Speed factor ndm is the product of rotational speed n (rpm) and the bearing pitch diameter dm (mm), expressed in mm·rpm, used to evaluate bearing speed capability in a more comprehensive way than rpm alone.
dm = (d + D) / 2, where d is the bore diameter and D is the outer diameter. Example: bearing 6308 has d = 40, D = 90 → dm = 65 mm; at 3,000 rpm → ndm = 195,000 mm·rpm. See ndm limits by application type in bearing speed limits guide and shop deep groove ball bearings rated for your target ndm. According to the SKF Rolling Bearings Catalogue, ndm is the standard parameter for comparing speed capability across different bearing types and sizes — a small ball bearing running at 20,000 rpm and a large ball bearing running at 2,000 rpm can have equivalent ndm values if their size ratios are appropriate.
ndm Limits by Bearing Type
The ndm limit depends on bearing type, lubrication method, and cage design:
| Bearing Type | ndm with Grease (×10³) | ndm with Oil (×10³) |
|---|---|---|
| Deep groove ball (6xxx) | 300–600 | 600–900 |
| Angular contact ball (7xxx) | 350–700 | 700–1,100 |
| Cylindrical roller (NUxxx) | 200–450 | 400–700 |
| Tapered roller (302xx) | 150–300 | 300–500 |
| Spherical roller (222xx) | 100–200 | 200–350 |
| Ball bearing, Si₃N₄ ceramic | 800–2,000 | 1,500–3,000 |
Exceeding the ndm limit: cage temperature rises beyond control → cage deforms or fractures → sudden bearing failure.
How Size Affects ndm in Practice
This is a point engineers frequently overlook: when you increase bearing size to raise the C load rating, the actual ndm decreases because dm increases. An illustrative comparison:
Shaft 40 mm, speed 8,000 rpm. Two options:
- 6308 (d = 40, D = 90): dm = 65, ndm = 520,000 — below the grease limit of 600,000 → acceptable
- 6316 C3 (d = 80, D = 170, with adapter): dm = 125, ndm = 1,000,000 — exceeds grease limit
The larger bearing has higher C but is not suitable at this speed with grease lubrication. Solution: stay within the ndm limit or switch to oil lubrication.
Practical Example: CNC Machine Tool Spindle
A CNC spindle uses bearing 7210 B (d = 50, D = 90, B = 20 mm, C = 35.1 kN) at a maximum speed of 12,000 rpm. dm = (50 + 90) / 2 = 70 mm. ndm = 12,000 × 70 = 840,000 mm·rpm.
The ndm limit of 7210 B with grease is approximately 600,000 — exceeded. Solution: switch to oil-jet lubrication — the ndm limit rises to 1,100,000. At a precision machining facility in Dong Nai, after switching from grease to oil-jet, spindle housing temperature dropped from 85°C to 62°C at the same speed — and bearing service life doubled.