[N&W] Ed King explains eccentric cranks

nw-mailing-list at nwhs.org nw-mailing-list at nwhs.org
Thu May 6 22:03:46 EDT 2004

[Ed King takes Ben Blevin's challenge to educate the rest of us:]

Ben - Without getting into far deeper technical aspects than this list is
interested in:

1.  The Y-3, in common with most N&W locomotives, used piston valves
arranged for inside admission (that refers to the steam going to the
cylinder from the valve)

2. With inside admission of the steam, the movement of the valve must FOLLOW
that of the piston by approximately 90 degrees - in other words, when the
piston starts its stroke, the valve will be moving in the OPPOSITE direction
from the way the piston is going to move

3. In order to produce the proper motion, the eccentric crank - that's the
thingy out on the end of the main crankpin that is connected to the valve
gear - must be arranged so that its connection with the eccentric rod -
that's the rod that goes from it to the valve gear - is 90 degrees BEHIND
that main crankpin as the wheel rotates in the forward direction.  Well, in
actual practice with these engines, the crank-eccentric rod connection will
be a little more than 90 degrees because the Baker Valve Gear's connection
is above the longitudinal centerline of the drivers . . .

Tell you what.  If you loosen the crank on the crankpin and turn the drivers
until the crankpin is at the very lowest point of its circle, and run a
straghtedge from the center of the driver axle to the bottom of the rod in
the valve gear that connects to the eccentric rod, and put the eccentric rod
connection to the eccentric crank on that straightedge you'll have them as
close as it can be set, assuming that the manufacturer has got the rods and
cranks the right length.  In other words, on the straightedge will be a)
main driving axle center; b) eccentric crank-eccentric rod connection; c)
eccentric rod-valve gear connection.  If you've got 'em in a straight line -
and the crank must be pointing toward the front of the locomotive - you'll
have them set as close as you can get them to correct.  And that'll work
with any direct valve gear - Walschaerts and Southern, too (Southern was
always direct).  Indirect injects questions we need not consider here, and
N&W folks living today would have been most likely to see it on the front
engines of Z-1a 2-6-6-2s.

If I can figure this out, I don't know why the folks that design these
models can't.


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