line balancing considering ergonomics.pdf

8/14/2019 line balancing considering ergonomics.pdf

1/67

1

Faculty of Technical Sciences

Assembly Line Balancing Problem Considering Ergonomic Factors

Graduation Project

Course: System Management

Skopje October, 2011


2/67

2

Faculty of Technical Sciences

Assembly Line Balancing Problem Considering Ergonomic Factors

Graduation Project

Course: System Management

Mentor:

Prof. Aleksandra Porjazoska KujundziskiPerformed by:

Nihal Kemer, 07.08/3

e-mail:[email protected]

Skopje, October, 2011


3/67

3

CONTENT.........................................................................................................................................................3

1. ASSEMBLY LINES..............................................................................................................................101.1. BASIC PRINCIPLES OF ASSEMBLY LINE...................................................................................121.2. A CLASSIFICATION OF AL..............................................................................................................14

1.2.1. ACCORDING TO THE NUMBER OF PRODUCTS OR MODELS...........................................14

1.2.2. ACCORDING TO THE TASK DURATION................................................................................15

1.2.3. ACCORDING TO THE LINE SHAPE OR LAYOUT..................................................................15

1.2.4. ACCORDING TO THE WORKPIECES FLOW............................................................................17

1.2.5. ACCORDING TO THE LEVEL OF AUTOMATION...................................................................19

1.3.1. A TYPICAL ASSEMBLY LINE......................................................................................................20

2. ERGONOMIC APPROACH...................................................................................................................22

2.1. EFFECTIVE ERGONOMICS.............................................................................................................23

2.2. GOALS OF ERGONOMISC...............................................................................................................25

2.2.1. CONTINIUOUS IMPROVEMENTS AND BREAKTHROUGHS................................................25

2.3. WORK RELATED MUSCOSKELETAL DISORDERS..................................................................26

2.3.1. STANDING AND WALKING POSTURE DURING ASSEMBLY OPERATIONS..................30

2.3.2. UPPER EXTREMITY ASSESMENT TOOLS...............................................................................32

2.3.3. HAND VIBATION EXPOSURE ON AN ASSEMBLY LINE......................................................33

3. THE RELATION OF TASK ASSIGMENT TO EXERTION FREQUENCY, DUTY CYCLE,NORMALIZED PEAK FORCE, VIBRATION ACCELEATION AND VIBRATIONDURATION...................................................................................................................................................39

3.1. DETERMINATION OF HAND ACTIVITY AND PEAK FORCE MEASURES BY TASKASSIGMENT.................................................................................................................... .............................39

3.2. DETERMNATION OF HAND-ARM VIBRATION MEASURES BY TASK ASSIGMENT.......41

4. OPTIMIZATION MODEL...................................................................................................................42

5. NUMERICAL EXPERIMENTS AND DISCUSSION........................................................................48

5.1. MANUFACTURING TASK DESCRITION AND PARAMETER ESTIMATION.....................48

5.2. RESULTS FROM DIFFERENT COMBINATION OF ERGONOMIC CONSTRAINTS..........51

6. CONCLUSIONS...................................................................................................................................56

APPENDIX A..............................................................................................................................................58

APPENDIX B..............................................................................................................................................60

REFERENCES...........................................................................................................................................65


4/67

4

LIST OF FIGURES

FIGURE 1.1: FORD S CAR ASSEMBLY LINE .....10FIGURE 1.2: PRECEDENCE GRAPH .....12FIGURE 1.3: SINGLE -MODEL LINE ......13FIGURE 1.4: MIXED MODEL LINE ......14FIGURE 1.5: MULT-MODEL LNE........................................................................................ 14FIGURE 1.6: SERIAL LINE (A N ASSEMBLY LINE OF VCR UNITS OF SONY ) ..15FIGURE 1.7: TWO-SIDED LINES (THE ASSEMBLY LINE OF THE TOYOTA LEXUS , CANADA ) 15FIGURE 1.8: PARALLEL LINES ..16FIGURE 1.9: U-SHAPE LINES ..16FIGURE1.10: CLOSED LINE ..17FIGURE 1.11: SYNCHRONOUS LINE ..17FIGURE 1.12: ASYNCHRONOUS LINE ... 17FIGURE 1.13: FEEDER LINES 18FIGURE. 1.14: MANUAL LINE ... 18FIGURE 1.15: ROBOTIC LINE 17FIGURE 1.16 THE TASK DESCRIPTION AND PRECEDENCE OF ASSEMBLY TASKS WITH VIBRATIONPROFILES 19FIGURE 2.1: CONCEPTUAL MODEL OF THE DEVELOPMENT OF WORK-RELATED MUSCOSKELETAL DISORDERS 27FIGURE 2.2: COST ANALYSIS BY OCCUPATIONAL INJURY AND ILLNESS TYPES IN CANADA 27

FIGURE 2.3: FORMER BATCH WISE PRODUCTION(LEFT) AND ONE-PIECE FLOW PRODUCTION (RIHT) IN FINAL ASSEMBLY .29FIGURE 2.4: HAND ACTIVITY LEVEL(0- 2)CAN BE RATED USING THE GUIDELINE 32 FIGURE 2.5: A REPRESENTATIVE WORKSTATION .34FIGURE 2.6: CENTER FREQUENCY OF THIRD OCTAVE BANDS 36FIGURE 2.7: THE STRUCTURE OF THE RESEARCH 38FIGURE 3.1: THE TLV S BASED ON HAL AND NPF. 40FIGURE 5.1: THE TASK DESCRIPTION AND PRECEDENCE OF ASSEMBLY TASKS WITH VIBRATION PROFILES 48FIGURE 5.2: THE ACCELERATION AND THE COORDINATE FIGURES OF TWO SCREWDRIVERS ..50FIGURE 5.3 R ESULTS FROM DIFFERENT CONSTRAINT COMBINATIONS .52FIGURE 5.4: HAL TLV OF LBMC, LBMCH, LBMCV AND LBMCHV 52FIGURE A.1:ONE-THIRD OCTAVE BAND SPECTRA 3,4 CM DIAMATERAUTOMATC SHUT-OFF SCREDRIVER ..58Figure A.2:ONE-THIRD OCTAVE BAND SPECTRA FOR THE 2,5 DIAMETERCLUTCH SCREWDRIVER IN THE SLIPPAGE OF THE CLUTCHCONDITION58


5/67

5

LIST OF TABLES

TABLE 2.1: FORMER AND NEW SITUATIONS IN FINAL ASSEMBLY AND PRINT ASSEMBLY AND THE MAIN NEW ELEMNTS ............................................................... 29TABLE 2.2: HAND ACTIVITY LEVEL(0-10) IS RATED BASED ON EXERTION FREQUENCY AND DUTY CYCLE................................................................................... 32 TABLE 2.3: MEAN AND EIGHT HPUR VIBRATION EXPOSURE DURING RUN AND CLUTCH CYCLESFOR POSITION REQUIRING OPERATION OF POWER SCREWDRIVERS ........................................... 35

TABLE 3.1: TLV S FOR EXPOSURE OF THE HAND TO VIBRATION IN EITHER X, Y, Z AXIS .................................................................................................................................. 42TABLE 4.1: REFERENCE TABLE OF HAL WITH M = 5 .................................................... 45TABLE 4.2: THE SET OF RANGES OF EXERTION FREQUENCIES ................................ 46TABLE 4.3: THE SET OF RANGES OD DUTY CYDES ...................................................... 46TABLE 4.4: THE CONVERSION OF INTEREDIATE VALUE TO HAL ....................... 46

TABLE 4.5: THE CONVERSION OF DAILY VINRATION TO ACCELERATION TLV .... 47TABLE 5.1: DATA EXERTION NUMBER, CYCLE TIME AND NPF................................. 49TABLE 5.2: MALE POWER GRIP STRENGTHS IN NEWTON(N)..................................... 50TABLE 5.3: ACRONYMS FOR THE LINE BALANCING MODELS .................................. 51TABLE 5.4: RELATIVE VALUE OF ERGONOMIC MEASURES COMPARED TO LBMC CASE .................................................................................................................................. 54TABLE 5.5: VIBRATION RESULTS ................................................................................... 55TABLE B.1 HAND ACTIVITYRESULT OF LBM ............................................................... 60TABLE B.2 HAND ACTIVITYRESULT OF LBMCH.......................................................... 61

TABLE B.3 HAND ACTIVITYRESULT OF LBMCV.......................................................... 62TABLE B.4 HAND ACTIVITYRESULT OF LBMCHV ....................................................... 63Table B.5 DETAIL VIBRATION RESULTS ....... 64


6/67

6

NOMENCLATURE

The mathematical symbols used in this study are introduced as follows.

