d being fixed, a can revolve while c swings about its axis. For this we must always have the conditions
a + b + c ^> d a + d + c > b
a + b - c < b a + b c ^b
and a the smallest of the four links ; the letters here standing for the lengths of the links between the centres of the pins.
The parallelism of the cylinder pairs makes all the centroids plane figures, and all the axoids cylinders. In its applied forms the link a is always known as a crank, and from this we may call the chain a cylindric crank-quadrilateral, or, more concisely, a
quadric (cylindric) crank-chain. The mechanisms obtained by fixing one or other of the links will then be called quadric (cylindric) crank mechanisms or trains. The designation cylindric requires to be retained, as we shall presently become acquainted with crank mechanisms of another kind. The mechanisms occurring are four in number, their contracted formulae being (C″4)d , (C″4) b , (C″4)a (C″4)c We may take them briefly in order. The mechanism (C″4)d. "We have met with this mechanism often enough to be now tolerably familiar with it. Its links possess such totally distinct functions that we may venture to use for them distinct names, this will enable us make our descriptions shorter and more exact. We shall call
- a the crank
- b the coupler
- c the lever
- d the frame.[1]
The characteristic of the mechanism (Fig. 201) is that it has both a
crank and a lever among its moving links, the one turning while
- ↑ I propose to distinguish between links that can turn completely round their centres and those that can only swing to and fro by calling them cranks and levers respectively. I do not think this will lead to any confusion, and it often greatly simplifies the nomenclature of the trains, as will be seen further on. For brevity's sake I have used coupler instead of the more common 4 but much longer name connecting rod.
