Overheating

Diagnosing overheated bearings

Overheating damage may represent itself in many ways, such as babbitt discoloration, cracking, wiping or deformation. It may result from a wide range of problems, many of which stem from the lubricant flow rate, the viscosity, an interruption in the oil films and/or boundary lubrication. In the photo on the right, oil additives were plated out causing overheating.

The following additional conditions may also cause overheating:

• Improper bearing selection
• HP lift system failure
• Poor collar, runner or journal surface finish
• Insufficient bearing clearance
• Excessive load
• Over-speed
• Harsh operating environment

When there is evidence of overheating, we recommend that you contact our technicians to verify that the quantity and quality of oil flowing to the bearing is sufficient. To make an assessment, we will need a drawing or detailed description of the bearing assembly, the axial and/or radial loads, the oil viscosity, flow rate and supply temperature and pressure.

Repeated cycles of heating may produce thermal ratcheting, a type of surface deformation that occurs in anisotropic materials. These materials possess different thermal expansion coefficients in each crystal axis. If thermal ratcheting has occurred, as in the photo here, examine the shoes for the existence and depth of cracks. Remove the cracks and restore the original shoe surface. If this cannot be done, replace the shoes. Journal shoes typically must be replaced, but if the correction leaves the bearing within design tolerance, the bearing may be reused. The condition of the rotating journal, collar or runner surfaces must also be evaluated. They must be restored to original condition, either by lapping, hand stoning or replacement.

Preventing Bearing Overheating

Overheating can be prevented altogether at the design stage if potentially damaging conditions are known before installation. Some important considerations include:

1. Incorporating a directed lubrication bearing such as Kingsbury's Leading Edge Groove (LEG) technology
2. Increasing lubricant flow rates
3. Increasing bearing area to better distribute the load; e.g., in a thrust bearing, increasing the diameter; in a journal bearing, increasing the width-to-diameter ratio
4. Installing thermocouples or RTDs in the bearings (i.e. not the oil) to monitor the babbitt temperature in critical zones
5. Using a reliable high pressure lift system to ensure smooth startup

Contact a Kingsbury sales engineer to discuss your application during the machine design phase.