120

MODERN STEAM PRACTICE.

by valve, minus the lead, or i inch; this is the distance from E to F.Then set off E D, the lead equals y, inch, while the remainder ofthe radius of the eccentric circle, or B D, equals the outside coveror the lap of the valve. With the length of the eccentric rod as theradius from E, the point 2 on the valve can be determined; this,for sake of illustration, is measured from the edge of the valve,instead of the pin on the valve rod for taking the eccentric rod,and from the edge of the valve, as at 2, cut the eccentric path at2, and from the point D fix the point 3 on the valve; cut as beforethe point 3 on the eccentric path, join the line from 2 to 3 onits path; that line is the chord of the arc of supply by eccentric.It will thus be seen that the figures on the eccentric path cor-respond with the figures on the valve; thus the point 2 gives thelead, the point 1 the full opening of the port by valve, and at thepoint 3 the valve has returned and just covers the port, this beingthe point of cut-off, or no more steam is admitted into the cylinder,the remainder of the stroke of the piston being actuated by theexpansive force of the steam in the cylinder. It will thus be seenthat when the crank centre is at the commencement of the IN stroke,as at A, the port is open y 2 inch; when the crank pin centre travelsto G the port is full open, and the valve returning, until the crankcentre has travelled to C, then the cut-off takes place, the valvehaving closed the port; thus the expansion of the steam in thecylinder commences. This only provides for the IN stroke; thatfor the OUT stroke must be found in like manner, and it will beseen that with the same throw of the eccentric the opening of thesteam port by valve is less for the OUT stroke than for the IN stroke;consequently the lap for the OUT stroke must be greater than forthe IN stroke. When great nicety is required, the area of the porton the cylinder should be arranged for the OUT stroke, and thelength of the port for the IN stroke reduced accordingly, so as toget equal area of port for IN and OUT stroke, as likewise equal cut-off in the cylinder. Thus when the versed sine of the chord of thearc of supply is given for the crank path, and the versed sine ofthe chord of the arc of supply can be determined, it becomes aneasy matter for the Student to practically delineate the variouspoints of the crank and eccentric paths in relation to each other.

To find the versed sine of the eccentric rod, working to the for-mula V=R—(VR 2 —C 2 ) [V = versed sine, R = radius, C = semi-chord], we can take as follows:—Throw off the eccentric, or the