358 KINEMATICS OF MACHINERY.
81.
Chamber-crank Trains from the Swinging Block. Plate XVI., Figs 1 and 2, aiid Plate XVII.
The second of the four mechanisms which are obtained from the chain (C^P- 1 ) was the swinging block slider-crank, the mechanism in which the chain was placed on the coupler b, (CgP- L ) b . It has been employed as a chamber-crank train in a number of ways.
Probably the oldest form is that of the oscillating steam-engine, Fig. 2, PL XVI., invented by Murdock,* 1785. The former coupler ~b has become the fixed link, the crank a turns about the pin which was the crank pin ; the block c has taken the shape of the cylindrical chamber and the slide d that of the piston. The chambering can be expressed in the form ( F=t) d, c, while the special formula of the mechanism is (C^'P-Md. The valve-gear used by Murdock was so arranged that a fixed but somewhat elastic arm caused the rod of a common D slide-valve to move up and down parallel to the piston rod, so that the slide was always in the middle of its stroke when the crank was at a dead point. Later on the steam admission and exhaust were managed by means of suitably formed openings in the hollow pin or trunnion 3, or in some piece conaxial to and connected with it, and the same method is still applied in cases where the mechanism is to be employed as a hydraulic engine or a pump. We may call such an arrangement trunnion-valve gearing. If it be required to use the fluid ex- pansively a more complex gear must be used.
A comparison between Fig. 2, PL XVI., and Fig. 1, PL XIV. shows that apart from its valve-gear the oscillating steam-engine is simply an inversion of the direct-acting engine. The signs of the pair 4 are also reversed, so that the piston of the latter becomes here the " cylinder," while its " cylinder " is here the piston. If the chain had been inverted without this pair- inversion, movable steam and exhaust pipes would have had to be used on account of the motion of the chamber. If, however, the gearing were so altered that the ports could be carried through the piston-rod, it might be possible to arrange the admission and discharge by
- Muirhead, Inventions of James Watt, vol. iii., plate 34.
