From this line we could then draw the dotted circle g, and just meeting the line p, which would give the eccentric position.
To find the piston position, the arc y would require to be drawn by the same means as before.
Fig. 3313.
If the valve is to have lead, the diagram may be constructed as in [Fig. 3313], in which the circle has a diameter equal to the travel of the valve and the cut off is to occur when the piston is at r and the crank at h.
When the valve is at the end of its travel and has fully opened the port, the eccentric will be at d, hence from d we mark an arc g distant from d to an amount equal to the width of the steam port, drop the vertical m from g, and at its lower end v′ is the position of the eccentric centre at the point of cut off. Then draw a line p, distant from m equal to the lead, which will give at v the position of the eccentric when the crank is on the dead centre, and the valve is open to the amount of the lead. The lap is obviously the distance from the centre c of the crank shaft to the arc g.
We have here found all the points necessary except the point at which the valve will open the port for the lead, and this we may find by setting a pair of compasses to the radius b h (or to radius v v′, as both these radii are equal), and from v as a centre, mark at a an arc, which will give the crank pin position at the time the port first opens for the lead, or in other words it will give the position. The proof of the construction is, that if we set the compasses to the distance between the crank pin position on the dead centre and the point of cut off (or from b to h), we may apply the compasses to the points v, v′, which represent the eccentric position when the port is opened to the amount of the lead, and when the cut off occurs.
