August 1, 2011

Link and Pin Drawhead Redesign

Now that I have a few freight cars, I've been doing more test operating. I have noticed that when backing cuts of cars, the pushing forces are sometimes are causing my cars to derail. As I study what is happening I see that the coupler offset on "S" curves (such as when taking the diverging route on a turnout) is causing a strong sideways force on the cars. It can actually lift the car off the track and cause a derailment. John Armstrong called this "lurching" and he developed a phrase called the "coefficient of lurch." My links and drawheads are causing a high coefficient of lurch.

Original design with internal plate
The photo at the right shows the lurch, or offset I am getting. The problem comes when the link hits the backing plate inside the drawhead. I designed the drawhead with this backing plate as I thought I would need it to keep the link from sliding too far into the draw head. I actually have to solder in a separate piece for this plate. It is a simple matter to leave off the plate. I built some drawheads without the plate and did a test.

Sure enough, the cars back up much more reliably without the plate.

Modified design without backing plate

Without the plates the link can slide completely into the drawheads when the cars are being pushed. This allows the two drawheads to strike face-to-face. This reduces the lurch as it allows the cars to come closer together. It also reduces the sideways force on the cars.

The cars pull normally. Without the plate it is a bit harder to get the link positioned in the drawhead as they can now slide all the way in and slightly sideways, but this is not bad and worth the improved reliability.

Now I need to retrofit the rest of the couplers and remove the backing plates.

I did some more testing and determined that shorter links also work well. I can make up a batch of shorter links using brass wire. This will allow me to avoid retrofitting existing drawheads.

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