Another Crank pin proof of concept

The next round of experiments with Percy will require yet another new set of connecting rods. I’ve been wanting to experiment with a new style of connecting rod and crank pin for years and this seems like a good time to do it.

North American eight-wheelers seemed to have had two styles of crank pins. The first, I would call “blind” where the crank pin is wasted and captured by two transverse halves of a bearing that are wedged in place. The second style, I will call “nutted,” utilizes a nut to hold a washer and a longitudinally halved bearing in place.

Blind crank pin – North Pacific Coast Railroad Sonoma, California State Railroad Museum
Travel Town Museum, Los Angeles
Nutted crank pin on Stockton Central #1, Travel Town Museum, Los Angeles

622 had blind crank pins, and while the 1mm bolt heads I used to hold the rods on look about right for nutted crank pins, they’re not right for 622. Coming up with a way to model blind crank pins would also help all those British modellers who never seem to have enough space behind the crosshead guides for nuts on their leading drivers, and are always writing about thinning nuts down to the thickness of hair and holding them on with spit. Besides, this idea has been rattling around my head long enough: time for it to come out!

However, before committing to four crank pins, I decided to do one and see if the idea can work at all.

Main rod (front) crank pins in my era were up to about six inches in diameter, while connecting rod crank pins were about three to four inches. Because the connecting rods are on the outside, it makes sense to attempt to make the model reflect the smaller crank pins. The first attempt aimed for 1 mm crank pins; however, by the time a groove had been cut to hold onto the key, the crank pin was starting to get a little delicate, and it ultimately broke.

The next attempt combined a 1/16″ hole in the end of the rod with a 1 mm key. Despite the mixture of measuring systems, this was much more successful. Cutting the slot in the crank pin was a delicate operation, and I am surprised only one key was lost in fabrication — the little blighter is only about 1.5mm x 4mm.

Once assembled, the tab at the top of the key, together with one of the tabs at the bottom of the key look a lot like the wedge on a prototype locomotive. The other tab at the bottom of the key could be cut off after “final” assembly.

3 thoughts on “Another Crank pin proof of concept

  1. I’ve never really understood the term wedges in this context. In the photo of the North Pacific Coast RR loco, are the wedges the squarish brass plates that sit outboard of and in between the rods? Or are those the bearings, and the wedges are ???? Enquiring minds . . .

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