331  ISSN 1854 ‐6250 Journal ho me: a p em‐journal.or g Advances in Production Engineering & Management   Volume 15 | Number 3 | September 2020 | pp 331–344 https://doi.org/10.14743/apem2020.3.369 Original s cientif i c paper Simulation‐based time evaluation of basic manual  assembly tasks  Turk, M. a , Pipan, M. a,* , Šimic, M. a , Herakovič, N. a   a University of Ljubljana, Faculty of Mechanical Engineering, Slovenia  A B S T R A C T A R T I C L E   I N F O T h e p a p e r p r e s e n t s a s i m p l e s i m u l a t i o n m o d e l o f t h e l i f t i n g p r o cedure that c a n b e u s e d t o p r e d i c t t h e t o t a l t i m e r e q u i r e d f o r t h e s e q u e n c e o f b a s i c m a n u ‐ a l a s s e m b l y t a s k s d e p e n d i n g o n t h e v a r i o u s p a r a m e t e r s o f t h e l o a d a n d w i t h regard t o the w orkers’ health. Th e a i m of t he research is t o de termi n e t h e a p p r o p r i a t e n e s s o f u s i n g s i m u l a t i o n t o o l f o r ( r e ) s e t t i n g t i m e s tandards f or m a n u a l a s s e m b l y t a s k s . A n a v a t a r i n t h e s i m u l a t i o n m o d e l p e r f o r ms s equenc‐ es o f tasks with a handling m ass of u p to 20.5 kg. The indi vidua l t i m e s o b ‐ tained f rom the s imulatio n mo del were analysed a nd c ompa red wit h several t i m e p r e d i c t i o n m e t h o d s a n d v a l i d a t e d i n l a b o r a t o r y e n v i r o n m e n t . An a naly‐ s i s o f t h e i n f l u e n c e o f d i f f e r e n t l o a d p a r a m e t e r s o n t h e t o t a l t i m e w a s a l s o p e r f o r m e d . D e p e n d e n c y i s m o s t l y l i n e a r , s o f r o m t h e p r a c t i t i o n e r point o f view, we can pr edict with reasonable c ertainty the total t ime for a ny s e q u e nc e o f m a n u a l a s s e m b l y t a s k s f o r e v e r y s i z e a n d m a s s o f t h e b o x . B a sed on the results we c an c onfirm that simulatio n t ool J ACK is s uitable no t onl y f or e rgo‐ n o m i c a n a l y s e s b u t a l s o f o r s e t t i n g t i m e s t a n d a r d s f o r t h e w o r k ers. F urther ‐ more, wit h t he s imulati o n too l we analyse the process and get t he a ccurate results in shorter time compar ed to other mentioned methods. © 2020 CPE, Uni versity of M a r ib or. All rights re s erve d.   Keywords: Ass embly ; Manual task; Work‐job design; Time analysis; Jack s imulation ; Avatar *Corresponding author: miha.pipan@fs.uni‐lj .si (Pipan, M.) Article history: Received 20 April 2020 Revised 25 Se ptember 2020 Accepted 29 Se ptember 2020 1.Introduction Many tasks a t industrial a ssembl y w orkplaces still require manu al w ork t hat i ncludes a vari ety of a ctivities such a s loading and unloading, p ushing a nd p ulling , a n d c a r r y i n g t a s k s t h a t r e q u i r e m a n u a l h a n d l i n g o f g o o d s a n d m a t e r i a l s ( M M H ) [ 1 , 2 ] . When d esigni ng j obs and pro d ucts the aggregated i nformatio n on proce sses, t ools, m ac hines, s ubjects of w ork, tasks a nd o perators must b e tak e n into a cco unt, l imitati ons, which are oft e n co n f lic t i n g , m u s t b e m e t a n d a d e s i g n m u s t b e g e n e r a t e d , w h i c h w i l l b e a c c eptable for all parties inv olved [3]. I n o r d e r t o a d d r e s s workplace design f rom an e rgonomic and health p oint o f view, it is necessary t o predict the t i m e s r e q u i r e d f o r a w o r k e r t o c o m p l e t e i n d i v i d u a l w o r k t a s k s . These tim e s are impor tant for determining expected productivity , p l a n n i n g s t a f f a n d m a t e r i a l requirem e n ts i n the w o rkplace, conducting e rgono mic as sessments, reducing w or k‐related musculo skeletal d isorders ( WMSDs), e t c . [ 4 ‐ 7 ] . T h e y a r e u s u a l l y p r e dicted b y th e use o f Predeter m i ned Motion Time Systems (PMTS), s u c h a s t h e M e t h o d s T i m e M e a s u r e m e n t ( M T M ) a n d t h e M a y n a r d O p e r ation Sequ e n ce T ech‐ n i q u e ( M O S T ) [ 8 , 9 ] . D i g i t a l H u m a n M o d e l s ( D H M s ) a r e e f f e c t i v e design tools for visualizations, time a nal y ses and ergonomic evaluation of u ser and workplace in ter a ctions in terms of r each, clearance, visibility a nd c omfort. There is a g ap b etween the u se o f DHM tools for er gonomic assessment of w orkplac e s and the u s e o f D H M t o o l s f o r a d v a n c e d time a nalysis according to d if‐ Turk, Pipan, Šimic, Herakovič    332  Advances in Production Engineering & Management 15(3) 2020 ferent l oad parameters a nd a vata r poses. T o clos e t his gap, t he re a re s till challenges t o be a d‐ dressed [7, 1 0 ], and our r esearch is focused in this direction. In t his p a per we d escribe s tep by s tep a time a ssessment method ology for basic assembly tasks in a s equence ( m ul titaskin g, combined tasks) u sed in a n i ndustrial environment. W e com‐ pare t ime reports obtained in th e co nventi on al w ay w ith t h e MTM m e t h o d , t h e s i m u l a t i o n t o o l Sieme n s JAC K 9 .0, a labo ratory e xper iment and a new bio m ec hanic al ti m e prediction m o del de‐ veloped and presented by H arrari et al. [ 8 ] . I n o u r c a s e s t u d y w e f o c u se d o n w h e t h e r t h e s i m u l a ‐ tion done i n Siemens Jac k p roduces the same r esults a s the MTM m e thod o r whether t h ere are parameters ( trajectories o f h a nds, b anding r outi ne, dimensio n an d m a s s o f t h e l i f t i n g o b j e c t ) that l ead to d iffere nt t im es i n Siem en s Jack s imul ation co mpare d to t he M T M m ethod. I n addi‐ t i o n , t h e r e s u l t s o f t h e t i m e a n a l y s i s w e r e a l s o c a l c u l a t e d w i t h the new met h od N TPM d e velop e d by H arr a ri et al . [8 ] and verified b y a labor a tory e xperi m ent. O ther r esear c h ha s a l s o b e e n c a r ‐ r i e d o u t i n t h e f i e l d o f t i m e a n d e r g o n o m i c s a n a l y s i s a n d D H M s imulations o f th e lift ing proce‐ dure, both i n the studies of sin gle t a sk s and as well in co m bin ed tasks. Firstly, in the literature r eview, w e first concentr ate on indi vi dual t asks o f the li ftin g p r oce‐ d u r e a n d a n a l y s e s t h a t o t h e r r e s e a r c h e r s h a v e d e a l t w i t h , t a k i n g into a ccount different parame‐ ters, gender, mass of the l oad, e tc. Sec ondly, we wi ll di scuss studies on combined tasks. We w ill a l s o r e v i e w t h e c a s e s t u d i e s o f D H M s i m u l a t i o n a n d f i n a l l y f o c u s on the studies that a re m ost relevant to our case. Padula et al. [ 1 1 ] s t u d i e d t h e D H M s i m u l a t i o n o f t r u n k m o v e m e n t w h e n l i f t i n g th e lo ad t o dif‐ feren t h eigh ts. The e x pe riment w as p erform ed o n different popul ation groups ( fe male, male , students, an d workers with a nd w ith o ut m usculo skeletal s y m ptoms ). M