Drive for a door or window sash

The drive 10 comprises a piston 16 slidably guided in a housing 12, a spring 14, an output shaft 20 and a rack-and-pinion gear 18 with an out-of-round pinion 22 with teeth 24 and counter-teeth 26 on the piston side, wherein a tooth pair is designed such that when opening the sash a rising line of action results, which runs horizontally in the direction of displacement of the piston, and wherein the effective lever arm length of the pinion teeth decreases upon opening of the sash up to a first opening angle, then after a second angle the length increases with an increasing opening angle. The line of action may be produced by a profile shift varying when rolling over the curve. The pinion and piston each have an asymmetrical tooth and the output shaft is provided with an undercut. The piston may be hollow with internal toothing.