How critical is ATF Viscosity?

[MolaKule]

I thought this question should be preserved for posterity.


Quote:
How critical is viscosity to ATF operation, and how would a fluid that is more viscous or less viscous affect operation and/or shift quality?


Quote:
One would like the fluid to have the viscosity of water, which is 0.66 cSt@40C.

But, we need a finite amount of viscosity for the reason given below.

But no lubricant design so far has achieved that viscosity, although ATF lubricants have been developed as low as approx. 4 cSt for newer transmissions.

In transmission design, the pump, channels and piping are designed to pump a certain volume and flow rate at various rpms and temperature swings in order to achieve certain pressures.

Now too high a viscosity can cause cavitaton (especially at low temperatures) and energy loss in certain hydraulic system designs. The energy loss is a result of having to push too high a viscosity fluid around the system, thus reducing efficiency and increasing fluid temperature.

Now, why do we need a low viscosity fluid for hydraulic systems like AT's?


Quote:
We need a finite amount of viscosity for the reason given below.

It turns out the lower the viscosity of a fluid the less tendency to have turbulent flow. Turbulent flow induces foaming and reduces heat transfer. So the lower the viscosity, the less tendency to foam, less power needed to push the fluid around, and better cooling of parts.

I dont think a viscosity variation of 2.5 cSt is going to cause oprational changes, all other things being equal.

 

[mechanicx]

I agree. Dex VI and Maxlife seems to work fine in previous DexIII application in transmission that aren't going out anyway. Also with it's higher sheer stability it's not too thin at all.

 

[friendly_jacek]

Originally Posted By: MolaKule
I thought this question should be preserved for posterity.

One would like the fluid to have the viscosity of water, which is 0.66 cSt@40C.


Are you saying ideal is under 1 cst @40C?
Please explain why and why we don't have such thin AFT in use?

 

[MolaKule]

You want laminar fluid flow for the most part.

The lower the viscosity the lower the amount of energy lost in pumping the fluid and the lower the turbulence as explained above.

You need a miminum viscosity to provide a lubricating film and prevent internal seal leakage.

Lubricating fluids with that low a viscosity have yet to be developed that will provide a sufficient lubricating film, although AT fluids as low as 4cSt have been developed for some of the newer transmissions.

 

[Shannow]

Molakule,
as Reynolds' number is density(x)velocity(x)dimension/viscosity, lower viscosity always pushes you away from laminar and into turbulent flow.

 

[friendly_jacek]

I see now that you are saying that low viscosity is ideal for hydraulic fluid, but not ideal for ATF, as there lubrication needs.

So, the real question is what is the absolute minimum viscusity in transmission to meet the lubrication needs.

 

[MolaKule]

Quote:
Molakule,
as Reynolds' number is density(x)velocity(x)dimension/viscosity, lower viscosity always pushes you away from laminar and into turbulent flow.


Of course it is and I stand corrected and mispoke.

The Reynolds number for pipe flow, which determines turbulent flow or laminar flow is inversely proportional to viscosity, or Re = VD/v, in which the Velocity term V = (parameterXDiameter^2)/(32XviscosityXLength of pipe).

It was late at night and I mistakenly was looking at the Head Loss equation which is directly proportional to viscosity.

In terms of AT hydraulic systems, the flow velocity is low, and pressures relatively high, so the Reynolds numbers should be in the turbulent regime. Pipes and channels in an ATF hydraulic system are short.

Low viscosity fluids DO lessen power losses and reduce the onset of cavitation.

 

[MolaKule]

Quote:
I see now that you are saying that low viscosity is ideal for hydraulic fluid, but not ideal for ATF, as there lubrication needs.

So, the real question is what is the absolute minimum viscusity in transmission to meet the lubrication needs.


AT fluid systems ARE hydraulic systems, just a special type of hydraulic system.

In heavy equipment hydraulic systems, higher viscosity fluid is used because of sealing constraints on the cylinders and valving, and because the piping lengths are longer.

Current reasearch I have seen indicates that a fluid with approx 2.5 cSt viscosity@100C would be the lowest viscosity one could use and still maintain a EHD film. Of course, additive technology and sealing technology would have to be advanced to accomodate this fluid.

As I mentioned before, some European transmssions have already been designed for 4.0 cSt fluids.

 

[MolaKule]

Quote:
In terms of AT hydraulic systems, the flow velocity is low, and pressures relatively high, so the Reynolds numbers should be in the turbulent regime. Pipes and channels in an ATF hydraulic system are short.



