CHAPTER VI. 



TESTING OPERATIONS (Continued.) 

 TWISTING OR TORSION TESTS. 



108. Rotating shafts are subjected to twisting or 

 torsional loads, producing shearing stresses on planes at 

 right angles to the axes of the shafts, acting in a rotary 

 direction round these axes. The metals chiefly used for 

 this purpose are iron and steel, and it is customary to 

 subject small specimens of these metals to torsional stresses 

 in testing machines, similar to those they may be expected 

 to have to withstand when forming the actual parts of 

 machinery. Measurements and observations made on 

 specimens tested in this way yield more reliable informa- 

 tion as to the behaviour of the metal in question than 

 do tests in direct shear. 



In addition to simple torsion, many shafts are subject 

 to combined bending and torsion, as in the case of the 

 crankshaft of an engine. There are few instances of cases 

 where test pieces have been subjected to these combined 

 stresses, and these have only been tests to destruction, 

 where no attempt has been made to measure the elastic 

 strains. The reason for this is that, though not insuper- 

 able, the difficulties of strain measurement are great in the 

 case of combined bending and torsion. 



The theory of simple torsion has already been con- 

 sidered, and a definite relation found to exist between the 

 twisting moment on a circular shaft, the shearing stress at 

 the outer surface, the dimensions of the shaft, and the 

 modulus of transverse elasticity, G. There is also a rela- 

 tion found between the twisting moment, the dimensions, 

 the angle of twist, and the modulus of transverse elas- 

 ticity. In testing a shaft or torsion specimen it is possible 

 to hold it in such a way that a moment of known magni- 

 tude is imposed, and at the same time the angle of twist 

 or strain arising from this moment can be measured. In 

 some of the rougher commercial tests it is not usual to 

 measure the angular deflection, but simply the moment 

 which is required to produce fracture. 



