Abstract: The control knob includes an aperture with a D-shaped cross section. The aperture is formed by a relatively rigid planar portion and an arcuate portion which further includes a relatively flexible inner concentric portion and a relatively rigid flexible outer concentric portion. The relatively flexible inner concentric portion is attached to the relatively rigid planar portion by living hinges and further includes outwardly pointing stop bumps. The outwardly pointing stop bumps are spaced from said relatively rigid flexible outer concentric portion when no rotational torque is applied to the control knob with respect to the shaft and the structure allows torsional elasticity when a limited amount of rotational torque is applied to the control knob. However, when torque is applied which exceeds a threshold, the stop bumps are urged against the relatively flexible inner concentric portion and additional torsional elasticity and additional rotation is limited.