to organize all boxes upon the production line and let the chain driver take them into the main robot function which is splitting the box elements into pallet and collar. This action done by robot arm (Kragplockaren) by folding the collar and place it in collars area for packaging and transportation purposes, then pallet moved to pallets region for same goal. This picture shows how the wooden box looks like:

Collar

Pallet

Image 1. The wooden box

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According to Mrs. Norberg (Maintenance Technician of Dynamate Intralog AB), she defined empty boxes process as simple and complicated system; its simple while all steps are clear and there is no delay between different operations, and its very complicated in case of components failure occurred to palletizing robot that stop the whole production line. The company refers their time and money loses to machine wear-out, therefore they are depending quite much in maintenance strategies, and in such a case the corrective and preventive policy have been applied. The problem still in high rate and need more integrated and flexible maintenance approach in order to minimize time and money loses.

4.4 Robot performance and critical components

During the interview with Mr. Weli Zubair (Production and Logistic Manager of Dynamate Intralog AB), he states various kind of failures types encountered the machine components and he mentioned frequent stoppages because of these breakdowns. Furthermore, the observations show many disturbances blocking the robot efficiency in terms of time (availability), consequently the overall equipment effectiveness declined by limiting the operational time. Hence studying robot critical components are important to optimize system reliability and conducting best maintenance practices. According to Mr. Weli Zubair, components can be ranked and listed as following:

Photocell, gearbox, brake motor, guide rails, chain, turntable, and robot arm; the system performance proportionally affected by these components availability. Those elements are going to be clarified in details in later parts.

4.5 Maintenance strategies within Dynamate Intralog AB

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4.6 Corrective maintenance for essential components

During our visit to the company, we have tried to collect data as much as it’s possible about robot critical components and its stoppage time for approximately two year interval. As we mentioned above, the critical components selection based on interview with production manager that listed by him, consistency with his daily experience. First component was the photocell and its reflexes, this item exposed to internal and external errors done by human through accidental crash, or by environmental issues and not dismissed the electrical contact, in which all situations led to non processing system and breakdowns to other mechanical parts. Thus, photocell needs a certain type of corrective maintenance known as replacement by new, and this route considering money and time consumption, for instance every hour spent in corrective maintenance its costs 450 SEK and new photocell costs 1500 SEK.

Sequentially, detecting the photocell been broken or non-functioning takes around 30 minutes and 1 hour to be fixed or replaced although the spare parts reserved in storage and buffer areas, but the consequences from broken photocell involve other hindrance operations and components to be responding. For example, whilst photocell not running perfectly the whole process flows through production line deactivated and passed wooden box may damaged because of mistiming robotic movement, and in such a case if this wooden box damage and stuck in the robot arm need around 1 hour to remove it. The dataset about photocell divided into number of failures and their times to failure been collected through interview and it will be presented in appendix (3).

Second component is brake motor incurred mechanical failure such friction between brake cylinder and vanes caused by unstable screws that being running for long term, plus the erosion appeared on the surface of brake metal. Accordingly, this kind of failure evaluated as real problem because of time needed to repair the fault and in case of replacement by new one. In both cases it goes along 3 days to reactivate the production engine back because of outsourcing demand for brake motor elements as well as importing maintenance engineers who has capabilities dealing with complex repair. Nevertheless, detecting the failure takes around 1 hour to be located. Dataset been gathered and generated based on times to failure and number of failures. See appendix (4).

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type of failure affect other components indeed. Dataset about this component can be found in appendix (5).

Fourth component known as air bellows, the failure here surrounded by rusty conditions because of corrosion basis, and that can be known through air leakages. This is the hardest failure to detect because the system still processing and long farther in same state down to suddenly breakdown. That’s taking almost 5 hours to detect the failure and need 4-6 hours to restore the functional circumstances. Data set about this component shown in appendix (6).

