Q. Suppose the gear showed a tendency to move, what would happen?

A. The slide valve would be displaced, admitting air to the cylinder and returning the crosshead of the device to the position shown by the reverse lever.

Q. Describe the Nathan steam and air throttle valve for power reverse gear.

A. It is practically a three-way cock. Under normal conditions it is in connection with the line from which the air is taken for the operation of the reverse gear and the operating handle is sealed. If anything should happen to make the air supply unavailable, the handle is turned to the opposite position (breaking the seal), the air line is cut out and the steam line cut in so as to operate the reverse gear by steam.

CHAPTER LIV

MAIN AND SIDE RODS

Q. How is the pressure on the piston communicated to the wheel so as to make it rotate in the same direction, no matter whether the piston is making its inward or outward single stroke?

A. By the connecting rod and crank.

Q. What is the character of motion of the connecting rod?

A. The front end has a true reciprocating motion exactly corresponding to that of the crosshead; the rear end has a true rotary motion exactly corresponding to that of the crank pin; all intermediate points have motions combining the two classes, and with more or less of the reciprocating or rotary character according as they are nearer the crosshead or the crank pin.

Q. Is there any loss of power by the use of the connecting rod and crank, by reason of the fact that the angle at which the connecting rod acts on the crank and that at which it receives the pressure of the piston, constantly vary in each half rotation of the crank pin?

A. None whatever, except that due to friction.

Q. At what point in the rotation have the piston and crosshead the most power to cause the crank to rotate?

A. At that point (about mid-stroke of the crosshead) where the crank pin is about at the uppermost or the lowermost point of its rotation.

Q. How much power have the piston and crosshead to turn the crank pin when the centers of the wrist pin, the crank pin and the main driving axle are in the same straight line?

A. None whatever.

Q. How then is the engine kept going?

A. The cranks are quartering, so that when one is on the dead center the other is about at its maximum power.

Q. Is there no means of preventing this difficulty of having dead centers?

A. Quartering the cranks gets around it well enough. Q. What sort of a stress does the connecting rod get? A. When the piston is making its out stroke (toward the stuffing box) it is in compression; on the return or in stroke, in tension.

Fig. 202. Rod Ends.

Q. What is the most common shape of connecting rod? A. They are flat wrought-iron bars, larger at the crank-pin than at the wrist-pin end, and having a cross section either rectangular, or modified from the rectangular by milling out wide flutes to remove material from the lengthwise center line, where material gives the least strength.

Q. Why are they larger at the crank-pin than at the wrist-pin end?

A. Partly because the crank-pin should be larger than the wrist pin, and partly because experience has shown that that end is the more liable to break.

Q. What class of bearing have the wrist-pin and crank pins, in the rod ends?

A. There are two classes. In one the rod is enlarged into a stub end having a П-shaped strap by which halfbrasses are held in place around the pin, and which may be set up as desired. In the other, the pins turn in bushes hydraulically pressed into the eyes in the rod ends, and which have no capability of adjustment; in fact cannot be taken out except at the shop.

Fig. 203. Rod Ends.

Q. How is the adjustment of the brasses effected, with the ordinary stub end and strap?

A. There are keys by which the brasses may be closed up on the pins, up to that point where their faces touch; then to get any more adjustment they must be taken out and their faces filed off.

Q. In this latter case what is the shape of the hole in which the pin rotates, after the brasses have been thus planed off or filed off and set up?

A. Its outline is that formed by two circular arcs, each rather less than a semicircle.

Q. How are the crank-pin journals oiled?

A. By metal cups attached to the straps, where the stub-end type of rod is used, or to the enlarged head of the rod where solid bushings are employed. Sometimes also, in the stub-end type, there are on the under side of the straps recesses or "cellars" for oil, which is dashed up against the pins, through holes in the under strap leg.

Fig. 204. Rod Ends.

Q. What material is employed for the brasses? A. Sometimes brass, in other cases bronze; these being sometimes plain, but generally supplied with babbitt plugs or strips cast therein to lessen friction and wear.

Q. When a main rod has one key back of the crosshead pin and another back of the main crank pin, what is the effect on the effective rod length when both keys are tightened by reason of the brass-wear?

A. It will be left practically the same.