ON SPHERE AND ROLLER MECHANISM FOR TRANSMITTING POWER. 491 



The Material and Construction of Spheres. 



The production of balls at a low cost, and yet suitable for standing 



the heavy pressures without excessive wear, has been one of the chief 



difficulties encountered. 



Fig. 10. 



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In first small models the balls were made of boxwood, lignum-vit83, 

 ivory, oak, gutta-percha, india-rnbber, and brass, of which the last was 

 most satisfactory. In the large machines, lead, mixtures of lead, zinc, 

 and tin, cast iron, phosphor bronze, and hollow unhardened cast steel have 

 been tried. Of these, hollow cast-iron balls have proved the best. With 

 solid cast iron it seems almost impossible to get spheres quite free from 

 flaws ; as many as sixteen balls, cast by four different founders, only left, 

 on turning up, two which were fairly good. But this material stands the 

 wear and tear much better than was expected. In one case a hollow 

 cast-iron ball 8" diameter has been in frequent use in a ^-ton hoist 

 (figs. 3 and 4) for eight months ; careful callipering fails to show any 

 loss in diameter. 



Unhardened cast steel is too soft. Hardened and ground cast steel 

 and phosphor bronze are too expensive. . 



It might be mentioned here that, though with absolutely rigid material 

 the surface of contact between the balls and the wheels would be reduced 

 to a point, yet according to the pressure and material a considerable facet 

 is formed. The amount by which the centre of the surface of contact is 

 depressed is, however, extremely small. Thus in a 6" ball in contact with 

 a plane surface this depression is only -0017" for a facet i" diameter. 



