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Offline jerdenberg  
#1 Posted : 25 January 2022 17:35:28(UTC)

Joined: 10/01/2005(UTC)
Posts: 994
Location: Den Helder, Noord-Holland
On my contorted oval layout, four longish trains usually run without any problem, but on a few occasions a spontaneous uncoupling occurred between two specific cars after several "safe" rounds along the oval. When I finally bothered to look at the problem in detail, I found two separate issues – one with an Athearn ready-to-run 50-foot boxcar, the other with the replacement coupler of a Märklin 40-foot boxcar from one of the US car sets.

The first picture shows the Athearn boxcar (at right) coupled to a Bachmann Silver Series boxcar. The coupler of the Athearn car is clearly raised relative to that of the Bachmann car.


In the second picture, the Athearn car is coupled to the Märklin car [which had one coupler replaced by a Kadee(?) to make a transition between a set of cars with Märklin couplers and another set with knuckle couplers]. Here, again the Athearn coupler is raised, but the height difference is aggravated by the fact that the coupler on the Märklin car is drooping.


It is evident that the couplers on the Athearn car sit too high; comparing the relative locations of trucks and car bodies between the Athearn car and the other ones suggests that the trucks of the Athearn car are too far from the car body (in other words, the car body sits too high, see the roofs). So, even replacing the couplers with offset ones will not solve the real problem.

The drooping of the coupler on the Märklin car results from the thickness of the NEM mount of the replacement coupler: 1.50 mm (as opposed to 1.65 mm for a Trix knuckle coupler or a Märklin coupler, and 1.75[-0.05] in the NEM specs). I'm not 100% sure that the drooping coupler is a Kadee (it is not identifiable) but I cannot recall ever having purchased knuckle couplers of another brand, and two Kadees I had in storage do have the 1.50 mm.

The difference between the offending coupler and a Trix coupler is clear in the last picture, where the car with the drooping coupler is paired with a boxcar from a Trix car set:


In view of the above, it was the coincidence of two issues that caused the spontaneous uncoupling; either of the offending cars coupled to a well-behaved car stayed within safe bounds on my flat oval.

Replacing the offending coupler on the Märklin car was easy enough (and solved the drooping problem), but adjusting the Athearn car is too much for now [hoping that coupling it to well-behaved cars will prevent uncouplings, even though the height difference between car bodies looks a bit strange].


Figomima division, UP; mostly figment of my imagination yet.
Offline DTaylor91  
#2 Posted : 25 January 2022 23:07:48(UTC)

Joined: 31/08/2007(UTC)
Posts: 411
Location: Kennesaw, GA
If you haven't yet acquired a Kadee coupler height gauge, that may help you figure out where adjustments are needed the most.

That said, with Kadee couplers that do not match perfectly with another are far as height is concerned, the spontaneous uncoupling is usually caused by vertical play with each coupler. If you couple two cars together, and start slowly pulling the away from each other, you will see the pressure cause the higher coupler to be torqued upwards, while the opposite happens with the other. If the pressure reaches the point to where the couplers drift up or down too far, they slip away from each other.

You could remedy this somewhat by:

Using very thin sheets of styrene or ABS to add a shim to either the top or bottom of the coupler box to limit the amount of vertical movement of the coupler head. Add a shim to the top side of the coupler box on couplers that tend to rise under torque, to prevent them from lifting too far, and do the opposite for couplers that sit too low.

You can also mitigate this a bit by placing cars with mismatched coupler heights toward the rear of the train, which should reduce the force on the couplers.

Hope this helps
Offline jerdenberg  
#3 Posted : 26 January 2022 16:32:03(UTC)

Joined: 10/01/2005(UTC)
Posts: 994
Location: Den Helder, Noord-Holland
Don, thanks for your suggestions.

Indeed, the spontaneous uncoupling between the Athearn and the Märklin car happened because of the unfortunate match of a coupler that is too high on the Athearn car, and a coupling with too much vertical play on the Märklin car. While the latter (due to the shaft of the Kadee coupler being too thin) could be adjusted by a shim in the coupler box, I decided to do away with the Kadee coupler and use a Trix coupler instead: at 1.65 mm shaft thickness (vs 1.50 for the Kadee), the Trix coupler reduces vertical play sufficiently to avoid the drooping. The combination of the unaltered Athearn car and the "Trixed" Märklin car has now run for about an hour without uncoupling.

As altering only the couplers of the Athearn would involve purchasing Kadees with the correct offset, but would not remediate the incorrect distance between trucks and car body, I keep that car as is for now; the deviation in coupler height is not large enough to cause uncoupling with well-behaved cars, so I can run it anyway. Real adjustment by reducing the distance between trucks and carbody (removing material from the truck and from the mounting pin on the body) is for later.

The thin Kadees (## 18 and 19) I have were purchased something like a decade ago. I wonder if present-day Kadee replacement couplers are still too thin. Strangely, on the Kadee page for #18 couplers, the technical drawings do not specify the thickness of the shaft, although they do show the correct inside height of the coupler box at 1.75mm.


Figomima division, UP; mostly figment of my imagination yet.
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