What is the speed of precision crossed roller bearings and what are the factors affecting the speed?

Precision crossed roller bearings are designed for high rigidity, accuracy, and load-carrying capacity in a compact space, not for high speed. Their speed capability is generally considered low to moderate compared to other bearing types like deep groove or angular contact ball bearings.

A typical speed for a medium-sized (e.g., 100-200mm bore) crossed roller bearing under standard grease lubrication might be in the range of 500 to 2,000 RPM. However, this is a very rough estimate, and the actual permissible speed can be significantly higher or lower depending on numerous factors.

The most accurate way to define bearing speed is with a speed factor (dn or ndm value), which combines the bearing size and its rotational speed. For crossed roller bearings, this value is often limited to a range of 100,000 to 300,000, whereas high-speed ball bearings can exceed 1,000,000.

Precision Crossed Roller Bearings Speed

Instead of a simple RPM value, engineers use a speed factor to create a more universal metric that accounts for bearing size.

Speed Factor (dn Value)

n: Rotational Speed (in RPM)

d: Bearing Bore Diameter (in mm)

Sometimes dm (mean diameter) is used, which is (bore + outside diameter) / 2.

dn Value = d x n

Why is this used? For a given RPM, a larger bearing has rollers that travel at a much higher surface speed, generating more friction, heat, and centrifugal force. The dn value represents this peripheral velocity and is a better indicator of the bearing's physical speed limit than RPM alone.

Typical dn Values for Crossed Roller Bearings:

Grease Lubrication: 70,000 to 150,000

Oil Lubrication (Oil Bath/Mist): 150,000 to 300,000+

Example:

A crossed roller bearing with a 100mm bore (d=100) is lubricated with grease.

Its maximum dn value from the catalog is 130,000.

Maximum allowable speed (n) = dn / d = 130,000 / 100 = 1,300 RPM.

If the same bearing were lubricated with circulating oil and had a dn limit of 250,000:

Maximum allowable speed (n) = 250,000 / 100 = 2,500 RPM.

This shows how lubrication alone can nearly double the speed limit.

Key Factors Affecting the Speed

The "limiting speed" listed in a manufacturer's catalog is a reference value calculated under specific, ideal conditions. The actual achievable speed in an application is determined by the interplay of the following factors. The ultimate limit is almost always heat. If the bearing generates heat faster than the system can dissipate it, the temperature will rise, leading to lubricant failure and bearing seizure.

1. Lubrication (The Most Critical Factor)

Lubrication reduces friction, dissipates heat, and prevents corrosion. The type and method have the biggest impact on speed.

Grease Lubrication:

Pros: Simple, clean, low maintenance.

Cons: Poor heat dissipation. At high speeds, the grease can be "churned," which generates excessive heat and reduces lubrication effectiveness. This severely limits the speed.

Oil Lubrication:

Pros: Superior cooling and lubrication properties, allowing for much higher speeds.

Methods (from slowest to fastest):

Oil Bath: The bearing is partially submerged in oil. Good, but can cause churning like grease.

Oil Drip/Mist: A controlled amount of oil is supplied, reducing churning and friction.

Oil-Air / Oil-Jet: An atomized mix of oil and air is precisely directed at the rolling elements. This is the best method for high-speed operation as it provides maximum cooling with minimum friction.

2. Load (Magnitude and Type)

Higher loads increase the pressure on the rollers and raceways, generating more friction and heat.

Magnitude: A lightly loaded bearing can run faster than a heavily loaded one. Catalogs often provide "speed correction factors" based on the applied load.

Type: Crossed roller bearings handle radial, axial, and moment loads. Complex combined loads, especially heavy moment loads, create more complex stress distributions and friction, reducing the permissible speed.

3. Preload and Clearance

Preload: This is an internal force applied to the bearing to increase rigidity and eliminate play. While essential for precision, preload increases rolling friction and heat, thus significantly reducing the speed limit. High-preload bearings run slower.

Clearance: A small amount of internal clearance allows the bearing to accommodate thermal expansion at higher speeds. Bearings intended for faster operation are often set with a slight clearance rather than a preload.

4. Heat Dissipation

The bearing's speed is limited by its ability to reach a stable operating temperature.

Housing & Shaft: Materials with high thermal conductivity (like aluminum vs. steel) and a larger surface area can dissipate more heat, allowing for higher speeds.

External Cooling: Adding cooling fins, fans, or a liquid cooling circuit to the housing can dramatically increase the speed capability by actively removing heat.

5. Seals

Contact Seals: These seals ride directly on the inner or outer ring, providing excellent protection against contamination. However, they create significant friction and heat, drastically lowering the speed limit.

Non-Contact Seals (Shields or Labyrinth Seals): These have a small gap and do not make contact. They generate virtually no friction, allowing for much higher speeds, but offer less protection against fine dust and liquids.

Open Bearings: No seals offer the highest speed potential but require a clean operating environment or external sealing.

6. Cage / Separator Material

The cage separates the rollers to prevent them from rubbing against each other.

Steel Cage: Robust and common, but heavier.

Brass Cage: Good performance, often used in larger bearings.

Polymer (e.g., PEEK, Polyamide): Lighter weight, lower friction, and can dampen vibrations. Polymer cages often allow for higher speeds because they generate less centrifugal force and friction.

7. Mounting Accuracy

Precision bearings require precise mounting.

Out-of-Round Housing or Shaft: This will deform the bearing rings, creating uneven load distribution, localized hot spots, and high vibration, all of which reduce speed and bearing life.

Misalignment: Poor alignment of the shaft and housing also leads to uneven loading and premature failure.

Conclusion

To determine the permissible speed of a specific precision crossed roller bearing, you must:

Consult the Manufacturer's Catalog: Find the reference speed and dn value for that specific part number.

Analyze the Application: Consider your specific load, lubrication method, required preload, and environmental conditions.

Apply Correction Factors: Use the manufacturer's guidelines to adjust the reference speed based on your unique operating conditions. The final allowable speed will be a "system property," not just a bearing property.