Proceedings 19th Triennial Congress of the IEA, Melbourne 9-14 August 2015

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Development and testing of a screening approach for the evaluation of forceful operations in industry

Schaub Karlheinz, Wakula Juri, Berg Knut, Oberle Marius

Institute of Ergonomics, Darmstadt University of Technology, D-64287 Darmstadt, Germany

In 2006 to 2008 the project „Force Atlas for Assembly Operations" was carried out (Wakula et al. 2009a, b; Schaub et al. 2014). Maximum isometric action forces of the whole body and finger-hand system were measured from male workers (n=273) in real working postures in assembly tasks. Based on the collected force data and an analysis of existing approaches (e.g. EN 1005-3), a “classical” force assessment approach for shop floor and planning analysis was modelled with respect to the state of art in ergonomics and the results of the own lab studies (Wakula et al. 2011). Different tasks of force exertions - homogeneous as well as heterogeneous ones can be analyzed using this approach. The modelling resulted in a paper and pencil method and an EXCEL spread sheet Practitioner Summary: With respect to the strong interest from the field of practice also a screening approach „forceful operations“ had been worked out and realized based on paper -pencil- solution and an EXCEL spread sheet in 2013 and 2014. This instrument allows the user to calculate a “score” instead of the “force index” from the “classical” approach. It derives maximum recommended force values from maximum isometric action forces taking into account specific task and worker related parameters. The approach was tested in major German automotive OEMs. Process engineers, specialists from health & safety and ergonomists gave positive feedback regarding the screening approach. Keywords: force exertions, screening tool, muscular-skeletal disorders (MSD), standards & guidelines.

1. Introduction Physical work has remained a wide spread phenomenon in many areas of life (industry, agriculture, private households, logistics), even in today's service-oriented society. Frequent complaints and impairments of the human musculoskeletal system, including occupational diseases; indicate the use of forces beyond a level of tolerability. National German approaches (e.g. Schultetus et al. 1987) and international “classical” approaches for the ergonomic evaluation of action forces (e.g. EN 1005-3) are of limited applicability in various branches (automotive, truck, aircraft and marine industries) where the geometry of the product imposes ergonomically unfavorable postures during force exertions – e.g., trunk twisting and / or bending, overhead or one-handed operations. However, most of the currently available force data were collected in upright body postures (Rühmann & Schmidtke 1992, Rohmert et al. 1994, DIN 33411-5). Therefore the research project "Force Atlas for Assembly Operations" was realized in 2006 to 2008 by the Institute of Ergonomics, Darmstadt University of Technology (IAD) and partners from industry. The project focussed on maximal static action forces of the whole-body for typical real-work life (two-handed) symmetrical postures as well as on forces of the finger-hand-arm system. Action forces encountered in industrial environments (nine automotive companies and the suppliers) were collected from a sample of 273 industrial male workers in Germany during 1.5 years (Wakula et al., 2009a,b and Wakula et al. 2011). The data and results from laboratory studies have been used for modeling the “classical” assessment approach (Wakula et al. 2011). 2. Model and Methods 2.1 Modeling the force evaluation tools The evaluation of action forces exertions have a long tradition in Germany (based on the work of Burandt, 1978 and Schultetus et al. 1987, described by Schaub et al. 1997) and also worldwide (e.g. EN 1005-3:2002+A1:2008, ISO 11228-2:2007, described by Schaefer & Schaub 2006, Schaub 2006).