Finally, control list component has over-flooded fat problem suit a sliding collars and that’s actually a serious problem been observed tangibly through our visit, also the wear-out and corrosion appeared clearly. When this problem occurred the robot arm stop working however attempted to reinstall again, and required the worker to stop machine then try to remove the collar manually and put it in collar area then restart the machine. This mission takes 5-10 minute to reuse the robot arm, but main problem was control list and here the maintenance technician has to come and repair or replace the control list. The time needed split into two branches either item is available in storages and this operation to repair required 2-3 hours or they have to wait at least 2 days to receive such a component. Data set of this component shown in appendix (7).

4.7 Preventive maintenance for essential components

Mrs. Norberg focused on preventive maintenance practices within Dynamate Intralog AB; she splits the preventive maintenance schedule into two periods, weekly and monthly. She has talked about other component entailed by this scheduled maintenance but she gave us information about the critical component that been ranked by Mr. Weli Zubair.

4.7.1 Weekly preventive maintenance

Photocells and reflexes have to be cleaned every Thursday by a cloth for safety reason;

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Image 2. The photocell in palletizing robot

Brake motor has to check every Thursday by measuring the distance between the vane

and brake cylinder, the measurement has to shown 0, 2 mm through using feeler gauge otherwise the screws around the vane must adjust the difference between them. This operation longs from 15 minutes – 1 hour but the whole machine being stopped. The replacement screws take place every 2 weeks and the whole robot being stopped for almost 1 hour, and the screws cost 100 SEK.

Image 3. The brake motor

Photocell

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4.7.2 Monthly preventive maintenance

Gearbox undergoes among monthly check to notice the oil leakage and to check if

there is chaotic sound release from it, in such a case the mechanical technicians has to monitor such a situation. The robot still working during this kind of observations and inspections, but in extreme cases the machine has to stopped, for example the sound test showed unusual resonance waves. The inspection takes 30 minutes, besides the oiling for 20 minutes.

Image 4. The gearbox

Air bellows checked monthly by monitoring the air outlet paths if there is no air

leakage through those channels besides checking the cracks that would happen throughout chemical reactions, for instance corrosion and erosion. The inspection longs 30-50 minutes by stopping the machine.

Image 5. The air bellows

Control list criticality considered because of its role importance, the control list has to

be checked from surface cracked or lubricant troubles, for that reason the machine has to be stopped form 30 minutes- 1 hour and dried from over-flood fat, then pour new viscosity oil.

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Image 6. The control list

4.8 Corrective and preventive maintenance related cost

The data have been obtained about corrective maintenance cost covered the following categories; labour and new item cost. (The stoppage time cost will be discussed in

analysis part).

According to appendix (3) the photocell shows 43 number of failures through 5190 hours, the labour cost as mentioned before was 450 SEK and replacement cost is 1500 SEK.

Then, Brake motor appendix (4) shows 3 number of failures during 5109 hours, the labour cost approximately 500 SEK and parts cost about 2000-6000 SEK for instance vane or cylinder damage, but in case of replace it with new item cost 8000 SEK. After that, the gearbox repair may cost approximately 6000 SEK and can be less when its minor fix such as small parts within the gearbox, labour cost at least 500 SEK, the number of failures doesn’t occurs so often which is only 2 times see appendix (5). On other hand, the air bellows take place quite much in failure number it reach 6 times, and the labour cost 500 SEK beside the repair and replacement costs that ranging from 1000 SEK- 5000 SEK. At last, control list required more attention because of its importance related to robot arm, the labour cost is 450 SEK and the replacement or repair cost ranging from 1000 SEK- 4000 SEK.

The preventive cost classified into time spent in preventive maintenance in terms of money and the stoppage time cost that are going to be considered in analysis part as well.

