To reverse the motion of a plain Slide Valve engine, remove the cover
of steam chest and place the engine on the dead center. Observe the
amount of lead or opening that the valve has on the steam end, then
loosen the eccentric and turn it around on the shaft, in the direction
the engine is wanted to run, until the valve has exactly the same
amount of lead at the other end; then turn the engine to the opposite
center, to determine whether the lead at this end is the same as at
the other; then place the crank at half stroke top and bottom, and see
that the port openings are equal in both positions, and replace cover.
Q. What is dead center?
A. The dead center of an engine is the point where crank and piston rod
are in an exact line.
Q. What is a half stroke?
A. It is the point reached by the piston after traveling exactly
one-half its travel.
Q. What is the meaning of “lost motion”?
A. Lost motion is looseness of the connecting rod at crank or
wrist-pin, or looseness of the link or link block, cross-head and
guides, main crank shaft and pillow block, caused by friction and
wearing away of the metals and the neglect to take up the wear.
The lost motion in all parts of an engine should be taken up
frequently, to keep it running smoothly, but care must be taken not to
tighten bearings or keys too tight.
Q. What is lap and lead?
A. Lap is the position of the valve which extends or laps over the edge
of the ports when the valve is in its central position. That on the
inside of the D is the inside or exhaust lap, while that at the ends of
the valve is the outside lap and affects the admission and cut-off.
Lead is the amount of opening which is given to the port by the valve
when the engine is on the center.
Lead on a valve is the admission of steam into the cylinder before the
piston completes its stroke.
Q. How much “lead” should a valve have?
A. There is no general rule for the amount of lead that would be best
suited for all makes of engines. It must be determined by the design or
construction, speed and work required, to produce the best results for
economy and quietness in running.
Q. What is a throttle engine?
A. A throttle engine is one in which the speed is controlled by
throttling the steam with a governor, as opposed to an automatic engine
in which the speed is regulated by varying the point of cut-off at the
valve.
Q. What is the difference between a stroke and a revolution?
A. A stroke is the movement of the piston, from one end to the other of
cylinder. A revolution takes two strokes of piston.
Q. How are steam packing rings put on the piston head?
A. Remove the back cylinder head and take the piston and rod out of
the cylinder, and stand it “head up.” Then place the inside of the
ring, opposite to the opening, against the side of the piston head next
to you; gradually press the ring open with your hands, and it can be
easily slipped over the head and put in place.
Q. How is a piston put into cylinder?
A. It is always entered from the back end in horizontal engines. The
rings (if steam packing) should be carefully placed in position and
compressed by the hand if piston is small, but if large, curved blocks
of wood or a band of sheet iron can be used to support them until they
enter cylinder. When the piston rod passes through the stuffing box, it
should be supported at outer end to prevent cutting.