Indexes

f = The one-third octave band number

h = Hand number; h=1, 2

i, j = Manufacturing task

o = The direction of vibration, o= X, Y, Z

p = The range number of

q = The range number of

s = Worker or station; s=1,2,.,M

u = One dimensional cell value fort he table of hand activity level

Parameters

CN = The maximum Colomn Number

CT = The cycle time of the assembly line

= The HAL value corresponding to u

K = Weighting factors for acceleration calculation

NC = The number of cycles finished per day which equals to WT/CT


7/67

7

RN = The maximum row number

WT = The total working time per day

= The f-th frequency acceleration

= The frequency -weighted,rms acceleration of task i,directon o

= The frequency-weighted acceleration

= 5% up force time of one task of hand h

= The number of exertions of hand h during one task

= The processing time of task i

= The duraction of vibration of task i

Variables

1( = The indicator variable;one if belongs to , zero otherwise

1( ) = The indicator variable;one if belongs to , zero otherwise

= The total daily vibration exposure duration of at station s

= The equivalent,frenquency-weighted component acceleration of station s,direction o

= Column number in the table of duty cycle

= duty cycle of workers s

= Exertion frequency of worker s


8/67

8

= Hand activity level of worker s, hand h

= The limit of the equivalent,frequency weighted component acceleration of worker s, direction o

= Peak hand force of worker s, hand h

= Peak hand force of worker s, hand h

= Row number in the table of exertion frequency

= The intermadiate variables for calculating HAL; = +RN. (

= The binary variable of task assigment fort ask i, station s

= The binary variable of hand activity level of each hand of each worker in each station

z = Objective function value


9/67

9

ABSTRACT

Musculoskeletal disorders (MSDs) related to the workplace are among the most costly problems in todays society. The low-back, neck, shoulders, and upper limbs are the body parts most subject to risk. Among the factors associated with the risk for developing work-related MSDs are individual, physical workplace, organizational, and psychosocial factors.After the ergonomic intervention the sick leave due to low back problems decreases, so thatthe company benefits economically from such sick leave reduction because of lowerreplacement costs, learning costs, and productivity rates of new workers.Also, the properly planned and implemented ergonomics result in significant economic

benefit due to productivity and quality increases.The conventional approaches to decreasing work related musculoskeletal disorders (WMSD)risks in the assembly lines include slowing the work-pace or applying job rotations. Theseadjustments usually focus on individual assembly workers at the station level but not the

work allocation among the workers at the whole assembly line level, and thus may decreasethe line productivity. To avoid these negative effects, some research started consideringergonomic characteristics at the line level, such as balancing ergonomic burdens by properwork assignment among workers.

Thus, the aim of this diploma work was to make a survey of a literature which integratesergonomic factors in resolving assembly line balancing problems, especially consideringergonomic measures for upper extremity musculoskeletal disorders. A suitable model caneffectively control the exposure levels in the upper extremity by proper work assignment,

compared to the conventional approaches, and does not decrease production ratesconsiderably.The linear models allowing ergonomic and productivity measures to be integrated as amixed-integer programming model for assembly line design, are especially attractive.Linear models are developed to link work-worker assignment to the measures of hand activityand hand-arm vibration. As productivity measures, conventional assembly line design criteria areconsidered, such as cycle time and the number of workers. In addition, these linearizationmethods can be generalized in order to incorporate ergonomic measures in tabulated forms intoassembly line design problems.

This literature survey will act as a baseline for our future research considering similarergonomics problems.


10/67

10

1. ASSEMBLY LINES

An assembly line is a flow-oriented production system where the productive units performingthe operations, referred to as workstations, are aligned in a serial manner. The workpiecesvisitstations successively as they are moved along the line usually by some kind oftransportationsystem, e.g. a conveyor belt. Each workstation repeatedly performs a set oftasks in order to produced or manufacture a specific product. Tasks require certain time to be

processed and are related among one another according to the existing technologicalconstraints.

Originally, assembly lines were developed for a cost efficient mass-production ofstandardized products, designed to exploit a high specialization of labour and the associatedlearning effects.

The most famous example of an assembly line is the production plant of Henry Ford (Figure1.1), and the main focus of ALBP is how to distribute the entire workload to the workstations of an assembly line [1].

.

FIGURE1.1: Fords car assembly line

T-model components were manufactured in the first moving line using the ideas of workdivision to decrease the production cost per unit and to allow massive production. However,

the work division ideas and this kind of configurations date from much earlier times. TheVenetian Arsenal (considered the world first factory) for instance, developed methods of


11/67

11

mass-producing warships which were much faster and required less wood. At the peak of itsefficiency in the early 16th century, the Arsenal was able to produce nearly one ship per dayon a production-line basis not seen again until the Industrial Revolution. In 1799, Eli Whitneyintroduced the assembly lines in the American manufacturing system. In 1901 Ransom Eli

Olds patented the first assembly line concept and his Olds Motor Vehicle Company was thefirst factory in America to mass-produce automobiles. Was until 1913 when Henry Ford perfected the assembly line concept; nowadays, the first assembly line for building cars isattributed to him [2].

Since the times of Henry Ford and thefamousmodel-T,however, product requirements andthereby the requirements of production systems have changed dramatically.

Although, assembly lines are most commonly found in the automotive industry, many othersectors are also organized in assembly lines. This is the case for most daily life goods, as, forexample, the final assembly of electrical products such as coffee machines, washingmachines, refrigerators, radio, TV and personal computers. More recently, assembly lineshave gained importance in low volume production of customized products as well as inservice systems [2].

Assembly lines have many advantages and disadvantages:

The benefits that assembly lines provided for businesses are mainly the following:

-

Provides a regular flow of material; allows using of power looms and human capacityat the highest level; aims to minimize blank of duration; distributes empty times between work stations properly; minimizes production costs.

- In addition, in order to respond to diversified costumer needs, companies have toallow for an individualisation of their products. For example, German carmanufacturer BMW offers a catalogue of optional features which, theoretically,results in10 32 different models. Multi purpose machines with automated tool swapsallow for facultative production sequences of varying models at negligible setup costs.This makes efficient flow-line systems available for low volume assembly-to-order

production and enables modern production strategies like mass-customization, whichin turn ensures that the thorough planning and implementation of assembly systemswill remain of high practical relevance in the foreseeable future.

Among the disadvantages one can account:

- There are low-skilled workers on assembly lines and employees are doing the same jobs consistently because of this there is monotony. In addition, rates of change indemand is directly linked to the efficiency of the production system. Also, due to thehigh level of automation, assembly systems are associated with considerableinvestment costs. Therefore, the (re)-configuration ofan assembly line is of criticalimportance for implementing a cost efficient production system.


12/67

12

- Configuration planning generally comprises all tasks and decisions which are relatedtoequipping and aligning the productive units for a given production process, beforethe actual assembly process can start. This includes setting the system capacity (cycletime, number of stations, station equipment) as well as assigning the work content to

productive units (task assignment, sequence of operations).

1.1. Basic principles of assembly line

Processing tasks: a processing task i (task, here after) is an indivisible working unit which hasassociated processing time t i. The total work required to manufacture a product in an

assembly line is divided into a set of n tasks.

Workstations: are the line component where tasks are processed, and can involve a human orrobotic operator, certain equipment and some specialized processing mechanisms.

Cycle time ct: is the time available in each workstation to complete the tasks required to process a unit of product -the production rate is equal to 1/ ct units of product per time unit.The cycle time is also defined as the time interval between the processing of two consecutiveunits.

Precedence relations: are defined by the technological precedence requirements thatdetermine the partial order in which tasks can be performed in the assembly line. A task

cannot be processed until all its immediate predecessors have already been processed.Precedence relations are normally represented by a precedence diagram.

Figure 1.2 shows an example precedence graph with n=9 tasks having task times between 2and 9 (time units) [1].

FIGURE 1.2: Precedence graph

Workstation load S j: is the subset of tasks assigned to workstation j.


13/67

13

Workstation time t(S j): is the sum of the times t i of all tasks assigned to workstation j.

Workstation idle time It j: is the difference between the cycle time and the workstation load.

Line balancing: is the process of distributing the n tasks among the m workstations in such away that precedence constraints and other constraints are satisfied; aiming at optimizing agiven efficiency measure. Classical objectives seek to minimize m for a desired cycle time ct,or to minimize ct given m.

There exists a great variety of configurations involving assembly lines, which arecharacterized according to diverse criteria. Amongst others, these include the layout andshape of the line, the number of products and models being processed in the line, types ofworkstation and the variability of the task processing times [1].

1.2. A classification of assembly lines (AL)

A classification of assembly lines is usually done based on the number of products or models produced, tasks durations, shape or layout of the line, the flow of the workpieces and thelevel of automation of the line.

