viscosity degradation

[staley1770]

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

 

[Oldhydroman]

I'm surprised neither your oil supplier nor your pump supplier had the answer here. It's a fairly well known process called mechanodegradation. The high shear rate that the hydraulic system imposes on the oil causes some of the heaviest [largest/longest chain] hydrocarbon molecules to be broken into smaller molecules. This causes a permanent loss of viscosity.

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

 

[IceStationZebra]

I've heard it called shear-back, but same thing. I would think that this is more prevelant with conventional oils that have more VI improvers than high end conventionals, or even full synthetics, which don't need as much help. Another thing to look for would be contamination or high oxidation. ISZ