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).

Cheers,

Julian

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.

Cheers,

Julian

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.