1.2.1. According to the number of products or models

Single-model line: is the classical configuration in which a single model of a unique producttype is produced (Figure 1.3).

FIGURE 1.3: Single-model line


14/67

14

Mixed-model line: several variants of a basic product, referred to as models, are producedsimultaneously in the line (see Figure 1.4). The production process does not involve setuptimes since all models require basically the same manufacturing tasks. Units of differentmodels are produced in a mixed sequence [2].

FIGURE 1.4: Mixed model line Multi-model line: different models with significant differences amongst one another are

processed in the line. Therefore, sequences of batches are processed, containing either thesame model or a group of similar models, involving intermediate setup tasks (Figure 1.5) [2].

FIGURE 1. 5: Multi-model line

1.2.2. According to task duration

Deterministic: all task processing times are fixed and known with certainty.

Stochastic line: task processing times may be significantly affected from different sources ofvariability such as, for example, the ability or motivation of human operators. Therefore, the

processing time of one or more tasks is considered to be probabilistic [2].

Dependent line: tasks processing times are not fixed but dependent, for example, on the typeof workstation to which the task is assigned, on theoperator or on the processing sequence.

Dynamic line: processing times vary over time and can be reduced in successive cycles dueto improvements in the assembly process or due to learning effects (for example, whenoperators become familiar with the tasks [2].


15/67

15

1.2.3. According to the line shape or layout

Serial lines: products units are processed throughout a group of workstations that areconsecutively arranged in a straight line such as, for example, a conveyor belt (Figure 1.5)

FIGURE 1.6: Serial line (An assembly line of VCR units of Sony)

Two-sided lines: consist of two serial lines in parallel, in which pairs of oppositeworkstations (left-hand side and right-hand side) process simultaneously the same workpiece.This configuration is commonly found in the automotive industry (Figure 1.6). Some taskscan be assigned only to one side (e.g. mount the left car wheel), some tasks can be assigned

to either side (e.g. install the hood ornament), and some tasks must be assigned to both sidesof the line simultaneously (e.g. install the rear seat) [2].

FIGURE 1.7: Two-sided lines (The assembly line of the Toyota Lexus, Canada)

Parallel workstations: in this case two o more workstations are put in parallel; hence, thework pieces can be distributed between several workstations that perform an identical set of

tasks.


16/67


17/67


18/67

18

FIGURE 1.13: Feeder lines

1.2.5. According to the level of automation

Manual lines: in manual lines the tasks are performed by human operators. These lines arecommon when workpieces are fragile or are of special importance. Harley Davidsonsmotorcycles, for example, are 100% assembled by hand as shown in Figure 1.14.

FIGURE. 1.14: Manual line FIGURE 1.15: Robotic line(http://encarta.msn.com/media701765960/RobotAssemblyLine.html (visited onFebruary 2004)

The characterization of line, to a great extent, determines the type of balancing problem thatis to be solved. For example a single-model, serial, paced, deterministic line merely entailsthe aassigment of tasks to the workstations -the simplest balancing problem. However,for


19/67

19

other line configurations the balancing problem comes toget her with a problem ofsequencing the models, wheras a multi-model line also implies a lot sizing problem. Paralellines involve a decision problem concerning the number of lines that needs to be installed.Robotic lines (Figure 1.15), on the other hand, involve the assignment of both tasks androbots to the workstations.

Asynchronous lines requires of the positioning and dimensioning of buffers:

- vand whenever feeder lines are considered, the production rates of the available lineshave to be synchronized [2].

It is evident that industrial systems involve a great variety of characteristics and problemvariations. However, due to their complexity, most literature studies on production and

manufacturing have addressed problems which do not consider many of the requirements andconstraints present in real systems. However, in the last years a considerable effort has beendone towards filling the gap between problems addressed in research works and real-worldapplications [2].

1.3. Assembly Line Balancing Problems (ALBP)

The distribution of the entire workload to the work stations of an assembly line, as the mainfocus of assembly line balancing problems (ALBP) have been extensively studied [1,3-6].The assembly line balancing problems, firstly proposed by Helgeson originated with theinvention of the assembly line. Figure 1.15 shows a typical assembly line.

FIGURE 1.16 Typical assembly line


20/67

20

1.3.1. A typical assembly line

As previously mentioned, the Assembly Line Balancing Problem (ALBP) consists inassigning a set of indivisible tasks to an ordered sequence of workstations in such a way that

precedence constraints are maintained, the workload of each workstation does not exceed thecycle time and a given efficiency measure is optimized. The term balancing arises from thefact that the workload of each workstation is to be balanced. Since a perfect balance (i.e., anidentical load for all workstations) is rarely achieved, workstations idled time becomes amain optimization objective. On the other hand, as the assembly line global cost is influenced

by the number of workstations, the classical objective of assembly line balancing problem isminimize the number of workstations for a given cycle time, which is referred to as time-oriented line balancing . Furthermore, production rate can be maximized by minimizing thecycle time of a given number of workstations. Problems that seek to minimize costs are

regarded to as cost-oriented line balancing. On the other hand, profit oriented are thosewhich implicitly consider the profit attained by the line.

The objectives of ALBP are to minimize cycle time, reduce number of workstations, or levelmanufacturing workload. Often, more that one efficiency measure is to be optimized. Thefollowing are some of the minimization criteria that have been considered in literaturestudies: throughput time (i e., the time interval between lunching a workpiece and finishingthr finished product from the line), cost of machinery and tools, inventory cost, dead time,number of buffers, line stoppage time, and variances in workstation times. Somemaximization objectives include production rate (which is equivalent to minimize the cycle

time), line efficiency and profit. A further objective considers that the workload of eachworkstation needs to be as similar as possible [4].

1.3.2. Procedures to Solve Assembly Line Balancing Problems

During the first forty years of the assembly lines existence, only trial-and-error methodswere used to solve the assembly line balancing problems. However, there have beennumerous methods, both, exact and approximate, developed to solve the different forms of

the ALBP. The first mathematical formulation was firstly established by Salveson. Gutjahrand Nemhauser showed that the ALP problem falls into the class of NP-hard combinatorialoptimization problems. This means that although guaranteeing an optimum solution, exactmethods have a problem size limitation, measured in terms of computing time; therefore, theycan only be applied to problem instances with small or medium number of assembly tasks.Approximate methods (i.e., heuristics and metaheuristics) have been developed in order toovercome such a limitation, and aiming at providing good solutions that are as near as

possible of the optimal solution [4].

The exact procedures applied so far are [2]:


21/67

21

- Mixed (integer) linear programming which is successfully used in description of theassembly line balancing problems, but it is not efficient in considering the real-worldscaled problems.

- Dynamic Programming (DP) procedures basically transform the problem into a multi-

stage decision process by breaking it into smaller sub-problems, which in turn aresolved recursively; then the optimal solution of the sub-problems are used to constructthe optimal solution of the original problem. The main drawback of these proceduresis their large memory requirements.

- Branch and bound (B&B) technique - which finds the optimal solution byexploring subsets of feasible solutions. Sub-regions are formed by branching thesolution space. A bounding process is recursively used to find lower or upper boundsof the optimal solution in each sub-regions.

Two main types of approximate methods are:

- Heuristic methods bases on logic and common sense, where, at each step of the procedure, one element of the solution is chosen according to a given criteria, forexample, positional weight (the summation of the task time and the processing timesof all its successors), maximum task time, maximum total number of successors,minimum earliest and latest workstation and minimum slack, etc., until a completesolution is obtained. The simplest method randomly generates solutions, evaluateseach one of them and keeps the best of all solutions obtained. The main drawback ofclassical heuristic methodsis a failure in a local optimum.

Metaheuristics methods- have been developed to overcome the previous limitations.These procedures are developed to find an initial solution and local search algorithms tomove to an improved neighbor solution. In contrast to local search approaches, metaheuristicsmethod do not stop when no improving neighbor solutions can be found. They allowmovement to worsening solutions in order to avoid premature convergence to a localoptimum solution.

During the past decades numerous optimal approaches have been developed to solve ALBPwith different characteristics, including parallel, U-type, mixed-model, two-sided, etc.

Anyway, the assembly line consists of series of work stations, in which particular operations(set of tasks) are executed repeatedly. The objective of ALBP is toassigns the set of tasks tosuccessive workstations in order to minimize the number of workstations needed for a givencycle time or maximizing the productivity [6]. Task assignment is one of the most decisionsin assembly line balancing [7]. However, assembly work mainly involves activities such asfitting, adjusting, handling and quality control. The physical stresses thereby arising inautomobile assembly are caused mainly by unfavourable postures, application of forces,manual materials handling and repetitive tasks. These are complemented by stresses on thesensory organs and nerves during performance of test procedures. Vehicle geometry isresponsible for forced working postures in the form of, for example, (lateral) bending andtwisting of the trunk and extension of the arms. Other factors can include the need to apply


22/67

22

strong forces, unfavourable load-handling situations and extreme joint angles, in some casesaggravated by heavy stress on the finger-hand-arm system from application of strong actionforces and repetitive movements. Consequently, the stress accumulations occurring inassembly work are muscular (and cardiovascular), accompanied by biomechanical stresses onthe spinal column and unilateral dynamic stresses on the hand-arm system.