Preventive maintenance cost for photocell within 1 week is 75 SEK and no need for machine stoppage by cleaning the dirt upon photocell. In paradox with brake motor

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that need to stop the whole robot to make preventive maintenance and it costs up to 500 SEK. The gearbox required only vision inspection to be surely about functioning conditions, and does not required any spending cost in this activity, because it might be included through daily work instructions. The air bellows require 225 SEK even as considered monthly and it’s very sensitive to be checked correctly. Finally the preventive maintenance cost for control list component may range around 500 SEK divided into labour and lubricant cost.

4.9 Production list for palletizing robot

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5. ANALYSIS

This chapter comprising the data that have been collected about maintenance practices and their correlations to costs, the assumptions been conducted in this part to obtain systematic calculations.

5.1 Maintenance cost calculations

As we mentioned before the data been gathered within 5190 hours interval of time, the assumption here related to production unit whereas each box equal to 1 SEK, and the dataset record 43, 3, 2, 6 and 7 failures number of components respectively. The total maintenance cost contains of two main factors which are corrective and preventive maintenance cost, so the calculations will take each component one by one, starting with photocell, the formulas consists of the following combinations:

(1) Photocell

 Corrective cost: Labour cost /h + Stoppage time cost + Replacement item cost Labour cost /h = 450 SEK

Stoppage time/ one failure number = 2.5 hours New photocell cost 1500 SEK

Labour cost during 5190 hours = 450×2.5×43 = 48,375 SEK

New photocells cost through 5190 hours = 1500×43 = 64,500 SEK

Stoppage time = 2.5×43 = 107, 5 hours and in terms of money, as the assumption induce the time variable by each box cost 1 SEK, then Stoppage time cost = 89×1×107,5 = 9,567.5 SEK

Therefore, Corrective cost = 48,375+64,500+9,567.5 = 122,442.5 SEK

 Preventive cost: Labour cost + Stoppage time cost+ Replacement item cost Labour cost during 5190 hours = 150×5190\60 = 12,975 SEK

Replacement item cost = 1500×35= 52,500 SEK

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(2) Brake motor

 Corrective cost: Labour cost /h + Stoppage time cost + Replacement item cost Labour cost /h = 450 SEK

Stoppage time/ one failure number = 30 hours New parts cost 6000 SEK

Labour cost during 5190 hours = 450×30×3 = 40,500 SEK New parts cost through 5190 hours= 6000×3 = 18,000 SEK

Stoppage time = 30×3= 90 hours, then Stoppage time cost = 89×1×90= 8,010 SEK Therefore, Corrective cost = 40,500+18,000+8,010 = 66,510 SEK

 Preventive cost: Labour cost + Stoppage time cost+ Replacement item cost Labour cost during 5190 hours = 450×5190\60 = 38,925 SEK

Replacement item cost = 100×5190\120= 4,325 SEK

Stoppage time = 173 hours, which mean = 89×1×173= 15,397 SEK Therefore, preventive cost =38,925 +4,325 +15,397 = 58,647 SEK

(3) Gearbox

 Corrective cost: Labour cost /h + Stoppage time cost + Replacement item cost Labour cost /h = 450 SEK

Stoppage time/ one failure number = 30 hours New parts cost 6000 SEK

Labour cost during 5190 hours = 450×30×2 = 27,000 SEK New parts cost through 5190 hours= 6000×2 = 12,000 SEK

37  Preventive cost: Labour cost + Stoppage time cost+ Replacement item cost Labour cost during 5190 hours = 56.25×5190\60 = 4,865.625 SEK

Replacement item cost = 0 SEK

Stoppage time = 28.6 hours, then Stoppage time cost = 89×1×28.6= 2,551.3 SEK Therefore, preventive cost = 4,865.625 + 2,551.3 = 7416.9 SEK

(4) Air bellows

 Corrective cost: Labour cost /h + Stoppage time cost + Replacement item cost Labour cost /h = 450 SEK

Stoppage time/ one failure number = 11 hours New parts cost 2500 SEK

Labour cost during 5190 hours = 450×11×6 = 29,700 SEK New parts cost through 5190 hours= 2500×6 = 15,000 SEK

