Near-net gears come from the forging press in almost the exact shape of the finished gear. Gear teeth are forged with an envelope of material around the tooth profile, unlike near-net blanks, which are forged cylinders into which the teeth must be cut by rough hobbing. Near-net gears have teeth forged with a thin envelope of excess steel, which is removed by either a single-pass grinding or hobbing operation.
According to Chris Carman, president and chief operating officer at Presrite Corp. (Cleveland) Presrite is one of a few companies worldwide that produce forged tooth gears at near-net shapes. Presrite is currently supplying gears to customers with as little as 0.5 millimeter stock allowance per flank using only the forging operation. According to the company, tests show that forged gears have as much as double the field life of conventionally produced gears in part because of the lack of stress induced into the gear as it is forged.
Near-net gears can be produced using any carburizing or induction hardening steel in five basic configurations: spiral bevel, helical, straight bevel, spur gears with 0.04-inch plus stock and spur gears with 0.004- to 0.012-inch stock. The near-net gears can be produced in diameters up to 17 inches with stock allowances ranging from 0.1 to 1.5 millimeter. The specifications for various gear configurations include:
Spiral bevel gears can be produced up to 17 inches in diameter, with 0.02 inch minimum stock per flank. A maximum spiral angle of 35 degrees and a pitch range of less than 7 diametral pitch (D.P.) can be achieved.
Helical gears can be produced up to 10 inches in diameter and up to 90 pounds in weight, with 0.02 inch minimum stock per flank. A maximum helix angle of 25 degrees and a pitch range of 4 to 12 D.P. can be achieved.
Spur gears 0.04 inch plus stock can be produced up to 16 inches in diameter and up to 300 pounds in weight, with 0.04 inch minimum stock per flank. A pitch range of less than 5 D.P. can be achieved. This type of gear requires a finish process of hobbing, hobbing and shaving, or hobbing and grinding or skiving.
Spur gears 0.004- to 0.012-inch stock can be produced up to 10 inches in diameter and up to 6-inches maximum face width, with 0.004- to 0.012-inch stock per flank. A pitch range of 4 to 12 D.P. can be achieved. This type of gear requires a finish process of grinding or skiving. A net root is possible.
Straight bevel gears have properties similar to spiral bevel gears.
Net spur gears that are totally finished are possible but must be used in a slow rotating application, rather than a high-speed application. Presrite produced net spur gears for applications such as master clutch hubs, reverse idler gears, and hand winch-drive gears.
Forging Near-Net Gears
The manufacturing process begins with steel bar stock, usually turned and polished to improve the surface, and cut to the exact weight. The exact weight is critical because the amount of steel must completely fill the die to produce the complete gear profile. Prior to forging, billets are heated between 1,700 to 2,250 F in an electrical induction furnace that is controlled by an optical pyrometer to + or - 25 F.
In a single stroke, standard mechanical forging presses, ranging from 1,600- to 6,000-ton, form near-net shape gears with the complete allowable material envelope, or stock allowance. The purpose of this first operation, which forms a "pancake," is to break the scale off of the billet and size the outside diameter to just under the size of the root diameter in the gear die. Next, an operator positions the billet into the finish die. After forging, a hydraulic knockout system immediately extracts the gear from the finish die.
After the raw forged gear is hydraulically ejected from the die, it is placed in a trimming nest where the hole is punched. It is then allowed to cool to ambient temperature, which usually takes up to 24 hours. Once cooled, it is ready for turning.
If grinding is the final operation, a complete and consistent lower material envelope of grinding stock, 0.1 to 0.3 millimeter per flank, is ensured by cold drawing the forged near-net gear through a finish sizing die. In the sizing operation, the gear is placed into a sizing die, where it is flooded with oil and pressed through the die. This operation is also capable of providing a finished protuberance and root configuration to the geometries supplied by customers, thereby eliminating requirements of grinding the root area. And in some cases, the cold drawing process is capable of producing a net tooth geometry, eliminating gear finishing altogether.
Consistent grain flows
The near net gear forging process is not only cost advantageous when compared with machining or casting because of the ability to achieve near net shapes, but it also offers certain engineering advantages derived from the grain-flow direction and its effects on properties of the finished components.
For the gear machined from a hot-rolled bar of steel, the remaining texture of the rolling will have one preferred direction along which the properties of the material (strength, fatigue) will be higher than in other directions. However, the machining process of the complex shapes will inevitably expose other grain directions to the service loads and will result in the stress concentrations not necessarily aligned with the preferential grain flow of the original bar. Custom grain orientation can be available from directionally solidified castings, however, castings always have internal defects because of shrinkage and gas entrapment that will reduce the strength unless advanced, and costly, casting technologies are used.
During the forging process, the grains on the surface of the bar become elongated and follow the shape of the gear tooth, creating a texture that never exposes non-banded structure on the tooth surface, even after subsequent finish machining. This significantly improves bending fatigue properties.
Another advantage of forging is the ability to custom engineer microstructure and properties following hot forging without subsequent heat treatment. This is made possible by the use of micro-alloyed steels that can achieve high tensile strength through air cooling after forging, hence eliminating the heat treatment. As a result, potential distortion can be eliminated, costs reduced and the part processing sequence simplified.
"Since we launched our near-net gear program more than 10 years ago, we've found that as we improved our process, we are now able to supply our customers with near-net gears in half the turnaround time and at half the cost," says Fisher. "And, clearly there is an increase in demand from the industry for near-net shape forged gears."
Gears have as little as 0.5 millimeter stock allowance per flank using only the forging operation.
Tests show that forged gears have as much as double the field life of conventionally produced gears in part because of the lack of stress induced into the gear as it is forged.
Near-net gears can be produced using any carburizing or induction hardening steel in five basic configurations: spiral bevel, helical, straight bevel, spur gears with 0.040 inch plus stock and spur gears with 0.004 inch to 0.012 inch stock.