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heat treatable and have a much better index of machinability corresponding to some of the carbon steels. Table 1.23 lists mechanical properties of typical gear steels. Table 1.24 presents relative machinability of various steels.

14.2 Non-Ferrous Metals

The commonly used non-ferrous materials are the aluminum alloys and bronzes. Zinc diecast alloys are used also. Non-ferrous metals generally or selectively offer good machinability, light weight, corrosion resistance and are non-magnetic.
14.2.1 Aluminum as a gear material has the special feature of light weight, and moderately good strength for the low weight It is also corrosion resistant and easy to machine. A major disadvantage is the large coefficient of thermal expansion compared to steels. Many aluminum alloys differ in ease of forming, machining and casting. Aluminum alloys respond to cold working and heat treatment Mechanical properties for several alloys are given in Table 1.25.
14.2.2 Bronzes have long been used for gear materials. They possess favorable frictional and wear properties when mating with steel gears. They are particularly advantageous in worm meshes and crossed-helical meshes because of the large amount of sliding. Bronzes are extremely stable and offer excellent machinability. The material can be cast, but bar stock and forgings are superior. Chief disadvantages are the high specific weight (highest of the gear materials) and relatively high cost.
There are many bronze alloys, but only a few are extensively used for gears. These are the four alloys listed in Table 1.25. This table also lists brasses that are used for low load fine pitch gears.

14.3 Die Cast Alloys

 Many high-volume low-cost gears are produced by the die-cast process. Most are produced in alloys of aluminum and zinc, and a few in bronze and brass. Properties of alloys suitable fOr gears are given In Table 1.26.

14.4 Sintered Powder Metal

This is a process of molding fine metal powder and alloying ingredients under high pressure and then firing to fuse the mass. It is a high-production means of producing relatively high-strength gears at low cost. Metals used for gears are iron-based mixtures, bronzes and brasses. Powder metals are expensive, but offsetting this the scrap losses are very small. Properties of sintered powder alloys suitable for gears is presented in Table 127.

14.5 Plastics

Plastics gears offer quiet operation, wear resistance, damping, lightweight, non-corrosiveness, minimum or no lubrication and low cost. On the debit side, they are difficult to machine to high precision and are subject to large temperature-induced dimensional changes and instability. Gears can be directly finish molded with teeth, entirely machined from bar and plate stock, or cut from molded blanks.
Phenolic laminates have bases of either paper, linen, or cotton cloth with relative strengths in that order. They offer relatively good strength and in cotton-canvas base are suitable for large gears and high loads. Properties for gear phenolics are given in Table 1.28.

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