ar tinez et al . focused on the study of g ender diffe r ences in u p p er l imb tec h nique duri ng a li fting t a sk of a 6 or 12 k g bo x from h ip t o eye level [12]. Othe r researchers focused on studyin g t h e i n f l u e n c e o f b o x w e i g h t o r handling h ei ght on the b i omech a nical exposure o f workers [1, 8, 9 ] , w h i l e o t h e r s i n v e s t i g a t e d the correlation between manual h an dling a n d injurie s [ 13 ‐16], a nd s o m e of th e m foc u sed on th e m a x i m u m a c c e p t a b l e w e i g h t o f a l i f t ( M A W L ) a n d l i f t i n g f r e q u e n c y [1 7, 18]. At this point, i t must f i r s t b e e m p h a s i z e d t h a t o u r c a s e i s f o c u s e d o n l i f t i n g a n d l o w e r i n g t a s k s , w h i c h a r e o n l y p a r t o f the MMH tas k s, a nd t hat we h ave not c oncentrat ed on a part o f t he MMH that includes, f or e x‐ ample, p ushi ng a nd p ulling t asks. Secondly, our study is b ased on the c o mbination of t he b asic tasks of M MH, as w e named it a s sequence o f tasks or l ifting p r ocedure. I n the are a o f combine d tasks it i s ne cessary to mention the studies for Straker et al . [19]. T h ey c ombined basic manual handling a ctivities such a s pulli ng, lifting, c arrying, l owerin g and pushing and investigated h ow the risk of such combined tasks could be assessed. T he a im of the study wa s to com p a re the risks assessed in s ingle manual h andling t a s k s w i t h t h e r i s k s o f c o m b ination tasks according to M axi‐ m u m A c c e p t a b l e We i g h t s ( M A W s ) . T h e y c o n c l u d e d t h a t t h e r i s k a s sessment of c ombin e d manu‐ al handling t a sks using MAW measur es c annot b e performed accura tely w h e n using th e risk a s‐ sessments o f isolated s ingle tasks. I n [20] they focused on r at ings o f dis c omfort, exertion and heart rat e a nd c oncluded that c omb i nation t a sk d iscomfort S u m, Rating o f Perceived E xertion and he art r a t e m easur e s were differe nt to me asur es of the co mpo n e nt of s i ngle t asks. Different DH M tools are used t o spee d up a m an u a l workplace and to use " w hat‐if" scenarios for time a nd e rgonomic a nalys e s. M any researchers have investig ated D HM t ool i n differen t sit‐ uations a nd i ndustrial fields. Se veral studies have r eported on t h e u s e o f D H M s i n t h e a u t o m o ‐ tive, a e rospa c e and o ther industries [ 2, 10, 21]. There is s o m e r e s e a r c h t h a t u s e s D H M t o o l s a s s a f e t y t r a i n i n g m e t h o d s [ 7 ] . T h e r e a r e a l s o r e p o r t s o f d i s a d v a n t a g e s i n t h e u s e o f D H M s o f t w a r e for exampl e when w orki ng w ith the workers with d isabilities [22] . T h e s t u d y f o c u s e s m a i n l y o n working environments w here m anua l work is presented with the a i m of c r e atin g new c l assifica‐ tions o f disabilities r elat ed to a manu facturin g en vironment. T h e p r o b l e m is t h a t t he w e l l ‐ k n o w n e r g o n o m i c s o f t w a r e p a c k a g e s o f D H M d o n o t i n c l u d e w o r k e r s w i t h d i s a b i l i t i e s f o r t h e i r s e r g o ‐ n o m i c a n d t i m e a n a l y s i s . O t h e r s t u d i e s , m o r e r e l a t e d t o o u r c a s e study, h ave focus e d on work cycle time and time p rediction models [ 6, 8 , 9, 23]. In m any ca ses, t he P MTS method d oes not accurately t ake into a ccount the physiological and biomechanica l aspec ts in time p r edictions. Especially in cases of l ifting, carrying and lowering o bjects, the PMTS m et hod predicts a s horter time p eriod t h an a w orke r is capabl e o f p e rfor min g witho ut h e a l th consequences in the future. Simulation‐based time evaluation of basic manual assembly tasks   Advances in Production Engineering & Management 15(3) 2020  333 The most r el evant studies regarding our research on time p r e dict i o n m o d e l s c a n b e f o u n d i n [8, 9, 24]. Th e [9] co vers t he M TM e xperiment an d MOST a n a l y sis , but com p ared to this r esearch it d oes not i nclude c omp u ter simulati ons. The foc u s is on the wh o l e b o d y o f t h e w o r k e r a n d t h e a i m w a s t o d e v e l o p a n e w t i m e p r e d i c t i o n m o d e l . T h e [ 8 ] d e a l s w ith the design o f a workplace with m anual material h andling tasks. I t considers both p roducti vity a nd e r g on omics. I t includes the DHM si mulation t ool a nd p roposes a new ti me p rediction mode l to b e used in our s tudy. T he same n ew t i m e predicti on m o del w a s used i n [ 24], as a p aramet er f or o ptimization of t he productivity. In [ 24] aut hors present s a n inno vat i ve f ramew o rk for formulating workplace de‐ sign a s an o ptimizati on problem that m axi m izes p roductivity whi le m aint ainin g e r g on omic a s‐ sessment val u es b elow co mmo nly us e d thresholds. Based on th e literature r eview de scribed above, w e can conclude that the DHM simulation t o o l i s m a i n ly u s e d a s a n e r g o no mic assessment tool, rather t han a s a to o l t ha t c a n be u s ed i n t h e process of w orkplace d esign bas e d on t ime an alysi s a nd d ifferen t load's p ar ame t ers. T h e refor e , t h e m a i n i d e a of o u r r e s e a r c h i s t h e us a g e o f t h e DH M f o r b o t h , e r g on om i c an d t i m e ‐ ba s ed a n a l y ‐ s i s , w h i c h c a n b e p e rf o rm e d i n m u c h s h o r t e r t i m e c o m p a r e d to o t her ment ioned appro a ches a nd with equal time pr ediction accura c y , is useful and reliable met hod. 2. Materials, methods, and experimental work  In t his section the meth o d ology, t he m ethods o f s t udy and the a nalysis of o ur r esearch a re p re‐ s e n t e d s t e p b y s t e p . T h e r e s e a r c h i s d i v i d e d i n t o t w o p a r t s , a s s h o w n i n F i g . 1 . T h e f i r s t p a r t i s a comparison of the t otal t imes o bt ain e d with f our methods ( M TM, Jack t ool, NTPM, laborator y e x p e r i m e n t ) a n d a s e c o n d p a r t i s a s t u d y o f t h e i n f l u e n t i a l p a r ameters. B esi d es the o ver v iew, the methodol ogy is d ivided i nto three su b‐sections, the first p a rt i s a c a s e s t u d y p r e s e n t i n g t h e b a s i c assembl y o p e rations. Th e next s ubsection is a c omparison o f t he m e t h o d s a n d a n e x p l a n a t i o n why and ho w different b asic o perati ons c an b e compar ed w ith eac h other using differ e nt m eth‐ ods. The t hird part deals with t he i nfl uential parameters of th e bo x. 2.1 Overview of the research approach  The ov ervie w o f th e res e arch a ppro a c h a n d t h e s t e p s o f t h e c a s e s t u d y a r e s h o w n i n F i g . 