Evening star., 1926-03-07
No, 140-Valve Gear Variety.

Valve gear is the engineers term
for the valve action of an automobile.
Even in its simplest form the mechan-
ism that opens and closes the ports in
the cylinders is complicated If there
doesn't appear to he much by way of
operating mechanism the motorist
soon comes TO know that metallurgy
and design play an important role IN
keeping the valves functioning prop
erly.
Smith was beginning to find that
there still is considerable question as
to the proper kind of valve gear for
an automobile engine. This idea took
more more definite shape as he be
came familiar with the several va
rieties Briefly classified. he found the
following examples of action:
Poppet. Sleeve-valve. Rotary valve.
Piston valve. Slide valve. Pressure
valve. These were found in four-cycle
engines and were applicable to any
number of cylinders.
Poppet and sleeve-valve arrange
ments were the only ones in general
use The remainder were expert
mental. with every possibility how
ever, of their rapid adoption.
Smith was particularly interested in
the various kinds of pOppet valve
gear. lie had figured he knew the
whole story when dividing them into
the L-head and valvein-head variety,
but he still was to become familiar
with the overhead camshaft type.
Duty of the Camshaft.
"All poppet valves" ] explained,
need some mechanism to raise them
or to push them down. depending
upon whether they are in the pockets
al the sides of the cylinders or in the
heads of the cylinders. This arrange
ment calls for a camshaft, which is a
smaller shaft run by and paralleling
the crankshaft. This smaller shaft
has knobs on it. or cams as they are
called, which raise the tappets and, in
turn, actuate the valves themselves If
the valves are at the side Of the en-
gine the tappets can be very short
and the valve gear comparatively sim
pie. In the T-head type. where the in
take valves were on one side and the
exhaust valves on the other two cam
shafts We're necessary. However this
Is an antiquated arrangement and you
need not consider It.
When the valves are to he i'm the
head several arrangements are pos
sible. The most common Is to put
rocker arms on the top of the engine
operating these from long push rods
which ride on top of the tappets. The
camshaft is just where it would he in
the simpler L-head ARA nEement. A
system that is growing in popularity
because of its success in aviation en-
gines is the overhead camshaft. Here
the camshaft runs parallel with the
crankshaft but on top of the engine
This gives more direct action for the
overhead valves. The arrangement
also provides for excellent fabrication
but such valve gear must be carefully
designed and made in order to prevent
camshaft vibration and knocking.
Sleeve Gear Differs.
Sleeve valve gear is found i'm the
Knight type engine. Here the gear is
all inclosed in the crankcase and en-
gine base and is well sealed against
exposure. The sleeves. which slide up
and down between the pistons and
their cylinder walls. and which open
and close the two ports in each cylin.
der at just the right moment for prop
er operation of the engine, are &C
tuated by rods connected with a
smaller crankshaft that parallels and
is geared to the main crankShaft. One
of the newer arrangements is a sin