I posted something similar in the Suspension/Differentials forum but since the Dana-28 is unique to the Aerostar I thought this might be a better forum.

Yesterday, while chasing down a running gear whine, I noticed the front differential was hot to the touch - to the point of not being able to touch the area around the pinion bearing for more than 5 seconds. I then checked the rear and found it was similarly hot but slightly less than the front. Full fluids in both. Ambient temp was in the 80's and this was following an unloaded, easy highway drive of 25 miles.

Out of curiosity, does anyone know what normal differential temperature operating ranges would be?

 

Under normal operating conditions, you can expect them to be anywhere from 100-150 degrees F. Obviously, that's hotter then you'd want to touch. I have observed axle temperatures on RWD cars in the 250-300 degree temperatures while testing the differential at highway speeds with a minispare tire, so there can be a bit of a range depending on conditions. The max it should be allowed to get is around 300, because above that the additives in the axle oil tend to break down.

The availability of cooling air and the sump volume impact this as well. The front axle is right behind the air dam lip and bumper cover, and it's tuck up into the suspension fairly well, so these things conspire to reduce air flow over the axle. The rear, on the other hand, has pretty good air flow over it because it hangs a fair ways down from the body. It also has full width axle tubes, which act as an oil cooler to an extent. The longer tubes also allow a greater volume of oil then the front axle has. More oil allows greater transfer of heat energy from the differential and gearing before raising the average temperature as much. This is why the rear runs cooler then the front.

For the record, we typically maintain an oil temperature of 55 degrees C during bench dyno tests. That translates into 131 degrees F, so that should give you a ball park idea...

 

Bravo! Just exactly what I wanted to know and from the guy I hoped would chime in on this. Thanks Rick, your expertise is greatly appreciated on this forum.

For the record, I guess I am surprised differentials run this hot. Obviously there is a lot of friction going on despite the constant lube bath.

 

The vast majority of the friction isn't from the diffs - when driving straight down the road, they spin as one lump with nothing dynamic going on internally. Even in a turn, the gearing in the diff might rotate at, oh, 10 RPM. No - by and large the heat is generated by the ring and pinion gears. Their hypoid gear teeth have a mesh that is something like a 85% sliding contact, meaning that the mating teeth slide across each other rather then roll together like spur gears do. This sliding contact creates a lot of friction and pressure on the tooth itself (this is why axles usually require a hypoid gear oil for durability, which is why the AWD front's ATF spec is still shocking to me - but I won't go there now).

The steady state temp around 100 degrees F is generally all from friction in the r&p gears. The extra 100 degrees that is added with the minispare I mentioned before, however, is additional heat that is generated by a limited slip differential being forced to spin at 30 RPM or so for a long period of time...

 

This is fascinating reading.

It seems all the more remarkable that ring and pinion gears last as long as they do. I've never had to replace one which begs the question: Is there some rule of thumb as to R&P replacement guidelines?

Am I correct in understanding that hypoid gearing is used mostly because it is a quieter arrangement than spur cut gearing?

 

anyone ever burn out from overheating and lube failure, the front ring and pinion on an AWD Aero?
what about adding a moly additive to the Mercon which has litte or no moly? and a viscosity thickener?
anyone have long term experience running 75w90 in a Aero front diff?
I drain and change mine every 30k miles and put in Mobil 1 syn Mercon, 2 qts, cheap security. Every drain looks like thick worn out motor oil, dark and no Mercon visible characteristics left. No cuttings or fines in drain.
Transfer case drain always pretty Mercon red, smells and looks like new on white paper.

 

I put 75~90 in mine by mistake not knowing it required Mercon. it was in there for 6 months and never gave me a problem. It looked good when I changed it too mercon.

 

I maintain that the front axle will be fine with gear oil. I've asked the engineers I know at Ford Truck about it the best answer I got was that it allowed the AWD specific parts to use a common lube. Hypoid gears will run better in hypoid gear oil, and a more viscous oil (like 75W-90) will provide better lubrication and impact protection.

Aero, hypoid gears are a special case of helical cut gears. Helical gears are used (in transmissions especially) because they are quieter the spur gears, but also because the helical tooth allow a longer engagement between teeth which helps strength.

