Evaluation of the effect of two-handed tool design characteristics and orientation of the workpiece on comfort, discomfort, usability and torque

Abstract

Introduction: Despite the mechanization of most industries and the electrification of tools, a significant portion of activities still requires the use of hand tools. In fact, working with hand tools is considered the primary level of interaction for the operator in the workplace. Given the limitations of humans when facing systems and tools, the characteristics of the hand tools used by humans often attract researchers' attention. Hand tools designed with ergonomic standards can reduce occupational injuries in the upper limbs and provide comfort for the user while ensuring high-quality products for consumers. Moreover, improving the usability and quality of hand tools enhances efficiency and productivity in work. In light of the above, the present study focuses on examining the impact of hand positioning, force application direction, and posture on the level of comfort, discomfort in the forearm and fingers, usability, and torque when working with T-shaped wrenches, which are among the widely used tools in industries. Methods:This study was conducted in the laboratory of the group located in the Faculty of Health at Tabriz University of Medical Sciences and Health Services. In this study, the effect of independent variables on dependent variables was measured and assessed. In total, there are 24 combinations for each individual (2 postures for the forearm in supination and pronation, 2 hand arrangement positions in Clock 3&9 and Clock 6&12, 2 movement directions (clockwise and counterclockwise), and 3 workpiece orientations as vertical or 90 degrees, horizontal or 0 degrees, and 45 degrees). Measurements were taken in different conditions of workpiece orientation and movement levels using a torque meter, which was calibrated before the start of the study. Two maximum strength measurements were recorded for each condition (with a two-minute break between consecutive measurements), and the average of the two recordings was used for analysis. These two measurements are considered valid if the difference between them is less than 10%, otherwise, the strength measurement was repeated. Results: Forearm supination posture, clockwise movement direction, horizontal direction of the work piece and placement of 6 & 12 hands provided the maximum power of the hand torque as well as the maximum usability of the system and the feeling of comfort. The greatest amount of discomfort in using the system is related to the pronation posture of the forearm, the vertical direction of the work piece, the direction of counter-clockwise movement, and the placement of 3&9 hands. The highest score of feeling of comfort is also reported in supination posture, horizontal direction of the workpiece, arrangement of 6&12 hands and clockwise movement direction. The findings of this study showed that the torque, usability of the system and the feeling of comfort are influenced by the direction of force, the direction of the part and the placement of the hands, and the posture of the forearm has no significant relationship with the power of the hand torque. The direction of applying force, the direction of the part and the placement of the hands and the posture of the forearm play a major role in the severity of hand discomfort. Also, the examination of the results shows the significant impact of the main effects of all 4 factors and a large number of 2nd and 3rd order and also 4th order interaction effects. The significance of interactions means that the effect of a factor on the dependent variable depends on the levels of the factor or other factors. The results of this study can be used in redesigning hand tools, workstations and improving usability by researchers and manufacturers with the aim of reducing injuries and increasing productivity.