. Farm machinery and farm motors. ntric from the shaftand turn it backward untilthe lead is again the sameas before. The distancewhich the eccentric is to beturned backward should be180° plus twice the angle^^^- 259 of advance (Fig. 259). Ihe valve does not need to be moved on the rod, northe rod lengthened or shortened. The only caution neces-sary is to be sure that the lead is always on the end thepiston is on when the engine is on center. An engine running backward or under will do just asmuch w^ork as one running forward or over,«but when itis running over the pressure of the crosshead is
. Farm machinery and farm motors. ntric from the shaftand turn it backward untilthe lead is again the sameas before. The distancewhich the eccentric is to beturned backward should be180° plus twice the angle^^^- 259 of advance (Fig. 259). Ihe valve does not need to be moved on the rod, northe rod lengthened or shortened. The only caution neces-sary is to be sure that the lead is always on the end thepiston is on when the engine is on center. An engine running backward or under will do just asmuch w^ork as one running forward or over,«but when itis running over the pressure of the crosshead is alwaysdown, while wdien it is running under the weight of thecrosshead and connecting rod is down, but the pressurecaused by the steam on the piston and the angle of theconnecting rod and piston rods will be up; hence thereare two forces working in opposition at the crosshead,and this will cause an up-and-down pound. Not onlythis, but if an engine runs over, this force will all be ex-erted upon the engine bed and not the Hminn} 378 FARM MOTORS 524. Reversing gears.—Since the simple engine cannotbe reversed without stopping and using time, engineswhich have to be reversed often and quickly are providedwith reversing gears. That is, they are arranged so theycan be reversed with a lever. There are two generalclasses of reversing gears, the double-eccentric and thesingle-eccentric. 525. Hooking up an engine.—Some engine makers des-ignate their reversing gears as expansion gears. Suchgears are simply reversing gears which can be used sothat the steam works on expansion. Reversing gears areactuated by means of a lever which works in a the lever is in one half of the quadrant steam isadmitted so that the engine runs under, and when in theother half the engine runs over. These gears are gen-erally so constructed that if the engineer wishes his fly-wheel to run in a direction away from him he moves thelever in the direction the wheel turns, and if he wishesth
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