viscosity degradation
viscosity degradation
(OP)
Oil analysis results from a machine service shows the oil viscosity is ~50cSt. This has been consistent from a number of samples taken over a 12month period
The oil used is an ISO68 oil and a sample taken out of the drum prior to filling the machine reported viscosity at 61cSt. I'm told for compliance the viscosity of stocked oil is allowed to be ±10% so this is right on the lower limit. However another sample was taken ~1 week after a complete oil change and it was again down around 50cSt
What can cause this viscosity degradation? The duty on the machine is not extreme; average operating pressure is only around 60bar although there are some peaks up to 300bar, and average operating temperature is 40°C. From the data we have the oil temp has never been above 44°C
So far the oil manufacturer and pump supplier have not come up with anything useful so I would appreciate any ideas anyone has
The oil used is an ISO68 oil and a sample taken out of the drum prior to filling the machine reported viscosity at 61cSt. I'm told for compliance the viscosity of stocked oil is allowed to be ±10% so this is right on the lower limit. However another sample was taken ~1 week after a complete oil change and it was again down around 50cSt
What can cause this viscosity degradation? The duty on the machine is not extreme; average operating pressure is only around 60bar although there are some peaks up to 300bar, and average operating temperature is 40°C. From the data we have the oil temp has never been above 44°C
So far the oil manufacturer and pump supplier have not come up with anything useful so I would appreciate any ideas anyone has





RE: viscosity degradation
Your oil datasheet might give a value for "shear stability" and this is the clue. A super refined oil with a particular viscosity resulting from the chain molecules all being around the same size will be less susceptible to shear thinning than an oil with a wider range of molecule sizes (for which the viscosity is determined only by the average molecule size).
High viscosity index oils (HV or HVLP types) have a viscosity index improver additive which is also sensitive to shear thinning. The major oil manufacturers have put a lot of effort into making their additives shear stable, if yours is a budget range oil then you might have the old style compounds.
If the resultant [shear thinned] viscosity is too low for you then you need to start out with a higher grade and let it thin down to the required value in service. If all you wanted was an explanation then you're good to go.
DOL
RE: viscosity degradation