516. Q.—­How are the eccentric straps constructed?

A.—­The eccentric hoops are generally of wrought iron, as brass hoops are found liable to break.  When formed of malleable iron, one half of the strap is forged with the rod, the other half being secured to it by bolts, nuts, and jam nuts.  Pieces of brass are, in some cases, pinned within the malleable iron hoop; but it appears to be preferable to put brasses within the hoop to encircle the eccentric, as in the case of any other bearing.  When the brass straps are used, the lugs have generally nuts on both sides, so that the length of the eccentric rod may be adjusted by their means to the proper length; but it is better for the lugs of the hoops to abut against the necks of the screws, and, if any adjustment be necessary from the wear of the straps, washers can be interposed.  In some engines the adjustment is effected by screwing the valve rod, and the cross head through which it passes has a nut on either side of it, by which its position upon the valve rod is determined.

517. Q.—­Will you describe the eccentric rod and valve levers?

A.—­In the engines in use before the introduction of the link motion, the forks of the eccentric rod were of steel, and the length of the eccentric rod was the distance between the centre of the crank axle and the centre of the valve shaft; but in modern engines the use of the link motion is universal.  The valve lever in locomotives is usually longer than the eccentric lever, to increase the travel of the valve, if levers are employed; but it is better to connect the valve rod to the link of the link motion without the intervention of levers.  The pins of the eccentric lever in the old engines used to wear quickly; Stephenson used to put a ferule of brass on these pins, which being loose, and acting like a roller, facilitated the throwing in and out of gear, and when worn could easily be replaced, so that there was no material derangement of the motion of the valve from play in this situation.

518. Q.—­What is the arrangement of a starting lever?

A.—­The starting lever travels between two iron segments, and can be fixed in any desired position.  This is done by a small catch or bell crank, jointed to the bottom of the handle at the end of the lever, and coming up by the side of the handle, but pressed out from it by a spring.  The smaller arm of this bell crank is jointed to a bolt, which shoots into notches, made in one of the segments between which the lever moves.  By pressing the bell crank against the handle of the lever the bolt is withdrawn, and the lever may be shifted to any other point, when, the spring being released, the bolt flies into the nearest notch.

519. Q.—­In what way does the starting handle act on the machinery of the engine to set it in motion?

A.—­Its whole action lies in raising or depressing the link of the link motion relatively with the valve rod.  If the valve rod be attached to the middle of the link, the valve will derive no motion from, it at all, and the engine will stop.  If the attachment be slipped to one end of the link the engine will go ahead, and if slipped to the other end it will go astern.  The starting handle merely achieves this change of position.