All About Engines

IIO All About Engines against the flanges, and prevents steam from leaking through underneath the rings. Just as a soft packing became useless for pistons, so also it is going out of use for stuffing boxes—at any rate in engines using superheated or high- pressure steam. A good form of metallic packing is that made by Lancaster and Tonge, shown in Fig. 62. Here the stuffing-box contains blocks of metal fitting closely round the piston rod and held in con- tact by spiral springs. For pressures above 120 lb. on the square inch an additional length of packing of a different kind is used, as in Fig. 63. In this case there is only one spiral spring to hold the blocks in place, and the blocks themselves are built up in such a way as to form rings of triangular section. These are pressed closely against the rods by the action of the short spiral springs at the back. The pressure on the rod or spindle is much more uniform over the whole area of contact with metallic, than with the softer, packing, and the friction is less. Balancing Before considering expansive working it will be interesting and profitable to examine some of the forces which are set up within the engine itself. These forces arise from two causes. In the first place the piston, piston-rod, cross-head, and con- necting-rod are all moving forwards during one stroke and backwards during the next one, changing their direction of motion twice in every revolution. If the engine is making 200 revolutions per minute there