Limit speed of bearings—requirements for bearing selection based on rotational speed

Every bearing has its own maximum allowable speed. The rotational speed plays a crucial role in determining the temperature rise of the bearing. If a bearing’s rotational speed exceeds its maximum allowable speed, it can lead to increased bearing temperature, drying out of the lubricant, and even cause the bearing to seize up. Therefore, when selecting an appropriate bearing type based on the bearing’s rotational speed, it’s essential to understand the specific requirements for choosing bearings according to speed.

Bearing rotational speed:

The rotational speed of a bearing is primarily limited by the temperature rise caused by internal friction and heat generation within the bearing. Once the rotational speed exceeds a certain threshold, the bearing will no longer be able to continue rotating due to damage such as burning.

Requirements for bearing selection based on rotational speed:

1. Ball bearings have higher limiting speeds and greater rotational accuracy than roller bearings; therefore, ball bearings should be preferred when operating at high speeds.

2. Under the condition of identical inner diameters, the smaller the outer diameter, the smaller the rolling elements will be. Consequently, the centrifugal inertial force exerted by the rolling elements on the outer raceway during operation will also be reduced, making such bearings more suitable for operation at higher speeds. Therefore, at high speeds, it is advisable to select bearings from the same diameter series but with smaller outer diameters. If using a bearing with a smaller outer diameter fails to meet the required load-carrying capacity, you can either install additional identical bearings in parallel or consider using bearings from a wider series.

3. The material and structure of the cage significantly affect the bearing’s rotational speed. Solid cages allow higher speeds than stamped cages, and bronze solid cages permit even higher speeds.

In general, for applications involving higher rotational speeds, it is advisable to select deep-groove ball bearings, angular-contact bearings, or cylindrical roller bearings. For applications with lower rotational speeds, tapered roller bearings can be used. The maximum allowable rotational speed of tapered roller bearings is typically about 65% of that for deep-groove ball bearings, 70% for cylindrical roller bearings, and 60% for angular-contact ball bearings. Thrust ball bearings have a low maximum allowable rotational speed and are suitable only for applications involving relatively low speeds.

Note: Depending on the type and brand of lubricant, some lubricants may have excellent performance in certain aspects yet are not suitable for high-speed rotation. Therefore, if the bearing speed exceeds 80% of the maximum speed specified in the dimension table, please contact the bearing manufacturer.

Limit speed of the bearing:

The limiting speed of a bearing refers to the maximum rotational speed at which frictional heat does not cause burning and continuous rotation is still possible. Therefore, the limiting speed of a bearing depends on various factors, including the shaft’s design, dimensions, tolerances, lubrication method, quality and quantity of the lubricant, material and design of the retainer, and load conditions.

The maximum permissible speeds for various types of bearings when lubricated with grease or oil (oil-bath lubrication) are listed in the respective bearing dimension tables. These values represent the maximum speeds at which standard-design bearings can operate under typical load conditions (C/P ≥ 16, Fa/Fr ≤ 0.25 approximately).

Experience has shown that even when bearings operate under the most favorable load and friction conditions, they should not exceed their maximum speed—either due to technical limitations or because maintaining stable operating conditions over a certain period would be prohibitively costly.

The极限 speeds listed in the product catalog apply to basic bearing designs. When the极限 speed exceeds the reference speed, the operating temperature may significantly exceed the reference value. Under these conditions, it may be necessary to conduct appropriate measurements (such as whether the cage, seals, preload, and lubricant can withstand high temperatures). If these measurements are still insufficient, it will be necessary to verify the internal clearance of the bearing as well as the precision of the bearing housing and shaft neck, and make appropriate adjustments to meet more demanding operating conditions.

In addition, it is essential to consider whether the materials used in the bearing system meet the requirements for the bearing’s operating temperature and its intended service life. When the steady-state operating temperature exceeds the maximum temperature recommended for the bearing material’s stability grade (for example, for the SN grade, the recommended maximum temperature is 120℃), bearings with a higher stability grade may be required to maintain the mounting stress and the internal clearance within the bearing.

For greases, other factors should also be considered, such as the lubrication conditions of the cage guide surfaces at the operating temperature and the grease’s consistency.

Some open-type ball bearings have very low friction; therefore, the reference speeds listed in the product tables may exceed the limit speeds. Consequently, it is necessary to calculate the adjusted reference speed and compare it with the limit speed, then select the lower of the two values. In certain special cases—for example, with some cylindrical roller bearings—if a different type of cage is chosen, the bearing can operate at speeds higher than the standard limit speeds specified in the tables.

If a bearing needs to operate at speeds higher than the maximum speeds listed in the product catalog, it is essential to address certain factors that currently limit its speed. This may involve making modifications to the bearing itself, the lubrication system, or the application. Such modifications could include improving the rotational accuracy of the bearing, changing the material and structure of the retainer, adjusting lubrication conditions, or altering the heat dissipation method.

In certain applications, other factors are more important than the limiting speed. For example:

1. Low rotational speed. At very low speeds, it is difficult to form an elastohydrodynamic lubricating film at the contact interface between the rolling elements and the raceways. In such applications, it is advisable to use lubricants containing extreme-pressure (EP) additives or solid lubricants.

2. Reciprocating oscillation. In this type of motion, the direction of bearing rotation changes before completing a full revolution. Since the bearing’s rotational speed is zero at the instant it reverses direction, it is impossible to maintain a fully developed fluid dynamic lubrication film under these conditions. To ensure the formation of a boundary lubrication film capable of supporting the load in such scenarios, lubricants containing effective extreme-pressure (EP) additives should be used. Hybrid ceramic bearings perform well even under insufficient lubrication conditions, making them particularly suitable for applications involving rapid acceleration, deceleration, and load reversal (direction changes), where they exhibit excellent performance. Typically, under reciprocating oscillation operating conditions, it is not possible to specify a particular rated speed or modern rotational speed. This is because the maximum achievable speed depends not on thermal equilibrium but rather on the inertial forces acting on the bearing. During each reverse rotation, the rolling elements may slide a certain distance due to inertia, leading to wear between the rolling elements and the raceways. The permissible rates of acceleration and deceleration depend on factors such as the mass of the rolling elements and the cage, the type and amount of lubricant used, the operating clearance, and the bearing load.

Correction of the Limit Speed:

When the load condition is such that C/P < 16 (i.e., the equivalent load P is greater than approximately 6% of the basic dynamic load) or when the axial load in the combined load exceeds 25% of the radial load, the limiting speed must be corrected using Equation 1-1.

a = f1 * f2 * n

na: Corrected maximum speed r/min

f1: Correction factor related to load magnitude (Figure 8-1)

f2: Correction factor related to the synthetic load (Figure 8-2)

n: Maximum speed under normal load, r/min

Precautions for High-Speed Rotation of Bearings:

When bearings are used in high-speed rotation—especially when the rotational speed approaches or exceeds the maximum rated speed specified in the dimension table—the following precautions should be observed:

1. Use high-precision bearings;

2. Analyze the internal clearance of the bearing (taking into account the reduction in internal clearance caused by temperature rise);

3. Analyze the material and form of the cage (for high-speed rotation, it is advisable to use a machined cage made of copper alloy or phenolic resin; alternatively, there are also molded cages made of synthetic resins that are suitable for high-speed rotation).

4. Analyze lubrication methods (suitable methods for high-speed rotation include circulating lubrication, jet lubrication, oil mist lubrication, and oil-air lubrication, among others).