come basically in two sorts, the half beam and walking beam
engine, with sizes ranging from colossal, weighing hundreds of tons in
full size, to the model engineers version which can be around the size
of your hand.
They started off initially as rocking beam engines to provide pumping
facilities for mines, using atmospheric pressure on a large piston with
a partial vacuum on the other side of the piston to
drive the piston along its stroke to raise the water, with the weight
of the pump 'buckets' below ground providing the downwards part of the
The one above is a later version (the
generally known walking beam
engine) using higher steam pressure, as boiler pressure
raised owing to the better production methods and boiler
materials. This one also has a flywheel allowing roatation motion to be
used, whereas the very early originals mostly had a rocking
beam with no flywheel, used simply as a push-pull engine for
the pumping action.
They would have low pressure steam entering the cylinder from below the
piston with a rocking beam engine as the weight of the linkage falling
down the pumping shaft raised the piston in the cylinder and draws in
the steam. Once at the
top of its stroke with the cylinder filled with low pressure steam
there would be cold water sprayed into the steam which quickly
condensed causing a partial vacuum inside the cylinder below the
piston. Air pressure above the
piston then forced the piston down its stroke causing the pumping
lift the water from the mine shaft.
These early engines had the top of the piston rod fixed to the beam so
that the piston had to rock slightly in the cylinder, meaning the
piston seal was rudimentary, much like a bicyle pump is at present,
using a rubber seal with water above it to keep the air out as the
vacuum was created.
They were not very efficient or powerful owing to air pressure
not being much higher than the partial vacuum created below the piston
needed very large diametered pistons to achieve much work. But the
larger the diameter the greater the difficulty with sudden cooling of
the steam, meaning they were very slow acting.
beam linkage was eventually modified using what is known as parallel
motion, the walking beam, to keep the con-rod acting in one plane, and
from this era boiler pressures were increasing so that was the
beginning of the industrial revolution for the whole world when
flywheels were incorporated for factory and other industrial usage.
Half beam engines like this
one on the left were a modification to full beam engines
because of space saving needs in that the beam is supported at
one end above the cylinder with a pivoting column at the other end and
the drive is taken from the beam to a flywheel at around one quarter to
one third the way along the beam from the cylinder.
In use the beam itself pivots on the arms running from the top of the
triangular support above the cylinder giving a strange, but very
pleasing motion to the whole engine.