The high pressure steam which
is controlled by the regulator in the dome is piped to the cylinders which in
turn drive the wheels. More early designs often had the cylinders mounted
between the wheels which made maintenance much harder. More simple designs
use slide valves, as shown in the animation below.
Greater efficiency is
obtained by the use of piston valves.
Water condensate will collect
in the cylinders when the locomotive is not in motion. Water cannot be
compressed to it is essential to be able to remove it. For this reason, drain
cocks are fitted to the bottom of the cylinder bores, at each end.
These can be opened by a control in the cab and are always used when the loco
moves off. The clip below shows an engine move off with the drain cocks
At the outer end of the
piston rod is the crosshead. This slides backwards and forwards guided by one
or two bars. This a a major point of wear and the replaceable bearings in the
crosshead are called 'slippers'.
a single slide bar portrayed in a working model
The reciprocal movement must
now be transferred to the wheels.
The cylinders need to be
lubricated with a special steam oil. This can be introduced by the use of a
displacement lubricator. It was introduced in the United Kingdom in 1860 by
John Ramsbottom. It operates by allowing steam to enter a closed vessel
containing oil. After condensing, the water sinks to the bottom of the
vessel, causing the oil to rise and overflow into delivery pipes. In a steam
locomotive, it was often positioned in the cab where the rate of oil feed
could be observed. Alternatively, two displacement lubricators (one for each
cylinder) might be positioned on either side of the smokebox.
Mechanical lubricators are
far better and are generally operated by a mechanical linkage attached to an
appropriate position on the valve gear.