Human factors and behavioral science: Effects of shape and size of knobs on maximal hand-turning forces applied by females

Outside plant craftspeople use knobs to apply turning forces on clamp mechanisms that hold field equipment temporarily in place. A study was performed to develop a set of data that provide guidance for determining knob size and shape characteristics most appropriate for various outside plant working conditions. Forty female participants applied maximal isometric turning force to each member of a set of twenty experimental knobs that systematically varied in shape and size. In half the trials the participants applied force with greased hands and in the other half used nonslip compound. In addition, two arm-wrist positions were observed. In general, triangular knobs allow more hand torque to be generated and require significantly less material than square, pentagonal, hexagonal, or circular knobs of comparable size. However, this effect depends upon the arm-wrist position and grip conditions. A 3.5-inch turning diameter is desirable when both cost and performance are considered.