12.26 Material

The material of the gears can be selected from the material database. The strength is dependend of material type, treatment and quality.

12.26.1 Material treatment

There are different possibilities for heat treatment: through hardened, nitrided, induction hardened and case hardened materials:

• Through hardened: annealed, normalized or quenched and tempered. Carbon content ranges from 0.30 to 0.50%. Alloy content ranges from plain carbon steels (e.g. MSI 1040) for tiny gears, to Cr-Ni-Mo alloys (e.g.AISI 4340) for large gears. The best metallurgical properties are obtained with quenched and tempered steels. Hardness ranges from HB = 180 for lightly-loaded gearsets, to the limit of machinability (approximateby HB = 360) for highly-loaded gears.

  Good tooth accuracy (typically Q = 10 acc. AGMA2000) can be obtained by hobbing the teeth after heat treatment, eliminating heat treatment distortion from the generated tooth forms. Hardenability must be adequate to obtain the required hardness at the root diameter.

• Nitrided gears are quenched and tempered to obtain the desired core properties, then the teeth are cut and finished, followed by the nitriding process. fle gears are placed in an ammonia gas atmosphere where nitrogen is absorbed into the surface bayers of the gear teeth and forms hard fron nitrides. Because nitriding is performed at the relatively low, temperature of 950-1050 ^∘F, and there is no quench, the distortion due to heat treatment is small. Surface hardness ranges from HB = 432 for alloys such as AISI 4340 to HB = 654 for Nitralloy 135M and 2.5% chrome alloys. The practical limit on case depth is about 0.025 in, which limits the application of nitriding to pitches finer than approximately P_nd = 8.
• Induction hardened gear teeth are heated by electromagnetic induction from a coil or inductor and are immediately quenched. Because only the surface layers of the gear teeth are hardened, heat treat distortion is minimized. Very tight controls of every step of the process are necessary for satisfactory results, and it is best for high-volume production where the process can be optimized. Several gears from each production run must be destructively inspected for case depth to ensure that the induction hardening is properly controlled. Carbon content of induction hardened gears is usually 0.40 or 0.50%. Plain carbon steels (e.g. AISI 1050) may be used for small gears, while alloys such as AISI 4350 may be used for large gears.
• Carburized gears are first cut, then heated in a carbon atmosphere (usually gas carburizing) which causes carbon to diffuse into the surface layers of the gear teeth. The gears are either quenched from the carburizing temperature or cooled, reheated and quenched later. Most gears are tempered at 300-400 ^∘F after carburizing and quenching. Carbon content of carburizing steels range from 0.15 to 0.25%. Low alloy steels (e.g. AISI 8620) are used for small gears and moderate loads while high alloy steels (e.g. AISI 4820) are used for large gears and high loads. Minimum surface hardness ranges from HB = 615 to HB = 654. Because carburized gears are subjected to a drastic quench from a high temperature the distortion is large, and grinding is usually required to obtain acceptable accuracy.

12.26.2 Material quality

Material quality strongly influences pitting resistance and bending strength. For high quality material, the following metallurgical variables must be carefully controlled:

• Chemical coposition
• Hardenability
• Toughness
• Surface and core hardness
• Surface and core microstructure
• Cleanliness/inclusions
• Surface defects (flanks and root flllets)
• Grain size and structure
• Residual stress pattern
• Internal defects, seams or voids
• Microcracks
• Carbide network
• Retained austenite
• Intergranular oxidation
• Decarburization

There are three basic grades of material:

• Commercial quality typical of that obtained from experienced gear manufacturers doing good work. Modest level of control of the metallurgical variables.
• High quality typical of aircraft quality steel with cleanliness certifled per AMS 2301 or ASTM A534. Close control of critical metallurgical variables.
• Premium quality typical of premium aircraft quality with cleanliness certified per AMS 2300 or .ASTM A535. Absolute control of all metallurgical variables.

12.26.3 Own input of material data

Using the plus button next to the material list the material values can be entered directly by the user. You have to be careful choosing the values since they are not checked by the software. Important for the calculation are the allowable stress numbers s_ac{σ_Hlim} and s_at{σ_Flim}. The youngs module is needed for the hertzian stress and the yield point for the static strength. The hardness value is only used for documentation.