American Flyer Steamers Running Faster In Reverse: Theory and Solutions

[Anonymous]

Recently, a user inquired about an American Flyer steam engine running faster in reverse than forward. Having seen this problem before and never finding a solution, I set out to get to the bottom of this with a 322AC that was  displaying the same issue. Here are my findings:

 

Ok, I just went back through this particular 322AC. The cause of the faster running in reverse was partly caused by the armature being incorrectly shimmed. 

I determined this by turning the locomotive upside and using my thumb to apply torque to the drivers to simulate the load placed on them during operation.

When I rolled them toward the rear of the chasis to simulate running forward, the armature pulled in towards the chasis which caused the armature stack to not be inline with the field stack.

Rolling the drivers toward the front of chasis to simulate reverse pushed the armature back away from the chasis and towards the brush holder. However, the armature stack now lined up perfectly with the field stack. That, along with reduced pressure on the worm end, explains the faster running in reverse.

Solution:

I fixed this alignment problem by inserting another thrust washer between the front of the armature and the shaft bushing in the chasis. This moves the armature away from the chasis so that it is always properly aligned with the field. 

The motor now runs better in the forward direction, but this "faster in reverse" issue is also an inherent design flaw for steam locomotives built before October 1949 according to a footnote in the factory service manual.

Prior to October 1949 production, the armature has nothing as produced to limit it's rearward travel towards the brush holder. This results in the armature's commutator contacting the ends of the brush tubes. This creates a larger contact area, which allows more current to flow, so the motor runs slightly faster in reverse.

After October 1949, the armature was revised to include a brass oil slinger mounted above the commutator.  The brush holder was redesigned to accomodate the oil slinger.  The oil slinger limits the rearward travel of the armature, preventing the commutator from contacting the brush tubes.

[Major]

I agree with your assessments on the problem and solution.  Many times previous owners have lost the washers that were on the worm end of the armature permitting too much forward movement. Fortunately replacement washers are available.  One other condition that can cause the faster in reverse condition are brushes that are too worn or weak or broken brush springs.  As you said when the loco is running in reverse the armature moves backwards against the brush plate increasing pressure and contact area. With worn brushes or weak springs this movement backwards increases brush pressure increasing speed.