Ergonomic evaluation of manually-operated peanut butter mills

Abstract

Previous studies showed that the manually-operated peanut butter mills available on the market had technical problems related to the design and operation of the machines. One such problem was that the mills were too heavy for women to operate, resulting in limited operational time and ultimately, low total output. An ergonomic study of the original and modified versions of the mills was conducted at the University of Zimbabwe to verify the previously identified problems and develop appropriate and lighter mills for manual operation. A body discomfort assessment and heart rate measurement were used to determine stress endured by 12 women, as a result of operating the mills. Medium to high levels of discomfort were experienced in the lower back, neck, chest, lower arm, upper arm and shoulder. The heart rate readings showed that the mills currently available on the market were highly stressful (138 beats/minute), exceeding guidelines for safe manual operations. The modified mills, incorporating variable feed control devices, can be adjusted to ensure that stress levels are within recommended levels. The ergonomic study also established that for feed rates of 1.1 and . 1.5kg/hour, the mills produce peanut butter of acceptable fineness to the subjects, in one pass without over stressing the operator. In a separate field experiment using the same subjects, extremely high stress levels ( 150 beats/minute) were recorded with the traditional stone-mill.