ECCENTRIC RODS

Q. Which type of engine has the proportionally longest eccentric blades?

A. The four-wheel connected.

Q. What are the disadvantages of long eccentric rods? A. (1) Trying on the straps; (2) liable to spring and get the valves out of square, especially should the latter get dry.

Q. In standard American engines, which eccentric blade is attached to the top of the link?

A. The forward.

Q. Which eccentric comes next to the box?

A. There is no rule; sometimes the forward, sometimes the back-up.

Q. What is the general rule as regards the lead of the cranks?

A. The right leads the left 90°.

Q. Why are short eccentric blades used?

A. To avoid a long curved rod around the forward driving axle, or an intermediate rod around the axle, hung on links or guides at the rear.

Q. Which of these two evils is the greater?

A. The latter.

Q. What is the disadvantage of a long curved rod? A. Its weight, and its friction on the eccentric at high speed; also that it is liable to spring.

Q. What is the disadvantage of an intermediate rod? A. All the disadvantages of the other, with those coming from its extra joints.

Q. How can the evil of long eccentric rods, curved over the axle (as on many ten-wheelers) be remedied?

A. (1) By putting the link just back of the front axle, and using a curved transmission bar from the link to the rocker, just ahead of the leading axle-which gives two curved rods instead of one; (2) by using very short eccentric rods between main and forward axles; (3) as on the Southern Railway and the Plant System, by a straight direct transmission bar leading back to the rocker (Fig. 193); (4) as on some of the Brooks consolidated for the Long Island and the Lake Shore roads (Fig. 194).

Q. Why were eccentric blades formerly made adjustable, and why are they no longer so made?

A. It is no longer customary to make them adjustable by means of slotted holes at the eccentric end. They were formerly made adjustable so they could readily be moved to equalize the cut-off in case the engine became lame through wear or other causes. The practice was discontinued, as it was hard to keep the blades from slipping on the large modern engines.

THE TUMBLING SHAFT

Q. What is the most desirable tumbling shaft position? A. When it holds the hanger so as to guide its vibra

Radius of Link 53"

Fig. 194. Eccentrics and Rods.

tions in arcs practically parallel to the center line of motion. Also, it must be far enough above or below the central line of motion to keep it from being struck by the eccentric rods when the gear is moved from one motion to another.

Q. Why not curve the eccentric rods?

A. That would produce the desired results, but introduce into the design an element of weakness.

Q. What point must be noted in connection with the hanger?

A. It must be of such length that the link end will not

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strike the tumbling shaft in either forward or backward gear.

Q. What is the usual proportion between the tumbling shaft and hanger lengths?

A. The tumbling-shaft arm is usually at least as long as the hanger.

Q. Suppose that the boiler or other part prevents the tumbling-shaft arm from going far enough up to prevent the link being placed in full back gear, what will have to be done?

A. There are two remedies; one to put the tumbling shaft below the link motion, the other to lengthen the rocker so as to lower the entire motion.

CHAPTER LIII

THE REVERSING MECHANISM

Q. What is the reversing mechanism on British locomotives?

A. The reversing mechanism commonly consists of a hand wheel on a shaft having a worm gear which moves an arm attached to the reach rod.

Q. On American?

A. A lever.

Q. Where is the reverse lever usually placed, and why? A. On the right side of the cab, because most engineers are right-handed.

Q. How is it held in place?

A. By a latch, worked by a trigger lying alongside the handle of the lever; the latch working in notches on the upper side of the quadrant. (See Fig. 195.)

Q. What is the usual arrangement of the notches in the reverse-lever quadrant?

A. They correspond to such positions of the gear as will cut off the steam at a given number of inches of piston stroke; as 6, 9, 12, etc., or 6, 8, 10, etc. Besides these, there is one notch corresponding to mid-gear.

Q. What will be necessary in the second case?

A. To change the relative rocker-arm positions in order to keep their motions proper.

Q. Which of these two methods is the better?

A. The second, as the greater the rocker-arm length the less the valve-stem vibration and the link-block slip. Q. Of what material are the rocker arms usually made?

A. Wrought iron.