Getting the balance right!

It is intended to avoid the very time consuming and expensive work required to balance Tornado’s coupled wheels. This was achieved by the traditional method of making weights to hang on the crank pins which represented the total mass of the rotating motion parts and a proportion of the reciprocating motion parts. Tornado’s wheels had cast in balance weights which are ‘adjusted’ by drilling holes in the back of the weights.

The actual dynamic balancing was undertaken with the assistance of Dowding and Mills (now Sulzer) at Middlesborough using their large electrical machinery balancing machine as no steam loco wheel balancing machines exist in the UK. One of our long term supporters who balanced large marine diesel engines in his youth kindly offered to produce the calculations for the overall balancing and a series of drawings to guide us as to where to drill the lightening holes. The readouts obtained from Network Rail ‘Wheelchex’ equipment which is installed in several places on main lines throughout the country to measure wheel forces (mainly to spot freight wagons with severe flats on wheels) show that the track forces produced by Tornado during testing are as predicted by the balancing calculations. The pictures below show the weights attached to the crank pins and the drive pulley required by the balancing machine, the holes drilled in the balance weights and the wheels being balanced.

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For No. 2007 we are intending to achieve balancing entirely by calculation and by using built up balance weights (crescent shaped plates riveted on both sides of the spokes with predetermined quantities of molten lead/antimony alloy poured into the cavities between the spokes and plates). This was standard GWR/LMS/BR practice. However instead of using a dynamic balancing machine to determine the precise amount of lead required, we intend to do this by calculation. The starting point is the centre of mass of the wheels. The 3D CAD models will predict where this is, however as the wheels are castings which rarely turn out precisely to the shape of the drawing, we need to use other methods to determine the centre of mass. One is to 3D scan the wheels to produce a solid model from which Solidworks 3D CAD can compute the centre of mass. To back this up we will also determine the centre of mass of each wheel in the plane of the wheel by balancing it on a knife edge on the back of the wheel boss and repeating this with the knife edge at 90 degrees.

As all the other components (tyres, crank pins, coupling and connecting rods, pistons, piston rods and crossheads) are fully machined to close tolerances, their masses and centre of masses can be computed accurately, as can any material removed from the wheel centres during tyre and crank pin fitting, the existing surfaces in these areas being already machined. This removed metal will cause the centre of mass to move slightly, but this displacement can be accurately calculated. The final check on hammerblow (the minimising of which is what balancing is all about) using the ‘Wheelchex’ equipment during the main line testing.

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