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5" gauge loco

crank axle to crank pin relationship

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Peter Daw07/10/2017 13:35:00
7 forum posts

Hi can anyone put me right on this question i am thinking of building a 5" gauge 2-4-0 metro but one part of the drawings is not very clear to me which is the crank axle has the right hand crank leading but does not show the position of the crank pin in relation to it can somebody please put me right

regards P Daw

julian atkins07/10/2017 20:50:13
1190 forum posts
353 photos

Hi Peter,

The crank pin on the wheel should be 180 degrees out of phase to the position of the crank axle pin nearest that wheel.

(If the on the same phase you have what is known as Stroudley balancing which is quite rare and in fullsize required excessive balance weights on the wheels).



Peter Daw08/10/2017 09:48:39
7 forum posts

thanks julian

one question i forgot to ask was do you advocate the use of a lead ballast behind the balance weights ?

also can a 5" metro boiler be constructed single handed i used to be a welder fabricator in the past and also did a lot of brazing manufacturing vehicle components with oxy acetalene and gas flux so i am understand the process thanks for an anticipated reply

regards peter

julian atkins08/10/2017 10:07:30
1190 forum posts
353 photos

Hi Peter,

Generally wheel balancing on a 5"g loco is ignored. If your wheel castings have balance weights cast on them leave them as that.

The Metro boiler is a smallish 5"g boiler though the belpaire firebox makes it more complicated. There is no reason why it should not be built single handed.



Peter Daw08/10/2017 13:40:54
7 forum posts

hi julian

thanks for your help and advice very helpful

regards peter

duncan webster12/10/2017 15:30:54
1593 forum posts
18 photos

I’ve looked at the drawings of several full size locos, and they all conform to Julian’s advice on relative position of crankpins.

However, if we take a 4 coupled inside cylinder loco with 16” * 24” cylinders and 6 ft wheels, running at 120 psi cylinder pressure, the piston thrust is 10.8 tons. Assuming the tractive effort is shared between the 2 coupled axles, the coupling rod force is half of this, ie 5.4 tons, and the force at the wheel rim is 5.4 * 24/72 = 1.8 tons. With the rods set out as per Julian and the main crankpin at 12 o’clock, the combined force at the axlebox is (10.8 + 5.4 + 1.8) = 18 tons. In the same position with the pins in line, the force at the axlebox is (10.8 – 5.4 + 1.8) = 7.2 tons. This calculation is a little simplified as the forces are not in the same vertical plane, and I haven't allowed for the other cylinders/wheels. The loco dimensions/ pressure used do not affect the direction of the argument, having the inside and outside crankpins aligned would seem to give the axle boxes a much easier time. Yes it probably requires bigger balance weights, but outside cylinder locos managed, and you can put quite a bit of balance onto the inside crank.

Peter Daw17/10/2017 13:26:30
7 forum posts

hi duncan

thanks for your input quite interesting to read the technical side of the question regards peter

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