Should have read:

In terms of AT hydraulic systems, the flow velocity is low, and pressures relatively high, so the Reynolds numbers should be in the LAMINAR flow regime. Pipes and channels in an ATF hydraulic system are short.

 

[MolaKule]

Infineum ARTICLE on low viscosity ATFs

http://www.infineum.com/Documents/Insight/Insight 49/Insight_49_Transmission.pdf

 

[morris]

for YEARS i run a few TQs of rear end grease in my trans. flame me if you want but my trans will last longer than yours. NOTE: i will NOT read your flame. dont for get the early hydromatics used engine oil.

 

[Shannow]

How about you tell us what you have run (lubricant wise), in what transmissions ?

 

[morris]

727, 42re. any rearend grease reg at lest 80-90 w. i have only used 2 qts at a time. my next fill i plan on full rear end grease. a guy i know has a race car he runs tractor hydraulic oil, with no trouble

 

[MolaKule]

Quote:
727, 42re. any rearend grease reg at lest 80-90 w.


So what kind of transmission and vehicle, Morris, you're being very criptic for some reason. Is this a manual transmission?

The discussion is about Automatic Transmissions and viscosity. Are you saying you are running differential fluid in your AT?

What do you mean by rear-end grease? Most differentials use a hypoid differential lubricant, a fluid.

 

[Shannow]

Chrysler 727 torqueflite and Jeep 42 RE ?

A couple of quarts at a time in a fill of ATF ?

Hardly "cutting edge", or finely tuned 1950s tech...

"tractor fluid"...my fave (manual) transmission lube at the moment is Mobil Agri 424
http://www.mobil.com/USA-English/Lubes/PDS/GLXXENCVLMOMobilfluid_424.aspx

And it probably does fine in an older designed auto transmission, particularly at a couple of quarts, but it certainly isn't "rear end grease", and a few Saturday nights per month isn't long term no trouble.

 

[morris]

i never said that what i did was "cutting edge". it just works.

 

[Shannow]

I was talking about your transmissions...

the designs are "old faithful", and probably tolerant of quite a bit of "abuse"

 

[morris]

on my ranch there is ONLY ONE trans, chrysler. i know the newer ones arnt doing well. but thicker seems to do fine. been doing it for years. you use the right term, "hypoid differential lubricants," i just call them rear end grease. iam NOT telling you to do it, iam just saying it works for me. allso this has had info posted about, external trans filters. like auto zone 2210, magna-fine 3/8 tubing, i have one on my 42er. to soon to judge, but it cant hurt. an other on wix 5896 2/16 tubing. all so was posted here nicopp brake lines, it is easer to use then old style brake tubing. i havent used it yet, but sounds to be better than old type. i think advance auto has it.

 

[MolaKule]

Hi morris, I once had a mini-ranch near El Dorado.

So are you saying that you are really using something like a 75W90 hypoid differential fluid in an Automatic Transmission, your Chrysler 727 Torqueflite?

I had a neighbor who had a son who accidentally refilled his dad's 700R4 transmission in a Silverado with 75W90 gear lube. Son didn't listen very well and grabbed whatever was on the shelf to refill after his dad did a pan drop. Neighbor DID change AT oil every 4 engine oil changes or 20k miles. Dad was in a hurry and didn't check his son's work.

Neighbor hooked up his horse trailer to take some horses to the Butler County fair. He got halfway home after the fair and had to call me to hitch up my Burb and take his trailer on home, so horses could be watered and fed.

I came back and pulled him into town so his mechanic could drop it and open it up the next Monday morning.

Report: Oil pump toast, pins bearings toast, torque converter vanes pitted, actuator piston and bore pitted, valve assembly highly sludged with sticking valves. The only thing not destroyed miracously was the clutch plate materials.

I would have thought the sulfur in the GL would have completely dissolved the cellulose clutch face materials.

 

[morris]

sorry it took me so long to get back. well i dont like to bad mouth brands other than my own. i WILL bad mouth my brand more than anybody else. i think your friends trouble was to much load and not the 70w90. i THINK the type of transmission has somthing to do with the trouble. however i havent put that much load on my transmission. if he tryed to drive at 65 mph with that much load, well its JUST NOT A GOOD IDEA. if he didnt have a transmission cooler hes just asking for trouble. for sure i am risking a transmission this way. but iam NOT going to tell people to use my ideas. last summer i pulled a trailer with a very good load, in the trailer . AND the truck, but not over 40 mph, about 8-9 loads, no trouble. yep there are a lot of small ranches around hear, but iam a city boy. i wouldnt want to work that hard.