For every bearing, there exists its own maximum rotational speed. Rotational speed plays a pivotal role in determining the temperature rise of a bearing. Should a bearing's rotational speed exceed its maximum limit, it will lead to increased bearing temperature, lubricant drying out, and potentially cause the bearing to seize. Therefore, when selecting an appropriate bearing type based on rotational speed, it is essential to understand the requirements for bearing selection according to rotational speed.

Bearing rotational speed:

The rotational speed of a bearing is primarily constrained by the temperature rise caused by internal friction and heat generation. Once the speed exceeds a certain threshold, the bearing may become unable to continue rotating due to damage such as burning.

Requirements for Bearing Selection Based on Speed:

1. Ball bearings possess higher speed limits and rotational precision than roller bearings; thus, they should be prioritised for high-speed applications.

2. For bearings with identical inner diameters, a smaller outer diameter corresponds to smaller rolling elements. Consequently, the centrifugal inertial forces exerted by these elements on the outer raceway during operation are reduced, enabling operation at higher speeds. Therefore, for high-speed applications, bearings with a smaller outer diameter within the same diameter series are preferable. If a smaller outer diameter bearing fails to meet load requirements, additional identical bearings may be mounted in parallel, or bearings from a wider series may be considered.

3. The material and structure of the cage significantly influence bearing speed. Solid cages permit higher speeds than stamped cages, with bronze solid cages allowing the highest speeds.

For applications involving generally higher rotational speeds, deep groove ball bearings, angular contact bearings, or cylindrical roller bearings are recommended. For lower rotational speed applications, 60% of the rated speed may be used. Thrust ball bearings have a low limiting rotational speed and are only suitable for lower speed applications.

Limiting speed of bearings:

The limiting speed of a bearing refers to the maximum rotational speed at which friction-induced heat does not cause damage and continuous rotation remains feasible. Consequently, the limiting speed depends on various factors including shaft type, dimensions, tolerances, lubrication method, lubricant quality and quantity, cage material and design, and load conditions.

The maximum rotational speeds for grease-lubricated and oil-lubricated (oil bath lubrication) bearings of various types are listed in respective bearing size tables. These values represent the maximum rotational speeds for standard-design bearings under typical load conditions (C/P > 16, Fa/Fr ≈ 0.25).

Experience indicates that even under favourable load and friction conditions, bearings should not exceed their maximum rotational speed due to technical constraints or the prohibitively high cost of maintaining stable operating conditions over extended periods.

Where the limiting rotational speed exceeds the reference speed, operating temperatures may significantly surpass reference values. Under such conditions, appropriate measurements may be required (assessing whether the cage, seals, preload, and lubricant can withstand elevated temperatures). Should these assessments prove insufficient, verification of the bearing's internal clearance and the precision of the bearing housing and journal is required, with appropriate adjustments made to accommodate the more demanding operating conditions. Furthermore, consideration must be given to whether the materials within the bearing system meet the requirements for both the operating temperature and the desired service life. When the steady-state operating temperature exceeds the maximum temperature recommended for the bearing material (e.g., 120°C for SN series), bearings with a higher stability rating may be necessary to maintain the mounting stress and internal clearance.

When the stable operating temperature exceeds the maximum temperature recommended for the stability class (e.g., 120°C for SN class), bearings with a higher stability class may be required to maintain the mounting stress and internal clearance.

For grease lubrication, additional factors must be considered, such as lubrication conditions on the cage guide surfaces at operating temperatures and the grease consistency.

Certain open ball bearings exhibit exceptionally low friction, potentially exceeding the limiting speeds listed in the product tables. Consequently, an adjusted reference speed must be calculated and compared against the limiting speed, adopting the lower value. In specific instances, such as with some cylindrical roller bearings, selecting alternative cages may permit operation beyond the standard limiting speeds stated in the tables.

Should operation above the limit speeds in the product list be required, factors restricting rotational speed must be mitigated. This may involve modifications to the bearing, lubrication system, or application. Such modifications could include enhancing bearing rotational precision; altering cage material and structure; revising lubrication conditions; or improving heat dissipation methods.

In certain applications, other factors may be more critical than the limiting speed. For example:

1. Low speeds. At very low rotational speeds, it is difficult to form an elastic fluid lubrication film between the rolling elements and raceways. In such applications, consider using lubricants containing extreme pressure (EP) additives or solid oils.

2. Reciprocating motion. In this motion pattern, the bearing's rotational direction changes before completing a full revolution. As the bearing experiences zero rotational speed during the reversal moment, it is impossible to maintain a complete hydrodynamic lubrication film. In such cases, lubricants containing effective extreme pressure (EP) additives should be used to achieve a boundary lubrication film capable of withstanding the load. Hybrid ceramic bearings perform well under insufficient lubrication, demonstrating excellent capability in applications involving rapid acceleration, deceleration, and load reversal (direction changes). Typically, specific rated speeds or maximum speeds cannot be specified for reciprocating oscillation conditions. This is because the achievable maximum speed depends not on thermal equilibrium but on the inertial forces acting upon the bearing. During each reversal, rolling elements may slide a distance due to inertia, causing wear against the raceway. Permissible acceleration and deceleration depend on the mass of rolling elements and cage, lubricant type and quantity, operating clearance, and bearing load.

Correction of limiting speed:

When load conditions satisfy C/P < 16 (i.e., equivalent load P exceeds approximately 6% of the basic dynamic load) or when the axial load within the composite load exceeds 25% of the radial load, the limiting speed must be corrected using Equation 1-1.

a = f1 × f2 × n

na: Corrected limiting speed (r/min)

f1: Correction factor related to load magnitude

f2: Correction factor related to composite load

n: Limit speed under normal load conditions r/min

Considerations for High-Speed Bearing Operation: When bearings operate at high speeds, particularly when approaching or exceeding the limit speeds specified in the dimension tables, observe the following:

1. Employ high-precision bearings;

2. Analyse internal bearing clearance (accounting for clearance reduction due to temperature rise);

3. Evaluate cage material and design (for high-speed rotation, copper alloy or phenolic resin-moulded cages are recommended; synthetic resin moulded cages suitable for high-speed rotation are also available);

4. Assess lubrication methods (employ lubrication systems suitable for high-speed rotation, such as recirculating, jet, oil mist, or oil-air lubrication).