Stoppage time = 11×6= 66 hours, then Stoppage time cost = 89×1×66= 5,874 SEK Therefore, Corrective cost =29,700 +15,000 +5,874 = 50,574 SEK

 Preventive cost: Labour cost + Stoppage time cost+ Replacement item cost Labour cost during 5190 hours = 56.25×5190\60 = 4,865.625 SEK

Replacement item cost = 0 SEK

Stoppage time = 72 hours, then Stoppage time cost = 89×1×72= 6,408 SEK Therefore, preventive cost = 4,865.625 + 6,408= 11,273.625 SEK

(5) Control list

 Corrective cost: Labour cost /h + Stoppage time cost + Replacement item cost Labour cost /h = 450 SEK

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New parts cost 2000 SEK

Labour cost during 5190 hours = 450×3×7 = 9,450 SEK New parts cost through 5190 hours= 2000×7 = 14,000 SEK

Stoppage time = 33×7= 231 hours, then Stoppage time cost = 89×1×231= 20,559 SEK Therefore, Corrective cost =9,450 +14,000 +20,559 = 44,009 SEK

 Preventive cost: Labour cost + Stoppage time cost+ Replacement item cost Labour cost during 5190 hours = 112.5×86.5=9,731.25 SEK

Lubricant cost = 500 SEK

Stoppage time = 21.625 hours, then Stoppage time cost = 89×1×21.625= 1,924.6 SEK Therefore, preventive cost = 9,731.25 +500+1,924.6 = 10,233.1746 SEK

5.2 Distribution for the failure data

Figure 6. No. of failures for each critical component during 5190 hours

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Figure 7. The time at which the components have failed

A graph above shows failure time for critical components. X axis represents the number of failures while Y axis represents the time at which the component has been failed. Failures have been registered over a 5190 operating hours. It is obviously shown that Guide ways for example has registered three failures during these operating hours. Those failures registered in different hours.

Due to large Number of failures for the ‘photo cell’ component it has been taken out of the graph in order to clarify the other components.

5.3 Optimum time to PM actions

In order to find the optimum time for each components, we need to use the following equation 3.7. Under the following assumptions:

- The component as good as new after performing the maintenance actions. - Cf is incurred if the component fail before time=T.

- Preventive maintenance is performed with Cp on a component at time =T. - Time to fail follows a Weibull distribution.

T= (m.η) + γ

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Table 2. The value of m

Source: Dodson, 1994

Finding the value of m, substitute it with equation 1 we have the optimum time to replace, maintain the component. Table 4 bellow shows the optimum time for each component. For the value of B see appendixes (9, 10, 11, and 12).

Cc/Cp Ration cost β η m γ Opti T = (m.η) + γ Photocells and reflexes 1.786063 9.3 121.8 2.229 _{0 97.8054} brake Motor _1.134073 1.36 4178 _2.229 0 _9312.762 Gear Box _5.905432 1.26 4667 _0.574 0 _2678.858 Air Bellows _4.486046 1.48 4048 _0.746 0 _3019.808 Control list _4.30062 2.75 4570 0.521 0 _2380.97

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5.4 Maintenance schedule based on optimum time

Maintenance scheduling could be done based on the calculating the optimum time. Graph bellow shows if we could run PM actions for more than one component at the same time in order to minimize the total PM costs. Notice Photo cell has been taken out with respect to scale.

Figure 8. Current PM Time and Optimum time

It is obvious that there are some unnecessary PM actions which mean extra unnecessary cost. PM could be rescheduled and implemented in groups for multiple components.

Table 4. The optimum time to perform PM

Components Time to perform PM

Photocells and reflexes _97.8054

Break Motor 9312.762

Gear Box _2678.858

Air Bellows _3019.808

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6. RESULTS

In this chapter the final result of the thesis analysis presented. With respect to the data collected from the company.

