68 



ENGINEERING ON THE FARM 



Combined stresses. Many combinations of stresses occur 

 in structures and machines, such as flexure combined with 

 torsion, flexure with compression, flexure with tension. In 



Tens/on 

 Compress/on 



Jhear 



Fig. 95. Torsion, or twisting 



the designing of structures or machines due consideration 

 must be given to all the stresses present, and enough material 

 provided to supply sufficient strength. 



Temperature stresses. All solid bodies expand when 

 their temperature is increased and contract when their 

 temperature is decreased. If they are so placed that this 

 tendency to expand or contract is resisted by the parts to 

 which they are attached, temperature stresses are set up 

 within the bodies. A good example of this is the steel tire 

 on a wagon wheel, which is heated and shrunk on and remains 

 under tension when in service. 



Deformation. External forces produce deformations in 

 the bodies upon which they act. Tension produces elonga- 

 tion, compression produces a shortening, and shear produces 

 a detrusion or slipping. The amount of deformation in 

 each case is in proportion to the force producing it unless 

 the force is large enough to strain the body beyond the so- 



