622 gets connecting rods

I am not so naive as to believe that the connecting rods would drop effortlessly onto the crankpins. When I built #10, I bet I spent a month fussing over the wobbly bits, before finally declaring a truce and moving on. They were never perfect, but I felt they were probably good enough. After a week, the motion on #622 is still a little stiff but already better.

When I did drop the connecting rods onto the crankpins, I found they had hard tight spots, even when only one connecting rod was in place. Fighting dismay, I reasoned that this must mean the bearings were installed too close together or too far apart, despite my best efforts. How could that be?

One thing that could have caused this was accidentally flipping a wheelset. The rear bearings are particularly difficult to identify, and so I reversed the rear wheel. Things got a little better, so I tried reversing the front wheels too because left and right can get mixed up when you flip things over. The motion was better still. Then I tried more combinations and found that the original was no worse than any other.

That was a great time for a cup of tea and a think!

I theorized that perhaps the rods were crooked in the horizontal plane. Out came the callipers and I confirmed this was the case.

I carefully measured the distance from the back of a wheel to the front face of a connecting rod and main rod. Then I made sleeves to hold the connecting rods at that distance from the wheel faces. The front drivers got them too because main rods are a nuisance until the cross heads are live.

After that, only one crank pin was actually long enough. The others were 0.2 to 0.4 mm too short. I turned some special washers that fill in the extra space.

All the washers and sleeves are about 1.5mm in diameter and .5 to 1mm long. In other words, they are about as big as pieces of dust. Like dust, they display a remarkable affinity for the corners under my workbench. In fact, I’m pretty sure that one of the sleeves I found down there actually was a piece of dust; at any rate, it wouldn’t fit after its trip to the floor and had to be replaced with a spare.

Finally, all ten pieces were screwed in place, and I pushed the chassis along. There was still a stiff spot but only with both rods in place – classic out of quarter. I went around and found the one joint that had the least tolerance, and reamed it ever so slightly. No more binding.

As I say, it’s still a little tight, but I can no longer detect specific spots with my fingers. So I’m going to hold off on any more tweaking and see if the motor is sensitive enough to point them out.

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