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Formulas for Shaft Friction. By J. J. Flather. 



Among the various methods employed tor the long distance transmission of 

 power shafting has been used to a limited extent. 



In many of the earlier applications the motion was one of translation. Thus 

 in the transmission of power from the large overshot wheel at Laxey, on the Isle 

 of Man. trussed rods are used to transmit about 150 h. p. several hundred feet; 

 the rods are continuously connected and are supported on wheel carriers running 

 on iron ways. 



This method was adopted, on a very large scale, in the mines of Devonshire 

 for the transmission of power from large overshot water wheels to pumps fixed in 

 the shaft of the mine at a considerable distance higher up the valley. 



In one case the water wheel was 52 feet diameter, 12 feet breast, and its or- 

 dinary working speed was 5 revolutions per minute. The length of stroke given 

 by the crank to the horizontal or " tlat " rods was 8 feet; the rods were Sa-inch 

 round iron, and were carried on cast-iron pulleys. 



At Devon Great Consols, near Tavistock, there are altogether very nearly 

 three miles of 3-inch wrought-iron rods, carried on bobs, pulleys and stands, 

 whereby power for pumping and winding is conveyed along the surface to differ- 

 ent parts of these extensive mines from 11 large water wheels ranging up to 50 

 feet in diameter. 



In the transmission of power by rotating shafting supported in bearings 

 throughout its length, the friction of the journals is a very important considera- 

 tion, and effectually debars its use for long-distance transmission. 



This can be seen in the following formula?, which show the relation between 

 the horse-power required to overcome the friction of the shaft due to its weight 

 and velocity, and the horse-power transmitted by the shaft for a given diameter 

 and length corresponding to an angular distortion of ^^^^ degree per foot of length. 



If the contact between shaft and its bearing be a line contact only, the initial 

 load which produces friction will be P ; on the other hand, if the shaft exactly 



fits the bearing the friction load will be — P; midway between these lies a 



4 

 value, PXI.28, or — P, which will be here assumed as closely approaching con- 

 ditions of actual practice when the journal is well worn to its bearing. 

 Under these conditions the friction horse-power will be: 



_ Fv 4 W X V 



»• "~ 33000 ~ - "^ 33000 (^) 



