Part 12 by Jason Ballamy

Fabricating the yoke

A lot of steam pumps have a quite distinctive ‘yoke’ which joins the pump ram to the piston rod but allows the crank to turn within it to keep the movement of the piston in line with that of the pump ram. I spent quite a while thinking of the best way to make this part and finally settled on a fabrication rather than cutting from solid.

The first job was to mark out the basic shape on some 1/16" steel sheet and drill a few holes so the two sides could be bolted together. They were then sawn to approx shape before mounting on the mill to drill a hole to give a nice radius where the two legs join.

I could then saw out the waste and file to shape.
The yoke tapers from top to bottom so the two pairs of legs were slightly bent and then clamped onto the top block to see how they looked.

A steel block of about 5/8 x 1" was drilled to suit the holes in the legs and a 3/8" x 40 thread cut in the middle into which the stainless steel pump ram will screw.

This block was than screwed to a plate which was in turn fixed to the rotary table so that it could be shaped to the correct curves. First the inner edge.
Then the outer, leaving a flat for the ram to butt up against.
Changing to a smaller cutter a bit more shape was added and the two areas where the legs go thinned down, flip it over and do the other side.
Then I used some lengths of 3mm rod to rivet the parts together with some additional blocks where needed. The mating surfaces were centre punched to raise a burr and give a gap for the solder to flow into. The lower joints were then soldered.

A quick clean up with a file and the thin lines of solder visible along the joint lines show that it all flowed quite nicely.

I then made up a threaded mandrel on the lathe and screwed the yoke onto it and gently centre drilled, drilled and reamed 1/4"" in the top block for the piston rod to fit, These two must be inline for a smooth running engine.

Back on the mill using a dummy piston rod in the vice and packing at the other end a hole was bored for the little end boss.

The top was then soldered, cleaned up and a raised boss turned around the piston rod socket.

I had made the little end boss with a waisted centre section so gap between the two bosses would be the right width for the little end bearing when soldering and also drilled it undersize incase it moved while soldering. This was now bored and reamed to take the 1/4" wrist pin, you can just see the remnants of the waisted middle sitting on the vice.
While set up like this I milled a 1/16" slot from both sides ready to be filed out for the wedge that retains the piston rod.
A piece of 3/4" stainless was threaded and then turned down to 11/16" dia for the pump ram.
A bit of filing and filling then some primer and that is the Yoke completed.