It seems possible that the physical stresses are at least a contributory cause of diseases andthat these could consequently be classified as work -related diseases. Manual materialshandling and application of physical force in association with unfavourable body postures arethe principal sources of stress in physical work, involving an increased health risk and

possibly higher absence t imes from work. The potential causes of this include stresses caused by work tasks, physical and chemical environmental factors and organizational and socialworking conditions, for example:

workplace design (e.g. space and movement-related factors, information flow and safetyaspects),

the work process in the broadest sense (i.e. everything included in the definitions of micro-and macro-logistics in both the immediate workplace environment and the whole building or

plant),

stresses caused by noise, climatic conditions, lighting, dampness, drafts, mechanicalvibrations and noxious substances,

degree of freedom in planning work actions, responsibility etc., and also cycle times, shifttimes and shift plans,

work climate,

remuneration system and other job-related financial factors [8].

2. ERGONOMIC APPROACH

2.1. Effective ergonomics

Ergonomics is an applied engineering discipline used to design the work environment toreduce injury and illness and support human performance.

A successful ergonomics process can be defined as one that is sustainable, business driven,and injury-reducing. Thus, the three critical elements of an effective ergonomics process have

been identified are:

Risk reduction strategies: A proactive approach to ergonomics means that factors

known to contribute to injury or illness are identified and addressed early.


23/67

23

Observation is the most basic form of assessment, which by the aid of assessment tools,firstly including the identification of risk factors, and then implementation of thesolutions. The goal is to capture existing insight and information from the work force,and then quickly move to resolving problems. To take advantage of this simple

approach to risk assessment, companies provide basic ergonomics awareness trainingand develop a process for employees to identify known challenges and providesuggestions. Providing a simple mechanism (like a feedback form) can ensure that allinput is captured and reviewed [9].

So, the three basic steps of risk management are: recognition, evaluation, and control(REC). REC is a systematic approach that must involve a cycle of measuring,improving, and re-measuring to maintain a safe environment. The REC approach issimple and effective: identify the challenge through basic awareness, evaluate thedegree of risk, and then implement control strategies.

Those ergonomic risks that are not easily resolved require analysis with a higher levelof knowledge, usually from a specifically trained group of employees, engineers, orfacility managers. The assessment tools must provide quantitative data for identifyingand prioritizing ergonomic risks, and they should identify alternative jobimprovements. They must be simple enough to be used reliably by health and safety

professionals, engineers, and ergonomics committee members, but they should provideenough information to be useful. Efficient assessment tools help users to obtaininformation needed to make decisions with as little effort as possible. A structured riskfactor survey or office needs questionnaire is usually the best approach. Some

assessments make pre-approved, easy-to-implement solutions available, while othersmay require more advanced control strategies and an in-depth analysis to reduce risk[9].

The element fix once, repeat many or FORM, is applied in avoiding the designamnesia. Namely, many companies are focused just in fixing the existing ergonomics

problems, ignoring the future. This is known as design amnesia, a vicious circle inwhich known problems are replicated in new designs even after effective solutions tothe problems have been defined and implemented. Since the designers have a greaterunderstanding of technical requirements and not in people requirements, the designamnesia often occurs.Exclusion of the design amnesia is possible by the application of two simplemechanisms: education for the specialists to recognize ergonomics problems and easyaccess to the effective design solutions.With the application of FORM strategy, products, manufacturing equipment, andoffice workstations optimize human performance, and one can prevent designmistakes from the start. Design requires informed decision making to formulate the

best solution while balancing a wide range of trade-offs. The ergonomic quality of thedesign is often overlooked for those who must use the product or equipment. This is

simply due to the lack of education for technical personnel in the principles ofergonomics and human performance [9].


24/67

24

Management of human capital is the third element of effective ergonomics.This element quantifies the bottom-line impact of improvements based on a healthyworkforce and improved productivity. With a good design, people perform moreefficiently and reliably and are less likely to get hurt. Too many business managers,however, the benefits of good ergonomics are not so clearly defined.Active support from senior management is critical to the success of an ergonomics

program, so the managers should consider ergonomics as:- A good investment the investment in ergonomics is justified by the reduction ofthe workers compensation cost.

Consider ergonomics exactly like any other important business function [9].

2.2. Goals of ergonomic

Specifically, some of the goals of ergonomics are:

Applied in the equipment design, as for the workplace, intend to maximize productivity by reducing operator fatigue and discomfort. Benefits provided byergonomics application in assembly systems design are first of all linked to the

reduction in occupational injury risks and in the improvement of health condition andmotivation of human resources. To accommodate differences in strength and body size among different workers. To remove barriers to productivity and performance. Continuous improvements and breakthroughs.

The reduction of injuries and illnesses related to inadequate workplace design requires a riskmanagement approach combined with the application of engineering solutions [9].

Risk management begins with the identification and measurement of workplace ergonomics

risk factors. After risk factors in the workplace are well understood, equipment andworkstations can be redesigned to minimize exposure to the risk factors. The application ofengineering solutions to reduce exposure to ergonomic risk factors requires a two-prongedapproach: retrofit solutions are applied to existing work stations and equipment, and thedesign of new workstations and equipment encompasses ergonomic criteria (fit once repeatmany) [9].


25/67

25

2.2.1. Continuous improvements and breakthroughs

In order to optimize the processes, ergonomics should rely on continuous and breakthroughimprovements. Continuous improvements are inexpensive, incremental changes, and made ona daily basis in order to drive out waste in operations.

The power of continuous and breakthrough improvements through the application ofergonomics principles could be seen in the case of vehicle assembly plants of Toyota inGeorgetown, Kentucky, United States. Namely, the plant faced with significant designchanges between two model years, resulting in potential ergonomics risk and inefficiencies,

particularly with the rear spoiler installation. The company hired a consulting firm to conductan ergonomic risk assessment, work measurements analysis, and make recommendations forimprovements [9].

The job process included nine specific detailed components, ranging from removing the trim panels, to attaching the spoiler to a final check and cleanup.

The ergonomics teams risk assessment and operator interviews identified significantergonomic risk. The operation, which required workers to climb inside the trunk, includedsuch risks as extended reaches, deviated postures, and mechanical stress to numerous body

parts. The team also performed a work measurements analysis of the entire operation. Thecycle time of the operation totaled 20.4 minutes, with drilling, trunk preparation, and clean upaccounting for more than half of that.

The easiest of the recommendations was to decrease tool reach and design new hand tools forthe wire harness installation. Tools were removed from the suspension system, placed in amobile tool cart, and angled 15 degrees, reducing the awkward postures and extendedreaches, resulting in reduced cycle times. A newly designed tool allowed operators to installthe harness from outside the trunk, thus eliminating wasted motions, reducing ergonomicrisk, and saving1.8 minutes [9].

The recommended breakthrough improvements included changing drill bits and designing anadditional trunk part. By switching from a standard drill bit to a multi-flute end mill bit,operators were able to drill a hole through multiple layers of sheet metal and eliminate theneed to vacuum up metals havings. This recommendation significantly reduced awkward

postures and resulted in a time savings of 8.9 minutes.

Due to the added weight of the spoiler, the trunk lid would not stay open, which resulted inthe operator climbing into the trunk to replace the torsion rods with higher tension rods.Rather than replacing these rods, they designed a compression spring that was inserted into

both hinges of the trunk, increasing the force required to closethe trunk lid. This eliminatednon-value added motions and wasted material, significantly reduced ergonomic risk, andresulted in a savings of 2.0 minutes.

The impact of these changes resulted in greater operation efficiency and time savings at thehighest-risk operations. Toyotas $9,000 investment resulted in increased through put and


26/67

26

less waste. The task cycle-time was reduced from 20.4minutes to 7.7 minutes, a 62 percentreduction, translating into a projected annual savings of $262,000 in direct labor cost.

Long-term savings include decreased medical and workers compensation expenses, lowerabsenteeism, and reduced training of new associates [9].

