440
STEPHENSON’S PATENT
engines which have no fly-wheels are always in pairs, like locomotives, except in
some of the smallest vessels.
The connecting rod being inclined below the piston rod during the back stroke,
and above it in the fore stroke, requires a moving joint at the end which takes hold
of the cross head; and the cross head is made spherical at that part, to prevent any
lateral strain that might arise during the motion from the crank not being accurately
in the line of the piston rod, or the axle at right angles to it. The varying
position of the connecting rod also renders the guides for the piston rod necessary to
resist the great oblique strain upon it caused by the inclined positions of the con
necting rod tending to force it upwards in the back stroke, and in the opposite
direction during the fore stroke. This oblique strain is diminished by increasing
the length of the connecting rod; which is therefore made as long as possible, in
order to diminish the friction of the guide blocks, leaving only a small clearance beyond
the crank and head of the piston rod at their extreme positions. Other modifications
of the plan are also used to preserve the parallel motion of the piston rod, such as a
single square bar placed on each side with the edges at top, termed diamond guides,
and having sockets on the ends of the cross heads sliding upon them; but the other
plan is found to be most advantageous; in small stationary engines similar plans
are also sometimes adopted. But in stationary and marine engines generally, the
motion of the piston is maintained in a straight line by various combinations of
rods, forming a parallel motion, which has less friction than guides, and is more
convenient in those cases. The strain is also diminished by the piston rod being
connected with the end of a beam instead of directly with the connecting rod, which
has a much less irregular motion.
The two cranks are thus made to revolve uniformly by the action of the steam
upon the pistons in the cylinders, and move with them the axle and the wheels
fixed upon it. The wheels are made to revolve in the same direction that they
would turn if the engine were running forward ; and they cannot turn round without
either slipping round upon the rails, or rolling forward upon them and moving the
engine with them. If the adhesion of the wheels upon the rails is greater than the
resistance of the engine to being moved, and the pressure of the steam be sufficient,
the wheels will roll forward upon the rails; and the engine will be propelled, and
be able to draw after it a load, the resistance of which is equal to the excess of the
adhesion of the wheels upon the rails above what is required for moving the engine
itself. The adhesion of the wheels is not always the same ; it is the greatest when
the rails are most clean, and are either quite dry or quite wet; and it is least
when the rails are dirty, and greasy with being partially wetted. For this