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Can transmitted torque alone determine the radial bearing load from a belt drive?

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Separate transmitted effort from the force on the shaft

In a simplified belt drive, torque depends on the difference between tight-side and slack-side tensions multiplied by pulley radius.

The force acting on the pulley depends on the vector combination of those tensions. Knowing only the torque therefore does not reveal the belt pretension or the full pulley loading. JTEKT's load-calculation guidance explicitly accounts for belt tension and operating effects when obtaining shaft loads. The bearing reactions need another step: the pulley position, support spacing and other forces must be included. Treating “torque divided by radius” as the final bearing radial load can omit a substantial part of the support-loading problem.

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Use a two-tension example to make the missing information visible

Take an illustrative pulley radius of 0.05 m, with belt tensions of 600 N and 200 N.

The torque magnitude from their difference is (600 − 200) × 0.05 = 20 N m. If the two belt spans pull in approximately the same direction, their resultant pulley force is approximately 800 N, not the 400 N tension difference. Different span directions require vector resolution. These assumed values demonstrate the distinction; they are not a belt-design recommendation. Once the actual pulley force is known, use its location and the shaft-support geometry to calculate the individual bearing reactions, including any other loads on the shaft.