All About Engines

8 All About Engines steel connecting rod serves to communicate any motion of the piston to the crank shaft of the engine. Now, if steam is admitted to the cylinder behind the piston, the latter will be forced forwards, the push will be communicated to the crank, and the crank and the shaft will turn until the crank pin is exactly in line with the piston rod and on the farther side of the shaft from it. The piston will now be at the front end of the cylinder, and no further motion in the same direction will be possible. But if steam be admitted between the piston and front cover of the cylinder, the piston will be forced backwards, and the pull on the connecting rod will bring the crank round another half turn. So that if this pro- cess be repeated, if steam is admitted alternately behind and in front of the piston, the shaft will probably continue to rotate in the same direction. So far as the description has carried us, however, there is a possibility that the crank, having made one half turn, will make the next half turn back- wards. Moreover, with a single cylinder engine there would be a tendency to stop on the “ dead points,” when the piston rod, connecting rod, and crank were in a straight line. For in that position the connect- ing rod merely pushes or pulls the crank-pin, and exerts no turning effect on the shaft. This difficulty is overcome by the flywheel. A heavy wheel requires a good deal of force to start it from rest, or to increase its speed, and when once it is moving much force is required to decrease its speed or to stop it. The first stroke of the piston sets the flywheel moving, and