Iron casting was the early choice for manufacturing gears, particularly in larger sizes because cast iron exhibits the advantages of having a hard wear surface provided on a casting surface as it is removed from a mold, and of holding acceptable dimensional tolerance during cooling. However, it suffers the disadvantage relative to steel of being more brittle and less resistant to damage from impact loads. Steel, although more difficult to cast than iron because higher temperatures necessitate use of sand molds and because distortion during cooling is less predictable and usually greater than for iron, became the material of choice for use in gears because of greater resistance to damage from shock loads exhibited than is true for cast iron. However, the complexity of machining, grinding and heat treating steel to provide gears of good quality renders the manufacture of cast steel gears relatively expensive. In addition, the as-cast grain structure of such gears lacks the resilience and strength exhibited by a more refined and densified grain structure produced by forging.
U.S. Pat. No. 2,654,944 discloses stamping or otherwise forming sheet metal into a cup-like disc configuration having an upturned rim, and thereafter heating and placing the article flat in the socket of a stationary die which is configured with a crenelated periphery surrounding the rim of the disc for causing teeth to be expressed from the rim into the crenelations when the disc is worked by a sleeved tool in which the core member fills the cavity defined in the cup-shaped disc and the sleeve member is impressed onto the edge of the rim of the disc. The process is not disclosed for use with steel, and its use with sheet metal, which is defined as having a thickness of less than 0.1875 inch, generally precludes heat treatment such as case hardening of the product made.
A design for strengthening spur gear teeth is disclosed in U.S. Pat. No. 2,862,400 in which a web is provided in a cast gear extending between the gear teeth in a radial plane at midline along the length of the tooth edge, the tooth portions on either side of the web being disposed in staggered arrangement.