Every machine changes dimensions when it reaches operating temperature. Shafts grow longer and fatter. Housings expand. Bearing internal clearance shrinks or grows depending on what is hotter and what material everything is made from. These changes are small in absolute terms but large enough to turn a good bearing installation into a bad one.
This guide covers the thermal expansion mechanics that matter for bearing applications: how to estimate dimensional changes, when thermal growth causes fit problems, and how to design around it.
Coefficients of Thermal Expansion for Common Materials
The coefficient of thermal expansion (CTE) tells you how much a material grows per degree of temperature rise per unit length. For bearing applications, the materials that matter are carbon steel, stainless steel, cast iron, aluminum, and bronze. The key numbers are:
Carbon steel: 11 to 12 µm/m/°C (6.0 to 6.7 µin/in/°F)
Stainless steel: 16 to 17 µm/m/°C (8.9 to 9.5 µin/in/°F)
Cast iron: 10 to 11 µm/m/°C (5.5 to 6.1 µin/in/°F)
Aluminum alloys: 22 to 24 µm/m/°C (12.2 to 13.3 µin/in/°F)
Bronze: 17 to 19 µm/m/°C (9.5 to 10.6 µin/in/°F)
The key insight is that aluminum expands at roughly double the rate of steel. A bearing outer ring is always steel (CTE ~12). If the housing is aluminum (CTE ~23), the housing grows away from the ring at nearly twice the rate. The fit loosens with every degree of temperature rise.
ΔD = D × CTE × ΔT
Example: 50mm steel shaft, ΔT = 60°C
ΔD = 50 × 12×10-6 × 60 = 0.036mm
That 0.036mm is enough to significantly change a bearing press fit.
Thermal Growth Fit Impact Calculator
Calculate thermal expansion of shafts and housings and see the impact on bearing fit. Enter material, dimensions, and temperature change to see dimensional growth and resulting hot-running fit.
Choosing Bearing Clearance for Temperature
Standard clearance (CN) bearings are designed for moderate temperature differentials and normal shaft fits. When the shaft fit is tight (m6 or tighter) or the operating temperature is elevated, the internal clearance consumed by the tight fit plus thermal growth can approach zero. A bearing with zero or negative clearance runs with preload, increasing friction and temperature, which further reduces clearance in a feedback loop.
C3 clearance provides 30 to 50 percent more internal clearance than CN. It is the standard choice for: electric motors, pumps, fans, and any application with shaft fits tighter than k5, operating temperatures above 100°C, or both. Most motor manufacturers install C3 bearings as standard.
C4 clearance provides even more margin and is used for high-temperature applications (above 150°C), very tight shaft fits (p6), or applications where significant differential thermal growth is expected. It is less common and may have longer lead times.
CN (standard): Normal fits, <80°C, shaft fit j5 to k5
C3: Tight fits m6+, temps 80-150°C, or electric motors
C4: Very tight fits, temps >150°C, aluminum housings
C5: Extreme temperature (rare, specialty applications)
Thermal Growth and Shaft Alignment
Thermal growth does not just affect bearing fit. It changes the alignment between coupled machines. A motor-pump set aligned perfectly cold will go out of alignment when the pump heats up to operating temperature. The pump centerline rises as the casing expands. The motor stays closer to ambient because it is typically cooler than the pump.
The solution is hot alignment: deliberately misaligning the cold machine so that thermal growth brings it into alignment at operating temperature. This requires estimating the differential growth between the driver and driven equipment and offsetting the cold alignment by that amount.
For bearing applications, misalignment causes uneven loading across the raceway width, concentrating stress at one edge. Self-aligning bearings (spherical roller, self-aligning ball) tolerate this to a degree, but rigid bearings (deep groove ball, cylindrical roller) do not. Misalignment from thermal growth is a common hidden cause of premature bearing failure in pumps and fans.