■ FIGURE 12
stepper motor shaft and for the exhaust damper shaft. The
stepper motor link arm is connected to the exhaust
damper link arm using a long 3/16 inch push rod, (Figure
14). Any diameter rod will work as long as it does not flex
while in the ‘push’ mode. There is very little load on the
exhaust damper itself, so strength is not necessarily a
primary consideration. See Figure 15 for a blueprint of the
link arms. Weld a small block of metal to the center of
both arms and drill them for a 1/4 inch shaft hole. Next,
drill a hole in the block itself for a small set screw that will
hold the arm to the shaft. Notice the additional holes
drilled in the link arm; these are used for travel adjustment.
The outer holes will give more travel/per step than the
inner holes. Four holes in both the stepper and the exhaust
■ FIGURE 14
link arms provide plenty of adjustment. The push-pull tube
is inserted two inches from the center shaft on both arms,
ensuring a good range of travel for the damper. Notice in
the blueprint the adjustment bolt for the closed limit switch;
this bolt is not needed on the exhaust damper link arm.
Two limit switches are actuated by the stepper linkage
arm. The first switch limits wide open travel of the damper
and the second limits fully closed travel.
Notice again in Figure 14 the red limit switch has an
adjustment bolt attached to the linkage arm. This bolt is
used for setting the fully closed limit switch position.
The easiest way to make this adjustment is to start a
fire and when it is hot and stable, begin closing the damper
until smoke begins back-feeding out of the flue (into the
■ FIGURE 13
November 2008 39