2.3. Work related musculoskeletal disorders

Several work activities, in particular those associated with repetitive movements and withconsiderable level of stressor with extended assumption of uncomfortable posture,might becorrelated to the insurgence of Muscolo-SkeletalDisorders (WMSDs= Work related Muscolo-SkeletalDisorders) [10].The lowback, neck, shoulders, and upper limbs are the body partsmost subject to risk (e.g., Roquelaure et al., 2006;Viikari-Juntura &Silverstein, 1999).Among the factors associated with the risk for developing work-relatedMSDs are individual,

physical workplace, organizational,and psychosocial factors, like repetitive motion, forcefulexertions, vibration and awkward postures. [11,12]. Figure 2.1 illustrates a systems model forconceptualizing the various elements of a work system that may affect musculoskeletalhealth. Working conditions can exert loads on workers which can lead to musculoskeletalstrain. In this model, these various elements interact to determine the way in which work is

carried out, and the effectiveness of the work in achieving individual and organizationalneeds and goals. At the center of this model is the individual with his/her physicalcharacteristics, perceptions, personality and behaviors.

The individual has technologies available to perform specific job tasks. The capabilities ofthe technologies affect performance and also the workers skills and knowledge needed for itseffective use. The task requirements also affect the skills and knowledge needed. Both thetasks and technologies affect the content of the job and the physical demands the job puts onthe person. The tasks, with their technologies, are carried out in a work setting that comprisesthe physical and the social environment.


27/67

27

FIGURE 2.1: Conceptual model of the development of work-related musculoskeletaldisorders

There is also an organizational structure that defines the nature and level of individualinvolvement, interaction and control, and which determines workload and working hours. Allof these work considerations combine to define the loads on the person. It is the personscapacity to deal with these loads that produces misfit that can lead to musculoskeletal strain.The ability to ` balance these loads determines the extent of misfit and potential strain [12]. Itis known that these kinds of disorders involve high costs linked to absence, medicalinsurance, and rehabilitation, Figure 2.2. In addition to the overall increase of WMSDs, there

is substantial evidence that ergonomics improvements result in financial gains for companies[11].

FIGURE 2.2: Cost analysis by occupational injury and illness types in Canada

Therefore, the prevention of musculoskeletal disorders in the workplace often focuses onevaluation of jobs in terms of physical job requirements and the development of solutions toreduce the frequency, duration and / or intensity of the biomechanical stressors during work.

As it was previously mentioned, ergonomics job analysis (EJA) is performed systematically:


28/67

28

- Identify hazardous work situations that present peak exposures (e.g., extremely heavylifting);

- Determine whether exposure is beyond conventionally recommended levels (e.g.,working in knee ling postures for more than 2 hours per 8-hour shift);

-

Rank order jobs for intervention;- Evaluate whether interventions actually reduce exposures associated with jobs.

Common questions that arise in ergonomics practice include:

- Is greater than 50 lb ( 23 kg) ever handled during a job?- Do workers in a job spend more than 25% of the work time in trunk flexion?- Which tasks or jobs are the most hazardous?- Did the lifting aid reduce the occurrence of manual materials handling or awkward

body postures?

To obtain exposure information on various physical and environmental factors of a job,observational approaches either in real-time or video are often used. Such approachesallow many potential risk factors to be evaluated quickly; are generally less costly thaninstrumentation methods used in research; and may reduce the risk for information bias thatappears to be present in self-reporting of ergonomics exposures.

One popular observational job analysis approach involves the identification or evaluation ofergonomics risk factors from video recordings. The advantage of this approach overobservational assessments made in real time is that videos can be replayed, or played in slow-motion or freeze-frame for a more careful assessment of exposures. This may reduce the riskof misclassifying exposures. On the other hand, video-based observations are generally moretime consuming than other observational approaches [13].

Many ergonomic measures reduce the exposure to musculoskeletal disorder risks. It isdifcult, however, to determine the effectiveness in reducing the actual MSD rates, because:

There often is no discrete onset,

- The exposure and morbidity history accumulated before the intervention may stillaffect outcomes after the intervention,

- The recurrence of an earlier episodeafter the intervention may cloud the benets, and The number of workers involved is often too smallto provide statistical evidence [11,12]. A

primary purpose of workplace interventions to control musculoskeletal disorders is to reducethe stress load to eliminate strain. As discussed below, this can be done by modifying theelements of the work system shown in Figure 2.16. Another tactic to control musculoskeletaldisorders is to increase the capacity of the individual to handle greater loads, therebyreducing the possibility of a misfit. There are a variety of actions that have been applied inthe workplace for eliminating or reducing the occurrence of occupational musculoskeletaldisorders. These include engineering redesigns, changes in work methods, administrativecontrols, training, organized exercise, and using personal protective equipment to reduceexposures. In addition, work hardening and medical management are used to improve the


29/67

29

capacity of the person, or to treat symptoms. Some of these actions have been evaluated inresearch studies using both laboratory and field settings [12]. An example for the workplaceinterventions would be a replacing the large tables with two parallel workstations, where themajority of the required assembly steps are performed by two operators, and a third station inthe series, where the products are finished and packed by one operator (Figure 2.3). [11].

FIGURE 2.3. Former batchwise production (left) and one-piece flow production (right) infinal assembly.

All three workstations, with the main elements are presented in Table 2.1 were ergonomicallywell designed.

TABLE 2.1.Former and New Situations in Final Assembly and Print Assembly and the Main

New Elements


30/67

30

In this case, it was considered that the decrease in physical load due to the variety ofworkstation adjustments to be significant, and on that basis it was estimated that up to30% ofthe work-related MSDs might be prevented. Prior to the intervention the sick leave

percentage of 7% was observed, while after the intervention, it was decreased to 6.6 %. As a

result of these changes, the company made a comparison between the costs made to replaceworkers because of sick leave before and after the intervention. It was observed that prior tothe intervention, each worker under observation) had 1 day of overtime per month tocompensate for sick leave for the entire population of workers, but after the intervention theneed for overtime was totally eliminated. This equals 50,558 of annual savings due toreduction of number of sick leaves (based on gross salaries and a 30% overtime bonus). Atthe same time not just the cost-effective benefits are observed, but the increased productivityas well. Productivity increased significantly both in print assembly (by 20%) and finalassembly (40%) [11].

2.3.1. Standing and walking posture during assembly operations

Standing is a common working position required by most occupations. Standing with inconfined areas for extended periods of time intensifies musculoskeletal fatigue and bodydiscomfort. During the last two decades, several health problems connected to prolongedstanding have been reported in literature. These include lower extremity fatigue, pain,

swelling and discomfort, venous blood pooling, low-back pain, and whole-body fatigue. Highincidences of low-backpain have been associated with prolonged standing of 4 hours or more per day. Prolonged standing can also cause the joints in the spine, hips, knees, and feet to become temporarily immobilized or locked [14].

This immobility can subsequently lead to rheumatic diseases due to degenerative damage tothe tendons and ligaments Machine operators, assembly-line workers, and others whose jobsare characterized by prolonged standing commonly report these problems. Ryan (1989)reported high incidences of lower extremity pain and discomfort among checkout personnelof supermarkets, who are confined to stand in restricted areas. The impact of such amplified

fatigue, discomfort, and musculoskeletal disorders may lead to reduced productivity apartfrom other physiological and psychological stresses [14].

Various solutions to reduce these problems have been proposed in literature. Most studieshave relied on ergonomic measures such as flooring and external aids such as mats, shoes,and sit/stand chairs. Standing posture is another influence able measure that has not receivedmuch attention. This study seeks to find if a dynamic standing posture produces less fatigueand more comfort than a stationary standing posture. Dynamic standing posture is anergonomic posture in which a worker intermittently walks while he is on the job. Itsstationary counterpart, stationary standing posture, is one in which the worker does notengage in walking breaks. A dynamic standing posture reduces the amount of time spentstanding within a confined area, by engaging the legs in an intermittent motion within a wide


31/67

31

floor space. Several medical problems have been associated with long-term standing and aneffective standing posture may offer a viable solution [14].

Fatigue related to standing has been quantified by objective measures, includingelectromyography muscle responses, performance measurements, skin temperature, andvenous pressure. During prolonged standing tasks, postural movements are performed todiminish discomfort caused by psychological and physiological factors. Adjustments to body

position are performed naturally to maintain comfort. These displacements are performedunconsciously and cannot be associated with any external source.

Center of pressure (COP) is a common variable to study postural changes. COPdisplacements, which follow a fractal stochastic process, have been shown to correspond tothe responses of the postural control system. Frequency of postural changes indicates fatiguesince they are performed to avoid discomfort and fatigue. A positive relationship fromneuromuscular fatigue to posturals ways has also been reported. Balasubramanian et al. [14]during their study used COP extracted features to evaluate fatigue.

