If you experience a premature bearing failure, you should not only look at your lubrication practice, but also your installation practice. When using an induction heater, make sure that the heating is stable and even; fast is not always good, especially if you don't know what happened to the temperature of the outer diameter (OD) of the bearing. Remember that the inner ring heats up faster than the outer ring. Precision bearings are the main candidates.

In the image above, you can see on the left that a single temperature probe is placed on the inner diameter (ID) of the bearing. When the temperature of the inner diameter reaches 110 degrees Celsius, the temperature of the outer diameter is only 60 degrees Celsius. If the clearance between the ball or roller bearing and the inner and outer diameter raceways is not enough, this can be a huge problem to deal with the temperature difference. In these cases, the bearing balls or rollers are squeezed between the outer diameter and the inner diameter, and the bearing is damaged even before installation.

The amount of internal clearance reduced by the difference in expansion of the two rings can be determined using the following formula:

δt ＝α×ΔT ×Do

δt = The amount of gap reduced due to thermal difference (mm)

α = Linear expansion coefficient of bearing steel 12.5 ×10-6/°C

ΔT = temperature difference between inner ring and outer ring (degrees Celsius)           

Do = outer ring raceway diameter (mm)

The outer ring raceway diameter (Do) value can be approximately calculated using the following formula:

For ball bearings and spherical roller bearings,

For roller bearings (except spherical roller bearings),

d=bearing bore diameter (mm)

D=Bearing outer diameter (mm)

Radial internal clearance of deep groove ball bearings (provided by NTN Bearings Corporation)

Interchangeable radial internal clearance of cylindrical roller bearings (provided by NTN Bearings Corporation)

So, let us look at an example of a deep groove bearing:

α = 12.5 × 10-6/degree Celsius

C2 = 2-20 μm (0.002 – 0.020 mm)

δt = (12.5 × 10-6) × (50) × [0.20 (100 (4 × 180))]

The built-in gap is only 0.002-0.020 mm. Admittedly, this is the worst-case scenario, but it emphasizes the need for careful temperature control.

On the right side of the top image, you can see that two temperature probes are used, one is mounted on the inner ring and the other is mounted on the outer ring. These induction heaters allow you to monitor two probes at the same time, not only allowing you to reach the desired ID temperature, but also allowing you to control the maximum temperature difference between ID and OD, thereby preventing the bearing balls from being squeezed.

This concern is also a problem of gears. The large temperature difference between the inner diameter and the outer diameter can cause cracks in the film between the inner diameter and the outer diameter, and damage the gear even before the gear is installed.

Understanding the clearance of each bearing and the maximum temperature difference between ID and OD is the key to successful, worry-free induction heating. Be sure to use a high-quality induction heater that allows you to monitor and control these two temperatures while heating. In addition, it must be completely automatically demagnetized and a report generated at the end of the heating cycle.

When an induction heater is used to heat the bearing, the rolling elements deform to absorb the internal clearance that is reduced due to the thermal difference. The bearing does not bear the load, and there will be no additional internal clearance loss due to interference fit during installation. This is why you never spin a hot bearing. The thermal bearing should go directly from the heater to the shaft and should not rotate until the thermal balance is restored. Having said that, the bearing can only withstand such a large deformation to absorb this temperature difference. Although the high temperature difference can cause irreparable damage to the bearing, it is not the only factor that needs to be considered.