1 . T h e s t u d y i s d i v i d e d i n t o t w o p a r t s . T h e f i r s t p a r t i n c l u d e s a t i m e a n a l y s i s o f t h e t o t a l t i m e o f t h e l i f t ‐ i n g p r o c e d u r e . T h e l i f t i n g o b j e c t i s t h e c u b e ‐ s h a p e d b o x w i t h e venly distributed weight. T h e di‐ mension (height/width/depth) o f the box is 4 00 mm and a mass is 13.5 kg. The lifting height i s 80 0 m m . Th e results of t he t ime an al ysis w ere ob tained w ith four d i f f e r e n t m e t h o d s ( J a c k s i m u ‐ lation, M TM m ethod, N TPM [8], l abo r atory exper i ment). T he s tudy i s only a simul a ti on study, w h i c h a i m s t o t e s t t h e d i f f e r e n c e i n t h e r e s u l t s d u e t o t h e d i f fer e nt time prediction models. The results were c ompared to d etermin e i f s i m u l a t i o n i s a s u i t a b l e t ool f or d esigning w ork t a sks. W e know that t he simulatio n tool e nables "what‐if" s cenarios, whic h only fac i litates and speeds u p t h e p l a n n i n g o f w o r k t a s k s , b u t i f t h e t o o l s w e r e a l s o s u i t a b l e f or d esign, i t would mak e i t easier t o s e t t i m e s t a n d a r d s f o r w o r k e r s . T h e s i m u l a t i o n w a s c a r r i e d o ut i n the Sieme n s Jack p rogram based on th e MTM meth od. For co mparison, w e calculated time s ta ndard s f or t he s equence o f t a s k s w i t h t h e c l a s s i c M T M m e t h o d a n d t h e N T P M m e t h o d , w h i c h e x tends certain times for the e x e c u t i o n o f t a s k s b y t h e w o r k e r w i t h t h e a i m o f n o t c a u s i n g i n j u r i e s o r W M S D s . T h e r e s u l t s were a lso verified b y a laborato ry e xperiment wi th 10 health y s tudents. A ll subjects w ere re‐ cruited on a voluntary basis. The t o tal ti me o f liftin g procedure TT [ s ] ( m a n u a l m a t e r i a l h a n d l i n g p r o c e s s ) c o n s i s t s o f walking, b anding, applying f orce , lifting, c arrying (walking w it h b o x ) , p u t t i n g t h e b o x ( l o w e r i n g ) and posin g i n neutral po sition. (1) where t walk , t band , t apply , t lift , t carry , t lower , and t pose , are the times req u ired t o walk, to b and fo r the mass (box) r e ach it a nd g rasp it, to apply force to the mass, to l i f t t h e m a s s , t o c a r r y t h e m a s s ( w a l k i n g w i t h m a s s ) , t o l o w e r t h e m a s s ( p u t a n d r e l e a s e t h e m a s s ) a n d t o p o s e t h e b o d y i n n e u ‐ tral position. Turk, Pipan, Šimic, Herakovič    334  Advances in Production Engineering & Management 15(3) 2020 I n d i v i d u a l t i m e s h a s d i f f e r e n t d e p e n d e n c y a m o n g p a r a m e t e r s . F o l lowing e quati ons ( Eq. 1 – Eq. 8) show functional dependency for each time, which partic ip ate in E q. 1 . , , (2) where d walking [ m m ] i s d i s t a n c e w a l k w i t h o u t o b j e c t , d turning [ m m ] i s d i s t a n c e w h e n t u r n i n g w i t h ‐ out obj e ct a nd v [ m / s ] i s v e l o c i t y o f w a l k i n g / t u r n i n g . W a l k i n g a n d t u r n i n g d i s t ances are exclu‐ sive, so we use d walking for task walk an d d turning for task turn the b ody. Δ θ ,ω , ; (3) where Δθ TB [ ° ] i s t r u n k e x t e n s i o n a n g l e , ω rB i s angular velocity [ s ‐1 ] of b ody, d R i s reach distance [ m m ] t o w a r d s t h e o b j e c t , a n d dim [ mm] i s dimension of the b ox, which is i mportant f or g rasp operatio n. , , (4) where m [ k g ] i s w e i g h t o f t h e o b j e c t , dim [ m m ] i s d i m e n s i o n o f t h e o b j e c t a n d μ i s friction coef fi‐ cient between two mat eri a ls (skin, car d board). , Δ θ ,ω (5) where m [ k g ] i s w e i g h t o f t h e o b j e c t , Δθ TB [ ° ] i s t r u n k e x t e n s i o n a n g l e a n d ω r i s angular velocity [s ‐1 ] of b ody and o b ject w eight. , , (6) where m [ k g ] i s w e i g h t o f t h e o b j e c t , d [ m m ] i s d i s t a n c e o f c a r r y i n g ( w a l k i n g w i t h o b j e c t ) , a n d v [m/s] is velo c ity of walki ng. , Δ θ ,ω , (7) where m [ k g ] i s w e i g h t o f t h e o b j e c t , Δθ TB [ ° ] i s t r u n k e x t e n s i o n a n g l e a n d ω r i s angular velocity [s ‐1 ]. “pos ” is p osition of t he o bject wh en r eleasing i t. P osition contains c l a s s o f f i t , c a s e o f s y m ‐ metry, e ase of h andling. (8) where “joint s l ocations” means the di ffer e nces b et ween c urre n t joint location and joint location of n eutr al po s e. Fig. 1 Overview of the inputs, the parameters and the results Simulation‐based time evaluation of basic manual assembly tasks   Advances in Production Engineering & Management 15(3) 2020  335 T h e s e c o n d p a r t o f t h e s t u d y i n cludes the total time a nalysis w i t h different param e ters o f th e load d uring the lifting and lowering p rocedure. The parameters we v ari e d are the d i mensio n ( h e i g h t , w i d t h , d e p t h ) a n d t h e m a s s o f t h e l o a d ( b o x ) . T h e t i m e a nalysis was perfor med by s imu‐ lation in t h e J ack progr a m, b y th e co nventi on al M TM m etho d and b y t h e N T P M m e t h o d . T h e l i f t ‐ ing hei g ht i s 800 mm. T h e a n a lysis covers f ive dif f e rent d ime n s ions o f bo xes, i .e. cubes without h a n d l e s . T h e d i m e n s i o n s o f t h e c u b e s (height = width = depth ) are 200 m m , 300 m m , 40 0 mm, 5 0 0 m m , 6 0 0 m m . T h e w e i g h t o f t h e b o x i s e v e n l y d i s t r i b u t e d . M a s s o f t h e b o x c o v e r s t h e f o l l o w ‐ ing v a lues: 4 . 5 kg, 9 k g , 1 3 .5 k g, 1 8 k g a nd 2 0.5 kg. The co mpa r i son was therefore made w ith 25 different co m bin a tions of p a rameter s . T h e p u r p o s e o f t h e f i r s t s t u d y on the review o f total working t ime using four m ethods a nd tools is t o dete rm ine whethe r the simula tion t ool i s suita b le f or work desi gn and t he d efi n ition o f the relevant s tandards/norms f or w o r kers i n ind u stry. This w ould significantly reduce t he t ime needed t o design a w ork p lace a nd m ake w o rkplaces m ore wo rker‐fr iendly. The purp ose of d e‐ termining th e relati onshi p betw een b ox d imensions, b ox w ei ght a nd t ime according to t he M TM, N T P M a n d J a c k m e t h o d s i s t o t e s t w h e t h e r t h e s i m u l a t i o n t o o l s h ortens o r lengt h ens the overall working ti m e whe n th e a v atar lifts or lowers a l arger or h ea vie r box (Study 2). 