Polyurethane prepolymers are described which are prepared from 2,4\u2032-diiso-cyanate diphenylmethane, a polyol component and an aliphatic diisocyanate. The gray-scale voltage generating circuit includes the common gray-scale voltage generating circuit and a plurality of individual gray-scale voltage generating circuits, wherein every individual gray-scale voltage generating circuit generates individual gray-scale voltage and corresponds to one of the displaying colors of the pixels of the LCD panel. The opposing brush has metal bristles that can dean golf shoe cleats and golf clubs. The ball-screw nut has an outer peripheral surface including a ball rolling groove for the spline nut, the ball rolling groove being formed over a range corresponding to the screw portion and the extended portion.