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Whereas traditional German methods focused also on individual characteristics (age, gender and training status), CEN and ISO standards use a more statistical approach and focus on “intended user” or “general working” populations and their distributions in force capabilities. All approaches calculate a maximum recommended force limit. The ratio of the force to be exerted and the maximum recommended force limit serves a basis for assigning a risk zone according to the 3 zone rating system as described in EN 614-1:2006+prA1:2008 (see 2.2). Gender specific effects on maximal force capacities are described at various sources (e.g. Rühmann and Schmidtke, 1992; Peeble and Norris, 2003). In general it is assumed that female force capacities are about 2/3 of male. There is one special consideration to the whole body force directions of ±A (Fig. 2a, b). A relative maximum is present at shoulder height, whereas a relative minimum appears at elbow height. So some 20 cm difference in working height has a substantial influence on force capacities (see Schaub, Berg & Wakula 1997a). As 20 cm is a quite common deviation in body heights in between the 5th female and 95th male body height percentiles a factor of 0,5 has been used for a conservative estimation for forces in directions ±A (Rühmann and Schmidtke 1992). When performing risk assessments at shop floor level according to the EC Framework Directive on health and safety at work (89/391/EEC) and relevant Individual Directives (e.g. on manual material handling 2006/42/EC) more easily applicable methods are desired, in order to keep the effort needed at a considerable level. To solve the problem for an easy applicable screening tool, the Federal German Institute of Occupational Safety and Health (FIOSH) had created “Key Indicator Methods (KIM)” (“Leitmerkmalmethode” Steinberg, Caffier and Liebers 2006) to meet the EU legal requirements of the Manual Handling Directive (90/269/EC). The Assembly Specific Force Atlas picked up this type of modeling which is described by Wakula et al. 2009 and Schaub et al. 2014. For the individual characteristics data from Rühmann & Schmidtke (1992) and Peebles and Norris (2003) had been considered. The classical approach is realized as described in chapter 2.2; the screening approach is still being tested in the field and minor alterations are likely to occur (chapter 2.3). Both approaches support a multi tasking in order to meet the requirements for practical applications at shop floor level. For homogenous force exertions (similar levels and directions of force exertions) the total frequency of exertions and the average weighted force levels are considered. This approach is closely linked to the NIOSH 1981 “multi-tasking” (NIOSH 1981). For heterogeneous force exertions (different levels and directions of forces exertions) the most severe case is considered in total and the other cases are considered incrementally in descending order. This approach is closely linked to the NIOSH 1991 “multi-tasking” (Waters et al. 1994). The classical approach – as well as the screening approach– is used for the evaluation of quasi-static force exercises of arm-shoulder or full-body system (whole-body forces starting at 40 N) on the one hand and the hand-finger system (starting at about 30 N) on the other hand. The approaches are applicable for the action forces in short-cycles of approximately 1 to 6 min. or 8 minutes at maximum. Both methods may also be applied for longer cycle times, as long as it is ensured that the action forces are uniformly distributed across the working activities and that there are no force peaks. Ideally the time of force exertions should be in the range 3 - 4 seconds, but not be less than 1 second. In effect durations below 1 second have a significant dynamic component and the evaluation based on the Force Atlas would be too high. Both methods should not be applied for long static force exertions of about 6 s and above (Kroemer 1977). The evaluation of force exertions are based on the 3 zone rating system (traffic light model), as described in EN 614:1995 (see table 1). Table 1: Risk model based on ISO 11228-2 and partly modified for the Assembly Specific Force Atlas

Evaluation “classical” approach “screening” approach

Risk zone Force index FI = Factual / Fmax_rec.

Total score TS = score F x score n (force points times frequency points)

green FI ≤ 0,85 TS ≤ 25 points

yellow 0,85 < FI < 1,2 25 < TS < 50 points

red FI ≥ 1,2 TS ≥ 50 points

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2.2 Homogenous vs. heterogeneous force exertions

The approach is based on the following four step model: 1. Selection of mode of analysis: (analysis of a planned task / work place or analysis of existing task / work

situation; 2. Calculation of the maximum recommended whole body force Fmax_rec.

The maximum “recommended” whole body force (Fmax_rec.) takes into account specific task and worker related parameters and will be calculated using following formula:

Fmax_rec. = F max. (PP) x P1 x P2 x T1 (x T2 x T3) whereas: Fmax.: Maximal static action force of the whole body (from table 2a or 2b related to defined working

posture and force direction) ; PP: Force percentile

P15 for planning analysis; P40 for analysis of existing work stations;

P1: factor for the age; P2: factor for the gender; T1: factor for the frequency of force exertions related to the task; T2: “Biomechanics” factor (considers muscular efforts for asymmetric exertions; does not apply

to forces of the hand-finger-system); T3: “Physiology” factor (considers frequent exertions in awkward postures; does not apply to

forces of the hand-finger-system); 3. Calculation of force index – FI = Factual / Fmax_rec. for the “classical” approach or the “Total Score (TS)

for the screening approach 4. Derive risk zone based on EN 614-2:1995 and modified from ISO 11228-2 (see table 1).

