in the third pair of drivers, and in front of and close to the pins in the second pair. Q. Why are the coupling-rod pins in Mogul and tenwheel engines smaller than on an eight-wheeler? A. Because in the former there is greater distribution of the pressure. Q. On this principle may consolidation engines have smaller coupling-rod pins than Moguls? A. Yes. Q. How are coupling-rod brasses usually keyed? A. With two keys at one end and one at the other, or with two at each end. Q. Why is the strap on the front end of the connecting rod usually rounded off at its end?. A. To give the strap clearance in the crosshead. Q. Should main-rod brasses be babbitted? A. They have been found to run cooler with than without babbitt, even where made of phosphor bronze. Q. Should side-rod brasses be babbitted? A. Yes, but it is not so often done with main rods. Q. What is the disadvantage of such caps? Q. Should the brasses extend to the edges of the strap? A. Yes, to exclude dust, and to prevent shouldering of the strap. Q. Where does the knuckle joint in a sectional side rod come into play? A. (1) Where the track is uneven, (2) on frogs, (3) in entering a turntable. Q. How many knuckle joints on a side have six-wheelconnected engines? A. One; usually back of the main pin. Q. Are any engines made without parallel rods? A. Yes; the Webb three-cylinder compounds shown elsewhere, in which the low-pressure cylinder drives one axle and the two high-pressure ones drive another. Q. In ten-wheel, Mogul, and consolidation engines, which rod usually takes hold of the inner journal of the main crank pin? A. The coupling rod. Q. What is the advantage of solid pressed-in bushings in coupling rods? A. There are fewer fitted parts, therefore less labor and expense in construction; fewer parts subject to breakage, and the only wear comes on the bush, thus enabling repairs to be effected more rapidly and cheaply than where bolts, keys, and straps require renewal; also there is insurance against being thrown out of adjustment by careless keying; prevention of too frequent adjustment where an engine is run by two or more crews in the same day. Q. The disadvantage? A. When worn it cannot be reduced to fit the pin, but must be scrapped, unless it can be bored out and used somewhere else on a larger pin where the same sized eye and external brass dimensions are used. Q. How is the bush kept from turning? A. By a hollow stud screwed clear through the rod at the eye and into which is screwed the oil cup. Q. What is the advantage in fluting a rod? A. The metal is put in the top and bottom flanges, where it will do more good than in the neutral axis. Q. What is the reason that a loose brass will cause heating as well as a tight one? A. Because of the pounding action. Fig. 211. Rod Ends. Q. How much larger should the hole in the brass be than the pin? A. Usually about 1/32 inch. Q. How is an engine to be keyed up, when the front end rod-straps are as shown in Fig. 210? A. The bolt must be loosened up first, else it cannot be keyed up. Q. In what position and condition should the engine be for setting up the front end main-rod brasses? A. On the lower quarter and no steam on, so that the rod hangs on the front section of the brass, and only the back brass section need be moved; further, the brass will be keyed to the largest part of the wrist pin. Q. For both ends? A. On the center, especially where the pins are out of round. Q. How about keying up side-rod brasses where there are three keys? A. They should be keyed while on the center. Q. Where there are two keys, one behind each pin? A. It makes no difference where the pins stand; both keys should be driven home to keep the brasses from working in the straps. Q. What is the disadvantage, in a consolidation engine, of having the main rod on the back driver and the eccentric on the second? B Fig. 212. Connecting Rod Strap. A. The back wheel has to move far enough to take up lost motion in its axle box as well as the lost motion in the main connection, parallel rods, and the connection of the side rod to the second wheel, before the valve motion feels it. Q. What is a disadvantage of solid-rod brasses? A. After heating they are always loose and have to be shimmed or renewed. Q. What is a common cause of trouble with rod straps? A. The brass seat is planed off the same width as the stub end; and when it is worn, liners must be riveted in. Q. How may this trouble be lessened? (See A. By slotting the seat a little narrower than the stub end, so as to give plenty of metal for wear. Fig. 212.) CHAPTER LV CRANKS AND CRANK PINS Q. What is a crank? A. A "mechanical movement" designed to convert reciprocating motion into rotary, as in a steam engine, or the reverse, as in a crank-driven pump. Q. Does the first half of an outward piston stroke correspond to the first quarter of the crank path? A. No; as seen in Fig. 213, in which the crank pin gradually in each circle falls behind at first, then regains comparative velocity; the difference being greater with the shorter connecting rod. Q. What is the effect on the piston velocity? A. To increase it during the first half of the outward stroke and the second of the return stroke and retard it during the second half of the outward and the first half of the return stroke. Q. What name is given to such cranks as are used on the ordinary English inside-cylinder locomotive? A. Center cranks; inside cranks; full cranks. Q. What name is given to such cranks as are used on the ordinary American standard outside-cylinder locomotive? A. Half cranks. Q. How are the inside cranks or full cranks of an English locomotive usually made? A. By forging a large mass on the axle, at the place where there is to be a crank, and slotting it out to form the crank, then turning the pin in place; or by hydraulically bending the axle to the required throw, and turning the pins in place. Q. What is the objection to the inside crank? |