What is the n×dm speed factor?

n×dm is the product of rotational speed (RPM) and mean bearing diameter dm = (d+D)/2 in mm. It is a key parameter for assessing whether a bearing can operate at a given speed with grease or oil lubrication.

Is there a universal n×dm grease limit?

No. Permissible speed and grease suitability depend on the exact bearing design, size, cage, seals, clearance, load, temperature, lubricant and heat dissipation. Use the current product table and lubricant data.

Can n×dm alone select grease or oil?

No. n×dm is a useful speed parameter, but lubrication selection also requires the exact bearing, load, operating temperature, viscosity, grease performance, relubrication method and thermal design.

Where should I obtain the limiting speed?

Use the limiting and reference speeds published for the exact bearing designation and execution by its manufacturer. A bearing-family name alone is insufficient.

Does a low n×dm prove the application is safe?

No. n×dm does not check load, minimum load, cage stress, lubrication film, temperature, clearance, sealing, alignment or critical speed. It is only one input to bearing selection.

Free Engineering Tool #053

Bearing Speed Factor n×d_m Calculator

Calculate mean diameter d_m and speed factor n×d_m. Compare the result only with data for the exact bearing and lubricant.

n × dmVerified FormulaManufacturer Limit Required

Results

Speed Factor n × dm

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Mean Diameter dm

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Speed Factor Formula

Interpretation

n×d_m is a speed parameter used in lubricant and relubrication guidance. It is not the bearing limiting speed and does not by itself decide grease versus oil. Use the exact bearing designation, execution, cage, seals, load, temperature and lubricant in current manufacturer documentation.

For outer-ring rotation or special arrangements, the manufacturer may specify a different speed parameter or correction. Follow that definition instead of assuming n×d_m.

The procedures described on this page were developed and field-tested by the Vibromera team using the Balanset-1A, a portable dual-channel balancer and vibration analyzer for on-site rotor balancing.