294 



APPLIED SCIENCE 



at an angle of 45, they are called miter wheels (Fig. 152). 

 The teeth are called teeth when they are of the same piece as 

 the body of the wheel, and cogs when they are of separate 



material. Wheels in whose rims 

 cogs are inserted are called mor- 

 tise wheels. 



FIG. 151. Worm Gear. 



337. Teeth of Gearing. 



Toothed gearing is employed for 

 transmitting motion from one 

 shaft to another. Under favor- 

 able conditions it is the most 

 economical of all means of transmitting power from one shaft 

 to another, but when the twisting effort is very irregular and 

 the space between the teeth great, much noise arises from 

 them, due to the teeth striking against each other. This 

 striking is called backlash. When backlash is excessive it 

 reduces the life of the wheel, but is seldom so great, except 

 in much worn teeth, as to be a 

 source of danger. It should be 

 remembered that impulsive (sud- 

 den) loads produce twice the strain 

 on the teeth that dead (steady) 

 loads of the same magnitude pro- 

 duce, and that an impulsive load 

 may strain the teeth up to, or be- 

 yond, the elastic limit of the ma- FIQ 152 _ Miter Gear 

 terial. If stress of this kind is 



repeated many, many times, the life of the teeth is greatly 

 shortened. 



When the position of gears requires that they be installed 

 so as to operate noiselessly, the teeth of one made of wood, 