2.2 Case study  O u r c a s e s t u d y i s a n e x a m p l e o f a s i m p l e a n d c o m m o n p r o b l e m f o r t he m anual assembly process in industry ( w arehous e s, man ual assembly area, logistics, etc.) . The defi nitio n o f th e pro b lem is a lifting procedure and its time a nal y ses. M ost manual a ssembly p rocess i n industry, e specially order‐picking systems us ed in practic e , are manual “ picker to p art” s ystems, and mor e t han 80 % of a ll orders p rocessed by w ar ehouses are picked m anually [ 25] . T h e o r d e r p i c k i n g p r o c e s s , a process in w hich humans are routed b y picking lists to i tems’ s t o rage l ocatio ns t o retrieve i tems f o r c u s t o m e r s , i s t h e m o s t l a b o r i o u s a n d t h e m o s t c o s t l y a c t i v i t y ( u p t o 5 5 % o f c o s t o f t h e p r o ‐ c e s s [ 2 6 ] ) i n a t y p i c a l m a n u a l a ssembly process. S ince walking p r e s e n t s u p t o 5 0 % o f t h e t o t a l t i m e a n d l i f t i n g i s m o s t e r g o n o m i c a l l y s t r e s s f u l t h e l o g i c a l w a y of i mproving l ifting p rocedure (walking, turning, l ifting, carry ing, e tc.) i s to s tudy time sp ent on p rocedure a nd try to reduce i t b y d e t a i l e d r e s e a r c h l i k e w e p r o p o s e d . T h e p r o b l e m d e f i n i t i o n o f liftin g procedure, w hich c on‐ tains walkin g, turning, b a nding, a ppl ying f orce, lifting, c arryi n g , l o w e r i n g a n d p o s i n g i s t o d e t e r ‐ mine i f simu lation tool i s appropriate tool t o asses s t ime of t he p rocedure. The object o f study is l i f t i n g p r o c e d u r e o f a b o x w i t h a m a s s o f m = 13.5 kg a nd d ime n sions o f A × B × C = 4 0 0 × 4 0 0 × 40 0 mm (height × width × depth) f rom the f loor t o the ta ble . I n the initi a l state, the worker stand s in fro nt o f t h e ta ble, f acin g th e t a ble, a nd the bo x st a nds on the worker's le f t . The m ain ta sk of the w o r k e r i s t o m o v e t o w a r d s t h e b o x , p i c k i t u p a n d p u t i t o n t h e t a b l e . W e h a v e s e p a r a t e l y p e r ‐ f o r m e d t h e c a l c u l a t i o n s f o r t h e t i m e a n a l y s e s w i t h t h e M T M m e t h od, th e new prop osed time prediction m o del and t he simulation e xperim ents w ith the so ft wa re t ool JACK. To b ring o ur case study closer to t he i ndust r ial en viron m ent where these activiti es a re c ommon (lifti ng, c a rrying, e t c . ) , w e s e t u p t h e l a b o r a t o r y e x p e r i m e n t a n d p e r f o r m e d t h e s a me s equen c e of m ovements. O ur c a s e s t u d y i n c l u d e s t h e n e x t s e q u e n c e o f t a s k s ( F i g . 2 ) . F i r s t , t he w orker turns his body b y 90° from i ts i nitial p osition. T hen the wor k er s tarts walking par a ll e l t o t h e e d g e o f t h e t a b l e t o w a r d s t o t h e b o x . H e w a l k s a d i s t a n c e o f 1 2 0 0 m m ( F i g . 2 a ) . T h e w o r k e r tak e s two lateral steps to achiev e a gap betwee n hi s feet t o lift t he b o x m or e easily. When t h e w o r k e r i s i n a b a l a n c e d p o s i ‐ t i o n , h e b e n d s t o t h e b o x a n d p r e p a r e s f o r t h e n e x t m o v e , w h i c h i s r e a c h i n g f o r t h e b o x ( F i g . 2 b ) . T h e w o r k e r p i c k s u p t h e b o x i n t h e m i d d l e o f t h e b o t t o m e d g e . H e g r a s p s i t w i t h b o t h h a n d s a n d applies a for c e corresponding to the load. The w o rker l ifts the b o x a n d r e g r a s p s i t u p a g a i n f o r easier c arryi n g. A fter t h a t, h e straightens his bo dy t o the ne u tral p osition (Fig. 2c). T hen he mak e s a 1 8 0 ° turn and w a lks back in a strai g ht lin e. Th e distan ce he walks i s 12 0 0 m m ( Fig. 2d). Turk, Pipan, Šimic, Herakovič    336  Advances in Production Engineering & Management 15(3) 2020 a) b) c) d) e) Fig. 2 Tasks from the sequence: a) turn the body by 90° and walk; b) bend the body and reach the box; c) l ift the box; d) turn the bod y by 180° and wal k; e) put the box on the table and take a neutral pose for the body T h e w o r k e r t u r n s h i s b o d y t o w a r d s t h e t a b l e . T h e n h e p u t s t h e b o x o n t h e t a b l e a t a h e i g h t o f h = 8 00 m m. T he p ositio n of t he b o x on the tabl e is not i mport a nt a n d f o r t h i s r e a s o n w e u s e t h e "Put" mo vement c o m mand a nd not t he "Position" movement c ommand. T h e " P u t " m o v e m e n t only places t he b ox o n th e table witho u t definin g a t ipping p oi nt, while the "Position" m o v em en t requires a s p e cific point t o b e de fin e d where the b o x is t o be pl aced. After p u tting th e b ox on t h e t a b l e , t h e w o r k e r r e t u r n s t o t h e n e u t r a l p o s e f a c i n g t h e t a b l e ( F i g . 2 e ) . T h e s t u d y w a s a l s o m e n ‐ tioned in an already p u bl ished paper [27]. MTM method We c alculated the time s equence of b a sic m ov em e n ts with the MT M m e t h o d b y f o l l o w i n g t h e instructions o f Kar g er [ 2 8 ]. For our study we m a in ly used body , leg, a n d foo t mo v e m ents , such as sidestep, turn b ody, b end , a rise , and walk. Other basic movemen ts t hat we u sed emphasize hand motions, like r each, (re)grasp, a pply force, move, and release. Some of t hes e e lements are used simultan eou s ly, so w e merged t he m t o get h er. For the time a n a lys es o f the s i multan eous m otions only th e ti m e for the indi v idual m o tio n is set so th a t it t akes th e gr e at est ti me [2 8 ]. New time prediction method (NTPM) In o ur c ase study we u sed the NT PM m ethod developed by [ 8] a nd calculated new times accord‐ i n g t o t h e i r r e s u l t s . T h e a u t h o r s o f t h e p r o p o s e d N T P M t o o k t h e M TM m odel a s a basis and up‐ dated it f or a ll movements in w hi ch the l oad was included. I n ou r c a s e w e t a k e i n t o a c c o u n t t h e individual t imes f ro m th e MTM m e t h ods for the move me nts: t urnin g, w alking, sidestepping, b e n d i n g , r e a c h i n g , g r a s p i n g , a p p l i c a t i o n o f f o r c e a n d r e l e a s e . F o r mov e m e n t s where the worker handles the mat e rial ( box), the exact weight o f th e box is take n into a ccount, a nd w e have c alcu‐ l a t e d n e w i n d i v i d u a l t i m e s f o r t h e l i f t i n g , c a r r y i n g , t u r n i n g ( c a rrying) a nd putting (lowering) m o v e m e n t b y E q . f r o m 9 t o 1 3 . I n a d d i t i o n t o t h e w e i g h t o f t h e b o x , t h e i m p r o v e d m o d e l a l s o takes into a c c ount t he a n g ular v elocity ( ω r ) and the trunk‐ext e nsion angl e ( θ r ), w hich w e have taken fro m t he simul atio n tool. ω 0 . 13766 2 ∙ 14 .3881 (9) Empirical o b t ained Eq. 9 presented th e correlatio n b etwe en t h e obj e ct m ass m [ k g ] , a n d t h e aver age angular velocity ω r [ s ‐1 ] . T h e r e a f t e r , t h e b o x l i f t i n g t i m e , t lift , was calculated (Eq. 10) u s‐ ing tru n k ext e nsion an gle Δθ T [°] a nd t he a ver a ge a ngular veloci t y ω r [s ‐1 ].  Δ θ ω (10) Empirical o b t ained Eq. 11 p resented t he c orrelati on betw een the o bject mass m [ k g ] , a n d v e ‐ locity of wal king with obj ect (carrying) v [m /s] . 