The linear drive unit incorporates a motor (58) in a housing, and a driving rod (64) with a hollow poriton by which it slides axially over the housing under the action of a transmission (62) between the motor and itself. The transmission incorporates a rotary body (92) with an external peripheral groove (160) and teeth (162). Rotary bodies (166) on a common axis each have peripheral grooving (172) and teeth (174), the grooving engaging on the inside the groove in the rotary of the transmisison on the outside it engages peripheral grooving 8164) on the inside of the rod hollow portion. The grooving is so shaped that there is no relative axial movement between the rotary bodies but the rod slides axially w.r.t. them.