In case of inhomogeneous force exertions Maximum “recommended” forces (Fmax_recommended) will be calculated using a formula based on model of Waters (1993) for multitasking manual materials handling.

Factual_ Forceful task _1 / (Fmax_ Forceful task _1 x T1 Forceful task _1new x T2 x T3 x P1 x P2) + (Factual_Forceful task_2new / (Fmax_Forceful task_2new x (T1Forceful task_1,2new – T1Forceful task_1new) x T2 x T3 x P1 x P2) + (Factual_Forceful task_3new / (Fmax_Forceful task_3new x (T1Forceful task_1,2,3new – T1Forceful task_1,2new) x T2 x T3 x P1 x P2) …

Frec = (Frec_Forceful task_1 + Σ ΔFrecommended_i) =

+ (Factual_Forceful task_n_new / (Fmax_Forceful task_n_new x (T1Forceful task_1,2,..n_new – T1Forceful

task_1,2,…(n-1)_new) x T2 x T3 x P1 x P2) P1 & P2: Factors for age and gender The relationship of maximum isometric whole body forces between female and male is well known in ergonomics. According to the study of Burandt (1978), Schultetus (1987), Rühmann and Schmidtke (1992) the general relationship is Fmax_female / Fmax_male = 0,5 – 0,67. T1-T3: Factors for frequency, “Physiology-Factor” and “Biomechanics-Factor” The frequency of force exertions has been considered according to the Schultetus (1987) method (T1). The data are almost identical to those of EN 1005-3, but the Schultetus data grid is more detailed. The “Biomechanics-Factor” T2 is new in the field of evaluating force exertions. It considers asymmetric trunk positions and one handed force exertions. This factor has been created for different types of force exertions based on the laboratory studies at IAD and IFA and expert ratings from practitioners from the shop floor (see fig. 2). When exerting forces in awkward postures like kneeling bent or above head, additional muscular load is needed for the force exertion. This is due to the fact that the operator has to enter and leave unfavorable postures before and after the force exertion or has to maintain these postures for a longer time while action forces are exerted. For this reason a “Physiology-Factor” was created in addition to the “Biomechanics-Factor”.

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The “Physiology-Factor” T3 considers these increased levels of internal muscular load, which is due to the awkward working postures combined with the force exertion. The “Physiology-Factor” is always less or equal 1 and decreases the maximal force capacities (measured action forces) due to the increased level of internal forces needed to enter and leave unfavorable postures before and after the force exertion or to maintain these postures for a longer time while action forces are exerted. The “Biomechanics-Factor” and the “Physiology-Factor” are a main result of the lab studies as described by Glitsch et al. (2008) and Wakula et al. (2009b). 2.3 Procedure for the “Classical” and Screening force assessment approach

The screening method is based on a similar procedure as the "classic assessment approach" (Schaub et al. 2009, Wakula et al. 2009b, Wakula et al. 2011). It is based on the fundamental philosophy that for increased stresses force and frequency scores will be granted. The higher the load situations, the higher the score(s). The procedure of the screening method is similar to the classical method. In a four-step procedure, the risk zone and actions to be taken (see Table 1) are derived. Table 2: Procedure for the assessment of whole body and finger-hand-forces with the screening approach.