5.229 51 26 05 0 .09390 34 72 44 ∙ (11) T h e box ca rry ing time t carry is given by E q. 1 2; d (12) where d [m m ] is th e car rying dista n ce, and v [m/s] is the carrying velocity in Eq. 11. Simulation‐based time evaluation of basic manual assembly tasks   Advances in Production Engineering & Management 15(3) 2020  337 A v e r a g e t i m e f o r b o x l o w e r i n g i s 1 3 % l e s s t h a n t h a t l i f t i n g u n der the same c onditio ns. Thus, using th e ch ange i n trunk angle Δθ T [ °] a nd a ngular v elocity for lowering ω r [s ‐1 ] , t h e l o w e r i n g time t lower wa s calculated as Eq. 1 3 :  Δ θ 1.13 ∙ ω (13) Simulation tool Jack W e p r o g r a m m e d t h e s a m e l i f t i n g p r o c e d u r e i n t h e s o f t w a r e t o o l J A C K w i t h t h e u s e o f t h e T S B (Task Simul a tion Builder) tool i n a virtual envir onment. T he T S B tool a u t omatic ally g en erat es t h e t i m e l i n e f o r e a c h m o v e m e n t s e p a r a t e l y . W e u s e d e i g h t d i f f e r en t movements ( g o, p ose, g et, apply force, reach, regras p , posi tion, put), some of which (e.g ., go) were repeate d several times. E v e n t h o u g h J a c k ( T S B ) u s e s t h e MTM method a s a basis, it makes s e n s e t o m a k e a t i m e c o m p a r ‐ ison betw een TBS and M T M results, b ecaus e i n a virtual en vir onm ent we p lace a n avatar i n dif‐ feren t postu res, which le a ds to a cha n ge i n durati o n for e ach t ask. Although t he n am es o f t h e individual m ov em ents d iffer in t h e M TM m e t h o d a n d i n t h e s i m u ‐ l a t i o n e x p e r i m e n t , t h e i r m e a n i n g i s t h e s a m e , s o w e c a n c o m p a r e t h e m . F o r e x a m p l e , i t c a n b e s t a t e d t h a t t h e m o v e m e n t " g o " i n t h e s i m u l a t i o n t o o l i s u s e d t o w a l k , c a r r y a n d t u r n t h e b o d y , and th e m o v e me nt " get" i s represent e d by th e M TM m ethod as bend in g and reach. Practical experiment In t h e l aboratory environ m ent we s et u p a syst em f or c onducti ng a practical e xperiment (Fig. 3). The exp e rim e nt i ncludes 10 h e althy students. Th eir me an (SD ) ant h ropo metric d ata were: age 26 ( 1 .5) ye ar s; body m a ss 75.5 (6.3) k g ; h e ight 1 76 0 (5 5) mm. All the subjects w ere rec r uited on a voluntary basis. A ll subje cts filled in a s creening ques‐ tionnair e to e nsure th at t hey wer e i n good h ealth (i.e. that t he y d i d n o t s u f f e r f r o m a n y o f t h e following: c hronic illnes s, heart c ondition, m usculoskeletal d i sorders, o r injuries). T hey al l s i g n e d a c o n s e n t f o r m a p p r o v e d b y t h e i n s t i t u t i o n a l r e v i e w b o a r d of t h e F aculty o f M e chanic al Engi neeri n g, U niversity of L j ubljan a . Th e experi ment w as c onduc ted 20 times, two times per student. T hey perfor med the follo wing m ovements: turning and wa lking fr om a s tartin g point t o the box th at i s 1200 m m a way fr o m t he t able ( Fig. 3 a), sidestep ping, b e nding to t h e b ox a n d reaching for it (Fig. 3b), apply ing a f o rce, lifting the box (F i g . 3c), turning and walking to t h e table (height 8 00 mm) (Fi g 3 d ) , putting t he b ox on the ta b l e a n d po sin g in the neutral position (Fig. 3e ). All 20 e xperi m ents w ere recorded for f urther a n a lyses. a) b ) c) d) e) Fig. 3 Practical experiment: a) turnin g the bod y by 90° and walking ; b) bending the body and reaching for the box; c) lifting the box; d) turning the body by 180° and walking; e) putting the box o n the table and taking a neutral pose of the body 2.3 Comparison of the method  All methods used i n our research a re b ased on the MTM metho d olog y , w i t h t h e e x c e p t i o n o f t h e practical experiment, which is a n indicator of the a ctual per f orm a n ce o f the s u b j e c t s . A s s h o w n i n T a b l e 1 , d i f f e r e n t t e r m s a r e u s e d f o r t h e b a s i c m o v e m e n t s i n a ll four m eth o ds. We c an s ee that the mai n d ifferenc e is only in t he n aming and the mer g ing of s e veral movements together; the purposes a n d m eanin g s are the sam e e veryw h er e, s o in s ummary w e c a n s a y t h a t t h e m e t h o d s are comp arable. The r e su lts of e ach m e thod w ere obtain ed i n diff e r en t w a y s : f o r t he MT M c l a s s i ‐ cal method, the results were c alculated using tables a nd r ec omme n d a t i o n s [ 2 8 ] . I n t h e J a c k simulation environment, a time re por t is automatically g enerated b a s e d o n t h e a v a t a r , i t s t r a j e c ‐ t o r i e s , a n d t h e p a r a m e t e r s y o u i n s e r t i n t h e p r o g r a m . I n t h e N T PM m eth o d, t he r esults w ere ob‐ tained b y cal c ulation according to the classical MTM method a nd t h e a d d i t i o n o f t a k i n g i n t o a c ‐ count the mass according to E q. f rom 9 to 1 3 . Turk, Pipan, Šimic, Herakovič    338  Advances in Production Engineering & Management 15(3) 2020 Table 1 Comparison and descr i ption of the basic movements f or all four methods MTM method NTPM Jack s imulation Practical experiment Turn body by 9 0° Same as MTM Go (turn) Turn Walk Same as MTM Go Walk Lef t s ides tep Same as MTM Pose Sidestep Ri ght si destep Same as MTM Bend Same as MTM Get Bend and reach Reach Same as MTM Gras p Same as MTM Apply fo rce Same as MTM Wait (time from MTM) Apply fo rce Ari s e Lif t Position (arise and reach) Lif t Move the box Regrasp Regrasp Turn body by 9 0° Carry Go (turn) Turn by 180° Turn body by 9 0° Walk Carry Go Walk Turn body by 9 0° Carry Go (turn) Turn Moving the b ox Lower Put Put Release the box Same as MTM Neutral pose Same as MTM Pose Neutral pose 2.4 Parameter combination  W e p e r f o r m e d 2 5 s i m u l a t i o n e x p e r i m e n t s ( 5 d i f f e r e n t m a s s e s o f t h e b o x m u l t i p l i e d b y 5 d i f f e r ‐ ent dime nsions o f th e b o x) w ith different co mbi n ations o f m a sse s and dimensions o f the bo x ( p a r a m e t e r s i n F i g . 1 ) . W e c o m p a r e d a l l v e r s i o n s u s i n g t h e M T M meth od, the NTPM a nd J ACK the simul a ti on tool, focusing on th e t o tal time. 3. Results and discussion  I n t h i s s e c t i o n w e p r e s e n t t h e r e s u l t s o f t h e t i m e a n a l y s e s f o r t h e j o b a n d t a s k . T h e s e c t i o n i s d i v i d e d i n t o t w o p a r t s . T h e f i r s t p a r t s h o w s t h e r e s u l t s o f t h e t i m e a n a l y s e s o f t h e i n d i v i d u a l tasks, w hich w ere achiev ed u sing a ll four m et hod s . The second p art prese n ts t he r esults o f the influential parameters obtained with all th ree methods: MTM, NT PM, and J a ck simul ati on to ol. 3.1 Time analyses of the individual tasks  All movements in a ll methods were u nified s o that t hey are c o mp ar able w ith each o t h er. In t he M T M m e t h o d w e c o m b i n e d s i m u l t a n e o u s m o v e m e n t s s o t h a t w e o b t a i n e d a t o t a l o f 1 1 m o v e ‐ ments whic h are t h e s a me i n all fo ur m eth o ds w e comp are d . The m ovements a nd t he c orre‐ sponding times are shown in T able 2 . T h e b o x w e u s e d i n t h i s p a rt o f the study has a dimension of 4 00 m m and we ig hs 13 .5 kg . Table 2 Comparison of the indi v idual time s of the basis movement for al l fou r methods MTM N TPD J ack E xperiment # Tas k des cription Time [s ] Time [s ] Time [s ] Time [s ] 1 Turn body by 9 0° 0 .67 0 .67 0 .2 0.97 2 Walk 0 .76 0 .76 1 .34 1 .97 3 Sidestep 0 .64 0 .64 0 .64 0 .78 4 Bend, reach and grasp the box 1 .30 1 .30 2 .54 1 .02 5 Apply force 0 .38 0 .38 0 .38 0 .59 6 Lift the bo x 1 .38 3 .50 3 .16 1 .55 7 Turn by 180° 2 .01 0 .39 0 .78 1 .47 8 Walk to the tabl e 0 .67 1 .10 0 .95 1 .37 9 Turn by 90° 1 .34 0 .18 0 .17 0 .61 10 Put the box o n the table 0.35 3.10 1.76 1.25 11 Neutral pose of the body 0.52 0.52 0.50 0.87 T otal time TT [s] 10.12 12.54 12.42 12.45 Simulation‐based time evaluation of basic manual assembly tasks   Advances in Production Engineering & Management 15(3) 2020  339 The t o tal ti me TT [ s ] f o r e a c h m e t h o d w a s c a l c u l a t e d b y E q . 