How to Save a Gearbox Running Hot At 170°C

I received a phone call from a paper mill. Maintenance staff of that mill were interested in trying to save a gearbox running at 170°C and operate in survival mode until the next scheduled maintenance period. Conditions of the gearbox are shown in Figure 1.

Figure 1. Warm-Cold Expander

1. Air temperature in this area – north side exposed to wet end of the machine 60°C measured 1 foot
2. Housing temperature near this bearing point – 170°C
3. Air temperature in this area – north side exposed to wet end of the machine 45°C measured above 1 foot
4. Air temperature in this area – south side exposed to wet end of the machine 45°C measured above 1 foot
5. Symptoms of lumpy greases near breather and surroundings
6. Housing temperature below this bearing point near – oil bath area this bearing point – 165°C

The Challenge

I must return the oil to its original condition within an hour to prevent further damage of rotating parts while the machine is running.

Result Achieved

The gearbox temperature was reduced from 170 to 80°C within three hours without stopping the machine. That gearbox was running at the same temperature until the planned shutdown two weeks after the incident.

Emergency Procedures

Gearbox oil volume is 40 liters (L). Oil used is Castrol Opti BM 220 gear oil, a mineral-based (viscosity index = 97, pour point = -16°C) extreme pressure (EP) product.

• To resolve the dryness of the gearbox, I topped-up 13 L of the relevant gear oil Castrol Opti BM 220 and checked the temperature. Temperature reached 140°C immediately.

• Added 13 L of gear oil plus 20 L of detergent cleaner, for a total of 33 L. When adding this mixture, I drained nearly 13 L of oil from the gearbox (Note: not filled to the 40 L recommendation initially to avoid wear debris entering the bearings.)

• Gearbox was in operation mode. After 10 minutes with this mixture the temperature dropped initially from 140 to 120°C, then began to level off.

• With the gearbox still running, some of the original detergent/oil mixture was drained off and checked for shiny particles, and a new, prepared mixture (30 L of oil with 10 L of detergent) was added.

• This procedure was repeated with 35/5 and then 37.5/2.5 ratios of oil to detergent to clean and flush the gearbox.

• Temperature drops to 90°C.

• Use of forced air fan further reduces temperature from 90 to 80°C.

• Suggested to use an oil cooling system, and PM for changing the sight glass at every planned shutdown to eliminate similar situations in the future.

• Advising root cause failure analysis to identify the source for foaming issue and installing oil level alarm system are best available options to add more values.

Logic to Save the Gearbox

Data collected at the time of emergency situation (refer to section on conditions of the gearbox) is important in making quick decisions. This one prevented unplanned downtime and resulted in saving a downtime cost of $45,000. Following are important data to consider:

• The difference between the gearbox housing temperature (170°C) and oil bath temperature (165°C) expresses the cooling efficiency of the oil in the gearbox. In this case, oil is cooling only 5°C. It means the rate of the cooling of oil is not meeting the demand of heat generated by the gearbox in the current situation. A possible cause is that oil is not reaching the bearing point at a designed flow rate. If it is a splash-type gearbox, then oil is not splashed enough. The reasons for not splashing enough are low level and high viscous property of the oil.

• The sight glass did not indicate the proper level. To confirm the flow of oil, the drain plug was opened. No flow confirmed that the oil is not at a flowing level or gearbox is dry.

• Symptoms around breather confirmed the oil is oxidized due to a foaming issue, thus the gearbox got dry.

• Checking vibration tells you how much current conditions further damaged the bearings. In this case, there was no vibration technician on-site. Based on the experiences of the lubrication technician and the author, they determined that the bearings were not in very bad condition.

Why do we need to add oil immediately?

When opening the drain plug in this case, there was no oil flow from the drain. This means the oil leaked through seals or became oxidized. There were no symptoms of a major oil leak. There were, however, symptoms of oil oxidation. The source may be a foaming issue or false indication of oil level at sight glass. Whatever the source may be, oil must be immediately added to resolve the dryness.

Top-up oil immediately - will it make the condition worse?

Immediate top-up will not make the condition worse; however, it is important how much oil is added. If you add to the level of bearing, wear particles can enter into the bearing spots at current condition of the gearbox. For a splash-lubricated system, the oil level should be such that the gear teeth at the bottom of the gear are submerged in the oil.

How much oil should be added?

In this gearbox, the normal recommended oil volume is 40 L.

I assumed that wear particles had settled in the sludge. They also might have settled on the walls by sticking to greasy lumps. Based on this assumption, it is better to add 30 to 33 L. The volume of mixture to be topped-up in this condition is based upon a combination of factors of the gearbox:

• Estimated wear/damage during this condition

• Oil volume for the gearbox

• Construction of the gearbox

• Lowest bearing seal height in the gearbox

• Visual inspection of oil that leaked through seals or breathers

• If you feel the level is high, you can drain at anytime.

Bearing in mind the recommended volume, 40 L in this case, prepare the mixture of new oil and detergent cleaner. First mixture to clean the worst condition should have 13 L of new oil and 20 L of detergent. Second mixture: 30 L of new oil and 10 L of detergent. Third mixture: 35 L of new oil and 5 L of detergent. Fourth mixture: 37.5 L of new oil and 2.5 L of detergent.

Why must we use a detergent-cleaning chemical?

We added enough relevant oil; in this case, Castrol Optimol Detergent System Cleaner.

Detergent cleaner will break the lumps into micron-level and will make the new oil and lumps a similar blend size. This will prevent the bearing from seizing by cage failure. Bearing cage will collapse if any bigger lumps enter into it.

• Detergent cleaner will lower the new oil viscosity and will make the new oil as flushing oil in this condition.

• Good filtering and settling properties of cleaning detergent will cause the wear particles to settle.

What type of detergent cleaner should we use?

Refer to your oil supplier to find suitable detergent cleaner.

Any special procedures?

Yes. The logic behind the procedure is the goal of maintaining the lubricity, flow property and filtering effect of the newly formed mixture (new oil plus detergent). For an optimum result, I followed this procedure:

When the temperature decreases 10°C, start draining 50 percent of volume of the mixture and add 50 percent of new oil and run for a while. The running mixture now has 25 percent of cleaning detergent in the circulating gear oil. Run for a while, drain 50 percent, and add 50 percent new oil. The mixture will now have 12.5 percent of detergent. Run for a while, then drain another 50 percent and add 50 percent new oil. The mixture will now have 6.25 percent. At this stage, the temperature should decrease to near 65 to 110°C based on the condition of the gearbox.

What to do with the drained oil?

• Look for shiny wear particles.

• Use a magnet to collect the ferrous particles.

• Send the drained oil for oil analysis if you think the gearbox can run at this condition.

This data will tell you the current condition of the bearing, helping you to make decisions on what steps to take from here, and how long the gearbox can run.

The author recommends that the unit would benefit from a cooler. This was installed and the gearbox now operates at 60 to 70°C, down from the previous 80 to 110°C.

Editor's Note

Normally when a detergent cleaner is used, it is important to flush the equipment with the final lubricating oil to remove all traces of the cleaning solution. In this case, the maintenance staff was interested in trying to save this gearbox and operate in survival mode until the next scheduled maintenance period.