Gear/Axle ENGINEERING for 90 vs 140?

[pgtr]

Yes I'm revisiting a topic several threads have touched on. I've seen a number of threads dealing with these two popular weights and why each is used and have watched them with interest.

First let's remove ambient temperature from the discussion and presume a fairly moderate nominal year round temp to simplify the discussion as a focus of the axle itself.

Here are some points I've seen entioned here as technical reasons for why a ...140 is selected or a ...90 is selected (in no particular order - these are not MY reasons - just a collection):

1) Shock loading (heavy pulling, hot rodding, etc...)

2) Size of ring gear
greater_than 9" == ...140
less_than_or_equal 9" == ...90

3) 'Built'* for ...90 or ...140

4) Sound quieting w/ ...140

5) Tolerances in axle design

6) Other...?

I figure it is some weighted combination of these but am yet to see what I would consider a conclusive and definitive rationale for a ...90 or a ...140 to be used in a given axle.

*Built... I mention this because while this doesn't really tell us anything - it gets mentioned a lot and it begs the question 'how' is an axle 'built' so as to require one or the other. For that matter, do engineers pre-select the weight of lubricant and then engineer their axle design around it as a design constraint? Or do they determine a weight of oil after the fact that best works with the axle as designed? What comes first, the chicken or the egg?

 

[crashz]

Ford went to 75w-140 in their light duty trucks back in 97 when the new style F150's came out. It was carried forward to the F250s and F350s and corresponding vans as well. Going with a synthetic gear oil allowed them to have a "lifetime fill" in the great push to reduce maintenance which seemed like a popular thing at the time. I beleive they had some durability issues with the 8.8 as well, and in an attempt to reconcile, the syn fluid was spec'ed. I beleive you hit a couple of good points though. The heavier fluid would quiet down the gear noise and would aid in protection against shock loading.

But keep in mind that the axles that Ford uses today are the same units that theu used 15 years ago. Back when 75-90 dino was spec'ed. I've got M1 75w-90 in my 2001 F150 and while I haven't seen and gains from it, it hasn't shown signs of failing either.

[ December 10, 2003, 10:39 AM: Message edited by: crashz ]

 

[Kestas]

I believe the higher weight is specified for towing (or racing) applications. I've seen controlled test data where towing significantly increases the temperature of the axle. The high multiweight component of the oil will protect against thinning from high temperature. Otherwise the lower weights should be sufficient for daily driving.

 

[MolaKule]

In addition to ring gear diameter, tooth design, angle of ground, etc, I think much of the design and spec of fluids has to do with these four major items:

1. Expected temperature range - Will high temps thin out 75W90 too much, will 75W140 cause too much fluid drag?
2. Pinion drive and wheel Bearing diameter - Larger bearings can handle thicker fluids, more torque.
3. Total Torque transmitted - Average and peak twisting forces. Angle of drive pinion to to ring gear, and type such as Limited slip, standard diffy, locker.
4. Fluid for life or short-term changes?

Rule of Thumb:
Start with a blended or synthetic 75W90 GL5 gear lube and work from there. If noisy, go up to 75W140; if not noisy and mileage suffers, drop down to 75W90.

[ December 15, 2003, 03:18 PM: Message edited by: MolaKule ]