sponding to crank-pin position at cut-off. A perpendicular from this to the diameter AB cuts it at a point corresponding to the wrist-pin position at cut-off. (Connect

ing-rod influence is neglected.)

Q. How does the Zeuner diagram represent the slidevalve movements and functions?

A. Referring to Fig. 168, AB represents the piston path and valve travel on different scales, the circle thereon the crank-pin path, and the eccentric-center path. Marking M and C as the crank position at points of ad

mission and cut-off respectively where there is lead, bisect the arc CM at F and draw the diameter OH and the perpendicular OE. On OF and OH draw circles, called the valve circles. Draw circles at the intersection of OM and OR respectively. Then OD or OS will be the required steam lap and OI or OV the exhaust lap. AS will be the maximum steam-port opening. AOM is the lead angle, EOG the lap angle; EOF the angular advance of the eccentric beyond 90° ahead of the crank. Making the arc EX equal to EF, X will be the position of the eccentric center when the crank is on the dead center A.

The piston or crosshead positions corresponding to the various angular positions are found by dropping perpendiculars from C, etc., to AB.

Q. Show on one diagram the influence of steam and exhaust lap, and lead.

A. In Fig. 169, AB represents valve travel and piston path. The radius of the small concentric circle is the exhaust (or inside) lap; that of the larger one the steam (or outside) lap; that of the small circle at the left, the linear lead. Then E, E,, the end of the chord tangent to the steam-lap circle, is the wrist-pin position when there is no lead, irrespective of the other elements; E,, E. when there is lead as shown. Release is at R, when there is neither lead nor exhaust lap; at R2 when there is lead but no exhaust lap; at R, when there is exhaust lap but no lead; at R, when there is both.

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Irrespective of the other factors, admission is at stroke end A when there is no lead and at S when there is lead. Compression starts at C, when there is neither lead nor exhaust lap; at C, when there is lead but no exhaust lap; at C, when there is exhaust lap but no lead; at C1 when there is both. (Connecting-rod influence not heeded.)

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CHAPTER LI

THE CROSSHEAD AND GUIDES

Q. What are the essential parts of a crosshead? A. A socket for the piston-rod end; a journal on which the connecting-rod may turn, and slides which may play between the guides.

Q. Which is it best to have cut by wear-the slides or the guides?

A. The slides or gibs.

Q. What is the objection to a wrist-pin cast in one piece with the crosshead?

A. It is difficult to true up.

Q. Why are crosshead pins made comparatively short and thick?

A. By reason of the lateral play between the driving

wheel hubs and their boxes making a twisting stress on

the pin, on curves.

A. It is usually cast solid therewith.

Q. How is the wrist-pin attached to the crosshead?

Figs. 171 and 172. Guide-bearer, Guides and Crossheads.

1. Guide-bearer. 2. Guide-bearer Knee. 3. Top Guide-bar.

4. Bottom Guide-bar. 5. Guide

fillings, 6. Crosshead. 7. Crosshead Gibs. 9. Crosshead Plate. 10. Crosshead Pin. 11. Crosshead Key.

Q. What class of crossheads may be used for compound engines having two cylinders on each side?

A. As shown in Fig. 170, having two sockets, one for each rod; the entire block being of cast steel in one piece and having its wearing-surfaces covered with block tin 1/16 inch thick.

Q. What is the purpose of crosshead guides?

A. To keep the piston in line with the cylinder axis.

Q. To do this, what is necessary?

A. That they be parallel with that axis and with each other, and at such hight as will bring the center of the crosshead pin in line with that axis.

Q. What forms are given to guides?

A. Their form is legion. (1) There may be only one guide-bar, above the piston rod and crosshead, and which is embraced by the latter, or (2) there may be two, one above and the other below, the crosshead having bearing surfaces on both, but not embracing either, or (3) two above the crosshead, or (4) two pairs, one pair above and one pair below the crosshead. Fig. 171 shows an arrangement in which the crosshead has four guide-bars, two upper and two lower, the wrist-pin center being about in line with the lower ones, as shown in the cross-section. Fig. 172 shows two, one upper and one lower guide, the wrist-pin coming about half way between them, as shown more clearly in the cross section. In Fig. 173 there is but one guide-bar, surrounded by plates bolted to the crosshead proper. In Fig. 174 there are two guides, having between them what is called the crosshead fillingpiece, bolted between the two crosshead cheeks.

Q. What name is often given to the distance-piece between the guides?

A. Guide filling-pieces.

Q. What class of guides is used, where one of the driving-wheels is opposite the guide-bars, as with Mogul and consolidation engines?