article: hydraulic fluid instead of engine oil

[1 FMF]

posting this since it may be useful for those folks having hydrostatic transmissions or other hydraulic equipment where the discussion/question is often around engine oil.


"Two Sound Reasons to Use a Multigrade Hydraulic Fluid"
- Why Hydraulic Fluid is Different from Other Lubricants
- The Benefits of Multigrade Fluid
- Ideal hydraulic fluid = 25 cSt
- The Problem with Multigrade Engine Oil

http://hydraulicspneumatics.com/hydrauli...ic-fluid?page=1

 

[RF Overlord]

I don't own or typically use anything that requires hydraulic fluid specifically, but I found that article interesting...thanks.

I had always assumed the opposite of what the article states; I thought hydraulic fluid was a simpler, less stressful application that didn't require fancy formulations and that engine oil would be overkill in a hydraulic application.

 

[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.

 

[1 FMF]

VI = viscosity index
higher VI means less viscosity change due to temperature,
the fluid is rated at 40°C (104°F) and at 100°C.
but during cold start or cold weather operating the fluid is colder than 40°C, and a fluid of lower VI is much thicker at colder temps which is bad. so the VI is most relevant at temperatures below 40°C where equipment can often operate for quite a while, and that's when a lower VI fluid is thicker and has greater impact on efficiency.

 

[IndyIan]

The cold running issue makes sense I guess, my tractor has a hydraulic oil cooler the same size as the radiator and placed in front of the rad, so I'd assume its got to be controlled by a thermostat, and that high temperatures need to be controlled.
Probably I should still switch to the high VI synthetic tractor fluid anyways as it takes a long time to warm up the tractor fluid in the winter to something reasonable before I start using it.

 

[Shannow]

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.