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Balacing a locomotive power unit

How is it done in practice?

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Tim Stevens15/01/2018 16:16:14
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1516 forum posts

A question following the recent article on making loco wheels - how are locomotives balanced?

I know the simple answer is that bob weights are built into the driving wheels. but ...

On each side of the engine, there is a need to balance the out-of-centre rotating mass of the wheel connecting rods, and the reciprocating masses of the piston, the valve mechanism, and various links and twiddly bits. If the wheels and their links are in perfect balance, there is nothing to reduce the fore and aft forces from the piston assembly, in time with the crank pins, or the valve gear, at some other angle depending on the design. On the other side of the loco, the same values of fore and aft forces remain, but 90 degrees out of phase and well out of line with the other side.

If nothing is done about this, there will be a double fore and aft pull on the drawbar, and the passenger's heads will all nod in unison, in time with the driving wheels. If too much is done about it, at some (high) critical speed, the wheels are going to leave the track each revolution, and at different times each side. Either way, the loco will rock too and fro, shaking the fireman's tea from his enamel mug.

Neither is a good idea, so in the real world (and in the model world) what is the formula, please?

Regards, Tim

duncan webster15/01/2018 16:52:38
3697 forum posts
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I've sent the acting editor of ME an article on exactly this topic. Hopefully he will publish it.

Tim Stevens15/01/2018 18:07:06
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1516 forum posts

Thanks, Duncan

I hope the Acting Ed does better than 30 x 106 when what was intended was 30 x 10 to the power of 6. As any schoolboy will tell you, 106 does not a million make. [ME 4578, p156, top of column 4]

cheers, Tim

Roger B15/01/2018 19:38:35
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172 forum posts
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This was one of the major problems with conventional steam locomotives. The various out of balance forces used to cause significant damage to the permanent way. The introduction of diesel and electric locomotives greatly reduced the damage and allowed greater axle weights and power outputs.

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