point
K. Consequently, as the side shaft rotates in the direction indicated,
the lever L will _begin_ to open the valve V when the cam is in the
position shown in fig. 29, reach a maximum opening at K, and finally
close when the cam has moved so that point E is now where F was. With a
cam of this shape, however, a considerable portion of the stroke would
have passed before the valve was raised any _appreciable_ distance off
its seat; it would only be fully open for an instant, viz., when K was
passing over R, and would begin to close again directly.

Moreover, if the engine were running at even a slow speed, the motion
imparted to lever L would be indefinite; and this, especially if the
governor is fitted to the air valve lever, as in fig. 25, is very
undesirable. Therefore, to obtain a definite opening we must set out the
cam, as shown in fig. 31. In this diagram the roller is shown standing
clear of the back of cam by about 1/16 in. A line MN is then drawn,
forming a tangent to both roller R and circle GHJ at points F and O
respectively. This gives us the opening portion of cam. Then from the
centre S with radius SF describe the arc FE (shown dotted in fig. 31),
and set off the angle required (ABD, fig. 30), as previously explained.
Through point E draw a line forming a tangent to circle GHJ, and produce
it towards P. This line gives us the closing portion of cam. The
distance W is of course variable, according to the amount of lift we
give the valve. By comparing these two diagrams it will be seen that in
both cases the valve will be opened the same length of time, but in
first case the motion will be indefinite and uncertain. In practice the
corners are rounded off somewhat, in order to obtain a steady motion;
and when the air cam is also the governing cam, it is advisable to round
off the opening face, as indicated in fig. 32. Upon the shape of this
face both the sensitiveness and the life of the governor gear depends.
If it is nicely rounded off, giving a gradual rise, very little tension
(or compression, as the case may be) of the controlling spring will be
necessary to give the required speed to engine; whereas, if the rise is
sudden, the spring will have to be screwed up tighter, and, if uneven
and lumpy (_i.e._, not a fair curve), the result will, of course, be
erratic governing.

[Illustration: FIG. 32.]

A certain amount of clearance should always be provided between the
roller and the back of cam (compar