Hypoid differs from normal helical gears by changing the centerline of one of the gears so that it is no longer on the same plane as the mating gear. In the case of an axle gear set, the centerline of the pinion is below that of the ring. This allows an even longer engagement still, and effectively shifts differential and ring upward in relation to the driveshaft, thereby increasing the amount of ground clearance under the axle.

 

thanks Torsen,
I respect your technical knowledge, experience and education.
I'm replacing with Valvoline Synthetic 75w90 and will post results.

Rick-any recommendations on syn vs dino fat hypoid oil, 75w90 vs 75w140, and putting in any additives such as extra moly?

 

For the front axle, I'd run synthetic due to the more limited cooling air flow. 75 or 80W90 is fine, I don't think there's any need for anything as viscous as the 140.

In the rear axle, the type of oil doesn't really matter much. I guess typical folks would favor a synthetic, just because of how long the change interval is. However, if you wind up changing the oil frequently for various reasons, then mineral based oil is fine.

As far as additives go, well, I guess friction modifier should be used if the axle has an LSD in it. But if it's an open, then the modifiers are of no benefit. One of these days I'll be putting a Torsen in my van. I've got it, I just haven't had the chance yet. I'll probably just use the same 80W90 synthetic oil that we use in all our truck axles here at work...

 

another reason for the front diff being hotter is that it uses the same gears as the rear, turning backwards. this means that the front drive is constantly running on the "coast" profile of the gears, the wrong side of the gears. some makers do use correctly cut gears for the front diff but not Aerostar

ken

 

I put in Amsoil 75w90 Extreme Duty Hypoid since it was on sale. Mobil 1 Mercon ATF that was in less than 2k was dirty already.
Gas mileage the same and top speed, over 100 mph GPS, still there.

 

This reminds me of something I read a long time ago in regard to the Ford 9" diff. It seems to be the only pass car diff that has a snout bearing to support the pinion. But at the same time. the Chevy crowd would curse it for being inefficient, while choosing it for its superior strength over a Chevy 10 bolt. I always thought it was because of the position of the pinion relative to the ring that caused it to generate more friction in operation, but allowed it to be mounted with a snout support. But from what Torsen Rick is saying, all hypoid gear sets will have the pinion positioned below the ring. Are ther any major differences between the Ford 9" and others that do not have the snout support?

Also, if you think your diff makes a lot of heat, you should look at one of the recent Cobra-R that used a gearotor mechanism in the diff to lock the two sides together. They actually installed an outboard oil cooler for it.

One more thing: I think a Torsen diff can also generate a lot of heat from the worm gears wheels working against each other. Rick can verify this after he installs his.

 

A couple of things: yes, all hypoid gears have offset centerlines. That is what defines a hypoid gearset.

The thing that sets the 9" apart is that 3rd pinion bearing. The meshing between the ring and pinion gear generates a substantial amount of sideways load on the ring and the pinion gears from the pressure angle's creating seperation forces. In a coast direction, this tried to push the ring gear sideways toward the end of the axle (this force can actually break the flange off the diff in a 5-2 heavy downshift if there's enough torque). It also puts a substantial bending load on the pinion. The 3rd bearing on the 9" pinion is specifically intended to support that bending load, and this is what makes the 9" so strong. The draw back is that it limits the amount of space available for the differential.

Lastly, LSD's only contribute to heat generation minimally. They only generate heat when differentiation occurs. If you think about it, the vast majority of time the gears in the diff are static, most of the time you aren't turning significantly. When it comes down to it, a clutch diff, a Torsen, a Gerotor, etc all contribute similar amounts of heat because all need to generate similar amounts of friction to have similar slip-limiting abilities. FWIW, the Cobra R needed a cooler because the diff works more on a race track (a lot more turning). But what is more significant is that the Cobra R's rear end was an IRS. Without the long tubes of the live axle, the oil volume in the axle is less. Oil is a great heat sink. Also, the tubes have something of a cooler effect. So the IRS is a double disadvantage in terms of axle temps. It wasn't because it had a Gerotor diff.

Anyway, I'm going now...

 

Since 2004 June, I use Amsoil 75W-140 on both rear and front dif.'s, I also use Amsoil for transmission. I don't notice any impact but I understand my system will benefit from those Hi-tech lubrication. I use Mobil1 & Motorcraft filter, however the oil turn to dirty very fast (about 2k miles or so). I am thinking about flush again then switch to Armsoil motor oil next time.
Fordboy49