1 . I t c a n b e n o t i c e d t hat th e terms of t he E q. 1 a nd t ask names are n o t the sam e . Th ese are o n ly w o rd d ifferences a nd not s ubstan‐ tive o n e s. H ere are the explanatio ns o f meanin g for each t er ms of Eq. 1 a ccording to task descrip‐ t i o n . T a s k # 1 “ t u r n b o d y b y 9 0 ° ” , # 2 “ w a l k ” a n d # 3 “ s i d e s t e p ” c ontributes t i me to t he f i r st t erm of E q. 1”t walk ” and h a s a functi onal dependence d etermi ned by E q. 2 . Task “ ba nd, reach an d grasp” c ontr i butes time t band t o t h e t o t a l t i m e a n d i s d e f i n e d b y E q . 3 . T a s k # 5 “ a p p l y f o r c e” is described by term t apply a n d E q . 4 . T a s k # 5 “ l i f t t h e b o x ” c o n t r i b u t e s t o t o t a l t i m e t h e tim e t lift a n d i t f u n c t i o n a l i t y i s d e s c r i b e d b y E q . 5 . T a s k n u m b e r e d f r o m 7 t o 9 m e a n s c a r r y i n g ( w a l k i n g w i t h object) and i t s contributions a re d efi n ed b y t carry a n d f u n c t i o n a l d e p e n d e n c i e s b y E q . 6 . T a s k # 1 0 “put t he b ox on the table” m eans t o lower the load ( object) and i s defined by term t lower a n d E q . 7 . The last t ask “neutr al pos e o f th e b ody ” contributes time to t e rm t pose and is d e scribed by E q. 8. T h e v a l u e s f o r t h e t i m e s o f a l l m o v e m e n t s a r e n o t c o m p l e t e l y c o nsistent w hen comp aring all four m ethod s . It is clear that the different movements contribu te d ifferent p roportions to the total tim e . Therefore, t he s ame mo ve ment has a g reater o r l e ss strong e ffect on the t o t a l time i f w e u s e d i f f e r e n t m e t h o d s . W e c a n s e e t h a t t h e m o v e m e n t s t u r n i n g t h e b o d y a n d w a l k i n g , w h i c h w e r e o b t a i n e d b y t h e e x p e r i m e n t a l s t u d y , c o n t r i b u t e a r e l a t i v e l y l a rge p r oportion to t he t otal t i m e c o m p a r e d t o t h e o t h e r m e t h o d s . T h e e x p e r i m e n t a l s t u d y w a s conducted in a l aboratory e n v i r o n m e n t . T h e s u b j e c t s w e r e i n e x p e r i e n c e d a n d w e r e o n l y i n s t ructed t o perform the opera‐ tions a t a pace that w o ul d not m ake them f eel uncomfort a ble if the s e quence o f tasks was re‐ p e a t e d o v e r a l o n g p e r i o d o f t i m e . I f w e e x c l u d e t h e s e f a c t o r s , t h e w a l k i n g t i m e w o u l d b e c l o s e r to the result s of t h e othe r meth ods. The next c omparison refers t o th e results obtained with the sim ul ation tool J ack a n d the NTPM. In th e N TP M we only focus o n the f ollowi ng m ovements: lif t, t urn by 1 80° with o bject ( c a r r y ) , w a l k w i t h o b j e c t ( c a r r y ) , t u r n b y 9 0 ° ( c a r r y ) , a n d p u t ( l o w e r ) – t a s k n u m b e r s f r o m 6 t o 10, b ec ause only thes e movements t a ke i nto acco unt the w e ight of t h e l o a d a n d a r e u p g r a d e d b y t h e M T M m e t h o d . W e c a l c u l a t e d t i m e s b y e q u a t i o n s f r o m 9 t o 1 3 w ith t h e f o llowing p arame t ers: m = 13.5 k g ; d walking = 120 0 m m; d turning = 20 0 mm ; and Δθ T = 4 3 . 6 ° . I t c a n b e s e e n t h a t t h e v a l u e s determined w ith these t w o met h ods for the times of th e m ovement s "li f t", a nd " put" (lower) are longer than the ti mes determined w i th the oth e r two method s (MT M and ex perimental s tudy). T h e r e a s o n f o r t h i s d i f f e r e n c e i s t h a t b o t h m e t h o d s ( N T P M , J a c k t o o l ) t a k e i n t o a c c o u n t t h e t r u n k ‐ e x t e n s i o n a n g l e , w h i c h a l s o i n c r e a s e s t h e v a l u e o f t i m e a n d e n s u r e s t h a t t h e w o r k e r h a s enough ti m e to lift and lower th e load s in an er gonomically c or rect way. The results of t he M TM m ethod in di cate that the worker s pends m ost of t he t ime on body turns compared t o the other methods . B oth in t he s imulation expe r i m e n t w i t h t h e J a c k t o o l a n d in the e xperimental stud y (assumi ng that th e t e st s ubjects woul d be tr a ined) less time is needed to t urn the body, becau s e in r eality w e combin e rotation and wa lking into a s imultaneous mov e m e nt f or which we need less ti me. Jack t ool compared t o other method s spend least time f or tas k “ turn t h e b ody”. The reason is t h a t J a c k d o e s n ' t h a v e a s e p a r a t e " T u r n " m o v e m e n t , b u t y o u h a v e t o use a "Go" m o v ement. This m o v e m e n t r e q u i r e s m o v i n g t h e a v a t a r b y a c e r t a i n d i s t a n c e , a n d not j ust turning the w h ole body i n t h e s a m e p l a c e . S o w e u s e d a d i s t a n c e o f 2 0 0 m m f o r t h e t u r n a n d a p r e d e t e r m i n e d s p e e d (specified i n the simulat i on tool) and thus o btai ned the task ti m e . F o r t u r n i n g t h e b o d y y o u s p e n d m o r e t i m e t h a n f o r w a l k i n g , b e c a u s e o f t h e h i g h e r c o m p l e x ity of t he k inemati c s of t he j o i n t s a n d h e r e i s t h e t i m e d i f f e re n c e . T h e s e c on d t a s k , w h i c h is a t contrary l onger i n comparison w i t h o t h e r m e t h o d , i s “ b a n d , r e a c h a n d g r a s p ” t a s k . T h e r e a r e t w o r e a s o n s f o r t h i s . F i r s t i s t h a t i t is n ecessary t o place the avat ar i n a c e rtain distan ce f ro m obj e c t and also t o "att ach" t h e h and for "grasp" movement to a c e rtain pl ace, w hich, how e ver, d iffers i n e ach simu lation and a f fect t h e task’s t ime. T he r epeatability o f this t ask is a chieved only th r o ugh exp e rience a nd l ong‐term use of s imulatio n tool. The second r easo n is t hat the task i tself i s predefined i n the simulation envi‐ ronment so t hat the avat ar l eans tow ards the o bjec t with s tretc hed legs, which did not s uit us in t e r m s o f c o m p a r i s o n w i t h o t h e r m e t h o d s . S o w e s u b s e q u e n t l y c h a n ged the a n gles o f certai n j o i n t s ( k n e e s , h i p s , a n d t o r s o ) a n d t h u s g a i n e d t i m e f o r t h i s t as k. W hen ch anging s uch a complex task, the si mulation tool determine longer time. Turk, Pipan, Šimic, Herakovič    340  Advances in Production Engineering & Management 15(3) 2020 The tot a l ti me f or a ll th e tasks obt a i n ed w ith th e MTM met h od is t = 10 .12 s; u sing t he J a ck simulation t ool i t is t = 1 2 . 