CHAPTER LXXXI

KNOCKS AND POUNDS

Q. What are the most usual causes of pounding?

A. (1) Lost motion in the connecting-rod brasses, between the driving boxes and the jaws, or (2) in the driving-box brasses; (3) side rods out of tram or with badlyworn brasses; (4) worn guides; (5) piston head touching the cylinder head; (6) spider getting loose on a piston rod; (7) a piston rod loose in the crosshead.

Q. Where will the pounding be in case of worn guides? A. At the crosshead.

Q. What is this liable to cause?

A. A bent piston rod.

Q. What is the best way to find out where a pound is? A. To put one of the cranks on the quarter, block the wheels and have the throttle opened a little, and the engine reversed with steam on; then each connection may be watched in turn as it comes and goes.

Q. Under what conditions will a crosshead pound? A. When the guides are worn very open.

Q. Under what circumstances will side rods pound on the centers?

A. When they are out of tram or their brasses are badly worn.

Q. Where is the most difficult knock to place on an engine?

A. That caused by a spider that has come loose on the piston rod; or that when the piston packing is too short.

Q. How can the knock caused by a loose spider be detected?

A. By the slight blow and the sharp click that is made when the engine is passing over both centers.

Q. How may a loose spider be detected?

A. By the sharp knock made when passing the front center.

Q. What is a very rare cause of piston pounding?

A. Where a thick cylinder has been rebored until there is no counterbore left and the piston head has worn a shoulder; the slightest alteration in its adjustment will cause the piston to strike this shoulder.

Q. Suppose that an engine pounds in full gear and the pounding cannot be stopped by either tightening or slackening the brasses, what should be done?

A. More lead, or more cushion, should be given.

Q. Why is it that engines will sometimes pound only in full gear?

A. Because there the lead is least, with the ordinary shifting-link motion.

Q. When pounding lessens when the engine is hooped up, of what is that the sign?

A. Of insufficient cushion.

Q. But why is the pounding less when the lever is in the center notch?

A. Because there cushion and preadmission are greater than with full valve opening.

Q. Of what may a neglected pound be the forerunner? A. Of a broken crank pin, cylinder head, etc.

Q. What should the engine runner do on discovering a serious pound?

A. First locate it, then report it, thus relieving himself of further responsibility in the matter and enabling prevention of an accident.

Q. What are the most usual causes of pounding?

A. Lost motion in connecting-rod brasses or between the driving boxes and the jaws, or in the driving-bex brasses; insufficient oiling of piston, main shaft, main crank pin or wrist pin; side rods out of tram or with badly worn brasses; worn guides; piston touching the

cylinder head; piston rod loose in either the crosshead or the piston head; too close wedges, loose knuckle pin or bushing, loose middle connecting brasses, wedge down or stuck; broken frame, loose cylinders or deck; loose pedestal braces where the frame is light; faulty fitting of shoes and wedges; loose oil cellars or driving brasses (either circle or gibbed), square-bottom spring bands, poorly-fitted spring saddles or anything that hinders free movement of the equalizers; springs rubbing the boiler, saddle striking the frame; wet steam or foaming; excessive back pressure; imperfectly balanced drivers; too much or too little steam cushion; loose cylinders; loose follower bolt. Q. What may be effect of lost motion in the valve gear? A. Rattling reverse lever.

Q. What, of valves out of square?

A. Jerks when hooked up near the middle.

Q. Do parallel rods pound?

A. No; they rattle.

Q. Do packing springs pound?

A. No; they click.

Q. When an engine pounds on the back center, for what should you look?

A. A loose spider.

Q. On the front center?

A. Loose driving boxes or wedges.

Q. On the quarter?

A. Flat spots on the tire.

Q. What is the sign of a loose spider?

A. A hard knock when the engine is on back center, so that the tendency is to push the spider off the fit.

Q. What could cause pounding in a shut-off engine? A. (1) New piston packing striking against an old shoulder in the cylinder; (2) faulty adjustment bringing the piston too far front or back and thus striking a shoulder.

Q. What would cause a piston-valve engine to pound when running shut off with the lever hooked up, although she does not pound when the lever is in the corner?

A. Over-compression in the cylinders, with valves having no relief.

Q. What style of valve is specially addicted to pounding?

A. The piston valve, by reason of the exhaust steam passing over its ends and exerting a force which will take up the lost motion in the gear suddenly; continuing the motion of the valve in the direction in which it was going.

Q. What is often the result of this pounding?

A. Crystallizing and breaking valve stems and other parts, and causing irregularity in the steam distribution, as the valve jumps and then stops.

Q. What causes a piston valve to pound when the engine is drifting with the lever hooked up?

A. Lack of compression, by reason of the valve being open for admission, and thus permitting the air which is being compressed to escape into the steam ways.

Q. How may this pounding be stopped?

A. By placing the lever in the corner or on the center; or better yet, by keeping the main-rod brasses filed.

Q. Explain the difference in the pound of a loose piston and that of a loose driving box?

A. The two are different, but sometimes confusing. The loose piston pound may be detected by watching the fit of the rod in the crosshead, and the sound is more to the forward end than that of a driving box.

Q. How would you locate a driving-box pound?

A. Spot the engine with the crank on the upper quarter on the test side; block the wheels, have the fireman "thump" the engine. Watch the boxes to see if (1) they rock in the jaws between shoes and wedge, or if (2) the journal shakes in the box. Then test everything along the line of rods and of boxes.

Q. At what part of the stroke does the main rod pound?

A. Only at the end.

Q. What is the cause of side rods pounding?

A. (1) Wedges slack, wrongly set up, or having the wrong taper; (2) engine out of tram.

Q. Will the side rod on a three-wheel connected engine pound any more than that on a two-wheel engine?

A. Not necessarily.

Q. Why will an engine in good condition pound harder when passing the forward than the back center, when running ahead?

A. When the pin passes the front center it pushes the main axle back against the box and jaw, which are moving forward with relation to the track; on the back center the push is in the same direction as the box movement.

Q. What is the best position to place a crank pin in order to find a knock?

A. The upward, because there it is freest to move.

Q. What is the objection to bringing the crank pin downwards to find a knock?

A. It would be necessary to move the weight of the engine in order to find a knock or a thump in a driving box or a frame.

Q. What is the objection to having the crank pin on either the front or the back center, in finding a knock or a thump?

A. In that position, moving the reverse lever lets steam in at only one end of the cylinder.

Q. What are special causes of pounding on the back centers of a worn road engine?

A. (1) Forward wear of shoes and boxes; (2) displacement of the valve stem, due to lost motion in the yoke, etc.

Q. What is the effect of this wear on distances between centers of parts?