A. There are two bars above the crosshead and none below or on the sides.

Q. What holds the guide-bars in place against the great vertical strains to which they are subjected?

A. They are bolted at the front end to the back cylinder-head and at the back to a guide-yoke attached to the frame of the engine, and usually, also, to the boiler.

Q. What is the guide-yoke?

A. A transverse plate or casting secured to the frames by angles or knees, holding and supporting the outer guide ends, and frequently having a brace to the boiler waist on each end, as well as an expansion plate for the boiler between the frames. It is also called spectacle' plate, motion plate, guide bearer, and guide cross-tie. The British call it a slide-bar bracket.

Q. What other name is often given to the guide-yoke? A. The guide-bearer.

Q. What provision is there for reducing to a minimum the wear of guides and slides?

A. The guides are hard and finely finished, and the slides fitted with gibs of brass or bronze between them and the guides; these being adjustable so that as they wear they may be set out to take up the lost motion. The gibs or wearing-pieces being softer than the guides, get nearly all the wear, which is desirable, because they are cheaper to renew; and may be set out quite readily, by liners or otherwise.

Q. Is there any provision for bringing the guide-bars nearer together when worn, or for other reasons?

A. Where they are double, one above and one below, or one pair above and one pair below, they are held at a fixed distance apart by end-blocks or distance-pieces; and these latter being removed and planed off to any desired extent allow of this sort of adjustment. Another way is to provide liners at first and to have them removed from between the end-blocks and the guide bars, as the gibs wear.

2

I

Q. Is the wear on the guides uniform?

A. No; not where, as is usually the case, the engine runs more in one direction than in the other.

3

Figs. 173 and 174. Guide-bearers and Crossheads.

1. Guide-bearer. 2. Guide-bearer Knee. 3. Top Guide-bar. 4. Bottom Guide5. Guide-fillings. 6. Crosshead. 7. Crosshead Gibs. 8. Crosshead Filling piece. 9. Crosshead Plate. 10. Crosshead Pin. 11. Crosshead Key.

bar.

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Q. Where is there the greatest strain on a slide-bar? A. At the center of length, by reason of its having less support there, and of the angularity of the connectingrod being greatest there.

Q. Which slide-bar gets the most wear in running ahead?

A. The upper.

Q. Why?

A. Because on the out stroke, towards the crank, when the connecting-rod is below the crosshead, it is in compression and throws the latter up against the slide; and on the in stroke (from the crank), when the connectingrod is above the crosshead it is in tension and tends to draw the latter up against the same bar.

Q. Which slide-bar gets the most wear in running backwards that is, tender first?

A. The bottom one, because on the in stroke the connecting-rod when below the crosshead is in tension and tends to drag the latter against the under side, and on the out stroke when the connecting rod is above the crosshead it is in compression and tends to thrust the latter against the bottom bar.

Q. When an engine is running ahead, using steam, does the crosshead run on the bottom guide-bars?

A. No; only when the engine is shut off or backing up. Q. Why?

A. Because, if the steam is pushing the piston in the cylinder ahead, and the main pin is above the center, the tendency would be to lift the crosshead off the bottom guides. The same would be true with the pin below the center and the steam pushing the piston back.

Q. What is one of the principal causes of trouble with crossheads?

A. They are too low, owing to the bottom guide-bar getting the most wear on down grades.

CHAPTER LII

THE ECCENTRIC MOTION

Q. Would it be possible to make the ordinary slidevalve engine reversible with only a single eccentric for each cylinder?

A. Not without great complication of mechanism; it is, however, done, as in the Walschaert gears, described elsewhere.

Q. To what does a link operated by two eccentrics correspond, as a mechanical equivalent?

A. To one operated by a movable eccentric.

Q. In what is it superior to a movable eccentric? A. In that its motion can be accurately adjusted so as to do away greatly with the irregularities in cut-off and exhaust closure, due to the connecting-rod angularity.

Q. Is there any other way by which the valves could be given to-and-fro motion from a rotating axle, than by eccentrics?

A. Yes, cranks might be used, the eccentric being in effect a crank, the pin of which is so enlarged as to include the shaft. Thus, ordinarily, the crank-pin is smaller than the shaft and at some distance therefrom; in Fig. 175, it is of the same size; in Fig. 176, the pin is larger than the shaft, but does not inclose it; in Fig. 177, the pin not only is larger than the shaft, but incloses it and has become an eccentric.

Q. If an eccentric is turned down half an inch in a lathe, how much will its throw be altered?

A. Not at all.

Q. What determines the throw?

A. The distance from the center of the axle to that of the eccentric sheave.

Q. What is the effect of boring out and closing the eccentric strap?

A. To change the travel of the valve.

Q. What is the remedy?

A. Lengthening the eccentric rod.

Fig. 175. Crank Axle.

Q. How tight should the keys of eccentric strap bolts be?

A. So tight that the nuts cannot work back more than a turn, else straps will probably break.

THE ROCKER ARM

Q. What would be the most simple way of getting the motion of the eccentric to the valve?

A. By an eccentric rod direct from the strap.

Q. Why cannot this be done in the case of a locomotive?

A. Because it is usually necessary to have two eccentrics so as to be able to reverse the engine, and to have a link to be able to alter the throw for the purpose of varying the period of admission and degree of expansion.

Q. With two eccentrics and a link motion, is the valve driven directly from the link?

A. No; there is a rocker arm to transfer the motion