Eagle River review. [volume] (Eagle River, Wis.) 1890-1927, 1896-10-01
Danger from Double Eccentrics.

Within the past ten years it has be-
come well-nigh a custom among al
classes of engine builders who manufac-
ture the four-valye type of engine to
operate the valve motion by the use OF
double eccentrics, one controlling the
steam valves and the other the exhaust
valves. To facilitate the working of the
valves by hand at times Of starting and
stopping it has been the practice to ar
range the connecting rod which carries
motion to the Wristpiate, or other device
from which the waves are operated, so
as to be unlocked at will. and use a
starting bar in the hand of the engine
driver for securing the desired move
ment The use Of double eccentrics neces-
states a provision for working the sec
ing mechanism is planned so as to be
readily brought into play when needed,
and as readily made secure when the
engine is in regular service.
Now there is an element of danger in
the separation Of the driving mechanism
of the two sets of valves. The word
element is employed because the mat
ter is by no means serious; but, such as
it is. there is good reason to bear it in
mind, for it has been the cause OF serious
trouble in at least one instance. Read
ers of this article who are familiar with
the details of enginerunning appreciate
the Importance OF keeping an eccentric
der. When one comes to think about
it, the eccentric is in reality a huge crank
pin, two or more times the diameter of
the main shaft Of the engine A slight
friction here means great deal. 1t the
surface Of this huge pin and the strap
around it bind together, or tend to this
condition ever so little, the effect is eX-
ceedingly marked The contrast between
the size of this surface and the size of
the eccentric rod which it drives is wider
than any other familiar feature in the
entire engine. Want of fabrication, or
too close a fit in the eccentric strap, as
those who run engines have found out
by sorry experience, are very serious
matters. II is right here that the dam
ger referred 70 creeiJs IN. When one
eccentric operates both sets of valves,
arrangement which occurs between the
wristplate and the eccentric is likely
to cause no further trouble than stop
ping the engine and repairing the de-
fective part But suppose that two &C.
centres are used, and one of them only
becomes disordered, and that one the ex-
haust? Suppose the unlocking device
gets deranged. the hook drops ont and
the exhaust valves stop working. The
steam valves continue to admit steam as
before. and the probability is that the
compression of this steam on the return
stroke will carry the pressure to a dam
gerous limit and bring strains upon the
main parts which will be destructive.
An instance of this kind has occurred
where the relief valves OH the cylinder
did all they could to relieve the pressure,
but this was not sufficient to prevent
the bed of the engine from being broken
square across. and the main pillow block
and shaft from being moved distance
of two inches away from the cylinders.
Double eccentrics have come to stay,
for they provide means for setting the
valves so as to get the best conditions
of service and of economy. Still they
bring new problems which must be con
fronted and solved.-Engineering Rec
ord