Bachmann DD40 splitting axle gears

[Jbub]

So when I was at the show two weeks ago I wanted to run my DD40 around the club layout pulling my stack train. I put it on the track, the sound turns on (sound value loco) I turn up the throttle and the front trucks wheels start turning and the rear trucks wheel do nothing. Yay!!  So tonight I thought I would do a little investigating. I pull the loco apart and start playing with the rear truck, turning the axle but it will only go to a point then bind. So I turn it the other direction and it will go to a point and bind again. Hmmmm, then I remember a post a while back about Bachmann using Nylon gears and that they can split.  So I pull the super un-detailed truck frame off and put on the opti-visor and see that not one, not two, and not even three, but all for axle gears are splitting. 

Here are some pics. Sorry about the photo quality.





The split on these three axles are almost noticeable with the naked eye and the 4th axle I could only see just the start of a split near the gear and couldn't get it to show up on camera.

The logical solution is to jump on Bachmann's website and see if they have some wheels with gears. NO DICE!! They only sell the entire truck assembly with and without the frame and they are sold out of both. Not my night.

So my question to you guys is this. Is there a way to get replacement wheelsets with the gear or is there a way to fix this.

Thanks in advance for any advice given.

[Chris333]

Is it a whole loco with no mods?  If so just send it back a tell them to replace it. Sound value can't be that old.

Mostly it's the older white gears that crack.

super un-detailed

[peteski]

Yes, the white plastic axle/gears were very crack-prone.  We were hoping that the black plastic was more not as bad, but you just proved it otherwise.  Or maybe you just have a really bad example. Maybe the half-axles were made slightly oversize causing extra stress?

It is also good to see that Bachman lubricates their locos very sparingly. 

[Jbub]

Is it a whole loco with no mods?  If so just send it back a tell them to replace it. Sound value can't be that old.

Mostly it's the older white gears that crack.
 

i think I bought this loco 4 years ago, so no it's not that old. As far as sending it back to them. I sent them an email about it so we'll see what they say. If all comes to worse the wheelsets from the 44/70 tonner  look to be the same.

[daniel_leavitt2000]

If it's just the 4 axles in the rear and the front ones are fine, my guess would be that the truck used wheels that went though the punch off center during manufacturing. I haven't seen any split gears in years and having all the axles on a single truck damaged like that makes me think it is not the same aging nylon Bachmann was infamous for.

[peteski]

If all comes to worse the wheelsets from the 44/70 tonner  look to be the same.

I don't think so.  the DD40 uses standard 40" (or is that 42") wheels and 44-tonner has tiny wheels (33"?).  Maybe the plastic axle tube/gear part is the same but I also doubt.  I think the 44-tonner's gear is smaller.

BTW, I like how you described the sideframes as un-detailed.  They are a bit like flat-chested women - not many bumps. 

[Doug G.]

Jeez, you would think knuckleheaded engineers would eventually realize it's NOT a good idea to stuff a comparatively incompressible material into plastic, which inevitably has weak spots. Somebody needs to tell them, "Plastic is not really the miracle, indestructible material you think it is.

Doug

[Chris333]

We are talking Bachmann here.

Many years back a bunch of On30 people filed some sort of legal action against Bachmann for cracked gears. I had 2 Bachmann On30 locos that were brand new and didn't run because of cracked gears. I got put on the list and about 2 years later they sent me a bunch of gears to replace everything. So now my railbus and Climax run... Yea!

[peteski]

Jeez, you would think knuckleheaded engineers would eventually realize it's NOT a good idea to stuff a comparatively incompressible material into plastic, which inevitably has weak spots. Somebody needs to tell them, "Plastic is not really the miracle, indestructible material you think it is.

Doug

A great majority of models are constructed using metal half-axles and plastic axle tubes.  When properly designed (using appropriate type of plastic and using carefully chosen dimensions), the design is reliable.  Heck, this type of design is used for non-modekl applications too.

[Doug G.]

Metal tubes OVER plastic axles is still a lot better. Heck, the wheel axle assemblies in a Con-Cor PA last forever.

Doug

[peteski]

Metal tubes OVER plastic axles is still a lot better. Heck, the wheel axle assemblies in a Con-Cor PA last forever.

Doug

Then there were the Mehano wheelsets with plastic axle/gear combo plugged into holes in the wheels.  And older Kato design similar to that. But those were all inside-bearing designs with wheel wipers. The outside bearing low-friction point-axle-end designs now depend on plastic axle tubes.

[mighalpern]

just curious
but would 3D printing be an option  ??  to make a new set with the hub made thicker and hopefully not as prone to splitting.
 Those Anycube Photon  guys are making some killer stuff

[u18b]

It isn't just the old white plastic the cracks.

I've worked with a dozen or so Bachmann Amfleet cars.....   and the level of cracking on the axles is astounding.

The F7 also uses black plastic.    The last F7 A/B set I worked with had ALL 8 plastic pieces cracked.

[narrowminded]

The material is definitely one thing but the fit is another and I have found the fit is excessively tight on units I've measured and fixed, beyond need or reason.  An interference fit in the .003" difference range on a part in the .047" range takes the material beyond the yield point and when that's been done there is nothing to be gained in the friction fit and in fact, as it yields, that friction fit begins to reduce all of the way to the fail point.  And when left in tension like that, in the yield range, over time the material is prone to cracking with some materials better or worse in that state.  (Hmmm, are we surprised yet?)   And if that's not enough, add the splines/ knurl to the length of the shaft that's pressed in too tight, creating stress risers at every peak and the only question becomes, "why didn't it break sooner?"

As far as the material goes, there are things in the material and also in the production process that can effect its properties.  Not unlike saying "steel" as an all encompassing description of that material when we know that's not true.  There are dramatic differences in that material's properties for a minor addition or subtraction of a few ingredients and/ or in the processing.  True for these plastics as well.  In general Delrin has very good properties for this service based on its properties and confirmed in years of good service in many applications, including our models.  What it can't do is patch a grossly fitted part nor maintain its properties if the manufacturing process isn't correct, especially proper heating and cooling of the part. 

Bottom line, based on my own experience with making press fit axles and gears, one half thousandths is enough, up to one and a half is probably safe and affords some reasonable manufacturing tolerance, and .003" or more, which is what I've found in cracked axles, is totally unnecessary and just invites problems with ample evidence of that. 

For my purposes using virgin Delrin bar and machining the parts takes a lot of the variables in material processing out of the equation, good when you don't have a way to really know what's going on with the material, and has good properties for the service including ability to withstand the press fit over time, and underscoring, a reasonable press fit.  That's my story and I'm sticking to it.

[Doug G.]

It isn't just the old white plastic the cracks.

I've worked with a dozen or so Bachmann Amfleet cars.....   and the level of cracking on the axles is astounding.

The F7 also uses black plastic.    The last F7 A/B set I worked with had ALL 8 plastic pieces cracked.

Yup, one of my Chinese Con-Cor Hudsons cracked its axles. I was able to repair it, fortunately.

Doug