The Walschaerts & The Radials

35640 Avenue F Yucaipa, California 92399

Upon several occasions in the past, the Walshaerts valve gear
has been referred to as a radial type gear. This misnomer must be
put to bed here and now. Let casual observers not be misled by the
combination lever as required on this gear, and which same has also
been applied to other types of valve gear. The Walschaerts is NOT a
radial type gear, and was never claimed to be. This gear was
designed primarily to afford accessibility and to avoid crawling
down and under for servicing and lubrication. But it is regarded as
a ‘sloppy’ gear, unlike all-wrist motion of a true radial
gear. While I do not wish to castigate this design, well do I
recall in the late ‘Teens when I worked in the Case factory and
the old railway roundhouse, there was carried in ‘Railway
Age’ an advertisement by the Pilliod Company of New York,
distributors for the Baker radial gear for locomotives, which
depicted the outlay of a Walschaerts gear with the comment, in
effect, ‘This compromising mechanical monstrosity was forced
upon the American locomotive industry!’ And final attempts at
uniform steam distribution in locomotives in this country included
those offered by Franklin and Caprotti, which utilized gear-driven
cams, operating poppet valves.

A true radial gear, by definition, is one in which a single
eccentric (or crank) is strap-connected to a radius arm, the
opposite or fulcrum end of which may be adjusted about a central
pivotal point in order to affect the point of stroke steam cutoff
and also control engine reversal, where the latter feature is
incorporated. The principle valve motion is taken off at a median
point of this linkage, or a point so actuated by it. Now, let us
see why the combination lever on the Walschaerts modified eccentric
type of gear is similar to that employed with radial gears, (this
is composed of a conjugate arm from the combination link off the
cross-head to the valve actuating rod).

In a conventional simple eccentric gear, wherein the valve is
designed with lap (and if it does not have lap it will of necessity
be very inefficient because of using steam full stroke, as on
simple throttle-reversing hoisting engines commonly used on
shipboard) the valve leads the crank position by approximately 120
degrees in the case of outside admission, common with D-valves, or
trails it by the same amount with inside admission as common
practice with piston valves.

Thus, if an engine is designed for reversible operation from a
single fixed eccentric, this eccentric must of necessity be set
effectively midway twixt both modes of operation; that is, 180 (or
zero) degrees from crank position. Actual geometrical differences
are accommodated through bell-crank linkages, etc. But a 180-degree
setting, operating in a geometrical configuration 90 degrees above
(or below) the crank-crosshead line of action would normally result
in a relative eccentric angle of 90 (or 270) degrees with the
crank. So a combination must be picked up through linkage with the
crosshead, wherein the resultant of two angles, zero and 90 degrees
for instance, is resolved as 45 degrees, the opposite end of a
fulcrumed lever from which would result in a 135-degree
displacement. This is satisfactory for the purpose when the valve
lap and lead are designed accordingly. And from this portion of the
discussion, it is apparent that this conjugate action applies to
both radial and Walschaerts motions.

Now, since we have divorced the Walschaerts out of the radial
family, it is of interest to consider the Joy radial valve gear as
applied in some locomotive practice, the closest relative to
traction engine design, since the primary gear takeoff power was
from a pivotal point in the main connecting rod between the
crosshead and the crankpin. This compares favorably with the Wolff
and similar radial gears. However, in many traction versions the
second radial (swing-able) arm was replaced by a sliding block.
While most of these blocks were straight, Case did, upon later
occasions, resort to a curved block to simulate radius action. In
these designs, the combination or conjugate action (radial and
crank) was taken off at a point between the eccentric and the
block. Another refinement to such radial gears was incorporated by
Reeves, probably the ultimate in this respect, by adding a
compensation to equalize one end of the cylinder against the other
(due to the angularity of the connecting rod, as also to that of
the valve motion as it is practically impossible to balance one
motion against the other in an out-of-phase displacement). In this
case (not Case) Reeves took advantage of the property of a
discordant bell crank linkage, wherein the takeoff speed of travel
of one leg of the bell is not the same as in the other leg.

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