dear all

we use 57cst lube oils for the journal bearings of our vertical hydro generators. recent oil analysis show an increase in viscosity of 66cst. what could be the problem? should the oil be changed? what is the recommended lub oil for sleeve bearings with combined thrust bearings?
regards
Radhakrishnan

I doubt the increase in viscosity by 20% or so will cause a problem directly. Especially if you don't see any abnormal temperatures.

However it seems indication of a problem. Either contamination or severe aging of the oil which can lead to sludging and other problems.

Regarding aging, normally I would expect you would see changes in acid number before change in viscosity (did you?). Also can evaluate aging with RPVOT and perhaps some other tests. Talk to your lab about whether you might expect to see changes in additives based on elemental analysis or whether other tests such as FTIR may shed light. Similar tests (acid number, elemental analysis, FTIR) may shed light on contaminants.

thanks pete
the test results available are
KV(cSt) at 40 deg C:65.67
TAN(mgKOH/gm): 0.28
Forming Characteristics(Tendency/Stability): 30/NIL
Demulsibility54degC/82degC(min.):40-40-0(30')

what is the conversion between SSU and cSt? the manufatures recommends oil with KV b/w 260 and 325 SSU
regards
Radhakrishnan

cSt = 0.22 * SSU – 180/SSU
260-325 SSU corresponds to 56.5 - 70.9 cSt
Note your specification for SSU should also indicate a temperature.
If your SSU is specified at roughly 40C (or 100F = pretty close), then the conversion above is good.
If your SSU specification is at 212F, then it won't work (let me know in that case and I will look up the other conversion)

If your not doing a baseline on your new (unused) oil you really don't know where your in service oil is. A general rule of thumb is +-10% of new oil viscosity to trigger an investigation. Baseline your new oil for ISO cleanliness, viscosity, acid number and additive concentration, and RPVOT.