grab lube from the sump and distribute it to the pinion bearings and coat the entire inside of the axle center section. This oil is also flung off the ring gear and differential toward the axle tubes. Since the ring gear is not centered in the vehicle (it is actually offset to the left), it tends to send more oil to the left-side axle tube. This oil then makes its way to the left wheel bearing area. Typically, the oil distribution to the left-side axle tube happens at ring gear speeds below 500 rpm or about 35 mph. The right-side axle tube and bearing do not get that much oil flow until the ring speeds exceed 750 rpm or about 50 mph. These bearings just need a small film of gear oil to survive when compared to the heavily loaded tapered roller bearings on the pinion and differential carrier.
No, this is not a test of Cherry versus regular Coke. Notice how the unvented Cherry Coke bottle (left) has sucked in or collapsed on itself. The internal trapped volume of air has decreased and caused the bottle to contract as your axle housing would, if it had not been vented. If the bottle were vented to the atmosphere, this contraction would not occur.
Venting
All gearboxes in your car must be vented to the atmosphere. This includes transmissions, transfer cases, and axles. The primary function of the vent system is to make certain that the axle housing is never exposed to vacuum or pressurization. You may be wondering, how can an axle become pressurized or draw a vacuum? Let’s try a little experiment.
The illustration shows that taking a room-temperature bottle and cooling it to about 40 degrees F causes it to contract a visible amount. This illustrates that air contracts when the temperature goes down. You already know air is denser when it is cooler. This is why cars run lower elapsed times in cooler nights compared to hotter day-time air. Now back to the axle situation: Imagine that you have been driving for some time and the internal temperature of the axle is 200 degrees F. Now if you drive through a puddle of water, the water in the puddle cools the axle significantly. If the axle were not allowed to draw in air, then the entire housing would be under a vacuum like the Cherry Coke bottle. The axle seals would have to resist this vacuum and not draw in outside air.
What if the seals drew in the outside air because the axle was not vented properly? As long as it was relatively clean air, then there is no problem. The problem arises when the axle draws in water from the puddle we just drove through or from rain that is falling. The same scenario is played out when you drive through water high enough to reach the door sills. This amount of water can submerge the axle vent fitting. If the water level is above the fitting, you are guaranteed to draw water into the axle.
Typical vent fittings (white) are actually pressed into the main axle center on the top of the axle housing. Also, notice the rare positraction lube fill label, which is usually long gone on older Camaros like this one.
There is also the other extreme. Imagine that your axle is at room temperature as you leave home. The axle temperature is steadily increasing as you drive, and the air inside is expanding. If the seals are not allowed to equalize with the atmospheric pressure, the expanding air will pressurize the seals. A number of potentially harmful events can occur when the axle is pressurized. The first is an obvious phenomenon—the pressurized air and oil can be forced past the seals. The second is that the increased pressure on the seals can actually cause the seals to apply more pressure on the sealing surfaces, and prematurely wear the seals and the mating surfaces. Improper or clogged venting can cause all of this.
These typical axle vents are known as jiggle-style caps because they have little metal caps that are crimped on the end of the fitting. This cap jiggles when you tap on it.
This close-up view of a jiggle-style vent illustrates a cap that has been installed on the rear cover of an axle. Notice that it is on the top of the axle, well above the sump level.
The inside of the same cover shows the little hole down in the packet, which feeds the vent fitting. There is also a little maze for the air to follow before it can get to the vent.
Quite a bit of work is required to find the correct lube level for an axle, as well as the correct placement of the vent port. If too much lube is in the axle, a couple of things can happen. Usually, the axle runs abnormally hot and lube can be pumped out of the vent. It is also important to place the vent in such a location that it is not exposed to direct splash. This can be quite difficult in gearboxes because the rotating components in the axle churn and sling the oil throughout the axle housing.
In order to help keep the oil from being pumped out of the vent fitting, the axle designers build in an indirect path to the vent. This forces the air to travel through small bypass passages to get to the vent. These tight bends also force the heavier oil to fall out of the airstream and eventually drain back to the sump. Again, make certain that the axle oil is not pumped out of the vent fitting.
Housing Reaction Loads
The axle housing needs to react to and, at times, resist many different load conditions. Some of these load conditions are very intuitive, while others are not. I will cover semi-float and full-float in Chapter 2, but at this point, we must realize that the axle housing needs to resist the vehicle weight and suspension loads. The axle housing essentially bridges the wheels and the chassis of the vehicle. There are springs and shocks attached either directly or indirectly to the axle housing. Therefore, the housing must be strong enough to handle these loads.
The blue arrows represent the directional loads applied to the suspension mount locations from the components (shocks and suspension control arms). (Dana Holding Corporation/Joe Palazzolo)
This shows the brake-force reaction load (blue arrows) that the axle housing must resist. (Dana Holding Corporation/Joe Palazzolo)
The axle housing also resists the brake reaction forces from the calipers or wheel cylinders. As the wheels travel along and the brakes are applied, the reaction force from the brake hardware is fed back through the axle housing to resist motion. This stationary point of the brake hardware must be strong enough to handle the brake loads under all conditions. If the mounting brackets and axle tube area are not strong to handle these reaction loads, then the brake hardware can come off the mounting points.
An axle experiences side-to-side rotation forces during hard acceleration. The arrows illustrate these rotational forces. The blue arrow in the center shows the pinion torque and the green arrows are the loads that torque develops at the wheels. (Dana Holding Corporation/Joe Palazzolo)
The last set of loads that the axle housing experiences comes from the reaction of the