Steps Description Action force of the whole body

Action force of the hand-finger system

1 a. Select the part of the body that exerts the force

b. Determine the frequency of force exertions per minute

c. Determine the actual force Factual to be exerted

2 d. Select the force percentile (P15 for planning analysis and P40 for analysis of existing situations

e. Determine max. Force Fmax

f. Select factor P1 for age (facultatively)

g. Select factor P2 for gender (facultatively)

h. Select the Biomechanics factor T2

i. Select the Physiology factor T3 j. Calculate Fmax_recommended

see table Fmax for whole body forces* dependent on - force direction - body posture (trunk, legs,

arms) see table factor for age* see table Fmax for whole body forces* see table Biomechanics factor* see table 4: Physiology factor Fmax_recommended= Fmax * P1 * P2 * T2 * T3 *)All tables in Wakula et.al.,2009a,b

See table for max. forces Fmax for finger-hand forces and gripping conditions* See table max. forces Fmax for -hand-finger forces* Fmax_recommended= Fmax * P2 *)all tables in Wakula et.al.,2009a,b

3 k. Select Force Score %Fmax = Factual / Fmax_empf. * 100

l. Select Score for frequency of exerting force T1

m. Calculate the total score TS

See table 2 for Score F see table 3 for Score n (as function from T1) TS = Score F x score n (tables for scores F & n are currently under revision)

table Score F table Score n (as function from T1) TS = Score F x score n (tables for scores F & n are currently under revision)

4 Identification of risk zones and measures to be taken

0 - 25 points „green“ > 25 – 50 points „yellow“ > 50 points „red“

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The following tables represent the current status of testing. Figures in the tables 2 to 5 may be a matter of change during the coming field tests.

Table 2: Scores F for a paper & pencil version of the Assembly specific force atlas, round to multiples of 5 N

%Fmax 3 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 Score F 0 1 2 4 5 7 9 11 12 14 16 18 20 22 24 26 29 31 32 35 37 Table 3: Scores n for a paper & pencil version of the Assembly specific force atlas

frequency / min 0,1 0,2 0,5 1 2 3 4 5 6 7 8 9 10 11 12 frequency / shift 48 96 240 480 960 1440 1920 2400 2880 3360 3840 4320 4800 5280 5760

score n 1,3 1,3 1,4 1,6 1,8 1,9 2,1 2,2 2,5 2,7 3,0 3,2 3,5 3,8 4,2

Table 4: Physiology factor T3 based on Wakula et al. (2009a,b) and Schaub et al. (2014) modified in the recent testings

   

frequency ≤ 4 5 5,1 5,2 5,3 5,4 5,5 5,6 5,7 5,8 5,9 6,0 ≥7 T3 1 0,9 0,9 0,8 0,7 0,7 0,6 0,6 0,5 0,5 0,4 0,4 0,2

 The frequency scores have been realized as the inverse function of T1. The traffic light colors are dependent on the height of point ratings in the screening method. Analogous to the KIM (Steinberg et.al. 2012) and EAWS (Ergonomic Assessment Worksheet) approach (Schaub, 2013) scores ≤ 25 shall represent a green area, scores > 50 a red area. The force index FI (force actually to be exerted Factual / maximum recommended force Fmax_recommended.) of the classical evaluation approach and the TS (Total Score) of the screening approach are calibrated to each other. A TS of 25 is equivalent to a FI of 0,85; a TS of 50 equivalent to 1,2 (see table 5).

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Table 5: 3 zone rating system for the classical and screeng approach (including a transfer function)

 

   

Figure 1: Relation in between the Forceindex (FI) and Total Score (TS)

The force factors cannot be easily calculated as they depend on the level of force exertion as well as on the frequency. This makes some computer aided support valuable.

Figure 2: Forceindex (FI) dependet on Frequency and % Fmax

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2.4 “MonKras” EXCEL spread sheet for testing the algorithms of the Assembly Specific Force Atlas

Especially for inhomogeneous force exertions an EXCEL based approach was implemented in order to avoid the neglecting (middle out) of highly demanding sub tasks. This second approach emulates the new NIOSH multi tasking (Waters et al. 1993) and allows identifying risky force exertions that should be redesigned with respect to the biomechanical load situation. The first approach estimates an average force level, which yields onto the total physical workload (e.g. energy expenditure). Whereas homogenous force exertions are suitable for a paper & pencil method, inhomogeneous force exertions need a computerized support. This software tool is not intended to be used as a regular analysis tool, but just as a mean to control and develop the evaluation algorithms and to have a high flexibility to adjust them. For a real software tool that will be realized at the end of the testing phase a much more increased usability will be presented. 3. Field test of the screening approach

