The present invention relates to a device for backlash-free cam control. Such devices are required, for instance, in the construction of photographic lenses in order to displace one or more lens members with respect to the objective tube, upon zooming or for making a distance setting. It is customary to provide one of the relatively displaceable parts with a cam guide curve and the other with a follower which slides along the guide curve. In order to obtain freedom from backlash between the parts the following possibilities are present in accordance with the prior art:
The control curve is developed as a groove having parallel surfaces and the follower which is forced into the groove bears an elastic sleeve. This has the disadvantage, however, that the follower moves in the groove with friction, which results in undesirably stiff movement of the device. The sleeves, which are generally made of plastic, are subject to considerable wear when placed under strong stress, which wear has a negative effect on the high accuracy of adjustment required for optical instruments.
It is possible to develop the follower as a roller and to apply it, free of backlash, against the guide curve by means of a spring which acts between the parts which are movable with respect to each other. Displacement devices which are made free of backlash in this manner can be moved very easily. In the case of steep curves, however, there is the undesired effect that upon termination of the adjustment force, and as a result of the greatly reduced friction, the follower may be moved by itself by the tensioned spring. In order to prevent this, the friction of the parts to be displaced must always be greater than the spring force which acts in the direction of the guide curve.
From West German Pat. No. 2,346,398 it is known, for the displacement of two lens systems in opposite directions, to apply each of the corresponding followers against its guide groove via a spring in such manner that upon the movement of the followers one spring is tensioned and the other spring is relaxed by the same amount. In this way, backward-driving forces are avoided and only slight frictional forces need be overcome upon the displacement. The device described can be used, however, only if two parts are to be displaced with respect to each other, in which connection the two courses of movement must be in a fixed proportional relationship to each other. Since two grooves and two springs which are adapted to each other are required, it is furthermore also expensive to manufacture.