ght springs
are used to hold the packing against the shaft and in some the pressure
of steam in the case does this. There is a pipe, also shown in Fig. 12,
leading from the main line to the packing case, the pressure in the pipe
being reduced. The space between the two upper sets of rings is drained
to the third stage by means of a three-way cock, which keeps the balance
between the atmosphere and packing-case pressure. The carbon rings are
fitted to the shaft with a slight clearance to start with, and very soon
get a smooth finish, which is not only practically steam-tight but
frictionless.

[Illustration: FIG. 12]

The carbon ring shown in Fig. 12 is the older design. The segments are
held against the flat bearing surface of the case by spiral springs set
in brass ferrules. The circle is held together by a bronze strap screwed
and drawn together at the ends by springs. Still other springs press
the straps against the surface upon which the carbon bears, cutting off
leaks through joints and across horizontal surfaces of the carbon. The
whole ring is prevented from turning by a connecting-rod which engages a
pin in the hole, like those provided for the springs.

[Illustration: FIG. 13]

[Illustration: FIG. 14]

[Illustration: FIG. 15]

[Illustration: FIG. 16]

The Safety-stop

There are several designs of safety-stop or speed-limit devices used
with these turbines, the simplest being of the ring type shown in Fig.
13. This consists of a flat ring placed around the shaft between the
turbine and generator. The ring-type emergencies are now all adjusted so
that they normally run concentric with the shaft, but weighted so that
the center of gravity is slightly displaced from the center. The
centrifugal strain due to this is balanced by helical springs. But when
the speed increases the centrifugal force moves the ring into an
eccentric position, when it strikes a trigger and releases a weight
which, falling, closes the throttle and shuts off the steam supply. The
basic principle upon which all these stops are designed is the same--the
centrifugal force of a weight balanced by a spring at normal speed.
Figs. 14, 15, and 16 show three other types.

The Mechanical Valve-Gear

Fig. 17 shows plainly the operation of the mechanical valve-gear. The
valves are located in the steam chests, which are bolted to the top of
the casing directly over the first sets of expansion nozzles. The
chests, two in number, are on o