As results, critical components have been determined based on an interview with the production manager. Those components are photocells and reflexes, Break motor, Gear box, Air bellows, and Control list. Failure data for these components have been followed a Weibull distribution. In addition estimating the value of beta and eata for each component are essential to calculate the optimum time.

The optimum time for the critical components have been determined to let PM actions take a place. Optimum hours are 98, 9313, 2679, 3020 and 2381 hours for photocells and reflexes, Break motor, Gear box, Air bellows, and Control list respectively.

Based on the calculation, schedule for those critical components has been made, aiming to reduce the total number of PM actions. A group of actions has been scheduled together as table 4.

The number of PM actions could be reduced by knowing the exact time, therefore unnecessary cost could be avoided. In the previous table, a PM could be performed after 2200 operating hours for Control list and Gear Box. Instead of apply PM every 900 hours. It is obvious that two round of PM unnecessary. There for those costs could be avoided, the total cost for PM actions for the whole components would be

Cost of PM for Component x1, 2,…n * Number of PM

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7. CONCLUSION

In this chapter the conclusion of our thesis which answer the problem formulation, criticism to our thesis and suggestions

Company experience is one method to identify the critical components. Using FMECA for example could make the analysis more accurate. Recording the failures of the machine is one step forward to enhance the availability of it. Analyse a historical data is the second step to improve the maintenance policy.

In order to avoid extra unnecessary cost Optimum time has been scheduled; PM actions should be implemented at the right time that could effective. By determining the optimum time a group of PM actions could be implemented at once.

The main aim for scheduling the PM actions is to identify the right time interval that should the PM actions performed. Based on analysis and knowing the behaviour of the system or component. Readability and availability of the data play big role in the result part and sometimes could affect a critical decision that the organization should make. Subsequently, the reasons behind the failure types are the core of analyzing the data by Weibull, and the way that trends through dealt with certain reasons of faults and not the all causes. Thus, FMEA is an integrated reliability analysis tool in order to cover all the critical failures and their consequences in the system. For instance the external factors may enhance the failure occurrence and the environment indeed, therefore, to manage better solutions these factors have to be considered.

Recommendations

The company needs to enhance their workers to have enough knowledge about those components and have best maintenance practices manual to monitor all the machine elements in case of stoppage and breakdowns.

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Appendix 1

QUESTIONNAIRE

1- What is the logistic connection between Dynamate and Scania along the supply chain?

2- Is there any lead-time or barriers through production process flow?

3- Is the problem classified into following categorizations:

a- Logistic b- Machinery and Maintenance c- Human resources

4- What is the production process according to robot main functions?

5- Is the robot stoppage time related to:

a- Failure components b- company policy such as just in time c- other reasons

6- Is the declined production whilst the robot deactivates involved critical components Breakdowns and what are these components?

7- What are the maintenance strategies and policies towards these components, and if there any analysis tools dealing with?

8- Is there any human factors and untrained worker may affect negatively the palletizing robotic system?

9- How much money and time were spending in corrective maintenance respect to failure components?

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Appendix 2

This table present the boxes number that been produced in week 6 Number of boxes that been produced Disturbance time or stoppages/minutes

First day of work is 22 from 26 boxes 70 minutes

Second day of work is 26 from 35 boxes 120 minutes

Third day of work is 26 from 39 boxes 60 minutes

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Appendix 3

Component No. of failures during 5190 hours

Photo cell 43

Failure No. at T, hour

50

Appendix 4

Component No. of failures during 5190 hours

Brake Motor 3

Failure No. at T

1 2407

2 3641

3 4753

51

Appendix 5

Component No. of failures during 5190 hours

Gear Box 2

Failure No. at T

1 3588

2 4799

52

Appendix 6

Component No. of failures during 5190 hours

Gear Box 2

Failure No. at T

1 3588

2 4799

53

Appendix 7

Component No. of failures during 5190 hours

55

Appendix 9

56

Appendix 10

57

Appendix 11

58

Appendix 12