While standing, the weight-transmitting area of the foot (ball of great toe and heel) iscompressed and deformed by the overlying pressure. This could inhibit the supply of blood,resulting in a deficiency of oxygen to tissue, and could cause discomfort or fatigue. For thisreason, the quantum of foot pressure or the pressure distribution at the foot/floor interfacemay be used as an objective indicator of comfort or fatigue. To analyze foot pressure, twoevaluation parameters of pressure were considered: (a) contact area as a function of pressurerange, which is a time independent measure of pressure; and (b) area pressure change root

mean square (aPcrms), which analyzes the dynamic behavior of pressure. aPcrms is also anindicator of comfort index (CI). Both these parameters were used to compare the CI betweenthe two standing postures. A positive inuence of the dynamic standing posture on standingcomfort can be used to improve industrial productivity [14].

A significant and positive difference between dynamic and stationary standing duringassembly work could be used to improve workplace ergonomics, industrial productivity, andoccupational safety. These results may also influence the ergonomic design of shop floors,where in a particular job, such as assembling, may be distributed. The inferences can begeneralized to all types of jobs where a prolonged standing posture is in practice [14].

2.3.2. Upper Extremity Assessment Tools

American Conference of Industrial Hygienists (ACGIH) is an association which is committedto prevent workers from occupational diseases. Hand activity levels (HALs) are introduced

by the ACGIH for mono- task jobs performed longer than 4 hours per day. Task whichrepresents duty in ergonomics is different from task representing the basic indivisibleelement in ALBP. Workers who repeatedly perform the same exertions every work cycle are


32/67

32

considered as acting mono-task by definition [7]. HAL are offered for assessing theWMSD risks in such cases. The scale for HAL was proposed by Latko et al. [15].

and this scale range is from 0 to 10. In this particular scale, 0 represents completely idleand 10 stands for the greatest level of continuous exertion. HAL is not only a function offrequency but also a function of work speed. There are two ways to determine HAL:

1. HAL can be obtained from Figure 2.4, which is based on the frequency of hand exertions,the idle time of hand exertions and the speed of motions and

2. HAL can be calculated from Table 2.2, which contains the exertion frequency and dutycycle.

FIGURE 2.4: Hand activity level (0-10) can be rated using the guideline

TABLE 2.2 Hand activity level (0-10) is rated based on exertion frequency and duty cycle

(% of work cycle where a workers force is greater than 5% of the maximum) [16]


33/67


34/67

34

experience and laboratory experimentation derived primarily from subjective human responseto hand-transmitted vibration and mechanical behavior of the hand-arm system. Exposurezones for hand-transmitted vibration are recommended for regular daily occupationalexposure over long periods of time in terms of acceleration measured in one-third octave

band spectra.

Accurate estimation of daily vibration exposure for individual workers on the production lineis difficult because of the highly variable operating techniques used between operators andanomalous contributors common to this operation such as cross threaded screws and back-upof co-workers who have fallen behind. Radwin and Armstrong [24] assessed vibrationexposure by sampling directly the vibration produced from the specific tools of individualoperators and prediction of daily exposure using in-plant data. They investigated the factorywhich produces small electromechanical appliances [24].

Twenty three workers were employed in the assembly operation where seventeen of these positions required the use of pneumatic screwdrivers. One position necessitated operation ofo two screwdrivers. The production rate was 2.9 units per minute which was below averagewith a production rate of 3.2 units per minute considered to be typical for the line. Fiveworkers were new and inexperienced. Three additional workers were positioned on the line to

back up the trainees.

The screwdrivers used were in-line pneumatic screwdrivers measuring 21.6 cm long, 2.5 cmin diameter, weight 450 g. And are push-to-start, torque controlled tools designed to startwhen the bit or socket is depressed and stop when this pressure is removed. A clutch

mechanism causes the rotor to slip when a screw is tightened to the set torque. The torque isin the 0.9 nm range and the tool was operated at 620 kPa air pressure [24].

FIGURE 2.5: A representative work station. Workers seated around a conveyor load theirscrewdrivers with screw scattered along a tray with a perforated bottom situated along side ofthe workbench. The loaded screwdriver is moved into position over a threaded hole with onehand while a wire with a crimped lug is positioned under the screw head with the other hand

and screw installed.


35/67

35

Like most air powered screwdrivers, these are supplied with bare metal handles.Consequently many workers cover their handles with adhesive tape in an attempt to make thehandles more comfortable. Most of the workers on the line wear light cotton gloves with thefingers removed. Many also cover their fingers with tape [24].

Figure 2.5 illustrates a representative work station. Hand and waist posture is not a problemsince workers operate the in-line tools on a horizontal surface at elbow height while seated ata workbench, where the bench height is 97 cm from the floor. Two experiments were

performed to assess the vibration exposure hazards. The first was an in-plant study toestimate the quantity of vibration exposure for each worker. The daily eight hour exposurewas predicted from observed vibration samples for each position on the assembly line. Thesecond experiment was a laboratory study to assess the quality of vibration exposure.

TABLE 2.3: Mean and Eight Hour Vibration Exposure During Run and Clutch Cycles forPositions Requiring Operation of Power Screwdrivers


36/67

36

The average running and clutch slippage time (Table 2.3) were computed for each workerwhose position required operation of a pneumatic screwdriver on the line. The number ofobservations ranged between 16 and 336 depending on the job of the individual operator. Thescrewdriver run time included engagement of the screwdriver during pick-up, screwdriveroperations and operation of the screwdriver when screws were defective (requires reverseoperation of the screwdriver to remove the defective screws).

The time that the screwdrivers operate in the run and clutch phases, are dependent on severalindividual operator factors. Operators load their screwdrivers by pressing the tool over ascrew which usually activates the push-to-start pneumatic motor [24].

The total eight hour exposure to vibration for workers on the line during the running portionof the screwdriver operation cycle ranged between 2.6 and 51.8 minutes (Table 2.3). The total

eight hour exposure to vibration produced during clutch slippage ranged between 1.2 and14.4 minutes.

Six of the 17 positions (positions 4, 5, 6, 7, 8 and 9) (Table 2.3) approached or exceeded 30minutes of total exposure to vibration produced during running. These jobs involvedinstallation of the greatest number of screws on the line. The position 14B required theinstallation of a screw that was longer than the others, so the mean exposure time in the runcycle was higher (258 ms).

FIGURE 2.6: Center frequency of third octave bands


37/67

37

One-third octave band recordings for the x, y, and z axes in the run and clutch screwdriverand in the run phases for the automatic shut-off screwdrivers, shown in Figures 2.6, wherewithin the ISO proposed two to four hour exposure boundaries. The greatest one-third octave

band frequency component was at 31.5 Hz. The maximum predicted eight-hour exposure for

the workers observed during the run phase was less than one hour. Thus, all the workersobserved were subjected to vibration within the ISO guidelines for vibrations producedduring the run phase of the screwdriver operation cycle [24].

Accurate estimation of worker vibration exposure is necessary when comparing alternativetool vibration signatures and using the ISO guidelines as a bench mark. Since hand tooloperation is subjected to a high degree of variability in many types of assembly operations,the need to implement vibration exposure measurements by direct sampling is indicated toaccurately estimate vibration exposure.

Zhan Xu [7] and his collaborators, in their research applied a new approach in order toimprove the conventional practice by which design an assembly line and reduce ergonomicrisks. A study was conducted to design an assembly line for consumer electronics appliance(blender) assembly. Their research develops more explicit integration of task assignment andergonomic measures using linearized formulations, which enables not only to integrateergonomic task characteristics directly but also to use efficient solution methods to optimizeassembly line design, in particular for upper body extremities. Moreover, these linearizationmethods can generalize ergonomic measures in table listed forms and incorporate them intoother assembly line design problems [23].The linear models developed in the research performed by Zhan Xu [7] help reduce WMSD

risks. The ergonomic measures incorporated with task assignment can be used to limit the peak force and vibration exposure levels of upper body extremities in all stations in anassembly line. The developed methodology can be extended to incorporate a variety ofergonomic characteristics of assembly tasks. This feature provides flexibility to considerother ergonomic constraints during assembly line design. Hence, the research by Zhan Xuwill help to prevent WMSD risks among assembly workers.

So, their attempts [14] were:

- To develop a methodology that incorporates hand activity and hand-arm vibration

measures into the task assignment models in assembly line design. In particular, to build linear functions integrating the exert ion frequency, duty cycle, normalized peakforce (NPF), vibration acceleration and vibration duration measures into the lineartask assignment model. Next, these linear models were integrated for assembly linedesign by using mixed-integer programming (MIP).

- To demonstrate the feasibility of the modeling methodology to identify the impact ofthe ergonomic consideration on assembly line design in terms of task assignment, andexamine the trade-offs between productivity and ergonomic conditions. The modelsdeveloped in this research will help control WMSD risks with reducing possible

negative impact on line efficiency.


