the machine slows down. With the stage valve in operation,
at a critical point the regulation is uncertain and irregular, and its
use has to be abandoned. The excess first-stage pressure will then be
taken care of by the relief valve, which is an ordinary spring safety
valve (not pop) which allows the steam to blow into the atmosphere.

The mechanical valve-gear does not often get out of order, but sometimes
the unexpected happens. The shop man may not have properly set up the
nuts on the valve-stems; or may have fitted the distance bushings
between the shield plates too closely; the superheat of the steam may
distort the steam chest slightly and produce friction that will
interfere with the regulation. If any of the valve-stems should become
loose in the cross-heads they may screw themselves either in or out. If
screwed out too far, the valve-stem becomes too long and the pawl in
descending will, after the valve is seated, continue downward until it
has broken something. If screwed in, the cross-head will be too low for
the upper pawl to engage and the valve will not be opened. This second
condition is not dangerous, but should be corrected. The valve-stems
should be made the right length, and all check-nuts set up firmly. If
for any purpose it becomes necessary to "set the valves" on a
1500-kilowatt mechanical gear, the operator should proceed in the
following manner.

Setting the Valves of a 1500-Kilowatt Curtis Turbine

We will consider what is known as the "mechanical" valve-gear, with two
sets of valves, one set of five valves being located on each side of the
machine.

[Illustration: FIG. 21]

In setting the valves we should first "throw out" all pawls to avoid
breakage in case the rods are not already of proper length, holding the
pawls out by slipping the ends of the pawl springs over the points of
the pawls, as seen in Fig. 21. Then turn the machine slowly by hand
until the pawls on one set of valves are at their highest point of
travel, then with the valves wide open adjust the drive-rods, i.e., the
rods extending from the crank to the rock-shaft, so that there is 1/32
of an inch clearance (shown dotted in Fig. 17, Chap. I) at the point of
opening of the pawls when they are "in." (See Fig. 22.) Then set up the
check-nuts on the drive-rod. Turn the machine slowly, until the pawls
are at their lowest point of travel. Then, with the valves closed,
adjust each _valve-stem_ to give 1/32 of an inch clearanc