R CL: V.

FIG. 29.-Reference Diagram, Walschaert Valve Gear.

If it will not shorten the effective throw of the link too much the connection from eccentric rod to link should be brought down as near the centre line of motion of the main rod as can be done; but this is a point for compromise.

The length of the long division of the combination lever should not be less than 24 times the stroke.

The length of the combination lever must be taken to suit the conditions under consideration in each case, so that the angle through which it oscillates will not exceed 60 degrees, but less is preferable. The required horizontal movement or travel of the connecting point F of the radius rod to the combination lever for a given maximum valve travel must now be ascertained, and is found by the following formula in which R and C are the same as above, namely:

These may also be laid out graphically as per Fig. 30 for outside and Fig. 31 for inside admission valve by drawing a circle with S as a centre and a as radius

(shown dotted in the figures).

Lay out crank radius R to the left from S=Sd and the lap and the lead dimension C=Se on same side of S for inside admission (Fig. 31), and on opposite side of S for outside admission (Fig. 30). Draw ef and Sh perpendicular to Sd when ƒ becomes the intersecting point between the valve travel circle and the line ef. Draw the line dj and where this intersects the line Sh which point we will call h (found by extending df in Fig. 31); Sh

FIG. 30.-Laying-out Diagram, Walschaert Valve Gear.

is then the desired dimension b in the formula, or one-half the required movement of point F, the total of which is represented by the full drawn circle in the figures.

This is a most important function of the gear upon which practically all the others depend and is rather complicated to find by plotting. With a correct suspension of the link-block it will have the same horizontal movement as the point F, and by limiting the angle

of the swing of the link to 45 degrees as a maximum we get the rise or depression of the link-block 'on either side of the link fulcrum the distance

Og

=

b* tan. d

(Fig. 29)

where O is the link fulcrum, d= one-half of the swing of the link in degrees, and b= half the travel of point F in the previous formula.

FIG. 31.-Laying-out Diagram, Walschaert Valve Gear.

The vertical location of the link fulcrum O should be, when practicable, on a line drawn through point F parallel with the valve-stem, and the eccentric rod connecting pin K to the link should be as nearly as practicable on the same level as the main axle in order to minimize the effect of the vertical play of the axle on the valve events, but on large engines it may be found necessary to lower fulcrum O and raise connection K to avoid excessive throw of the eccentric crank.

* As no suspension gives a perfectly equal drop of the block in both link positions, this formula is only approximate.

In locating the longitudinal position of the link fulcrum, consideration must be given to the lengths of the eccentric and radius rods so that both may be approximately of the same length. When these lengths fall below three and one-half times the total vertical sweep of the link-block the radius rod should be favored in preference to the eccentric rod. The exact position of the eccentric crank must be plotted as well as the longitudinal location of point K. The former must bear such relation to the main crank that it brings the link in its middle position when the main crank is on either side of its dead-centres and the connecting point K must be so located that it swings the link in the required angle d on either side of the middle position of the link; that is, in other words, the point K should be so located on the curve it must follow with fulcrum O as a centre that its deviation from the tangent of the eccentric rod to this curve is such that it, as near as practicable, compensates for the irregularities brought about by the angularities of the main and eccentric rods which in ordinary cases brings it from 2 inches to 5 inches in the rear of the tangent to the link drawn through the fulcrum O.

The locus of the suspension point of the radius rod lifting bar must also be plotted so that the link-block is at the same point of the link in its extreme positions at all cut-offs. This locus is a curve with its centre

.