_a._ All valve stems must be of 3/4-inch Tobin bronze and of the fixed
type, and must have the guard fastened on by one of the methods shown
in Figs. 9 and 10.
[Illustration: FIG. 9.]
[Illustration: FIG. 10.]
Other methods may be approved, in writing, if found by test and
experience to have especial merit.
_b._ These stems must be screwed into the seats on a straight, tightly
fitting thread, and the lower end then well headed over into a
countersink. The valve guard and nut must be of composition.
In Fig. 9 the upper part of the stem is slabbed off on two opposite
sides and fits a corresponding hole in the guard.
The guard, therefore, cannot turn. The outside of the special nut is
fitted on a taper to the inside of the guard, and the nut tapped out
to fit the 5/8 U. S. thread on the stem.
The action of the valve, whether with the spring or without, tends to
drive these taper fits together, producing a frictional lock similar
to that of a friction clutch; and although the nut may be loose on
the thread, it cannot possibly work off.
It will be apparent that the taper fit on the nut must be so made as
to always bear on the taper fit in the guard, and not bottom in the
guard.
It is believed that with the present screw machine practice in shops
of to-day these small parts can readily be turned out accurately and
cheaply in large quantities. The nuts and guards made in any one
shop must be exactly of standard dimensions, so that the product of
different periods will be interchangeable.
The taper should be about one inch to one foot. With this taper the
nut can be readily turned in or out, but there is friction enough to
hold the guard and nut together even if the spring is off.
In Fig. 10, the top of the guard is recessed in the form of a hollow
inverted pyramid of six sides, to correspond to a hexagonal nut. The
angle of two opposite sides of this recess, which should be about 75
degrees, will both surely lock the nut and still permit of its being
turned with a wrench.
The guard is kept from turning by slabbing off the stem, in the same
manner as described and shown in Fig. 9.
To facilitate the removal of the nut, the edges should be slightly
chamfered. An unfinished nut simply drilled and tapped is all that is
desired. Any hexagonal or square nut within the size of the tapered
recess will be locked.
With this construction, the nut cannot turn in either direction without
compressing the spring and is therefore locked, and, in the event of
the spring breaking or being left off, the nut is well protected in
its recess from the possible turning effects of water currents, and
experiments have shown that it will still stay in place.
With machine molding it will be possible to make these guards complete
in foundry, requiring no machine work further than a possible broaching
out of hole to fit the stem, as a fairly good fit is necessary.
While both of these devices are effective even though not tightened
down to a shoulder, they should be so tightened for greater safety and
to fix the lift at the half-inch limit.