Drive motor for cleaning glass surfaces

Claim 1

A drive motor for operating a cleaning device of the type having at least one wiper blade mounted for movement adjacent a glass surface to be cleaned and adapted to be driven by the drive motor across the glass surface to clean said surface, said drive motor comprising a hydraulic cylinder having a liquid inlet opening adjacent one end thereof and a liquid outlet opening adjacent the other end thereof, means for supplying liquid under pressure to said inlet opening and for receiving a return of liquid from said outlet opening, said liquid inlet opening having a flow area which is substantially greater than the flow area of said outlet opening, a slidable piston located within said cylinder for reciprocating movement, means connecting said slidable piston to said wiper blade of said cleaning device, said connecting means including rack and pinion means for converting reciprocating motion of said piston to rotary motion of said wiper blade, spring means between said piston and said cylinder for resiliently urging said piston toward said one end of said cylinder, said piston having a surface extending across said cylinder which surface is always in direct communication with said liquid inlet opening and continually exposed to the liquid under pressure at said inlet opening whereby said piston is movable against the restraint of said spring means toward said other end of said cylinder in response to forces exerted upon said piston by liquid pressure applied to said liquid inlet opening, said piston including an overflow passage extending therethrough for selectively equalizing the pressure on opposite sides of said piston, a valve member disposed adjacent said overflow passage, said valve member being operated by impact with a predetermined portion of said cylinder to open said overflow passage in response to movement of said piston in a working stroke from said one end to said other end of said cylinder, and said valve member thereafter being operated in the opposite direction by impact with a different portion of said cylinder to close said overflow passage in response to subsequent movement of said piston by said spring means in a return stroke from said other end to said one end of said cylinder, said overflow passage when open having a flow area substantially equal to the flow area of said liquid inlet opening and substantially greater than the flow area of said outlet opening, whereby the speed of movement of said piston is substantially the same during both said working stroke and said return stroke and is dependent upon the comparatively small flow area of said liquid outlet opening.