Abstract: The pump comprises a casing containing the pump shaft. The casing includes a lateral housing containing an axially slidable shaft. The slidable shaft carries a first pinion, while the pump shaft carries a second pinion which is firmly connected thereto. The slidable shaft is held in a first extreme position by a spring and can be axially displaced, against the force of the spring, to a second extreme position by pneumatic control means. The control means include a piston portion which forms part of the slidable shaft. One of the said positions is a working or pumping position in which there is a driving contact, e.g. between the sliding shaft and a driving shaft, between the first pinion and a driving pinion, or between a first and second pinion.