The teeth of a bevel wheel or bevel gears are always arranged at an angle to the shaft.
When the teeth of bevel gears form an angle of 45° they are called miter wheels.
Miter wheels to gear must be of equal sizes.
A crown wheel is a disc that has teeth which are on its side face; that is, teeth on a flat circular surface all parallel to the axis of the wheel.
A rack has teeth on a flat surface or plane all parallel to one another.
A gear cut by machine is called a cut gear. It has teeth with less clearance than cast wheels, which are not so true or perfect, and therefore require more clearance.
A worm with even a light load is liable to heat and cut if run at over 300 feet of rubbing surface travel. The wheel teeth will keep cool, as they form part of a large radiating surface; the worm itself is so small that its heat is dissipated slowly.
A worm throws a severe end thrust or strain on its shaft.
Steel Gears.—There is great economy in the use of cast-steel over cast-iron in gears; the average life of the former is nearly twice as great as of cast-iron gears. And, apart from their longer life and efficiency, there is less danger of breaking.
The most accurate teeth, strongest and most uniform in wearing, are to be found in steel gears cut from solid stock, or made by cutters of proper shape.