The screening approach has been tested at several OEMs from automotive industries. The test persons worked in the area of process engineering, health & safety and shop floor management. Several criteria concerning the design of the worksheets, the self explain ability and the practical relevance had been asked in a questionnaire. The designed parameters for the force evaluation had been rated as very positive; less than 20 percent saw a concrete need for the improvement of the methodology and information presented. 4. Conclusions Different tasks - homogeneous and heterogeneous - can be analyzed using the classical as well as the screening approach by means of paper & pencil (homogeneous tasks) and the EXCEL-tool MonKras (homogeneous as well as heterogeneous tasks). This instrument allows the user to derive maximum recommended force limits from maximum isometric action forces taking into account specific task and worker related parameters. The results from the project "Force Atlas for Assembly Operations" and the force assessment approach focuses onto designers, production and safety engineers, occupational medicine professionals and ergonomists and assists in the analysis, evaluation and optimization of force oriented working tasks. At the moment both approaches (paper & pencil and EXCEL based tools) are being tested in the field of whole body and finger-hand force exertions. Whereas the whole body-force approach seems to run quite stable, the approach for the finger-hand system seems to need more information about the coupling situation (mode of gripping, geometry of the parts of control actuators involved, etc.). Both aspects will be the focus for a testing in the field in the near future. Acknowledgments We thank DGUV and BGHM (German statuary accident injuries), who gave the financial support for this project. Thanks to our colleagues from BAuA, BGHM, DGUV and IFA for their valuable contributions and scientific support. Cordial acknowledgments also to our partners from industry and university and to our colleagues from the CEN and ISO "Biomechanics" standardization committees, who contributed with their work to the realization of the “Assembly Specific Force Atlas”. References Burandt, U. 1978. Ergonomie für Design und Entwicklung, Köln: O. Schmidt DIN 33411-1:1982 Körperkräfte des Menschen; Begriffe, Zusammenhänge, Bestimmungsgrößen (Human physical

strength of man; concepts, interrelations, defining parameters). English manuscript available. EN 1005-3:2002+A1:2008. Safety of machinery - Human physical performance - Part 3: Recommended force limits for

machinery operation.

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EN 614-1:2006/prA1:2008. Safety of machinery — Ergonomic design principles — Part 1: Terminology and general principles

Glitsch, U., Ellegast, R., Schaub, Kh., Wakula, J. und K. Berg (2008), Biomechanische Analyse von Ganzkörperkräften in unterschiedlichen Körperhaltungen. Dokumentation des 54. Kongresses der Gesellschaft für Arbeitswissenschaft (GfA), pp. 429-432

ISO 11228-2:2007. Ergonomics—manual handling—Part 2: Pushing and pulling. Kroemer, KHE. 1977. Messung der Muskelkräfte des Menschen. Forschungsbericht Nr. 161. Bundesanstalt für

Arbeitschutz und Unfallforschung. Dortmund Peebles L. and Norris, B. 2003. Filling 'gaps' in strength data for design. Applied Ergonomics, Volume 34, Number 1,

January 2003 , pp. 73-88(16) Rohmert, W., Berg, K., Bruder, R und Schaub Kh., 1994, Kräfteatlas Teil1: Datenauswertung statischer Aktionskräfte,

Berlin. Rühmann, H.; Schmidtke, H. (Hrsg.) 1992. Körperkräfte des Menschen: Dokumentation Arbeitswissenschaft 31. Köln:

Schmidt, pp. 333 Safety of machinery- Human physical performance- Part 3 2008: Recommended force limits for machinery operation;

German version EN1005-3:2002+A1:2008 Schaefer, P.; Schaub, K.: (2006) Recommended Force Limits for Machinery Operation: A New Approach Reflecting User

Group Characteristics. In: Karwowski, W.,.Handbook on Standards and Guidelines in Ergonomics and Human Factors, Mahwah/London 2006, S. 293-306. ISBN 0-8058-4129-6

Schaub, K.; Schaefer, P. 2006. Ergonomics of Manual Handling – Part 2: Pushing and Pulling. In: Karwowski, W.. Handbook on Standards and Guidelines in Ergonomics and Human Factors, Mahwah/London 2006, S. 271-291. ISBN 0-8058-4129-6

Schaub, Kh., Caragnano, G., Britzke, B., Bruder, R. 2013. The European Assembly Worksheet. In: Theoretical Issues in Ergonomics Science, Volume 14 (Issue 6) pp. 616-639. ISSN 1463-922X. 2013 [Online-Edition available from: http://www.tandfonline.com/doi/abs/10.1080/1463922X.2012.678...].

