Screwdriver Bit Head Design – Effect of Phillips, Straight, and a Hybrid Design on Torque, Axial Force, and Effort Ratio

Document Type

Conference Proceeding

Publication Date


Source Publication

Proceedings of the Human Factors and Ergonomics Society Annual Meeting

Source ISSN



Advancements in fastener technology have been complemented by the development of new types of screwdriver bits. While designs may vary, so may the force application requirements placed on the tool user. The primary objective of this experiment was to analyze the relationship between user applied torque and screwdriver bit design. A further objective was to utilize the results to develop an effort metric by which bits of different designs can be compared. Three types of screwdriver bit designs (straight, Phillips, and combination of straight/Phillips (ECXTM)) 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. The data suggest there is no difference in user torque exertion between the ECXTM bit, Phillips, and the straight bit designs in either direction, 2.61-2.97 Nm for pronation and 2.63 -2.85 for supination. Mean axial force was significantly less for the Phillips (67 N) than the other 2 bits (72 and 80N). Although there was no significant effect of bit head design on maximal torque and axial force, the data suggest that the Phillips bit design may allow subjects to exert less axial force, which would result in a higher biomechanical effort ratio. A greater effort ratio would produce greater torque for the same axial force or the same torque for a lower axial force. Mean effort ratio for the Phillips bit was 3.6 N/N (Sup) and 4 N/N (Pro) and approximately 3.0 for the other two combinations of bits and directions. Subjective assessment indicated that users overwhelmingly preferred the Phillips bit design


Published as part of the proceedings of the conference, the Human Factors and Ergonomics Society Annual Meeting, 2014: 1580-1584. DOI.