Q: What is the J factor and why is it editable?

J is the AGMA bending geometry factor. Unlike the pitting geometry factor I, it has no simple closed form: it is read from AGMA Figure 14-6 and depends on the tooth counts of BOTH meshing gears, the pressure angle and whether the tip or highest-point-of-single-tooth-contact loading is used. This calculator pre-fills a reasonable estimate from the tooth count, but exposes J as an editable input so you can enter the exact value from the AGMA chart or AGMA 908-B89. The pinion J is always smaller than the gear J.

Q: Does this calculator support metric (module) and imperial (diametral pitch)?

Yes. Switch between SI and imperial at the top. In SI you enter the module m in millimetres, face width in mm and power in kW, and stresses come out in MPa with the elastic coefficient Cp = 191 √MPa for steel. In imperial you enter the diametral pitch Pd in teeth per inch, face width in inches and power in horsepower, and stresses come out in psi with Cp = 2300 √psi. The two AGMA twins (2001-D04 and 2101-D04) differ slightly in the dynamic factor formula, which the tool handles internally.

Q: Is this an AGMA-compliant gear rating?

It implements the standard ANSI/AGMA 2001-D04 / Shigley Chapter 14 closed-form method for external spur gears with 20° or 25° full-depth teeth, and reproduces Shigley Example 14-4 to within 0.5%. It is intended for preliminary sizing, study and checking. A production gear rating should use exact J and I values from AGMA 908-B89, verified material allowables, and review by a qualified engineer. Helical, bevel, worm, internal and plastic gears, scuffing and thin-rim effects are out of scope.

Question 1

What is the difference between AGMA bending and contact stress?

Accepted Answer

Bending stress is the tensile stress at the tooth root that can break a tooth off (fatigue fracture); it is checked with the AGMA bending equation σ = Wt·Ko·Kv·Ks·(Pd/F)·(Km·KB/J) and a safety factor SF. Contact stress is the Hertzian pressure on the tooth flank that causes surface pitting; it is checked with σc = Cp·√[Wt·Ko·Kv·Ks·(Km/(dP·F))·(Cf/I)] and a safety factor SH. A gear pair must pass both. Because contact stress varies with the square root of load, SH is compared with the bending SF as SH² to see which mode actually limits the design.

Question 2

How do I know whether bending or pitting governs the gear?

Accepted Answer

Compare the bending safety factor SF with the square of the contact safety factor, SH². Bending stress is proportional to load, so SF is the load multiple that causes tooth breakage. Contact stress is proportional to the square root of load, so SH² is the load multiple that causes pitting. Whichever of SF and SH² is smaller is the governing (first) failure mode. In the classic Shigley Example 14-4 the pinion has SF = 5.62 and SH = 1.69, so SH² = 2.85 < 5.62 and surface wear (pitting) governs.

Question 3

What is the J factor and why is it editable?

Accepted Answer

J is the AGMA bending geometry factor. Unlike the pitting geometry factor I, it has no simple closed form: it is read from AGMA Figure 14-6 and depends on the tooth counts of BOTH meshing gears, the pressure angle and whether the tip or highest-point-of-single-tooth-contact loading is used. This calculator pre-fills a reasonable estimate from the tooth count, but exposes J as an editable input so you can enter the exact value from the AGMA chart or AGMA 908-B89. The pinion J is always smaller than the gear J.

Question 4

Does this calculator support metric (module) and imperial (diametral pitch)?

Accepted Answer

Yes. Switch between SI and imperial at the top. In SI you enter the module m in millimetres, face width in mm and power in kW, and stresses come out in MPa with the elastic coefficient Cp = 191 √MPa for steel. In imperial you enter the diametral pitch Pd in teeth per inch, face width in inches and power in horsepower, and stresses come out in psi with Cp = 2300 √psi. The two AGMA twins (2001-D04 and 2101-D04) differ slightly in the dynamic factor formula, which the tool handles internally.

Question 5

Is this an AGMA-compliant gear rating?

Accepted Answer

It implements the standard ANSI/AGMA 2001-D04 / Shigley Chapter 14 closed-form method for external spur gears with 20° or 25° full-depth teeth, and reproduces Shigley Example 14-4 to within 0.5%. It is intended for preliminary sizing, study and checking. A production gear rating should use exact J and I values from AGMA 908-B89, verified material allowables, and review by a qualified engineer. Helical, bevel, worm, internal and plastic gears, scuffing and thin-rim effects are out of scope.

Gear StrengthCalculator — AGMA Bending & Contact

Gear Strength
Calculator — AGMA Bending & Contact