4 2 s ; f o r t h e p r a c t i c a l e x p e r i m e n t i t i s t = 12.45 s ; and the total time obtained with the NTPM method is t = 1 2 .54 s. I n s u m m a r y , w e c a n s a y t h a t a l l m e t h o d s , e x c e p t M T M , g i v e a l m o s t the same r esult for the to‐ t a l j o b t i m e a n d t h a t w e c a n u s e a n y m e t h o d t o d e s i g n a n e w j o b w ith a sequence o f basic move‐ m e nts. I n m a ny ca se s PMT S (the MT M m e t hod) predicts a short e r t ime than t he worker i s able to w o r k w i t h o u t i n j u r y a f t e r s o m e t i m e . I n s u c h a c a s e , t h i s s t a t e ment p roves to b e correc t i n com ‐ parison with the o ther m ethods, as the M TM m ethod predicts 20 % l ess time t o per f orm th e same sequ e n c e of movements. I n industry it is ne ce ssa ry to pl an and design work processes a n d s equenc es o f jobs a s q ui c kly a s p o s s i b l e , b u t i t i s a l s o n e c e s s a r y t o a d a p t t h e t a s k s t o t h e w orkers a nd t o think ab out thei r well‐being. W ith the Jack s imulation tool, it i s easy a nd q uick t o check the ergon o mics a nd s uita‐ b i l i t y o f t h e j o b s f o r w o r k e r s u s i n g " w h a t i f " s c e n a r i o s . T h i s option allows u s to m odify t he vari‐ ous parameters o f the simulated obj e cts (weight, d imension, s ha pe) and avat ars (gen der, a n‐ t h r o p o m e t r i c c h a r a c t e r i s t i c s ) . F r o m t h i s p a r t o f o u r s t u d y w e c a n c o n c l u d e t h a t t h e J a c k s i m u l a ‐ tion to ol is a suitable met hod fo r designing jobs and workplace s when only the tot a l ti me and not t h e i n d i v i d u a l t i m e s a r e o f i n t e r e s t . I f w e w a n t t o o p t i m i z e t h e w o r k c y c l e f o r t h e s t u d y w e a r e discussing, it is necessar y to lo ok d eeper into the individual tasks and fi nd the r elations hips b e‐ tween the v alues o f th e i ndividual ti mes and th e individual t as ks, and de termine whi c h charac‐ teristics (e.g. steps, turning proc e dure, bending procedure, tr unk angl es, etc.) hav e t h e g r e at es t influence on a particul a r method. 3.2 Influential parameters  T a b l e s 3 t o 5 s h o w t h e r e s u l t s b y M T M , N T P M a n d J a c k s i m u l a t i o n . The aim o f t h e i nflue n tial parameter study was to d etermine how t he i ndividual meth od t ake s int o a ccount th e weight parameter and the load s ize para meter during the l ifting p rocedure. T he r esults s how the total t i m e s o f t h e e n t i r e s e q u e n c e o f a l l t a s k s . T h e l o a d t h a t a p p e a r e d i n t h e s t u d y i s a c u b i c b o x w i t h ‐ o u t h a n d l e s . T h e m a s s o f t h e b o x v a r i e s f r o m 4 . 5 k g t o 2 0 . 5 k g . T h e d i m e n s i o n s ( h e i g h t = w i d t h = d e p t h ) o f t h e b o x v a r y f r o m 2 0 0 m m t o 6 0 0 m m . T h e r e f o r e , t h e s t udy pre s ents 25 c o mbin ations of mass and dimens ional parameters. MTM method W h e n u s i n g t h e M T M m e t h o d , t h e m a s s a n d d i m e n s i o n s o f t h e b o x a r e t a k e n i n t o a c c o u n t w h e n c a l c u l a t i n g th e t o t a l t i m e ( T a b l e 3 ) . T h e t o t a l t i m e o f t h e t a sk s a n d t h e m a ss o f t h e b o x a r e l i n e a r ‐ ly d epende n t ( R 2 = 0.9 9 9 8 ; Fi g. 4 a). The same a pp lies t o the di mensio ns ( R 2 = 1 ; F i g . 4b ). T here‐ fore, if w e us e the MTM method, we c an p redict w ith reasonable certainty the total time f or this sequenc e o f tasks for eac h s ize a n d m a s s o f t h e b o x . T h e p r o b l e m with prediction is that these times ar e o f t e n to o short, w hich m ean s t hat a w o r k er i s not a bl e to p er form a ll movements with‐ out further d i scomfort. Table 3 MTM: to tal ti me analy si s fo r di fferent di m ensi o n s and masse s o f the box Mass o f the bo x [ k g ] 4.5 9.0 13.5 18.0 20.5 Dimension of th e box [mm] 200 9.32 9.60 9.89 10.17 10.34 Time [s ] 300 9.43 9.72 10.01 10.29 10.44 400 9.54 9.83 10.12 10.42 10.56 500 9.66 9.95 10.24 10.54 10.68 600 9.77 10.06 10.36 10.66 10.80 Simulation‐based time evaluation of basic manual assembly tasks   Advances in Production Engineering & Management 15(3) 2020  341 a) b) Fig. 4 Relations hip between the total t ime and the parameters : a) mas s, and b ) dimension using the MTM method NTPM method Table 4 shows the values o f the total times obtai n ed u sing t h e N T P M m e t h o d . I t c a n b e s e e n i m ‐ mediat ely th at t he d im en sions o f th e box are n o t included (Fig. 5 b). The r e lationship between t h e w e i g h t a n d t h e t o t a l t i m e i s l i n e a r ( R 2 = 0 . 9 9 7 7 , F i g . 5 a ) , s o f o r t h i s s e q u e n c e o f m o v e m e n t s we can predi c t the value of t he t ot al t ime for e a c h m a s s o f t h e b o x . To calculate the resul ts accord‐ i n g t o t h i s m e t h o d w e u s e s e v e r a l parameters (angular vel o city, trunk‐ext ension an gle) a nd t he mass o f th e b ox are used , which giv es more re alistic total ti me s co m p a r e d to the MT M m e tho d. Table 4 NTPM: total time analyses f or d i fferent di m ensi o n s and masse s of the box Mass o f the bo x [ k g ] 4.5 9.0 13.5 18.0 20.5 Dimension of th e box [mm] Not included 11.34 1 1.88 12.54 13.13 13.53 Time [s ] 11.34 1 1.88 12.54 13.13 13.53 11.34 1 1.88 12.54 13.13 13.53 11.34 1 1.88 12.54 13.13 13.53 11.34 1 1.88 12.54 13.13 13.53 a) b) Fig. 5 Relations hip between the total t ime and the parameters using t he NTPM: a) mass, and b ) dimension   Jack simulation tool The results in T able 5 s how that t h e t i m e c h a n g e s m i n i m a l l y w h e n the mass of the l oad changes. T h e r e f o r e , t h e m a s s o f t h e b o x h a s m i n i m a l e f f e c t o n t h e v a l u e o f t o t a l t i m e , w h i c h i s n o t l o g i c a l or empirical, as show n in Fig. 6 a – r e lationship b e t w een total time a nd t he ma ss pa ra me te r. Table 5 Jack simulation tool: t otal tim e analysis for different d imens ions and masse s of the box Mass o f the bo x [ k g ] 4.5 9.0 13.5 1 8.0 20.5 Dimension of th e box [mm] 200 12.06 1 2.20 12.34 12.51 12.57 Time [s ] 300 12.18 1 2.31 12.43 12.56 12.62 400 12.15 1 2.28 12.42 12.54 12.60 500 