�necessary restraint both against convergent and divergent motion is obtained. If we suppose further, that means are provided for preventing lateral motion, we have before us a closed pair.
This method of closure is not practicable in the cases commonly occurring ; possibly it seems here somewhat far-fetched ; we shall
�see presently, however, that it is in no way without precedent. In practice we use rather an easily arranged kinematic linkage between a and b. If, for instance, we attach both to a and to ~b (conaxially with their axoids or pitch surfaces), solid cylinders, enclose these
�in open cylinders, and connect the latter by a rigid bar (Fig. 135), the restraint in the direction N N of the normals is perfect (we may here, as also in the last case, allow the points of the teeth to be quite free, as they are no longer required for restraint). Instead of a closed kinematic pair, we have now a closed kinematic chain