Effects of Increasing viscosity of oil

[rota2]

Hello all,
We have only just discovered that our operators are filling a 320 oil into a gear box of planetary type , instead of the 220 oil recommended, what will be the effects of this?, the thought here is that no real damage will occur, in fact it may be better, I have my reservations , please enlighten me on this issue.
Thanks

 

[DaiWei]

Increasing viscosity can lead to increased temperatures within the gearbox, which can cause increased oxidation of the lubricant and premature failure. If you look at the lubrication data that should have been delivered with the gearbox, most manufacturers will provide charts/tables indicating not only the "preferred viscosity" but other viscosity grades for different environments, speeds, etc.
I have increased lubricant viscosity due to higher than original design running speed to provide sufficient film thickness to the gears/bearings.

Ken Culverson

 

[romke]

the original recommendation was problably based on the fact that a ISO VG220 oil would give the necessary lubrication and protection to the gears at their foreseen operating temperature - which would be for general industrial applications be between somewhere 55 and 75 deg C. if your particular application had a higher operating temperature using a higher viscosity oil might be a good idea (better protection of gears and bearings), when it was already running on lower temperatures the increased viscosity might give a higher operating temperature due to excessive churning of the oil and a certain loss of efficiency of the gears.

most gearboxes can run quite well on "one ISO VG grade lower" then recommended, the effect on the lubricating film thickness is related to the square root of the viscosity at operating temperature. in other words: if you want a lubricating film twice as thick, you need four times the original viscosity. be aware also that + or - 10 deg C in operating temperature has a far more profound effect on viscosity then changing from one ISO VG grade to the next higher or lower has.

in your particular application ISO VG220 is already rather viscous, most planetary gears require a lubricant in the order of ISO VG 100 - ISO VG150 since usually planetary gears, due to their design are not that heavy loaded, so ISO VG320 might be overdoing it quite a bit, unless when the gears run very slowly.