"What could be the cause of overheating on a gearbox that is using a polyalphaolefin (PAO) oil with an ISO viscosity grade of 320? The problem is worse when using a mineral oil with an ISO viscosity grade of 320."

Gearbox overheating can be linked to many different root causes. To perform a proper investigation into why this gearbox may be experiencing overheating, an in-depth list of data must be collected, including the following:

  • Operating temperatures
  • Ambient temperatures
  • OEM-specified lubricant requirements including viscosity grade (given the gearbox is running within the OEM recommendations)
  • Viscosity index of the lubricant used (for both the mineral and synthetic oils)
  • Load conditions on the gearbox and maximum/minimum speeds, etc.
  • Particle counts or contamination levels (any test results indicating ingressed particles or wear debris)

Without this information, a root cause cannot effectively be established. However, there are a few things to consider in this particular circumstance. The first is if the lubricant in use is the correct type and has the appropriate viscosity. If overheating is occurring, one obvious cause could be contact friction from lack of lubricant viscosity and lubricity. Polyalkylene glycols (PAGs), for example, perform better in worm gears because of their lubricity.

The next thing that must be determined is if the lubricant is breaking down and oxidizing. Does the oil smell or have a foul odor? Oil analysis must be performed to discover if the lubricant could be oxidizing and hindering its required capabilities. If this is the case, investigate why the lubricant is oxidizing. This may be due to the operating conditions, environmental conditions, lubricant choice or a combination of these factors.

Synthetic lubricants have better thermal and oxidation resistance and an improved viscosity-temperature behavior due to their higher viscosity index. Therefore, the fact that the overheating problem is worse with a mineral oil is no surprise. While several factors could be causing the gearbox overheating to escalate, none of these factors would be the root cause, since the problem existed regardless of the type of oil in use.

The best way to find a solution would be to perform oil analysis in order to determine the quality of the lubricant as well as the presence of any wear debris or contamination. Also, check the gearbox's OEM manual for the proper running conditions and lubricant specifications, and then make sure they match the actual conditions. These results should provide clues to help you diagnose the overheating problem.