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Problem with building 7mm 0-4-4 chassis


PhilH

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I'm trying to regain my modelling mojo by building a 7mm L&SWR O2 brass kit, however the chassis is proving to be extremely frustrating.

 

Things that I think are right:

Wheels are free running in bearings (it's a simple rigid chassis)

Quartering and wheel centres appear right as chassis pushes along freely with rods on when motor worm disengaged.

Coupling rod centres appear right - they push on crank pins freely.

No slop in rod end boss / crank pin fit.

 

At first all was well, I had it running slowly on two blocks with wheels off ground. Now however the non driven wheel will not go over centre on either back or front dead centres resulting in the situation displayed in picture below, when obviously it locks up.

 

post-6683-075437600 1284544379_thumb.jpg

 

Obviously something has gone wrong / broke / I've cocked up, but I can't for the life of me see what it is.

 

If anyone can shed any light on this I would be very grateful.

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If it is a rigid chassis and it ran well on blocks with the wheels off the ground but it doesn't subsequently run on the rails, then it seems as though the quartering has shifted. If the quartering has shifted, this would indicate something else is not quite right - the rod to chassis centres and/or the crankpin throws. Did you alter the tension on the crankpin nuts between the states when it was running well and when it was not running well?

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If it is a rigid chassis and it ran well on blocks with the wheels off the ground but it doesn't subsequently run on the rails, then it seems as though the quartering has shifted. If the quartering has shifted, this would indicate something else is not quite right - the rod to chassis centres and/or the crankpin throws. Did you alter the tension on the crankpin nuts between the states when it was running well and when it was not running well?

 

Thank you very much for the reply and you are right. I looked and looked and looked at it, got mad with it etc., all the time not noticing that on the last disassembly / assembly I had managed to get the driven wheel 90 deg. out on the quartering. How I didn't spot it I'll never know.

 

Oh dear and me a steam man as well.....

 

Thanks again.

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The wheels look like Slaters, which have a neat little square that fits over a square end to the axle; then one does up the retaining screw with a small hexagon key.

 

Please state whether the axles are correctly "quartered", i.e. one side's crankpins are a quarter of a turn ahead of/behind the other side. If they were half a turn out, then the lock-up photographed would happen readily.

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The wheels look like Slaters, which have a neat little square that fits over a square end to the axle; then one does up the retaining screw with a small hexagon key.

 

Please state whether the axles are correctly "quartered", i.e. one side's crankpins are a quarter of a turn ahead of/behind the other side. If they were half a turn out, then the lock-up photographed would happen readily.

 

thanks for replying, but if you read my post above I've found the problem which was exactly as you say. I must have looked at it dozens of times too without spotting it......

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Sometimes it is a simple case of not being able to see the wood from the trees - you can stare at something for ages yet remain convinced nothing can be wrong.

That is usually time for a good rest or a change of scene - on returning (or if new to the scene) it can all seem so obvious.

 

When I first looked at that photo it is glaring obvious that the forward wheel is not quartered correctly with respect to the driven wheel the pin is in the upper half of the wheel compared to the lower half on the driven. One of the biggest causes of this running out of quartering on a kit is slippage of the wheel centre on the axle - this can occur if the wheels are not tight on the axle and the axles are not perfectly square or the rods are of different lengths between pins. Usual cure is Locktite.

 

Anyway, glad it is sorted.

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