Radial load, lifespan and torque ratings

 

Torque Capability of a Planetary Gearbox as a Function of Output Shaft Loads

 

Planetary gearboxes are often selected based solely on the nominal torque values published in manufacturers’ catalogs. However, this approach can be misleading. A planetary gearbox does not have a single, fixed torque rating. In reality, the admissible torque depends on the required service life and on the actual operating conditions. Output shaft loads, in particular, have a major influence on the usable torque.

The type of transmission element mounted on the output shaft directly determines the radial and axial forces applied to the gearbox.

Among the most common configurations:

• Spur gears (in a rack and pinion drie or instance) generate radial forces dependent on their pitch diameter.

• Helical pinions introduce additional axial forces on top of the radial loads.

• Pulley-and-belt drives do not create axial forces, but belt tension generates a significant tilting moment that must be carefully considered during sizing.

• Wheels (as commonly used in AMR applications) apply a radial force offset from the reference position, typically the center of the shaft. In this case, catalog values for admissible radial loads are no longer applicable. Engineering validation is required to ensure proper dimensioning.

 

Sensitivity of internal gears to Misalignment

 

Gearing systems are extremely sensitive to misalignment. In any planetary gearbox, forces applied to the output shaft induce compression of the output bearings. This causes the shaft to either bend (in double-supported designs) or tilt around the bearings (in cantilevered configurations).

These deflections lead to additional stresses within the gear mesh: increased contact pressure between planet gears and the internal ring gear, as well as higher loads on the needle bearings inside the planets. As a result, the oil film between mating components breaks down at lower torque levels, accelerating wear and reducing service life.

Consequently, the torque that can be transmitted for a given lifetime decreases as external forces on the output shaft increase.

 

The torque-versus-load curve below clearly illustrates how admissible torque decreases as radial or axial forces increase.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Because no single torque value can be considered valid for all applications, we strongly recommend consulting us during gearbox selection. Our engineering team uses advanced calculation tools to provide reliable, application-specific recommendations and ensure long-term performance and safety.