38/67

38

The overall structure of the research was conducted as it is shown in Figure 2.7

First, the physical exposure in an assembly line for this research consists of five ergonomiccharacteristics: peak force, number of exertions, duty time, equivalent accelerations ofvibration and vibration duration. A guideline for industrial hygienists concerning physicalexposures of operators [16] was used for building the equations for the ergonomiccharacteristics. These numerical ergonomic representations mainly focus on upper bodyextremities.

Second, these ergonomic formulas are created as linear functions of task assignment. Theselinear formulas allow the easier integration of the ergonomic measures into linear assemblyline design models.

Third, an assembly line design model is built by incorporating the linearized ergonomicmeasures and conventional assembly line characteristics into an MIP model. The

conventional assembly line characteristics include task precedence, cycle time and thenumber of work stations. The objective function of the MIP model is to minimize the numberof workers in the assembly line.

Fourth, through numerical experiments, this research analyzed the effect of differentergonomic considerations by solving the MIP model with different combinations ofergonomic constraints. This analysis demonstrated the effectiveness of the new integratedapproach compared to a conventional assembly line model without ergonomic considerations[7].

FIGURE 2.7: The structure of the research


39/67

39

3. THE RELATION OF TASK ASSIGNMENT TO EXERTIONFREQUENCY, DUTY CYCLE, NORMALIZED PEAK FORCE,VIBRATION ACCELERATION AND VIBRATION DURATION

This chapter describes how to incorporate task assignment and upper extremity ergonomicmeasures by equations.

3.1. Determination of hand activity and peak force measures by taskassignment

Task assignment is the procedure of dividing the workload needed for assembling one product into several elements and distr ibuting them among work stations, and strongly affectsindividual workers ergonomic condition. Every task possesses ergonomic characteristicsaffecting the hands exposure levels such as the number of exertions, duty time and peakforce. Because a workers job in an assembly station is usually a set of several indivisibletasks, different task assignment (different sets of tasks assigned to each worker) results indifferent number of exertions and duty times for each worker.

The relations of the exertion frequency and duty cycle to task assignment are formulated aslinear Eqs. (1)-(2), respectively. In his research, Zhan Xu [7], assumed that one worker

charges one work stations job, and does not share the job with other workers. Thus, a workeris equivalent to a station in terms of task assignment, and they are used inter changeably inthis research. All tasks are performed in standing posture.

(1) (2)

Equation (1) represents the exertion frequency at hand h of worker s, defined as the numberof exertions per time unit. Eq. (1) expresses the sum of the numbers of exertions of all tasksassigned to workers divided by the cycle time of the station. In the equation, xis is the task-station assignment variable, hine is the number of exertions of task i, hand h, CT is the cycle

time in the assembly line. Thus, h sef represents the exertion frequency in station s, hand h.

Eq. (2) represents the duty cycle at worker s hand h, defined as the ratio of total time of duties

in a work cycle. Eq. (2) expresses the total duty time of all tasks assigned to workers divided by the cycle time of the station. In the equation, xis is again the task station assignment


40/67

40

variable, hidt is the duty time of task i hand h, CT is the cycle time in the assembly line. Thus,h

sdc represents duty cycle in station s, hand h. In these equations, number of exertion ( h

ine )

represents the number of energy costing motions, and duty time ( hidt ) is the duration where

the worker exerts more than 5% of the maximal force [16]. Both of them are measured basedon single assembly task.

Peak force is also an important factor contributing to WMSD risks [25-27].

A workers maximal force performed in one repeated job is represented as peak force and usually normalized as a dimensionless measure to fit the general population [28, 29]. If taskswith large amount of exertions and heavy normalized peak force (NPF) are assigned to aworker, this workers WMSD risk could be considerably high. The normalized peak forcelimit is related to the combined level of the exertion frequency and duty cycle (expressed ashand activity level at (Table 2.2)), and this relationship is explained below. In Figure 3.1, thehorizontal axis represents the hand activity level (HAL) and the vertical axis the NPF.According to a variety of workers strengths, an action limit (AL) is recommended as a safe

bound. One expression of the AL was proposed as shown in Eq. (3) [30] and is shown as thedashed line in Figure 3.1. The combination of HAL and NPF levels should be below the ALline. Thus, NPF is restricted based on the HAL as shown in Eq. (4).

FIGURE 3.1 The TLVs based on HAL and NPF.


41/67

41

(3)

(4)where h snpfl = NPF limitation of station s and hand h,

h shal = HAL value of worker s and hand

h, and h snpf = NPF value of worker s and hand h.

3.2. Determination of hand-arm vibration measures by task assignment

As we can see from the previous chapter vibration duration and vibration equivalentacceleration are also related to upper extremity WMSDs.

The daily vibration duration and its relation to task assignment are given in Equation (5).

(5)

Eq. (5) represents the sum of the vibration duration of all tasks. In the equation, xis is the task-station assignment variable, vt i = the duration of vibration of task i, NC is the number of workcycles per day in the assembly line, defined as the total daily working time divided by thecycle time. Thus, s TD represents the daily vibration duration in station s.

The frequency-weighted, equivalent, component acceleration of a set of tasks is expressed byEq. (6), and the largest equivalent acceleration among three directions is considered as thedominant acceleration used to evaluate the vibration level [16].

(6)

where o seqak , = the equivalent, frequency-weighted component acceleration of station s,

direction o, oiak = the frequency-weighted, rms acceleration of task i, direction o, TDs = the

total daily exposure duration of vibration of station s, and NC = the number of work cycles aworker completes per day [16]. Just for now, note that Eq. (6) is not a linear function of task


42/67

42

assignment. Because the vibration data of each task are measured in the realistic taskoperation, the data usually include the impacts of grip forces on vibration. Also, the effect ofstatic grip force on vibration is known not significant [31]. Thus, the TLVs of HAL andvibration acceleration can be evaluated independently [7].

The vibration equivalent acceleration TLVs are shown in Table 3.1. TLV for acceleration isthe limit of the largest acceleration in all three directions. Any equivalent vibrationacceleration in the station should not exceed the TLV. TLVs vary by the daily vibrationdurations in the station as shown in Table 3.1.

TABLE 3.1: TLVs for exposure of the hand to vibration in either X, Y, Z Axis [16]

* The total time vibration enters the hand per day, whether continuously or intermittently.

** Typically, the accelerations of one axis show dominion to those of the other two axes.

*** g = 9.81 m/s 2

4. OPTIMIZATION MODEL

This section describes the optimization model for assembly line design. This model is tominimize the number of workstations while considering production rates, hand activity andhand-arm vibration. The main decision variable represents the task assignment tostations/workers. The models include two types of constraints: ergonomics and conventionalassembly line design constraints. The ergonomic constraints include hand exertion frequency,duty cycle, normalized peak force (NPF), vibration acceleration and vibration duration. Theconventional assembly line design constraints include cycle time and task precedenceconstraints.

The optimization model is presented as follows.


43/67

43

Min z (15)

Subject to

(1)

(2)

(7)

(8)

(9)

(10)

(11)

(12)


44/67

44

(3)

(4)

(5)

(14)

(17)

(18)

(19)

(20)

(21)

The explanations of the models are as follows. The objective function, Eq. (15), is tominimize the number of workers (workstations). Constraint (1) indicates the exertionfrequency is derived from dividing the total number of exertions by the cycle time. Constraint(2) shows the duty cycle is linearly dependent on the duty time of each task. Constraints (7)can determine Table 4.1 s row index based on exertion frequencies, and duty cycles candetermine Table 4.1 s column index by Constraints (8). Constraints (7) (8) are explained asfollows: The equation (7) gives the row number ( ) in Table 4.1. The equation (8) relatesthe duty cycle ( ) attained from task assignment and the corresponding representative dutycycle used in the look-up table (Eq. (8) and Table 4.1). This equation gives the column

number ( ) in Table 4.1. Refer to Table 4.2 and Table 4.3 for the parameters in theequations. [7]


45/67

45

The row and column numbers obtained by Eqs. (7)-(8) are used to select the correspondingHAL value. The linear equations (9)-(10) determine value of the HAL at each station usingthe row and column numbers from Eqs. (7) and (8) and the HAL value table (Table 4.1). A

binary variable is defined to indicate the cell position in the HAL table. If the cell u isselected for station s then Then the equations (9) (12) determine the value of .Eq. (9) determines the address in the HAL table (Table 4.1). Variable alone can representthe addresses of HALs. Equation (10) ensures only a single cell is selected from Table 4.1.Eq. (11) finds the only cell in Table 4.1 that should be used ( u for which should be one).Eq. (12) assigns the corresponding HAL values using the value determined by Eq. (11).HT u represent the value of HAL when in Table 4.4. The linearization of theconventional look-up table based procedure is possible by converting the two-dimensionalrelations shown in Table 4.1 and development of new equations. [7]

TABLE 4.1: Reference table of HAL with m = 5 [16]

ACGIH TLV is applied to set the limits of ergonomic measures in this research. The tableconsiders the worst case for each unreachable entry of Table 2.2, so the results may beconservative. Such cases, however, will be rare.


