What Is the Speed Factor (ndm)?
The speed factor ndm is a simple metric that characterizes the speed-size regime of a bearing: ndm = n (RPM) x dm (mean diameter in mm). It is used to assess whether grease lubrication is adequate or whether oil lubrication (or oil mist) is required, and to estimate re-lubrication intervals.
Guidelines
ndm < 300,000: grease lubrication adequate. 300,000-500,000: high-speed grease or oil lubrication. > 500,000: oil lubrication or oil mist recommended. Ceramic hybrid bearings extend the grease-lubrication ndm limit by 30-50%.
Frequently Asked Questions
How do you calculate ndm?
ndm = n x dm, where n = rotational speed in RPM and dm = mean bearing diameter = (bore d + OD D) / 2, in mm. For example, a 6205 bearing (25mm bore, 52mm OD) at 3,600 RPM: dm = (25+52)/2 = 38.5mm, ndm = 3,600 x 38.5 = 138,600. Values above 500,000 generally require oil lubrication.