Schaub, K., Wakula, J., Glitsch, U., Ellegast, R., Berg, K., Bruder, R. 2010. The Assembly Specific Force Atlas. In: Mondelo, P.; Karwowski, W.; Saarela, K.; Swuste, P.; Occhipinti, E. (Ed.): Proceedings of the VIII International Conference on Occupational Risk Prevention. ORP 2010, Valencia, ISBN: 978-84-934256-8-5

Schaub, K., Wakula, J., Berg, K., Kaiser, B., Bruder, R., Glitsch, U., & Ellegast, R. (2014) The Assembly Specific Force Atlas. Human Factors and Ergonomics in Manufacturing & Service Industries; Available from http://onlinelibrary.wiley.com/doi/10.1002/hfm.20545/full

Schaub, K., Wakula, J., Berg. 2015. Erstellen und Testen eines Supplementes für ein Screening-Kraftbewertungs-verfahren für einen begrenzten Anwenderkreis im Projekt Kraftatlas-Weiterführung. In Proceedings of the GfA-2015 Congress, VerANTWORTung für die Arbeit der Zukunft –Beitrag B.1.19; GFA Press, ISBN 978-3-936804-18-8

Schultetus, W.; Lange, W. (Hrsg.); Doerken, W. (Ed.) 1987. Montagegestaltung: Daten, Hinweise u. Beispiele zur ergonomischen Arbeitsgestaltung. TÜV Rheinland

Steinberg, U.; Caffier, G.; Liebers, F.: (2006) Assessment of Manual Material Handling based on Key Indicators – German Guidelines. In: Handbook of Standards in Ergonomics and Human Factors. Ed. by W. Karwowski. Lawrence Erlbaum Associates. Mahwah, New Jersey, London 2006. S. 319-338. ISBN 0-8058-4129-6

Steinberg, U.; Liebers, F.; Klußmann, A.; Gebhardt, Hj.; Rieger, M. A.; Behrendt,S.; Latza, U. 2012. KIM Manual Operations (Leitmerkmalmethode Manuelle Arbeitsprozesse 2011). Report on the testing, validation and revision. F 2195. Dortmund/Berlin/Dresden 2012. ISBN 978-3-88261-722-1

Wakula, J., Berg, K., Schaub, K., Bruder, R., Glitsch, U., & Ellegast, R. 2009a. Isometric maximum forces of the whole-body in not upright postures and hand-finger-forces for the assembly-specific atlas. In 17th World Congress on Ergonomics (IEA) August 9–14, 2009, Beijing, China.

Wakula, J., Berg, K., Schaub, K., Bruder, R., Glitsch, U., & Ellegast, R. 2009b. Ed. von der Deutschen Gesetzlichen Unfallversicherung /DGUV: Der montagespezifische Kraftatlas [The assembly specific force atlas]. BGIA-Report 3 / 2009.) (Institute for Occupational Safety and Health of the German Social Accident Insurance [IFA]). Final report. Berlin.

Wakula, J., Schaub, K., Berg, K., Bruder, R., Glitsch, U., & Ellegast, R. 2011. Assessment approach for analysis of whole body action forces at assembly-specific workplaces in industry. In 10th International Symposium on Human Factors in Organisational Design and Management, April 4–6, 2011, Grahamstown, South Africa.

Waters TR, Putz-Anderson V, Garg A, Fine LJ. 1993. Revised NIOSH equation for the design and evaluation of manual lifting tasks. Ergonomics. 1993 Jul; 36(7):749-76.