Having put this locomotive to the side for some months - I have picked it up again. I HAVE to get it to work!
So I left this with the water pump ready to steam test. That test ended up with the pump full of condensate and unable to move. So I made automatic drain valves and fitted them to the cylinder and tried again. I couldn't understand why it had locked solid, but eventually (after a lot of head scratching) worked out that in clamping the pump up under the chassis I had distorted the shaft line so that the pump ram couldn't move. It just needed the fixing screws to be loosened slightly for it to work.
Lots of adjustment of servos, valves and got me to a point where everything was fine - except for the valve gear. I couldn't quite get the settings right for it to work properly - the motion was jerky.
So it was time to dismantle it and sort that out properly:
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There are really three issues.
Firstly my early attempts to make piston valves had ended with valves which were so tight in the valve cylinders that the motion had all been subjected to huge forces - and repeated attempts to stop them leaking, running on air, had basically worn out all of the bushes.
Secondly when I built the motion I was copying the full size drawings, as published in "the anatomy of a Garratt", rather than actually understanding how to design Walschaerts valve gear. I have to say that the valve timing seemed to be correct for the piston valves - so that approach probably worked. I just couldn't get the valves to stop leaking steam, so I made the huge decision to remake the valves into slide valves. However changing to slide valves which were constrained to fit inside the outline of the existing cylinder profile has made the design of the valve gear plain wrong!
I have spent more time studying "design procedures for Walschaerts and Stevensons valve gears" by Don Ashton. So I think I now understand what I need to do to get the valve gear to work! More of that in due course.
So I have the high pressure (rear) unit on its own on the bench:
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I have spent about a week replacing the bushes in the valve gear to eliminate backlash in the valve timing. That has improved things. However yesterday I realised that there was so much play in the wheel bearings that I was wasting my time. The play was greater than the measurement I was trying to make.
So I have stripped it down and re-bushed all of the wheel bearings. Bear in mind that when I started this project I really didn't know what I was doing, and I didn't have all the reamers, drills and taps etc that I have collected in the last 6 years, so I think the wheel bearings were too loose and lacked the smoothness required to begin with. This is partly why the bearings are now so loose!
So the bushes are remade and are a good fit - and they are reamed. Three of them are reassembled, onto the axles. So that is one axle with the cranks quartered and fixed in place, and the other axle with one crank fixed and one loose. The last crank to be fixed when the coupling rods are fitted.
So I am now re-bushing the connecting and coupling rods - which were also extremely loose.
I am not convinced that the original lengths of the coupling rods were correct (for smooth running), so I am working on getting that right at the moment.
Then I can go back to the valve gear.
Trevor