A coupling rod or side rod connects the driving wheels of a locomotive. Steam locomotives in particular usually have them, but some diesel and electric locomotives, especially older ones and shunters, also have them. The coupling rods transfer the power to all the wheels.
Side Rod Fit
The Live Steamer, July-August 1951
George Murray has suggested that a few words concerning the correct fits of rods and axle or journal boxes, would be welcome to the reader. I doubt if everyone will agree with me 100% as most builders have their own ideas, but here are mine anyway.
I have found that siderods work best when fitted on the loose side to the crankpins. My practice is to turn the crankpins 0.005 inch under standard size. This allows the driving wheels to drop into a low joint without making the siderods bind. In full size usage if the bushings are fitted too snugly they will run hot. I fit the bushing in the big end of the main rod somewhat a tighter fit - about 0.002 inch slack. This clearance can also be used where the crosshead wristpin runs in the front end of the main rod. When a locomotive, either big or little, is running over the track there are bound to be some high and low places that the drivers have to follow. This means that the crankpins have to twist, so to speak, and the only way they can do this in the rod bushings is through the clearance in these bushings.
Another advantage of fitting the side rod bushings on the loose side is that little errors are apt to creep in when the wheels are quartered. Added to this error is the fact that perhaps all the crank throws are not all just alike. Maybe the holes in the side rods did not all come out the same distance apart as the pedestal openings. It could be, too, that the holes in the journal boxes were not exactly in the fore and aft center line of the journal box.
This may sound like pretty poor machining, but everyone building our small steam locos are not toolmakers, nor do they all have facilities for doing these operations by precision means and we have to make do with what we have. I think I made all these mistakes in my first engine, but the side rods worked all right after I used a round file to make some of the rod bushings very much on the oval side.
- I would just used 660 bronze and lubricate appropriately.