Viscosity Question 2

[TGHNylon]

I have a gearbox at my plant which is running hot.
I cleaned the heat exchanger on the oil cooler and have good flow on the water and on the oil sides.
I noticed the oil in the box is viscosity grade 220 and then consulted the OEM literature which recommends VG 320.

My questions is: Would using a lower viscosity oil than recommended cause the box to run hotter?

Thanks for your help.

 

[unclesyd]

A short answer, YES.

 

[electricpete]

My simple way of thinking is that increasing viscosity generally increases friction heat generation, decreasing viscosity decreases friction heat generation but can increase wear.

Is that wrong?





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[unclesyd]

Usually a gearbox manufacturer will call out a lubricant that will lubricate the gears at the design conditions. They are several methods for doing this for gears, use the specified viscosity with the resultant ability to be transported by the the gears, not slung off. Another is to use a lighter gear oil with an additive package that contains additives to increase its ability to track the gears.

electricpete,
Your statement is essentially correct when using a higher speed gear box. The reason for my statement is for that type call out I assumed a large slow speed box that depends on the tackiness of the oil to track the gears, no auxiliary pumping.

 

[electricpete]

VG220 is still pretty thick. I have a hard time imagining that it would flow off during one revolution to the point there was not the tiny amount of residual oil necessary for a film. But that's just my thought, I've been wrong before.

You might check some other factors which can cause overheating.

A high oil level can cause overheating.
Overloading can cause overheating.
Misalignment or gear problems might cause overheating?

You might look at vibration data and lube oil samples to help diagnose the gearbox as well as comparing the operating conditions to the design conditions.

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[25362]

Although the main functions of a gear lubricant are wear prevention and friction reduction by providing a film between mating surfaces, in the case of enclosed gears the lube also has the function of carrying away the heat developed by friction.
This function requires greater thermal conductivities than supplied by conventional oils.

I was told the AGMA 250.2 standards describe the correct selection and provides a classification of gear lubricants.

 

[volpegrigia]

The temperature of the gearbox is a function of the energy lost within it. In order to remove heat, one has to increase the circulation (flow) and the size of the heat-sink used to remove the excess energy/heat. The sole change in viscosity should not be a great contributor unless the heat coefficient of the oil in question is radically different from the original oil. It has been preached for long time that a higher viscosity oil will improve the formation of elasto-hydrodynamic films hence the life of the contacts. This school of thought is somewhat being revised since it has been proven that higher initial viscosity tends to increase friction (any bearing catalogue will show the "churning" losses as a function of lubricant viscosity) and not reduce it. The viscosity of the medium selected should be what is necessary and not more otherwise the higher pumping losses will offset any benefits sinc ethe operating temperature will be higher and thus the viscosity will eventually drop to the point where it should have been. Please remember that the amount of lubricant needed to lubricate is a small fraction of what is used, with the majority of it used to remove heat and provide stability (long term) to oxidation.

 

[25362]

Beside all what has been said consider also the practicality of an external oil cooler, the oil circulated with a small gear pump.

 

[electricpete]

There may be some related info in these two links:

"What You Need to Know When Selecting Gear Oils"

http://www.machinerylubrication.com...hat You Need to Know When Selecting Gear Oils

"Lubrication Selection for Enclosed Gear Drives"

http://www.machinerylubrication.com...ubrication Selection for Enclosed Gear Drives

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[romke]

the temperature of the oil in a gearbox depends on load, ambient temperature, oil level and viscosity - whereas a lower viscosity usually gives a lower operating temperature. in your particular case a heat exchanger is mounted - suggesting that high temperatures are to be expected. can you tell us some more about the actual operating temperature and what the actual operating conditions are compared to the operating conditions the manufacturer of the gearbox had in mind when he made up his lubricant recommendations? or better still - what minimum viscosity the manufacturer assumes for trouble free operation? we then could advise to either use a thinner of more viscous oil in this particular case.