Analysis of Thermal Expansion Characteristics of Bearings: Principles, Effects, and Countermeasures

The performance and reliability of bearings are crucial in mechanical equipment operating at high speeds, heavy loads, or extreme temperature conditions. In addition to traditional load, speed, and lubrication analysis, thermal expansion characteristics have become a core factor that cannot be ignored. The thermal expansion of bearings and their surrounding components can directly change the internal clearance, contact stress, and even the neutrality of the entire system, ultimately affecting equipment accuracy, lifespan, and even leading to catastrophic failure. Therefore, in-depth analysis of the thermal expansion characteristics of bearings is an indispensable part of modern mechanical design.
####1. Basic principle of thermal expansion**
Thermal expansion refers to the physical phenomenon in which the size and volume of an object change when its temperature changes. For isotropic solid materials, their linear thermal expansion is usually described by the * * Coefficient of Linear Thermal Expansion * * (α), with units of/K or/° C.
The calculation formula is:
`ΔL = L0 * α * ΔT`
among which
-Δ L is the change in length
-L0 is the original length
-α is the coefficient of linear expansion
-Δ T is the temperature change quantity
For the bearing system, it is necessary to consider both the material characteristics of the bearing itself (inner ring, outer ring, rolling elements) and the supporting structure (shaft, bearing seat).
####2. The Key Influence of Bearing Thermal Expansion on System Performance**
The temperature of bearings increases during operation due to frictional heat generation, and their thermal expansion effect is mainly reflected in the following aspects:
1. * * Change in Clearance**
This is the most direct and significant impact. The initial clearance (installation clearance) of the bearing is the basis for its normal operation.
-Negative effect: Usually, the temperature of the bearing ring and rolling elements is higher than that of the adjacent shaft and bearing seat. Due to the expansion of bearing steel (α ≈ 11-13 × 10 ⁻⁶/° C), the interference fit between the inner ring and the shaft may be enhanced, and the clearance fit between the outer ring and the bearing seat may become tighter. This will result in a decrease in * * working clearance * *, and even a negative clearance (pre tightening). A small clearance can increase friction, exacerbate temperature rise, lead to poor lubrication, and ultimately cause bonding or fatigue failure.
-Positive effect: In certain situations, if the expansion coefficient of the bearing seat material (such as aluminum alloy, α ≈ 23 × 10 ⁻⁶/° C) is much greater than that of the bearing steel, it may cause relative relaxation of the outer ring and increase the working clearance. Excessive clearance can reduce rotational accuracy, cause vibration, and noise.
2. Contact stress and load distribution**
Uneven thermal expansion can alter the load distribution inside the bearing. For example, in cylindrical roller bearings, if the thermal expansion of the shaft is much greater than that of the bearing seat, it will cause the bearing to bear additional axial compression stress, resulting in uneven force distribution on the rolling elements and excessive load on some rolling elements, significantly reducing the bearing life.
3. Misalignment of Neutrality**
If the temperature field of each part of the system is uneven (such as high temperature at one end of the shaft and low temperature at the other end), it will cause asymmetric thermal expansion and bending deformation of the shaft and bearing seat, which will destroy the ideal alignment state of the bearing. This misalignment will generate additional torque and edge stress, accelerating wear and fatigue.
4. Changes in the nature of cooperation**
Thermal expansion can change the fitting properties between bearings, shafts, and bearing seats. Carefully designed interference fit may become too tight at high temperatures, causing tensile stress on the inner raceway or loosening after cooling; The gap fit may become too tight, causing the outer ring to be unable to move slightly inside the seat hole, unable to compensate for the thermal elongation of the shaft, and thus causing axial jamming.
####III. Analysis of Key Influencing Factors**
1. Material properties: The coefficient of linear expansion of different materials is the cornerstone of analysis. The comparison of common materials is as follows:
-Bearing steel (GCr15, etc.): * * α ≈ 11-13 × 10 ⁻⁶/° C
-* * Cast iron: * * α ≈ 10-12 × 10 ⁻⁶/° C
-Carbon steel/alloy steel (commonly used for shafts): * * α ≈ 11-13 × 10 ⁻⁶/° C
-Aluminum alloy (commonly used for bearing seats): * * α ≈ 23-24 × 10 ⁻⁶/° C
-Copper alloy: * * α ≈ 17-19 × 10 ⁻⁶/° C
-Ceramic (Si3N4 rolling element): * * α ≈ 3.2 × 10 ⁻⁶/° C (its low expansion characteristics are the advantage of hybrid ceramic bearings)
2. Temperature gradient (Δ T): The actual temperature at each point of the system is the key input. This needs to be obtained through thermal analysis or experimental measurements, including ambient temperature, frictional heat generation, adjacent heat transfer, etc.
3. * * Bearing type and configuration:**
-Fixed end free end configuration: In long axis systems, one bearing is typically used as the fixed end (bi-directional positioning) and the other as the free end (axially movable) to compensate for thermal elongation of the shaft. The selection of free end bearings (such as NU type cylindrical roller bearings) must consider their ability to move freely.
-Paired installation of angular contact ball bearings: Pre tightening force is extremely sensitive to temperature. The expansion caused by temperature rise will significantly increase the preload force, and a suitable initial preload level should be selected during design.
####4. Response strategies in design and operation**
1. Accurate thermal design calculation: During the design phase, the thermal expansion amount is calculated based on the expected maximum operating temperature and material expansion coefficient, and a reasonable initial clearance (such as C3 and C4 group clearances) and fitting tolerance are selected accordingly. For aluminum alloy bearing seats, a tighter fit is usually required than cast iron seats to compensate for their greater expansion.
2. * * Material selection matching:**
-In situations with large temperature differences (such as aerospace), low expansion alloys (such as Invar alloys) or ceramic materials can be used to make bearing components to maintain dimensional stability.
-Try to make the material expansion coefficients of the shaft, bearing, and bearing seat as close as possible to reduce the problems caused by mismatches.
3. * * Thermal Management:**
-Lubrication and Cooling: Using circulating oil lubrication not only provides lubrication, but also takes away a large amount of frictional heat, effectively controlling the system temperature. If necessary, a forced cooling system (such as a water cooling jacket or oil cooler) can be added.
-Thermal equilibrium analysis: Simulate the heat generation and dissipation of the system through theoretical calculations or finite element analysis (FEA), predict the temperature distribution in a stable state, and provide a basis for accurate design.
4. * * Adopting a highly adaptable bearing structure:**
-Using * * roller bearings * * or * * CARB ring roller bearings * * at the free end can better adapt to axial displacement and angular deviation.
-For high-precision spindles, a mechanism with adjustable preload is used to compensate for changes in preload force caused by thermal deformation.
####V. Conclusion**
The analysis of the thermal expansion characteristics of bearings is an interdisciplinary problem involving materials science, thermodynamics, and mechanical design. It is not an isolated parameter, but a core variable that dynamically affects bearing clearance, fit, neutrality, and load. Neglecting the thermal expansion effect often leads to equipment performing well in laboratory tests but failing prematurely in actual on-site operation.
Therefore, in the design and fault diagnosis of modern high-performance mechanical systems, thermal expansion must be quantitatively analyzed and effectively controlled as a key factor. Only through scientific material selection, precise clearance and tolerance design, and effective thermal management strategies can bearings maintain stable and reliable performance under complex working conditions, ultimately extending the service life of the entire equipment.