روش اندازه گیری زمان مبتنی بر تجزیه و تحلیل حجم کار ارگونومیک  

Production engineers are urged to take ergonomic considerations into account when planning for production. However, the means to do so are often inadequate with respect to training as well as support. There is a lack of methods to predict, at a reasonable cost, biomechanical load on an operator performing a task not yet observable on a product and in a workplace not yet existing. The purpose of this study was to develop an ergonomic complement to a modern MTM system called SAM that gives the production engineer a first insight into the future ergonomic quality of a planned production. A method was developed that requests the engineer to supply two additional pieces of information to the analysis: the zone relative to the operator's body in which the movement takes place or ends, and the weight or force involved in the operation. As method of comparison for validation purposes was selected the operator self-evaluation method VIDAR. The method was tested at the Torslanda final assembly plant of Volvo Car Corporation and at the ITT Flygt plant for large submersible pumps at three different balances and compared with two different methods of assessing biomechanical load. The results show that the method identifies the events causing high biomechanical load on the operator so that they can be redesigned. The suggested method has proven to be a useful tool and is being introduced at Volvo Car Corporation. Relevance to industry Under the concept of concurrent engineering the production engineer will have less time for experimenting with new production layouts. Given access to early design drawings of the product she or he must be able to decide on the future layout. There is a need for a tool giving the possibility to estimate simultaneously the consumption of time in the envisaged production, and the biomechanical load inherent in the planned tasks.

1.1. MTM in industry Work analysis methods were taken into use in the industrialized countries on a larger scale in the 1930s. A number of predetermined time systems (PTS) were developed, of which the method time measurement (MTM) method was developed by Maynard in the United States (Maynard et al., 1948). One reason why the MTM system became the one most widespread is probably due to the fact that it was made publicly available with no economical or judicial claims on behalf of the inventor. The MTM Association for Standards and Research was founded in the United States in 1951. The managing director of the Volvo Car Corporation, took the responsibility of introducing MTM in Sweden, and the first installation of MTM was made at the Volvo engine factory in Skövde in 1950. The work lead to the foundation of the Swedish MTM Society in 1955 as a result of an initiative from a sub-committee of the Swedish Academy of Engineering Sciences (IVA). The strong interest in the MTM system may be explained by the potential it holds in rationalization of work and the possibility for employers and unions to negotiate and agree on piecework contracts (Luthman et al., 1990). In Sweden, IVA decided to exclude physical workload from the agenda of all its work study sub-committees and consequently the research on production technology and work physiology was separated. During the late 1960s and early 1970s the MTM system was criticized and associated with Taylorism. Production technology was no longer in focus in an industry that was being more oriented towards market and product development. The piece-rate system was commonly replaced by time-rates. In the middle of the 1970s the Swedish MTM Society was dissolved and the previous general interest for MTM was often turned to its opposite (Luthman et al., 1990). However, in practice the MTM system and its modern versions are in widespread use in many companies and is used to calculate production times for line balancing, line pace setting and in calculation of business tenders. 1.2. SAM An analysis using the basic level of MTM, MTM-1, is a very time-consuming task. This led already during the 1950s to initiatives to combine MTM data in order to simplify and thus decrease time needed for analysis. The Swedish MTM-society took the initiative to a work that led to the development of MTM-2 that in 1965 was accepted by the International MTM Federation as an international standard. The work continued with the creation of MTM-3 that, however, never was widely accepted. During the 1970s a project was therefore carried out that resulted in the SAM system in order to shorten the time needed for analysis but eliminate the defects found in the MTM-3. SAM groups several MTM-1 movements into one SAM movement and excludes many special cases. The analysis is simplified resulting in shorter times both to learn how to use the system and to carry out an analysis, and the loss of precision is not discouraging (SAM-2 Kompendium, 1995). Similar systems have been developed elsewhere for much the same reason, such as MOST and MTM-UAS. Many of the modern developments of MTM derivatives have also been computerized (for a review, see Karger and Bayha, 1987). 1.3. MTM as base for ergonomic evaluation Studies to use the information received from MTM for ergonomic evaluations have been carried out in Germany since the early 1980s. Calculation of the distribution of the work task over the different body regions, giving the possibility to detect an uneven balance between load on the right and left hands during work by means of MTM-1, was done by Kühn and Laurig (1990). They concluded that the most appropriate is a “pragmatic ergonomic approach” and that this approach has a problematic process of validation but is acceptable because of its reproducibility. In the computer program ErgoMOST (H.B. Maynard and Company, Inc.) the methodology of MOST is used as an ergonomic survey tool. It is a special program, separated from the corresponding MOST software, in which it is possible to define e.g. joint angles, grip type, external forces and repetition in order to get an assessment of biomechanical load during a work sequence. However, the level of detail and thus the time needed to analyze a work sequence may be deterrent to a production engineer if not to an ergonomist. At the National Institute for Working Life in Sweden, Peterson et al. (1996) studied the relationship between some MTM activities and electromyographic (EMG) amplitude. They concluded that “…a SAM-analysis to some degree may predict exposure levels of some prime movers and stabilizers in the forearm and shoulder-neck during light manual tasks” but that the lack of information on “…e.g. work height, may weaken the predictive power of SAM activities”. 1.4. Purpose of present study Production engineers are more and more urged to take ergonomic considerations into account when planning for production. However, the means to do so are often inadequate with respect to training and support. The most frequently used methods for ergonomic considerations are company standards of, e.g., working heights and weights of objects handled, and checklists to remind the engineer of human limitations (e.g., Volvo Corporate Standard, 1994). There is a lack of methods to predict, at a reasonable cost, biomechanical load on an operator performing a task not yet observable on a product and in a workplace not yet existing. The purpose of the present study was to develop an ergonomic complement to SAM that gives the production engineer a first insight in the ergonomic qualities of a planned production. In a company that uses MTM or its derivatives (SAM, MOST, MTM-UAS, etc.) for the purpose of production planning and/or business offers, a large amount of work is performed to establish the expected time needed for a certain task to be performed by the operator. If the extra effort needed for an additional ergonomic evaluation is small, the incentive may be enough to include ergonomics in the overall evaluation of the planned process. The approach chosen was to base the development of the planning tool on SAM and on the Cube Model (Kadefors, 1994) for ergonomic assessment.

It is concluded from the present study that ErgoSAM can predict high load situations from the different tasks during the work sequence. Repetitive tasks are prevalent in manual industrial work and the source of many work related diseases, and ErgoSAM has the potential of being a tool for early detection of such risks. The method is proven to estimate correctly biomechanical load occurrence. When comparing different alternatives for technical and organizational design of a future production system, it will be possible to optimize with respect to productivity and biomechanical load simultaneously. The present system should be seen as a prototype, and the present paper as an evaluation of some basic properties of the ErgoSAM method. If ErgoSAM is to be used as a future tool by production engineers it is of vital importance to point out what type of musculoskeletal strains that are not identified by ErgoSAM. This may be done in the form of a checklist that is presented to the engineer in a suitable manner during the ErgoSAM analysis. To get a well-grounded insight in the usefulness of ErgoSAM there is a clear need for extensive continued validation e.g. many balances and many operators. Such work is presently being carried out by the National Institute for Working Life at Göteborg in cooperation with the two companies Volvo Car Corporation and ITT Flygt in Emmaboda, Sweden. Recently, based on these and more recent findings (Christmansson et al., 2000), Volvo Cars have decided to use the ErgoSAM method in their future planning work. It has been found that albeit there are imperfections in the method, it provides information in the planning stage that is not available in the present design tools.