y the
brass against. After keying up, test by moving the wheel to another
position and see if brasses are free on the pin. For the side or
parallel rods, always place the engine on the center for the side that
is to be keyed.

23. Q. How should the side rods on a mogul or consolidation locomotive
be keyed?

A. Place the engine on the center on that side, key up the brass on the
main pin first, work each way toward the ends of the rods, being careful
to keep them the proper length so they do not bind when passing either
center. Be sure that wedges are properly set up before keying the side
rods.

24. Q. What is the necessity for keeping the brasses keyed up properly?

A. If too tight, they will surely run hot; if too loose, they will pound
and injure the brasses as well as endanger the safety of the straps and
rod bolts. Very loose brasses can pound enough to get hot.

25. Q. What is meant by an engine out of tram? Out of quarter?

A. When corresponding wheels on opposite sides of the engine on
different axes are not spaced equally apart; where the axle of any wheel
is not at a right angle to the center line from front to rear of engine,
so they do not run square on the rails, or where the space between the
axle centers on opposite sides is not equal. This is sometimes indicated
by unequal flange wear and should be reported at once. Wheels are out of
quarter when the crank pin in one wheel is not exactly 90 degrees or one
quarter of a turn from the pin in the wheels on the other end of the
same axle. This is usually caused by slipping the engine with sand on
one rail only and the condition of engine should be reported at once.

26. Q. Describe a piston valve.

A. A piston valve is a cylindrical spool-shaped valve constructed with
packing rings much the same as the steam piston that moves through the
cylinder, except that a piston valve is double or composed of two
pistons connected by center rod or spool working in a bushing of equal
diameter. Steam and exhaust ports are cut through this bushing; steam
ports to the cylinder and exhaust port to the exhaust pipe. There is
also a steam port for live steam from the boiler. As the pressure on
this valve is equal in both directions it is practically balanced.

27. Q. What is a balanced slide valve? How is it balanced, and why? For
what purpose is the hole drilled through the top of the valve?

A. One in which the steam pressure on the top and bottom of the valve is