The effect of assembly tolerance on performance of a tape application task. A pilot study.
Artikel i vetenskaplig tidskrift, 2004
Task characteristics such as visual and manual demands influence the postures employed and the muscular load during task execution. According to ergonomic experts at a car assembly plant, these demands are difficult to predict during product and process development and can easily be underestimated. One such task with combined visual and manual demands is the application of deco-tape to the door frame of cars. This precision task requires the exact manual positioning of the tape and a careful visual control in order to continuously adjust alignment and to check for any air bubbles fastened under the tape. This task has to be carried out within limited time determined by the pace of the assembly line, thus requiring both high precision and a certain speed.
The present study aimed at the question: How do different degrees of assembly tolerance affect the execution of a precision assembly task?
Deco-tape fixation was transformed into an experimental task in order to study the effect of different degrees of precision demands.
Ten subjects without previous assembly experience completed 60 experimental trials each. In each trial a tape was affixed in an area indicated on a frame. Thirty trials required high precision with minimal assembly tolerance - that is the tape was affixed in an area that had the same widh as the tape. The remaining 30 trials were carried out with lower precision demands. - the indicated area provided an assembly tolerance of 3 mm at a tape width of 19 mm.
Task completion time, the movement path of the right hand and head, and the occurrence of quality defiences were registered.
The study showed strong effects of the change in assembly tolerance on the speed, postural behaviour and movementpaths during the task execution:
Tasks with high precision demands required significantly longer completion times. Higher precision demands lead to a working posture with less variation in the distance between the right hand and the head. During tasks with high precision, the movement path of the right hand was less economic - with smaller movement cycles of shorter duration - allowing for a more frequent control of results. It was concluded that precision demands play an important role for the speed, working posture and movement path during an assembly task. Considering these effects and their potential implication for the muscular load during task execution, it seems important to consider task characteristics such as precision demands in detail when designing products for assembly. A careful consideration of these task characteristics is essential when using simulation tools such as mannequins in extended phases of future product development process.
Relevance to industry
Current ergonomic guidelines often focus on the physical layout of a work place, while task characteristics such as visual and manual demands are considered to a lesser extent. However, these task characteristics may play an important role for assembly workers postural behaviour and the muscular load during work. Increasing use of simulation tools instead of physical prototypes in the product and production development process makes it even more difficult to evaluate task characteristics such as precision demands. On this background, more knowledge about the effect of task characteristics such as precision demands is needed. The present pilot study provides a detailed analysis of the effect of different precision demands on the work posture and execution of an assembly task - deco-tape fixation.