Gear Materials

Gear Materials

Gear Materials - A gear tooth may fail due to

(i) rapid wear, seizing or heavy pitting,

(ii) breakage of tooth under the bending effect of the load. This may occur if shock loading is applied to a gear made up of a brittle material.

Steel in one form or another is the most commonly used material for making gears, because it is capable of with standing rubbing action under load and secondly it is able to resist bending. Steel containing 0'4% carbon is the least expensive form suitable for gear purposes, and is widely used for light loading, conditions.

Increase of carbon to 0' 5 % increases the wear resistance of the steel by about 40 %, but makes it more difficult to cut accurately. Steel containing 3' 5 % nickel and 1 % chromium is used for more heavily loaded gears.

Case hardened steel is used to obtain the greatest load capacity in gears of given size. It contains 3'5 % nickel and other varieties contain both nickel and chromium. The wear resistance of any case-hardened steel is many times mole than that of the steels discussed in the above Para.

Cast iron is used for making gears which are not heavily loaded, and particularly if they are of large diameter. Its disadvantage is its low bending stress and brittleness and hence the teeth are liable to snap off under shock loading.

Cast steel is used for making large gears for heavy work, but there is now a tendency to build them up from rolled steel plates by welding.

For heavily loaded high speed gears such as are used for turbine speed reduction, the wheel teeth may be cut in a rim of high carbon steel, secured to a centre made from cast iron or mild steal plates by means of shrinking or bolting.

For the heaviest loading conditions, the worm wheel (gear) must, be made from phosphor bronze and the worm from case hardened steel.