26 M\ HINT DRAWING UTD DIBIQH 



as a solid shaft of the same material 7J inches in diameter, 

 but the former would have about double the strength of the 

 latter. Hollow shafts are also stiffer and yield less to 

 bending action than solid shafts, which in some cases, as in 

 propeller shafts, is an objection. 



- 



'<ting Moment. Let a shaft cany a lever, wheel, or 

 pulley of radius R inches, and let a force of P Ibs. act at the 

 outer end of the radius, and at right angles to it. The force 

 P produces a twisting action on the shaft, which is measure*! 

 l.y r*e product P X H This product I' x H is called thfl 

 ting moment or torque on the shaft ; and if P is in 11 . 

 and R in inches, the twisting moment is P x R inch-pounds. 

 The twisting moment in foot-pounds is got by dividing the 



ing moment in inch-pounds by 1 '2. 

 If the shaft makes N revolutions per minute, the horse- 



. l> X H x .'HUG X P N 

 power which is being transmitted is 12 x 



-2 X 3-1416 X T x N 

 or i2 y where T is the twisting moment in 



inch-pounds. 



Resistance of Sliafts to Torsion. The resistance of a shaft 

 to torsion is directly proportional to the cube of its diameter. 

 Thus if the diameter be doubled, the strength is increased 

 riLrht (,.>. 2 3 ) times. Let there be two shafts of the same 

 material, and having diameters D, and I)., ; and let the t\\i>t- 

 ing moments which they support when strained to the same 

 extent be T, and T 2 respectively : then T, : T, : : Df : I ' 



T,a 3 = T i 



cent of Resistance. The stress produced in a shaft 

 which is subjected to twisting is a shearing stress. This 

 stress is not uniform, being greatest at the outside of tin 

 shaft and diminishing uniformly towards the centre, wl 

 it is nothing. Let / be the greatest shearing stress on the 

 shaft and d its diameter. Then the moment of rcsistam 



1 From the author's Elementary Tert-B x>k of A ] .j 1 i ic.' 



