Year: 1878

In making steam motors of all kinds the


tendency has been of late to construct engines
| with high piston speed. BY high initial
speed in the motor, much Of the gearing


needed to obtain speed in the machinery that


it moves is dispensed with. The steam is


more thoroughly employed, and the size and


weight of the engine for q given power are
! reduced. The most important improvement


in this direction has sprung from radical
change in the construction of the steam cylin-


der and its moving parts. The cylinder is
increased in length and reduced in diameter,


and in place of one, there are two pistons,


designed to move in opposite directions. The
first piston has a rod passing through the


center of the second, and the second piston


has two rods, one on each side of the central


rod. Each piston has its piston-rod connec-


ted with crank on three-throw crank-shaft


These cranks ale placed opposite each other,


the single rod of one piston balancing the two


of the other. 1t will be observed that, by


this change in the construction of the eng inc,


the pstons must travel in opposite directions,


each making to-and-fro excursion over half


the two pistons are each at one end of the


cylinder, the steam may be admitted behind


them both, and thus drive them together.


When they meet at the center, the steam


enters at central port and drives them apart


until they reach their original positions, when


the excursion is finished and one revolution


of the crank is accomplished. The effect of
this disposition of the moving parts is to
create perfect balance in the reciprocating
action of the engine The pull of one piston


balances the thrust of the other 1t is no
longer series of pulls and thrusts, as in the
ordinary engine, but resulting motion of
the combined pull and thrust of the two pis-
tons. The advantages obtained by this bal
ancing of the pistons are found in the reduced
length and weight of the connecting rods and
the bed-plate of the eng inc, and the steadi-
ness of motion, as there is no pull or thrust
on the shaft and its bearings, higher speed
and greatly increased power. As there is no
straining between the cylinders and the shaft,
the engine rests securely on its bed. AS the
two pistons travel over only one-half of the
length of the cylinder in the same time, and
as the area of the piston surface is double
that of q sing]e-piston engine, the power is
more than doubled for a given speed. The
strain caused by the expansion of the steam
between the piston and the end of the cylin-
der, as in the ordinary engine, in the balanced
engine is spent on the two ends of the cylin-
der in one-half of the stroke, and between
the two pistons in the other half In either
case the strains are exactly balanced. Thus
the engine may run at a high speed without
producing troublesome jarring or shaking
of the motor. In marine work it is thought
the balanced engine will prove of advantage,
as it may be driven at high speed without
shaking the ship, and by giving the engine
higher speed, screw of more effective pitch
may be employed. The only disadvantages
found in this form of motor lie in the multi
plication of the connecting rods ana stuff1ng-
boxes, but these defects are more than bal |
anced by the steadiness and increased speed
and power of the engine.