3278
THEORY AND PRACTICE OF ENGINEERING.
Book II.
axle of the wheel, which as it turns pushes and draws the rod of the slide, does the duty ofa crank, converting the circular to alternate motion, as far as the slide is concerned, andalternate into circular, as regards the axle of the engine. The slide, however, not beingexactly in the same plane with the axle, the motion is not directly communicated to the
ECCENTRIC SLIDE VALVE, SIDE VIEW.
rod by the eccentric, but by a cross axle, and the two arms affixed to it; so that when theeccentric goes back, the slide rod is made to advance: there is an instant during the timethe slide is in progress that all the passages are shut, and this is whilst the piston ischanging its direction ; to obtain a maximum effect, the steam requires to be shut off beforethe piston has reached the extremity of the cylinder, and the aperture on its other sideshould be partly opened before its change of motion takes place.
The Drivers are those portions of the engine which serve to change its movement: thisis performed by disengaging the eccentric from one, and carrying it to the other; so thatthe director of the locomotive can at pleasure cause it to advance or retire. The driversare fixed on the axles on opposite sides of the eccentrics, which being pushed by meansof a lever can be thrown either into or out of gear; so that to change the motion it is only*
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requisite to lift one set of forked arms from off one set of weight bars, and replace them onthe other, which is performed by a handle, working on a fulcrum, and moving the arm attachedto it: this also works a small pin fitting into the oblong groove of another lever, andmoves the forked arms upwards; at the same time the lever is depressed by the pin,and with it the forked arms below. The piston-rods slide on guides, and preserve aregular horizontal motion, and from them is communicated a rotary movement to the axlesof the hind wheels, the alternate being transformed into circular motion by means of acrank on the axle.
There is great danger in cranked axles from their breaking, which with four-wheeledengines might occasion considerable damage; this has repeatedly happened, generally frombad welding: in one instance, on the London and Birmingham Railway, an engine wasdiscovered to have been running for some time with a broken axle, arising from the eccen-trics being keyed on to the weakest part of the axle, and only two-thirds of its sectionbeing* soundly welded. ,