46/67

46

TABLE 4.2: The set of ranges of exertion frequencies [16]

TABLE 4.3: The set of ranges of duty cycles [16]

TABLE 4.4: The conversion of intermediate value to HAL

Constraint (3) represents AL. Constraint (4) shows NPF of a worker should be always lessthan AL. Constraint (5) represents the total daily vibration exposure duration of worker s.

The acceleration limits are also shown in linear formulas. Eq. (6) is a linear form of Eq.(6)[16], defining the equivalent frequency-weighted, rms, component acceleration. Eq. (13)shows the equivalent accelerations of workers Should be smaller than the accelerationTLVs ( ). Equation (14) is derived from Eq. (6). Thus, constraint (14) ensures hand-arm vibration accelerations.


47/67

47

Constraints (17)-(20) are the traditional constraints in assembly line design. So called the

ghost task is defined so that the task requires all the other tasks should be done before it.Thus, the ghost is always in the last station. All characteristics in the ghost task are zeros, soit will not affect the solutions of models. Constraint (17) ensures that the last station containsthe ghost task for the precedence relation and the last station number is the total number ofassembly line stations (workers). Constraint (18) shows every task can only be assigned toonly one station once. Constraint (19) makes the sum of task times for each station under thecycle time. Constraint (20) makes sure the sequence of work stations does not conflict withtask precedence. This optimization function has the following characteristics. First, it is amixed-integer linear program. Thus, (1) the linear form of the constraints allows faster

calculation than other non-linear constraints found in the literature, (2) the ergonomic datatables used for the constraints can be conveniently replaced with more sophisticated tables orother ergonomic measure tables, and (3) other linear assembly line constraints can beincorporated in this optimization program easily. Second, the computational complexity ofthe optimization formulation is not significantly higher compared to general assembly linedesign formulation. [7]

TABLE 4.5: The conversion of daily vibration to acceleration TLV [16]


48/67

48

5. NUMERICAL EXPERIMENTS AND DISCUSSION

To verify if the models effectively control hand activity and vibration received by the upper-extremities in assembly line design, a numerical experiment were conducted by theresearchers. The solutions from traditional model and ergonomics considered model arecompared [7].

5.1 Manufacturing Task Description and Parameter Estimation

Task Description

1. Placing Control Box on Body Roof by Automatic Shut-off Screwdriver2. Placing Motor Base on Body Roof by Automatic Shut-off Screwdriver3. Putting Magnet on Motor Base by bare Hands4. Placing Motor Top on Motor Base by Automatic Shut-off Screwdriver5. Fixing Motor Top on Motor Base by Clutch Screwdriver6. Connecting Wires to Control Box by Plier7. Fixing Moto Base to Body Roof by Clutch Screwdriver8. Connecting Wires to Moto by Plier9. Connecting Fan with Motor by Wrench10. Connecting Body Bottom and Body Feet by automatic Shut-off Screwdriver11. Fixing Control Box on Body Roof by Clutch Screwdriver12. Pasting Label by Hands13. Fixing Body Bottom by Clutch Screwdriver14. Fixing Top Rod by Hands

The numbers in the circles represent assembly tasks. 1 and 2 represent vibrationProfiles 1 and 2, respectively.

FIGURE 5.1 The task description and precedence of assembly tasks with vibration profiles


49/67

49

A case study was conduct to design an assembly line for consumer electronics appliance(blender) assembly. The assembly process consists of 14 assembly tasks (Figure 5.1).

The hand activity data for these tasks are estimated from the assembly task analysis in IMSE898 and vibration data are estimated based on a study by Radwin and Armstrong, and Xu andHao [24, 32]. All tasks are assumed to be performed in standing posture. Data related to HALand vibration are shown in Table 5.1. Normalized peak force (NPF) is obtained by themethodology introduced in part which is passed above based on Table 5.2. The peak forceapplied in this case study is limited to grip force. These data were created by analyzingassembly tasks. The data is used as a sample set of data to initiate the model testing. Themeasured times, and an estimate of the performance times as input data for the numericalexperiments, are used. The data, in this research, based on one subject, and multiple subjecttests were be used in a future study to generate recommendations for practical use. [7]

TABLE 5.1: Data of exertion number, cycle time and NPF

Task No.

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Task Time 9.3 23.8 3.6 12.7 11.1 30.9 17.1 16.7 18.9 11.5 14.3 10.3 15.4 18.6(Second)

Number of 3 6 2 3 3 3 5 4 6 2 3 3 3 6Exertions (R*)

Number of 1 2 2 1 3 3 5 4 1 2 3 3 36Exertions (L)

Duty Times 5.8 7 2.2 8.3 6.7 3.3 11.4 9.1 4.0 7.4 10.3 1.8 5.5 5.7(R)(Second)

Duty Times 1.8 1 2.2 8.3 7.0 4.8 10.0 7.1 0.6 7.0 8.8 1.8 5.0 8.0(L) (Second)

NPF of Task 4.3 4.3 2.6 4.3 4.3 4.3 4.3 2.6 3.5 4.3 4.3 1.7 4.3 4.3(R)

NPF of Task 2.8 2.8 2.8 2.8 2.8 1.9 2.8 2.8 2.8 2.8 1.9 1.9 2.8 2.8(L)

VibrationDuration 5 4 0 3 2 0 4 0 0 7 2 0 3 0(Second)

Acceleration 1 1 - 1 2 - 2 - - 1 2 - 2 -Profile

R and L represent right and left, respectively.


50/67

50

TABLE 5.2 Male power grip strengths in Newton (N)

Dominant (right) Non-dominant (left ) Subject age Population

463.5* 398.9 18-65 Office workers

532.1 474.3 18-65 Laborers

556.6 514.5 18-65 Skilled

589.0 532.1 18-65 Semi-skilled

Data with * mark represent the data used by Z. Xu during the research [7].

FIGURE 5.2 The accelerations and the coordinate figures of the two screwdrivers.


51/67

51

The acceleration profiles were determined as follows. Because pneumatic tools causeconsiderable vibration in assembly line processes, this case study assumed the use of

pneumatic tools. It is assumed that two types of pneumatic tools, Clutch Screwdriver andAutomatic Shut-off Screwdriver, are used during assembling processes. Profile 1 represents a

task placing screw using Automatic Shut-off Screwdriver. This task contains loweracceleration vibration but longer vibration duration compared to those of Profile 2. Profile 2represents a task using Clutch Screwdriver tightening screws. The profiles of tasks are shownin Figure 5.1. This task causes relatively severe vibration acceleration and short vibrationduration. The accelerations and the coordination of the two screwdrivers are shown Figure5.2. The diameters of the two screwdrivers are assumed the same. All accelerations arefrequency weighted based on ISO 5349 and are explained in Appendix A. [7]

5.2 Results from Different Combinations of Ergonomic Constraints

Four line design models (LBMC, LBMCH, LBMCV and LBMCHV) were compared. Thesemodels were different in terms of the different combination of hand activity and hand-armvibration constraints included in the assembly line design formulations. The acronyms for themodels are shown in Table 5.3. These assembly line design models were solved by acommercial MIP software package CPLEX (IBM) by Zhan Xu. [7]

TABLE 5.3: Acronyms for the line balancing models.

Model Combinations of Constraints

LBMC Conventional constraints only

LBMCH Conventional and hand activity constraints

LBMCV Conventional and vibration constraints

LBMCHV Conventional , hand activity and vibration constraints

The result summarized is as a schematic diagram in Figure 5.3, and the detailed task stationassignment is included in Table B.1in Appendix B. The exertion frequency, duty cycle, NPFand its TLV are shown in Appendix B. The vibration acceleration and duration is shown inTable B.5 in Appendix B. As shown in Figure 5.3, LBMCHV (the model with all hand

activity and vibration constraints) is the only case in which all ergonomic exposures are below the TLVs. This is due to the added constraints in LBMCHV.


52/67

52

FIGURE 5.3: Results from different constraint combinations

Figure 5.4 represents the HAL & NPF values and TLVs. Table 5.4 shows the relative performance of each task. Table 5.5 shows dominant accelerations and TLV. The detailedanalysis shows the violation of constraints in each case. For example, Figure 5.4 (a) and (c)show at least one of HAL values exceeded the AL line in the models not considering handactivity (LBMC, LBMCV). Figure 5.4 (b) shows that all HAL values satisfy the TLVs.


53/67

53

HAL TLV of LBMC

HAL TLV of LBMCH

HAL TLV of LBMCV

HAL TLV of LBMCHVFigure 5.4 HAL TLV of LBM

line balancing considering ergonomics.pdf

Documents