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Brainstorming a better differential vent

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Old 02-15-2017, 08:28 PM   #1
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Default Brainstorming a better differential vent

Just thinking how the differential vent could be improved. Functions that I can think of:

Prevent vacuum in the differential
Prevent excessive high pressure, while maybe maintaining a slight positive
Keep dirt and debris out
Keep water out, even when completely submerged

Here's what I can think of. Sorry for the Paint drawing, it's all I have available here.




The differential would be connected to the left side of the... box. That system would be isolated from the right side, which is open to atmosphere.
The two sides are equalized at or close to the lowest expected temperature. So long as the differential is hotter than this, there would be slight positive pressure inside. Not enough to damage any seals, but enough to help counter increased pressure when submersed.
The vent to atmosphere would have a simple filter at the end - something that doesn't mind getting wet. If the entire assembly is submersed, water will flow into the right chamber of the box, but no further. This would also increase pressure on the outside of the membrane, further pressurizing the differential while it is submersed.

I don't know much about mechanisms like this. Is it feasible to construct something along these lines that is cheap, simple, durable, and relatively compact? Size would be determined by the air volume inside the differential when it is properly filled with oil, combined with an expected temperature range. Rough estimations put the capacity of the box at 1-2 liters.

One final consideration would be changes in atmospheric pressure due to elevation. Haven't figured out a hassle-free solution to that yet.

Thanks for the thoughts. I know this is overthinking, unnecessary, etc., but I like experimenting with things.
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Old 02-16-2017, 05:50 AM   #2
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Most of the time, water gets into the diff thru the axle shaft seals, not thru the breather. This is just a normal maintenance item that everybody just doesn't do. Jeeps have had a factory directive since they were first made that says anytime you go thru water above the axle centerline, you should check the diffs for water. When this was published, there weren't any jeeps with automatic transmissions, but they never mentioned the t/case either.

The Army had a few weird specs they wanted from *****'s when they accepted the contract for the jeep. The one that made me wonder why was the oil drain plug threads on the 4 cylinder had to be the same as a spark plug! Their reasoning was if a drain plug got lost or damaged, a spark plug would work. When I worked for Valvoline, I did the monthly stock inventory and we had 47 of those plugs and never used one! I had to count those loose plugs every month! AFAIK, they are still in the store!
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Old 02-16-2017, 12:56 PM   #3
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Hm ok.. I see 2 factors for why water would get in through the seals, assuming the seal isn't completely shot.

First, simply the pressure exerted by the water. If the tube which vents to atmosphere were at the same level as the differential, it would be able to equalize the internal pressure with the external. So, I would think, no water would be getting past a seal that was airtight without a pressure differential (pun slightly intended).

Second would be that a hot differential going into cold water will very quickly cool the air inside. If the vent were partially or completely blocked to returning air, there would be a partial vacuum inside. Since this thing would be self contained, that wouldn't be an issues.

On that last note... Since the air inside the system is always the same, that would mean no trace amounts of water would enter just from normal humidity. Lets say you drive him on a warm, humid day. With just a simple vent tube, as the differential goes from hot to air temperature the air inside contracts, so it draws in some of that still warm and humid air. When night comes, the differential and the air inside cool, and you get condensation that could mix with the oil.

On the other hand, depending on how the water interacts with the oil, I assume that over time much of this water could potentially evaporate and escape. If that is true though, lighter components of the oil could do the same, potentially changing the composition and viscosity.


That's interesting about the oil drain/spark plugs. I've always wondered why that wasn't the case with more areas, like fastener sizes. I usually work on older vehicles, so maybe there has been some loss/close enough replacement going on. But shock absorbers on my ZJ for example. Upper nuts are 14mm. Lower are 1/2". Why different measuring systems, and why not have them be the same size? My best guesses are cost saving and different manufacturers for different components.
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Old 02-16-2017, 08:08 PM   #4
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Yep that mix or metric and standard nuts and bolts aggravates the heck out of me, and since I use mine offroad I got to carry two full sets of tools in case of a breakdown. Not one of jeeps better ideas.
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Old 02-16-2017, 08:31 PM   #5
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Here's one item that fits this scenario. When people started building internal combustion engines in this country, the only manufacturer of spark plugs was in Europe, so the threads were 14mm. They still are, although the wrench sizes are 5/8 or 13/16 (16 or 21mm). I've only seen 1"X8 spark plugs on American made heavy equipment like a Waukesha flathead snow plow.

Last edited by dave1123; 02-16-2017 at 08:43 PM.
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Old 02-16-2017, 09:03 PM   #6
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this might be the solution!!!!!!!!

http://www.4wheelparts.com/Drivetrai...GI140207-1-KIT
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Old 02-16-2017, 09:46 PM   #7
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That's actually what got me thinking about this. Have a few concerns though.

Take my rear axle, a Dana 44 for example. It has an oil capacity of 1.9 liters. The fill level is a little less than halfway up the cover, so lets say 2.5 liters of air.

Now, for temperature ranges... lets say 250-400 Kelvin, which is about -10-260F.

If I remember right, for a quantity of gas, the ratio of volume to absolute temperature always stay the same. So 2.5/250=x/400. So, x, the volume of 2.5 liters of -10F air heated to 260F, is 4 liters.

That means we would need the bellows to hold 1.5 liters. That one looks quite a bit smaller. If you had a greater amount of air in the differential, or a greater temperature range, you would need more.

Now, the idea itself is appealing. It definitely is much simpler and less likely to fail. My biggest concern would be in how much of a vacuum is created when the thing fully contracts.

Finding a material that could expand and contract relatively freely while still being quite durable seems much simpler than the idea I was going on about though haha. I will look into that.
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Old 02-17-2017, 02:39 AM   #8
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This is a xj but you get the idea http://jeep-xj.info/HowtoBreatherExtensions.htm
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Old 02-17-2017, 02:47 AM   #9
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Nice! That's a much better version of what I have going on right now. Only real improvement that the bellows would provide over that is pressure equalization while the differential is submerged so water is less likely to push past the seals.
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