DETAILS OE SCREW ENGINES.

305

The semi-solid head is so termed from thefact that the cap and the portion on the rodare alike, as illustrated by Fig. 170. This

Fig. 170.

MESSltS. MAUDSLAY’S CONNECTING SOD HEAD.

design is the most symmetrical yet introduced,and the credit of the same is due to Messrs.Maudslay, who have combined strength andelegance in this case. The brasses are circu-lar, and prevented from turning around by thebolts, and laterally by the flanges. The boltsand nuts are retained by the set screws, essen-tialities with high speed engines. As theexample under notice is shown by two views,the remainder can be clearly understood.

Now, with reference to which is the bettertype of head, the flat or circular brasses,before deciding the question, attention mustbe devoted to the principle. The connectingrod is the only detail that does receive threedifferent motions at the same time con-tinuously in the marine engine. The small°r block end is guided by its connection,an d thus slides and oscillates simultaneously,w bile the crank end moves in a circle. Asthe crank pins are generally large in proportionto their length—for practical reasons, such asforging and turning the shaft, the head of ther °d is increased for that reason, rather than

any other. It is, of course, certain that thegreatest strain on the head is when the rod isat the greatest angle ; and, therefore, the sidedisplacement of the bolts are more liable atthat position. Now, as contact of surfaceproduces stability in all cases, it is obviousthat the longer the bolt hole in the headportion formed with the rod, the less thedisplacement or straining occurs. With theflat brass type the end of the rod is j shaped,and the thickness and the diameter of thebolt about equal; but with the semi-solid head,the length of the hole for the bolt is nearlytwice and a half of its diameter. Havingthus pourtrayed the advantage of the one typeover the other, the conclusion is, of course, infavour of that illustrated by Fig. 170. Mostall the marine engineers adopt the twotypes of heads under notice, as the plates inthis work illustrate. The strap and cotterend used by Messrs. Eennie is described inpage 45.

LUBRICATORS.

To reduce the friction of the surface inworking contact, lubrication is essential, andparticularly so where steam is introduced, orthe temperature of 'the surfaces above thenormal degree. The slide valve and thepiston are the details requiring lubricationwithin and connected to the cylinder, andtallow the unguent mostly used in a meltedstate. The immediate reservoirs are merelycups or cans fixed at the required localities,with stop cocks or valves for the admission ofthe tallow.

The lubricator for the piston is shown

2 Y