Friday, August 14, 2009

Riveting draft gear

SN 1005's excursion career saw it shipped all over northern California in freight trains. It is not built as strongly as freight cars, so it tended to take the brunt of the damage in rough handling and switching accidents. Needless to say, a lot of this fell on the couplers and draft gear. Previously, both ends' couplers and draft gear had been disassembled. Subassemblies had been repaired, including a complex steel forging in the shape of a "C" which wraps around the draft gear springs. This is riveted to the coupler body with two large 1-1/4" rivets.

These rivets are special. They are flat-head rivets. They must lay flush against the draft gear when they are done. Their holes are countersunk with a bevel, so the rivet is shaped like a flat-head screw. It's that way on both sides.

What makes a rivet a rivet (and not a bolt) is that it is heated to yellow-hot, inserted into the hole, and hammered. This makes the rivet expand fully into the hole, leaving no space. If the hole is irregular, the rivet fills it all, at least near the hammered end. Most rivets start with their familiar button-head already on one end. But this rivet is flat-head. It must expand into a countersunk hole. To get a good start, the rivet is turned with the correct shape of head. Even so, this will be heated, and will deform and expand to fill the hole.

One of the tricks with any riveting job, and this one especially, is to compute the correct amount of metal to be in the rivet to expand into the hole and yet leave the correct amount of metal in the head. With button head rivets there are familiar formulas. Ah, but what volume of metal will fill that beveled countersink? That required the sharp pencil and a fair bit of calculating.

Another way to make the rivet fit is to drill out the hole. This was more complicated because the coupler body is hollow, so each hole through the coupler body is not continuous steel. When the hole picks up on the other side, it doesn't quite align. "Misaligned holes" is a common problem when riveting. (Especially when riveting bridges, hold that thought.) We started with drill bits, stepping up in 1/64" increments, which made the developing hole tend to center itself. The coup de grace was delivered by a bridge reamer. It has a tapered shank, which centers on a slightly misaligned hole, and aligns it by enlarging it. Our finishing size was 1 and 5/32", or 1/32" over nominal rivet size.

Another problem - how do you hold the rivet gun onto a flat-head rivet? On a normal button-head rivet, the rivet itself does the job. The shop crew created a solution: The C-shaped bar was steel, so it could be welded. They found a pipe that would just fit around the rivet gun. They cut about a 2-inch length of that pipe, and tack welded it in the right place to guide the rivet gun over the rivet. Problem solved!

On the first coupler, this preparatory work was done Tuesday. On Wednesday, the rivets were driven. The rivet must easily go into the hole when it is heated. Heating makes it bigger. Which is the idea behind drilling the hole 1/32" oversize. It wasn't enough - on our first try, the hot rivet would not go in the hole. It was let to cool, and 1/32" was turned off it on the lathe. It worked the second time. Lessons learned, the second rivet went in fine. A fair bit of grinding followed, and riveting is now complete on one coupler. You can barely tell there are rivets there.

Are the rivets in shear? No, they aren't. Coupler draft (tension) and buff (compression) both go through a big spring that goes in the empty space you see. More on that later.

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