368

MODERN STEAM PRACTICE.

radius that determines the point C or centre of crank pin, when thesteam is cut off at five-eighths of the piston stroke. Thus the versedsine of the chord of the arc of supply to the cylinder can be found,namely, D E, which measures nearly xo inches. The diameter ofthe circle described by the centre of the eccentric can also be nowfound, as already described for direct motion; but as the lever fortaking the valve spindle may be shorter than the one for the slotlink, it is evident that the eccentric circle must have a greaterdiameter. Supposing this is the arrangement adopted, and thelength of the lever for the valve to be 12 inches, and that for theslot link 15 J/2 inches (those lengths can be only determined bylaying down in plan the valve and gear). When it is known thatthe versed sine of the chord of the eccentric circle for the arc ofsupply is the full opening of the port by valve minus one-half ofthe lead, we have the following, supposing the lead to be } 4 th part ofan inch: The full opening of the port by valve as already found isi - 5 inch minus ^ inch=i q.68 inch as the versed sine of the chordof the arc described by the eccentric circle for direct motion. Thediameter of the crank circle is equal to 48 inches, and the versedsine of the chord of arc of supply is 10 inches. We have therefore10 :48 : : i'468 = 7'04 inches diameter of eccentric circle for directmotion, or for levers of equal length; but as the levers for workingthe valve are of unequal length, we have 7D4 : 12 : : 15"5 = 9 'Oiinches diameter of the eccentric path or full travel of the valve.

Eccentric and hoop .-—-The eccentric is cast in two halves and boltedtogether, for the convenience of taking it to pieces or placing it onthe shaft. Were the eccentric placed on a plain shaft at the end,with nothing to interfere, it would be cast all in one piece; but asit is generally placed between collars turned on the shaft, at theside of the main cranks, the ring requires to be in two halves. Theeccentric sheave revolves freely on the shaft, and has a catch caston it, with a corresponding catch fixed to the shaft, so as to suit theforward and backward movements. The eccentric sheave is alsofitted with a back balance, so that when the engine is reversed byhand, the eccentric rod being out of gear and the sheave beingloose on the shaft, the latter is perfectly balanced, and preventedfrom revolving when the catch is not driving it. This is mostrequired for engines of great power, where the sheaves beinglarge would turn rapidly round, and being met by the catch wouldimpart a smart blow, tending to disarrange the gear. For small