Driving wheel spoke number

[jim.snowdon]

Engineering design is full of compromises between competing factors with, frequently, no single 'right' answer. Solutions are often drawn from experience with something similar rather than a blank paper design.

 

In this instance, fewer spokes requires a stiffer rim to resist the radial bending loads. But, fewer spokes mean they have to be thicker or stronger to withstand the shear forces as the tractive effort is transferred from the wheel centre to the rim and vice versa in braking. The depth of the spoke relates to the lateral stiffness of the wheel and its ability to transfer the lateral wheel/rail forces to the vehicle. And all the time, the weight of the wheel is critical, as it is unsprung, which directly affects the impact forces on the rails at vertical irregularities, especially fishplated rail joints, and in turn the forces transmitted through into the track foundation.

 

Trying to solve these varied problems is why, in the 20th century, engineers came up with variations, some quite marked, in the design of locomotive wheels. Some are well known - the Bulleid-Firth Brown wheel, with its corrugated 'waffle plate' centre, the Boxpok wheel, with its fat tubular spokes, the SCOA-P* wheel, with its fat channel section spokes, the Scullin wheel, whose centre was made by a pair of perforated discs. A particular feature of the B-FB, Boxpok and Scullin wheels was the provision of continuous support to the tyres; it was improved tyre support that was behind Stanier's adoption of new wheel designs that had a substantial triangular section rim, as flexure of the rim had been found to be a factor in tyres working loose.

 

 

*Steel Company of Australia

[PenrithBeacon]

6 hours ago, Compound2632 said:

 

To clarify why I saw 17 and 21 as examples of challenging numbers.

 

 

Were these not wooden master patterns made by the foundry pattern makers?

 

 

What evidence can you produce to show that this was the case in 19th and first-half-of-the-20th-century railway foundries? 

I had only a very limited experience of designing castings, but the drawing office I worked in did not do any work on designing patterns. That was left to the pattern makers who designed their products in an entirely vernacular way and occasionally the drawing of the finished casting produced by the drawing office was returned with sketched out amendments intended to ease the casting process. 

Those blokes were experts, a draughtsman would be unwise to argue!

[Pandora]

I was shown  a pattern produced for a steam locomotive driving wheel,  the pattern was produced with all the spokes originating or radiating from the dead-centre point of the axlehole,  then an expert pointed out the works drawing showed  the spoke centre as nearly the crankpin centre.