Help me understand how rear axle seals work.

[Great Lakes Greg]

Seems easy enough, right? Let me explain my confusion. I am working on a car with a bad leak on one side. Streaked wheel, soaked brakes, a real mess. I have heard over and over again, that the seals don't do much, because a spinning axle shaft flings the lube off before it makes it to the end of the bell. It then naturally drains back down the taper to the banjo housing. The only time the seal really works is when a car is parked at an angle and the lube has a way to run to the end of the axle. This is clearly not the case with the car I am working on. So, where is the flaw with the slinging lube theory?

[Tom Wesenberg]

Are you sure it's gear oil, and not old wheel bearing grease?
The original seals were leather and faced the direction to keep wheel bearing grease from entering the differential.
Modern seals should do a good job keeping the gear oil inside the tube, and the grease outside the tube.

[Mitch]

I never heard that axle shaft slinging theory. The axle housings are designed to drain the diff oil back to the center. Depending on what you find make sure that the bearing is riding concentric on the hub/ race. The races are known to flat spot on the bottom side, which can cause the wheel bearing grease to get past the seal. A worn hub can also cause a braking issue

[Larrys40]

The theory is that when using thecaxle housing fitting that the grease seal creates a block and resistance to force the grease through and around the axle to the hub and into the bearing. The reality is that it takes a lot of grease to do that. You are better off greasing hub bearings and inspecting them as well as the hub as a service operation .

If you have grease in the brakes you have a bad gun seal and way too much grease for past. Clean, replace/repair as needed.
larry

[Great Lakes Greg]

It is definitely smelly gear lube, and a lot of it. The grease fittings have not been used in the ten plus years I have known of the car, and the current condition just started after thousands of touring miles. I had not thought of the flat spot on the race as contributing to beating up the seal. I'll check that tonight.

[tbirdtbird]

There are two different seals involved, outboard one that you replace in the hub, and the inboard one can only be done with the trumpet off.

I have never found it necessary to grease the outer bearing very often at all, there is no place for the grease to disappear to, except into the brake cavity area and gum up your brakes.

Because of this statement
"It is definitely smelly gear lube, and a lot of it,"
I suspect your inboard seals are shot, they are there to contain the gear oil. The hub seal is there to contain bearing grease. Two different jobs. Look at a parts blow-up from one of the suppliers.

As far as the axle shaft slinging theory, I would add that to the category of the "made it up as I went along" theories.

Slinging or not, if there is a bad seal somewhere, oil or grease will most definitely find its way past it.

I will add that the axle area where the inboard seal rides is often worn down because the original leather had a lot of friction and wore down the shaft. You might be able to get a Redi-Sleeve or similar on there if this is the case

[Mitch]

Just to throw this out there
Make sure the diif is level with the fill hole and not over. I had a rear that got water in it. This not only caused it to be milky but it also leaked out the axle end..

[Larrys40]

Typically gear lube doesn’t get that far unless there’s a scenario like Mitch said or the car is parked in an angle to drain toward one end, etc.
using lube that is too thin also causes it to run quicker than it should as well.

As Mitch said I’d check the level and do what’s necessary to clean it up for service and see what happens with fresh 600w lube.
There are drain holes inside the axle housings so the lube will return to the banjo housing should it get to the axle housing .
hopefully the holes are clear.
i can’t say I’ve ever seen any fully clogged .
larry shepard