Factors affecting bearing friction force

Bearings are the "joints" in mechanical systems, whose core function is to support rotating components and reduce frictional resistance during motion. The magnitude of friction directly determines the operating efficiency, temperature rise, wear rate, and even the service life and energy consumption of the entire equipment of the bearing. Therefore, a deep understanding of the many factors that affect bearing friction is crucial for the correct selection, use, and maintenance of bearings. This article will systematically explore these influencing factors and their mechanisms of action.

1、 The main source of friction force
Before delving deeper into the analysis, we first need to understand that the friction inside the bearing mainly comes from the following parts:

Rolling friction: The resistance generated by the elastic hysteresis and slight slip of the material when the rolling element (ball or roller) rolls on the raceway.

Sliding friction: including the sliding between the cage and the rolling element/guide surface, and the sliding between the rolling element and the raceway in the non pure rolling contact area.

Lubricant friction: The internal friction of the lubricant (oil or grease) itself, as well as the fluid resistance caused by stirring the lubricant.

Friction of seals: Direct friction between contact seals and the outer ring of the shaft or bearing (sometimes even exceeding the friction of the bearing itself).

2、 Key influencing factors and their mechanism analysis
1. Load

Mechanism: Load is the most direct factor affecting friction. According to classical tribology theory, the rolling friction force is approximately linearly related to the load (F ∝ P). An increase in load will lead to an increase in the contact ellipse area, an increase in material elastic hysteresis loss, and thus an increase in rolling friction. Meanwhile, high loads can also exacerbate sliding friction components.

Characteristics: Both radial and axial loads can have an impact. For angular contact ball bearings and tapered roller bearings, small changes in axial load can significantly alter the internal stress state and friction force of the bearings.

2. Rotational speed

Mechanism: The effect of rotational speed on friction is nonlinear. In the low-speed range, friction mainly comes from sliding friction and rolling friction under boundary lubrication conditions. As the rotational speed increases, the lubricant is fully brought into the contact area, forming a fluid dynamic pressure oil film and reducing sliding friction. However, the stirring loss and internal friction of the lubricant will sharply increase, becoming the dominant factors.

Characteristic: Therefore, there exists an "optimal speed range" that minimizes friction.

3. Lubrication
This is the most complex and critical influencing factor, which can be subdivided into multiple sub items:

Lubrication method: Oil lubrication has a simple structure, but it causes significant stirring losses at high speeds; Oil lubrication (especially oil air and oil mist lubrication) is more suitable for high-speed applications, as it can effectively cool and control friction.

Lubricant viscosity: Viscosity is the most important property of lubricants. Low viscosity prevents the formation of a sufficiently thick oil film, leading to a sharp increase in metal contact, friction, and wear. If the viscosity is too high, the internal friction and agitation resistance will be too high, which will also lead to high temperature and high friction. Therefore, selecting the appropriate viscosity based on load, speed, and temperature is the core task.

Lubricating oil quantity: Insufficient oil quantity leads to insufficient lubrication; However, excessive oil volume (especially oil bath lubrication) can cause the bearings to operate in the "oil pool", resulting in significant loss of agitation energy.

Lubricant types: Both base oil types (mineral oil, synthetic oil) and additives (such as friction reducers, extreme pressure additives) can significantly alter friction characteristics. Fully synthetic oils typically have better temperature viscosity characteristics and lower friction coefficients.

4. Installation and coordination

Clearance/preload: This is an extremely fine and important parameter. Excessive internal clearance can lead to increased vibration and sliding of the bearing during operation, resulting in unstable friction torque. Negative clearance can improve stiffness and accuracy, but excessive preload can greatly increase rolling friction and temperature rise, which is a common cause of excessive friction.

Fit tolerance: If the fit between the bearing and the shaft or bearing seat is too tight, it can cause the bearing ring to expand (inner ring) or contract (outer ring), indirectly changing the internal clearance of the bearing and even causing deformation of the raceway, thereby increasing friction.

5. Bearing design and type

Bearing type: The friction mechanism between sliding bearings (such as hydrostatic bearings) and rolling bearings is completely different. In rolling bearings, the friction force of deep groove ball bearings is usually lower than that of angular contact ball bearings and tapered roller bearings (due to the latter having sliding friction); The rolling friction of cylindrical roller bearings is smaller than that of ball bearings.

Cage design: The material (steel, brass, nylon, PEEK, etc.), structure (crown, riveting, etc.), and guiding method (roller guide, outer ring guide) of the cage directly affect its sliding friction and inertial force with the rolling elements.

Sealing form: The friction force of contact rubber seal (RS) is much greater than that of non-contact labyrinth seal (ZZ). In applications that require low torque, such as electric vehicle motors, non-contact seals are often chosen.

6. Working temperature
Temperature affects friction in two ways:

Direct impact: Temperature changes can alter the viscosity of lubricating oil, thereby affecting the thickness of the oil film and internal friction of the lubricant.

Indirect effects: Temperature changes cause thermal expansion of bearing components, changing internal clearance or preload, thereby affecting friction. For example, temperature rise during operation may lead to an increase in preload force, forming a positive feedback vicious cycle of "friction temperature rise preload force increase friction further increase".

3、 Practical strategies for controlling friction force
Accurate selection: Select the appropriate bearing type, size, and clearance level based on the working conditions (load, speed).

Scientific lubrication: Following the principle of "correct viscosity, correct oil quantity, and correct method". Choose low viscosity oil for high-speed light load and high viscosity oil for low-speed heavy load.

Precision installation: Ensure appropriate fit tolerances and precisely adjust pre tightening force to avoid over tightening.

Thermal management: For high-speed heavy-duty applications, design effective cooling systems (such as oil cooling) to control operating temperature, stabilize oil film performance, and internal clearance.

Consider new technologies: In applications that require extreme low friction, ceramic ball bearings, low friction retainers (such as PEEK), or advanced low friction lubricants can be considered.

Conclusion
The friction force of a bearing is a complex result of the coupled effects of multiple factors such as load, speed, lubrication, installation, design, and temperature. In practical engineering, a factor should not be viewed in isolation, but rather systematically analyzed and balanced. By deeply understanding and effectively controlling these influencing factors, bearing performance can be maximized, equipment energy efficiency can be improved, service life can be extended, and reliable and silent operation of equipment can be achieved.