Modelling the relationships between physical risk factors and operator performance in short cycle

Abstract

Physical risk factors and their combinations are linked to the causation of Musculo Skeletal Disorders (MSDs) which has implications for on-site operator productivity. Current risk assessment methods have no element to account for productivity improvements associated with risk reduction. Engineering methods such as MTM are limited in their ability to consider the physical effects of physical risk factors. Productivity is an important benefit of many ergonomic interventions. Therefore, productivity benefits are not fully exploited when the results of ergonomic cost benefit analyses are communicated to management. The effects of on-site productivity loss are often much greater than productivity loss associated with absence from work. The purpose of this research was to develop a quantitative presenteeism model to estimate the productivity benefits of ergonomic interventions for short cycle assembly work.

Results of an industrial study revealed that in the companies investigated MSDs were prevalent and discomfort levels were high. MSDs and discomfort affect operator’s on-site performance and also result in absence from work. High levels of force, deviated postures were present in combination and for long durations of exertion.

A series of experiments were conducted involving repetitive grip exertions of the upper limb to investigate the relationship between physical risk factors, discomfort and productivity. Twenty seven participants completed the first experiment to investigate and model the relationship between force, posture, repetition, discomfort and productivity (Self Paced Cycle Time (SPCT)). In the second experiment (21 participants) the relationship between Self selected DCT, discomfort, force and duration of exertion was studied. Multiple regression analysis was used to develop a predictive model for duty cycle time based on force and exertion period, and this was a good fit to the data (R2 = 0.98,p <0.05). Profiles were generated presenting zones of acceptable self-selected duty cycle times based on force and exertion. Twelve participants preformed the third experiment to investigate the effects of grip type and posture on forearm muscle activity, the effects of grip type and posture on endurance and the effects of force, grip type and posture on performing a precision task (Fitt’s tap test). Eighteen participants were involved in the fourth experiment to profile risk factor effects over a range of grip types.

Data from experiments one, two and four were used to develop profiles of DCT, regression equations, data and models to populate the productivity model. The model presents a way of quantitatively estimating on-site productivity benefits from ergonomic interventions. Benefits communicated as DCT are easily understood and may be used to better estimate cost savings on the basis of productivity improvements.