Axle vents are something few people ever think about, but they play an important role in keeping the axle in operating condition. Axles generate a lot of heat running down the road. As the axle increases in temperature, the air pressure inside the axle likewise increases. To accommodate the increase in air pressure resulting from this heat, manufacturers add a vent tube to the top of the axle as means of releasing this pressurized air to the atmosphere. Without this vent, the pressurized air soon finds the easiest way out, typically a seal. This, of course, leads to an oil leak as axle lubricant follows the air out through the seal.
It is important to ensure that axle vents are clear and operating as they should. There is typically a cap on the vent that helps keep water from entering the vent. This part needs to be checked to make sure water and other contaminants stay out of the axle.
In times of rain and flooding something interesting may happen that makes proper vent operation even more important. When an axle at full operating temperature is suddenly submerged in water, such as in crossing an overflowing river or a flooded area, the axle temperature is quickly reduced by the cold water. When this happens, the air pressure is likewise quickly reduced, turning the vent into a potential straw that can quickly draw in quite a lot of water if the vent cap is not operating properly. When this happens, the oil level goes up and as the axle heats back up again, foamy oil may begin to blow out of the vent.
So, when inspecting axle fluid levels and axle leaks, don’t forget that little vent that keeps the axle breathing correctly.
This Tech Tip was contributed by Mitchell 1.