Fitting the Coupling Rods
Next up comes the coupling rods, and the first thing that needs attention are the crank pin holes. Up to now these have been sized to fit the pins on the chassis jig they were constructed on. They now needed to fit the AG crank pin bushes, which, by a feat of good management, are a bit larger.
I like to use a fine taper reamer for this job. I work on one hole until I have a comfortable fit on the bush, with just a smidgen of clearance. Then I mark my place on the reamer by wrapping a bit of masking tape around it at the point I have been reaming up to. Using that as a guide, I then reamed all the holes the same size. Remember to finish the reaming from both sides to eliminate as much as possible of the slight taper in the hole. It can be quite hard on the fingers to remove a lot of metal with a taper reamer, so you might want to drill up to close to the right size (1.5mm). But go careful because if/when the drill jams in the hole, it is very easy to wrap the coupling rod round the drill, which doesn’t do it a lot of good.
Once the crankpin holes are done, mount the rods on the wheels of two adjacent axles only. With the jointed rods mount the un-jointed section first, in the J10’s case, that’s the first and second axle. Check that those two axles revolve without binding. If you’re close, you can afford to ream all six of the crankpin holes in the rods out a touch more. If persistent jamming is evident, reaming out won’t necessarily fix the problem and you had better start looking for other issues or you’ll ream away too much. When those first 4 wheels revolve smoothly, connect up the last axle using the jointed section of the rods, repeat the process and see what you’ve got.
I don’t plan to go through all the ins and outs of sorting out quartering problems on this occasion, mostly because this one ran nicely first time, and so I didn’t have any problems to sort out. Sorry.
The next photo of the basic rolling chassis was taken at this stage.
You will note that I’ve used plain hexagonal 14BA nuts on the crank pins, as these are easier to deal with. I’ll swap to the proper threaded round crankpin nut later. I’ve still got to reduce the length of the crankpin bushes to suit the rods too. My fancy jointed rods are a touch to thick for the standard Gibson crank pin bushes and I had to invest in a pack of long ones.
Next steps and thoughts about the motor
The next step will be to fit the motor, and to run in the chassis a bit. I have a Mashima 1024 to go in. These are skew wound motors which I find quite as powerful as their larger cousins, and I think they run smoother. They do rev rather fast, which explains my attachment to high ratio gearboxes. Because of where the fixing screws are, you can only really fit it to the gearbox with the flats on the can top and bottom, which seems a bit of a waist. If you really have to have the flats sideways, you have to use the fixing holes above and below the worm gear. This means you can’t fit the top gear shaft and the worm wheel until after the motor has been screwed to the gearbox frame. This is a fiddle worth avoiding.
I don’t subscribe to the idea that you need as big a motor as you can possibly fit. What you must have is a motor that, when the load becomes too great, will spin the driving wheels rather than stalling. If the installed motor can take you past the limit of adhesion, then you can’t use any more power and a larger motor is a waste. For the sorts of layout and size of loco I’m aiming at, herculean feats of haulage and very heavy loco’s aren’t required. Hulking great motors aren’t required either. The ability to spin the wheels rather than stall is a useful safety valve for the motor and should prevent a burnout. If a chassis is a bit stiff, a considerable proportion of the motors power can be used up overcoming the stiffness, so, if a motor runs hot I’d make sure the chassis was running freely before I started wondering if a larger motor was required.
Once the motor has been run a bit and I’m sure the chassis is free running enough, it will be time to fabricate the pickups and to fit details like brake and sanding gear, guard irons, and an ash pan. Fitting pickups is likely to be the subject of the next J10 episode, but there may be a few CSB based diversions before then.
Last edited by Will L
on Sat Feb 05, 2011 9:22 pm, edited 2 times in total.