Effects of Bit Type on Maximum Torque and Axial Force Using Manual Screwdrivers

Mark Hickok, Marquette University


The screwdriver is a tool that has been among the most widely used hand tools for decades and continues to be used in the workplace to perform a variety of fastening tasks. Advancements in fastener technology have been complemented by the development of new types of screwdriver bits. While designs may vary, so do the force application requirements placed on the tool user. The primary objective of this experiment is to analyze the relationship between user torque and screwdriver bit design. A further objective is to utilize the results to develop an effort metric by which bits of different designs can be compared. In this experiment, three types of screwdriver bit designs (straight, Phillips, and combination of straight/Phillips (ECX)) were tested to determine how the design affects the amount and type of force applied by the user when performing a fastening task. The designs were tested to simulate fastener tightening and loosening operations. Sixteen participants were tested in this study. Although there was no significant effect, the data suggest that the Phillips bit design allow subjects to exert the maximum torque and the minimum axial force. This divergence suggests that the Phillips bit may have a higher biomechanical effort ratio, which is greater torque for the same or lower axial force. Finally, the data suggest there is little difference in user torque exertion between the ECX bit and the straight bit designs. Subjective assessment indicated that users overwhelmingly preferred the Phillips bit design. Bit designs requiring less axial force for the same torque exertion level reduce the overall muscular effort of the user, allowing work to be completed more efficiently and may reduce the risk of musculoskeletal disorder affecting the wrist, elbow, and shoulder. Results may also assist designers by allowing them to select fasteners that provide sufficient mechanical integrity of the design while maximizing user effectiveness.