Originally Posted By: IndyIan
I was wondering why the increased VI seems to matter so much in hydraulic efficiency? If a low VI hydro fluid has the same viscosity at operating temperature, as a high VI fluid, how can they get the 15% increase in productivity using the high VI oil?
My tractor has a hydrostatic trans and I'm considering switching to a much higher VI tractor fluid to keep that hydrostat happy, but my current fluid with a VI of 141 has nearly the same viscosity at 100C as the high VI(208) fluid.
For starters, I've been reading Brendan Casey's hydraulics newsletters for over half a decade, and he's always on the money. As I've always maintained, higher VI means that the viscosity is always "less wrong" at temperatures other than the design point
Hydraulic oils are rated on their KV40, so the higher VI oils will have a higher KV100 than their straight friends. Engine oils (and gear and tractor lubes) are rated against KV100, and the higher VI oils therefore have lower KV40s
Order a hydraulic oil, you are speccing a KV40...order a tractor oil you are ordering a KV100...that's why your viscosities are nearly the same...the 208VI will be much thinner at 40C...or 0C
His point against engine oils (and defacto, tractor oils) is that you don't want an oil with a VI bumped up by polymers, as they can be problematic. I have had experience where a previous engineer used an HV32 hydraulic oil in a machine prone to shear. Viscosity dropped to 24 in weeks, meaning that the KV100 wasn't even up to the straight 32 after it sheared down.
Looking at a machine like the excavator mentioned, there's factors at play. A pump running through relief will heat oil. A pump running through a working cylinder will do work, and not create heat. An oil that changes viscosity to a lower degree is more predictable, consistent, and yes, efficient in those circumstances...closer to optimum.
But you want a naturally high VI...like Castrol Carelube HTG.
edit...just freaky, he went to the same high school as my Dad according to the interwebs.