�illustrations of force-closed pairs ; they are kept in continual contact with their partner-elements the rails, by vertical down- ward closing forces.
In all these and similar cases force-closure presents itself ob- viously and naturally, and often greatly simplifies the construc- tion. This, however, is only one of the ways in which it occurs ; we must proceed to consider others.
�40. Force-closure in the Rolling of Axoids.
"While in the cases just mentioned the object of the closing force is essentially to prevent the separation of the incompletely formed elements, it may under certain circumstances have a much wider purpose. This occurs when the action of a force derived from it, friction, is used to complete the reciprocal restraint of the elements. An illustration of this is furnished by the friction wheels (Fig. 123) already mentioned in 37. Here the force-closure has not merely to hold the two cylinders in contact, but also to press, them so strongly together as to prevent sliding under a given tangential force, in other words, to cause the cylindric sur- faces, which here are the axoids them- selves, to roll upon one another.
By a more strict examination into this case, we can see that the closing force presses the small roughnesses of the surfaces together so as to make the cylinders work like spur- wheels, such force-components as tend to separate the wheels being
resisted by the closing force. It is this consequence of the pressing together of two bodies which is considered in mechanics under the name friction. 22
Tli is employment of force-closure occurs very frequently. Its application in the case of the driving-wheels of locomotives is im- portant in the highest degree. The whole development of our