448
STEPHENSON’S PATENT
to have some travel, in order that the port may be full open for some time; and
after having fully uncovered the port, the slides move or travel a little farther, not
beginning to close the port again until they have returned over the travel. The
motion is very varying as the eccentric drives the slide most quickly at the middle
of its stroke ; corresponding to the ends of the strokes of the piston, where the
quickest motion is wanted, to admit the steam for the next stroke; the velocity of
the slide diminishing rapidly towards the ends of its stroke, where it stops and
retrogrades. Many contrivances have been tried in stationary engines for working
the slides more suddenly, either by striking the spindle with tappets or projections
on a moving rod, or by means of different kinds of cams or eccentrics of irregular
shapes; with these plans travel of the slide would not be necessary, the port
being full open nearly all the stroke. But in a locomotive no plan can well be
adopted for working the slides which has a more sudden or irregular motion than
an eccentric, because of the very great rapidity with which the strokes have to be
made; which would soon cause the machinery to be deranged.
The piston and slides make two reciprocations or changes of motion during one
revolution of the driving wheels, and as these are five feet in diameter, they make
nearly 4 reciprocations per second when the engine is running at the rate of 20
miles an hour, and 8 reciprocations in a second when running at a little more than
40 miles an hour; the ordinary rate of working is about five reciprocations per second.
This extreme rapidity causes every change of motion to produce a violent blow to
the machinery, requiring that all the parts should be very well made and fitted to
gether, in order that they may stand the work ; the greatest strain is produced upon
the fixing of the piston rod into the piston, and upon the joints of the piston rod
and connecting rod. The brasses in the crank end of the connecting rod are
not keyed up quite tight, but a very little play is left, allowing them just to shake
when worked backwards and forwards; in order to prevent their heating by the
great rapidity of the motion, and expanding by the heat together with the crank
pin, making the joint very tight; they have sometimes expanded so much from the
heating in consequence of being keyed too tight, that the engine has been nearly
stopped by the great friction occasioned, and the brasses have been broken to
pieces.
All the moving parts require a constant supply of oil to diminish the friction; and
oil cups are fixed for this purpose upon all the principal moving parts, such as on
the ends of the connecting rods over the bearings, on each of the piston rod
guides, and over the piston rod and the slide valve spindle; the piston is oiled
by pouring oil into the cylinder by the cock in the cylinder cover, the bent end of