Every bearing has a speed limit, and most people find out about it the hard way: a seized bearing, a burned-out grease fill, or a machine that suddenly runs hot after a motor swap. The speed rating in the catalog is not a single number. It depends on the bearing type, the lubrication method, and how much heat the system can dissipate.
The index that captures all of this is ndm: the pitch diameter in millimeters multiplied by the rotational speed in RPM. A small bearing at high speed and a large bearing at moderate speed can have the same ndm, and the same thermal challenge. This guide explains what ndm means in practice, how different lube methods change the limit, and what to do when you need more speed than grease can handle.
What ndm Actually Represents
ndm is the product of the mean bearing diameter (in mm) and the rotational speed (in RPM). The mean diameter is the average of the bore and the outer diameter, sometimes called the pitch diameter. It represents the speed at which the rolling elements orbit the raceway center.
Why does this matter more than just RPM? Because a 6205 bearing (25mm bore, 52mm OD) at 10,000 RPM has an ndm of about 385,000. A 6310 bearing (50mm bore, 110mm OD) at 5,000 RPM has an ndm of 400,000. Both bearings face roughly the same thermal challenge even though one spins twice as fast. The larger bearing generates more friction per revolution because of its bigger contact area and heavier rolling elements.
Every bearing type has a maximum ndm for each lubrication method. Exceed it and the bearing generates more heat than the lube system can carry away. The grease breaks down, the oil film thins, metal touches metal, and the failure cascade begins. It can take weeks or it can take minutes, depending on how far over the limit you are.
ndm = dm × n
dm = (d + D) / 2 (mean diameter, mm)
d = bore diameter
D = outer diameter
n = speed in RPM
Example: 6208 bearing (40mm bore, 80mm OD) at 3,600 RPM
dm = (40 + 80) / 2 = 60mm
ndm = 60 × 3,600 = 216,000
Bearing Speed Limit Checker
Check bearing speed against conservative ndm limits by bearing type and lubrication method. Get OK/CAUTION/HIGH risk classification with maximum safe RPM for your configuration.
Why Lubrication Method Changes Everything
Grease-lubricated bearings have the lowest speed limits. Grease stays in the bearing cavity and relies on conduction through the housing to remove heat. At high speeds, the rolling elements churn the grease, generating friction heat that the grease itself cannot carry away. The base oil separates from the thickener, the thickener forms a crust, and the contact surfaces starve for oil.
Oil bath lubrication raises the limit by 30 to 50 percent over grease. The oil circulates by splash or gravity, carrying heat from the contact zone to the housing where it dissipates. Oil mist goes further: a fine mist of oil droplets in compressed air reaches the contact zone directly and provides cooling through evaporation.
Oil jet lubrication pushes the limit highest. A pressurized stream of oil is aimed directly at the contact zone between the rolling elements and the raceway. The oil absorbs heat on contact and drains away, often to a cooler and filter before recirculating. Oil jet systems are standard on high-speed spindles, turbo machinery, and large high-speed fans.
The practical takeaway: if your bearing is running near the grease speed limit and you cannot reduce the speed, switching to oil lubrication may double your available speed ceiling without changing the bearing itself. It is usually cheaper than redesigning the shaft or buying a higher-precision bearing.
Grease: ~300,000 to 400,000
Oil bath: ~450,000 to 550,000
Oil mist: ~600,000 to 700,000
Oil jet: ~800,000 to 1,000,000+
Roller bearings are 30-50% lower than ball bearings for the same lube method.
Which Bearing Types Are Fastest
Ball bearings are faster than roller bearings because the point contact between a sphere and a flat surface generates less friction than the line contact of a roller. Deep groove ball bearings and angular contact ball bearings have the highest speed capability of any standard bearing type.
Angular contact bearings in matched sets can handle very high speeds in spindle applications, especially with ceramic (silicon nitride) balls that are lighter and generate less centrifugal force at speed. These hybrid bearings are standard in CNC machine tool spindles running at 15,000 to 40,000 RPM.
Cylindrical roller bearings are moderate. Their line contact generates more friction than ball bearings, but they carry much higher radial loads. For applications that need both high speed and high load capacity, a cylindrical roller bearing with oil lubrication is often the best compromise.
Tapered roller bearings and spherical roller bearings have the lowest speed limits. Their sliding contact between the roller ends and the guide flanges generates significant friction heat. These bearings are workhorses for heavy loads and misalignment tolerance, but they are not designed for high-speed service.
Bearing Speed Limit Checker
Check bearing speed against conservative ndm limits by bearing type and lubrication method. Get OK/CAUTION/HIGH risk classification with maximum safe RPM for your configuration.
What Overspeed Failure Looks Like
A bearing running above its ndm limit does not fail instantly. The first sign is elevated temperature. The bearing housing runs 10 to 20 degrees hotter than it should. Infrared temperature surveys catch this early. If nothing changes, the temperature climbs further. Grease darkens and hardens. The bearing starts making a higher-pitched sound as the lubricant film thins.
The next stage is accelerated wear. The rolling elements and raceways develop micro-pitting as the oil film is no longer thick enough to fully separate the surfaces. Vibration levels increase. If the bearing has a cage, the cage may overheat and deform, causing erratic vibration spikes.
Final failure is seizure. The temperature spike is sudden and dramatic. The bearing locks up, scores the shaft, and may destroy the housing bore. On a motor or pump, seizure trips the overload or burns out the motor winding. The repair cost at this stage is multiples of a planned bearing replacement because the shaft and housing are also damaged.
The key lesson: bearing speed limits are not suggestions. Running even 10 to 15 percent over the ndm limit significantly shortens bearing life. Running 30 percent or more over the limit can cause failure in weeks rather than years.