Toothed gearing is employed for transmitting motion from one shaft to another. Under favorable 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 (sudden) loads produce twice the strain on the teeth that dead (steady) loads of the same magnitude produce, and that an impulsive load may strain the teeth up to, or beyond, the elastic limit of the material. 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, or rawhide, are let into and fixed in the iron rim of the gear. The gear so formed is termed a mortise gear, and is always the quicker running gear of the pair. It is in such a case as this that the teeth are termed cogs and they are usually made of hornblende or beech, both of which are compact in grain and take a smooth surface. Machine gears which are subject to much vibration and shock are frequently made of phosphor-bronze (an alloy of copper, tin, and phosphorus), gun-metal, steel, or malleable cast iron, because these materials have a high tensile strength and greater elasticity than ordinary cast iron.
Fig. 151. - Worm Gear.
Fig. 152. - Miter Gear.