The locomotive shop - Armstrong Standard Goods no 788 part 2
Entry posted by drduncan
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After the last entry about 788, I was ready to test fit the wheels and coupling rods to check that everything ran freely. (2811 is currently my chief problem child and I'll post about this when there is something positive to say.) I used a GW wheel press and quartering jig and was expecting this stage to be a complete doddle. Unfortuantely, the reverse was the case. Fitting the wheels and rods was striaght forward, but when the chassis was tested under finger power a tight spot was noticed. Worse, the crankpin nuts kept unscrewing as the chassis was pushed along, classic signs of qaurtering problems - but there couldn't be any quartering problems as a quartering jig had been used....
Cue head scratching and increasingly bad language as I tried to work out what was wrong.
I decided that more expert advice was needed, so I presented the recalictrant chassis for inspection at the South Hants MRC. Much helpful advice ensued, some of which was both timely and relevant! However, the sage of Fareham, Mr Richard Butler, hit the nail on the head and identified a possible casue. The SHMRC brains trust had, like me concluded that given how the wheels were quartered, it was unlikely to be a quartering problem and that the fault must lie elsewhere, but Richard quite literally put the chassis under the magnifying glass with the hornblock alignment jigs in place. He found that the hornblocks were frationally out of alignment.
This discovery left me vexed and discontented. After all, I had built the chassis using the hornblock alignment jigs and coupling rods to make sure everything was in the correct place. Yet, despite this, there was a problem. The question was how.
When I stopped cursing my inadequacies as a chassis builder, the sage of Fareham took pity on me and explained how the hornblock jigs could create a missalignment. The problem was the jigs themselves as they had tapered ends. The tapered ends mean that coupling rod holes will always fit at some point and therein lies the problem as the taper means that where the rods overlap at a joint, only one of them is a tight fit, it is possible for the outer one to be fracionally off centre - just enough to cause a missalignment and the resulting tight spot.
The only way forward was to remove one on the outer axles and the middle axle and their associated honblocks and rebuild it. Fortunately I had a solitary and very old Perserverence alignment jig which had parallel ends, which happily were the right diameter to fit the Alan Gibson crankpin bush - this one was used in the middle axle, while the taper ended jigs went in the outer axles where they was not overlapping rods to worry about. After short period of waving the soldering iron about I was ready to fit the wheels and rods. A short while after that I was ready to test the chassis (again) under finger power (again). It worked - no tight spots.
Now its decision time - strip and paint the chassis and wheels and then detail the chassis, or detail and then paint. My current preference is to strip the wheels off to ease painting and then finish adding the brake gear. Wahatver the eventual plan of attack, I'll keep you posted.
drduncan
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