12.15 1 2.27 12.40 12.53 12.57 600 12.36 1 2.48 12.60 12.72 12.79 Turk, Pipan, Šimic, Herakovič    342  Advances in Production Engineering & Management 15(3) 2020 a) b) Fig. 6 Relationship between the total t ime and the parameters using the Jack s imulati o n t ool : a) mass, and b) dimension F r o m th e re s u l t s o f th e a na l y s i s o f t h e d i m e n s i o n s o f t he b o x i t i s c l e a r t ha t th e d i me n s i on s a f ‐ f e c t t h e l e n g t h e n i n g / s h o r t e n i n g o f t h e t o t a l t i m e . T h e c h a n g e o f th e durati on o f a t ask happens b e c a u s e i t i s n e c e s s a r y t o a d j u s t a l l p o s i t i o n s o f t h e b o d y a n d j oints separately f or e ach task in o r d e r t o h a n d l e t h e d i f f e r e n t s i z e s o f t h e b o x e s . T h e t o t a l t i m e varies b ecause w e cann ot a djust the joints i n exactly th e s a me w ay, which is similar to the sit u a t i on in reality (Fig. 6 b). W e c a n c o n c l u d e t h a t t h e r e s u l t s o f t h e s i m u l a t i o n s h o w t h a t t h e m a s s o f t h e b o x h a s a l i m i t e d effect on the total time . Howe ve r, a more de ta ile d a na lysis shows that the c h ange in mass strong‐ l y a f f e c t s t h e e r g o n o m i c a n a l y s i s o f t h e p r o g r a m p a c k a g e ( L o w e r ‐ B a c k A n a l y s i s – F i g . 7 , M a n u a l H a n d l i n g L i m i t s , M e t a b o l i c E n e r g y E x p e n d i t u r e , N I O S H L i f t i n g A n a l y s i s e t c . ) . T h e r e s u l t o n F i g . 7 shows force (L4/L5 force) affecting th e lumbar v ertebrae 4 a nd 5 . I f w e w a n t t o o b t a i n a " c o r ‐ rect" correlation between the m as s a n d t h e v a l u e o f t h e t o t a l t i m e in a s imu l ation tool, we s hould try to pl a ce t he a vat a r in d iffere nt pos itions a nd, b a sed on th e ergonomic results, extend / shorten the tim e o f e a ch task a n d consequ e ntl y obt ain mor e re g ular v al u es for the t otal tim es. a) b) Fig. 7 Lower‐ba c k analysis – Results fo r L 4/ L 5 fo r ces [ N ] base d on different masse s of the b ox: a) 4.5 kg, and b) 20.5 kg 4. Conclusion  T h e s i m u l a t i o n m o d e l o f t h e l i f t i ng p rocedure is developed for prediction of the tot al time r e ‐ q u i r e d f o r b a s i c m a n u a l a s s e m b l y t a s k s w i t h d i f f e r e n t l o a d p a r a meters. The results of t he s imu ‐ lation model were c ompared with d ifferent ti m e p r ediction metho ds ( MTM, N TPM) a nd v erified by l abor ator y experiment. Workers i n m anual as sembly s uffer fro m rigor o us time standards. W i t h t h e i m p l e m e n t a t i o n o f a s i m u l a t i o n m o d e l , t h e t i m e s t a n d a r d s o f t h e t a s k c a n b e e f f i c i e n t l y redefined to make it w orker‐friendly. Our research c an b e divided into two p arts. The first part c onc erned the individual times a nd the total times determined w ith four m ethods, two of w hich a re time p rediction methods (MTM, NTPM), a not her on e is s i m ulatio n o f t he w ork e r obtain ed b y Siemens Jac k s imulatio n tool, and the last o ne i s a laboratory e xpe r i m e n t . T h e r e s e a r c h w a s c a r r i e d o u t o n b a s i c m a n u a l t a s k s , which cover only a part o f the MMH tasks. T he m ain objective was t o d e t e r m i n e w h e t h e r t h e simulation t ool is suitabl e not only fo r ergon o mic analys es but a lso for s e tting time standards for the work ers. T h e l iterat u r e shows that w orkers u n d er tradition a l standards do not h ave enou gh time to perform basic tas k s, r esulting in work‐related m usculos keletal disorders and injuries. As we f ound o ut in the case s tudy, the basis is the classical MTM method, whi c h gives th e shortest total time o f the lifting s e quence. Th e NTPM u pgrades the MTM m ethod by e x tending the times d u e t o t h e l o a d c o n s i d e r a t i o n , w h i c h r e s u l t s i n t h e w o r k e r h a v i n g m ore time to r e cover. T h e simulation t ool, which is a lso based on the MTM method, leads t o differences i n the total time v a l u e i n c o m p a r i s o n t o t h e M T M m e t h o d . T h e r e a s o n i s t h a t t h e T B S t o o l a l s o t a k e s i n t o a c c o u n t the trajector i es o f the body, arms, legs, and the way the avat a r grasp the load a nd a dds a ddition‐ Simulation-based time evaluation of basic manual assembly tasks al time on these tasks compared to MTM times. We also verified the sequence of the tasks with an experiment that showed very similar result as the simulation tool. From this we can conclude that simulation is a suitable tool for designing time standards for a sequence of basic methods, although the duration of a single task varies depending on the method used. The second part of the study focuses on the load of the lifting procedure. We studied the in- fluence of the load on the total time of the lifting procedure. Parameters of the load that were studied in more detail are the mass of the load and the dimensions of the load. As load we used a cube-shaped box with an evenly distributed weight without handles. The case study was per- formed as a sequence of tasks with 25 different load parameter combinations (5 different box masses x 5 different box dimensions). The case study was performed using three different meth- ods, MTM, NTPM, and the Siemens Jack simulation tool. Depending on the method used, we ob- tained different results. We found that the MTM method takes both parameters into account but, as reported above, gives too short times for a healthy working practice. The NTPM method only considers the mass of the box, while the dimensions are not considered. The simulation tool, on the other hand, takes the dimensions of the box into account, but the mass of the box has mini- mal effect on the duration of the tasks, which is in contrast to practice. To overcome this draw- back, we have found that the mass has a "correct" effect on the ergonomic result shown by the Lower Back Analysis, so the more detailed study of ergonomics implemented in the simulation tool can lead to a better correlation between the time of execution of each task and the mass of the load. Further research work will include a determination of this correlation, and how this can be used for an even more realistic estimation of the total time. Acknowledgement The work was carried out in the framework of the GOSTOP programme (OP20.00361), which is partially financed by the Republic of Slovenia – Ministry of Education, Science and Sport, and the European Union – European Regional Development Fund. The authors would also like to thank the Research and Development Agency (ARRS) of the Repub- lic of Slovenia, which made this study possible. References [1] Nogueira, H.C., Locks, F., Barbieri, D.F., Oliveira, A.B. (2018). 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