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 propshaft rotation against the wheels on the pavement through the axle gearing. As mentioned earlier, the propshaft is rotating in a counter-clockwise direction when viewed from the rear of the vehicle. This rotation and subsequent torque on the pinion is trying to rotate the axle housing. This rotation applies a downward force on the left wheel, and an upward force on the right wheel. So the load on the left wheel is increasing, and the load on the right wheel is decreasing. This is similar to the front-to-back weight transfer on the chassis during hard acceleration.
During hard acceleration, there is also a weight transfer from side-to-side across the axle based on the propshaft direction of rotation and torque reaction. We refer to this as tire jacking. Since our reference point is the ground and the wheels are attached to the ground, it may seem difficult to see what is happening. It is actually quite easy to see what is occurring, but the explanation seems backward. We see the right side of the vehicle squat down during hard acceleration. The right wheel is being pushed upward toward the vehicle body. However, since the wheels do not leave the ground and the ground is our frame of reference, it appears that the right corner of the car is lowering. If we were to put scales underneath the wheels during such an event (this is not recommended as you will probably ruin your scales), we would see that the load on the right wheel is decreasing, and the load on the left is increasing. Drag racers know this quite well, and try to combat the phenomenon by installing an adjustable air spring to apply a preload force to the lighter loaded tire on the right side of the vehicle.
These tire-jacking load conditions explain why when you take a right-hand turn it is so easy to spin the inside-right-side wheel under acceleration. During a turn, you are forcing the wheels to travel at different speeds, and under acceleration the right wheel is actually unloaded. The right tire has less contact force with the ground and the dynamics of the turn event are forcing the tire to travel slower than the left tire, so it is easy to make it slip relative to the ground. Now, try the same maneuver, but this time in a left-hand turn. The inside (left) wheel will not spin like the right did in the previous situation. As you apply more throttle to try and get the left wheel to spin, you are increasing the tire jacking load on the left wheel, and further decreasing its ability to slip. With enough power applied you end up spinning both wheels and the vehicle fishtails. Of course, I do not recommend this on public roads.
Reaction Loads on the Axle Housing
Now let’s talk about the reaction loads that the hypoid gear set applies to the entire axle housing. Chapter 6 discusses the gear reaction forces internal to the housing itself, but let’s now concentrate on the complete axle housing assembly. Imagine that the vehicle is at rest. The wheels are not rotating and the ring gear is stationary. As Sir Isaac Newton’s First Law of Motion states, an object at rest tends to stay at rest, and an object in motion tends to stay in motion at the same speed and in the same direction, unless acted upon by an unbalanced force. In other words, objects keep doing what they are doing. If they are stationary, they want to remain stationary. If they are moving, they want to stay moving (neglecting friction, of course). This sounds so simple.
Now, let’s apply it to the rear axle. The ring gear is stationary when the vehicle is not moving, and it wants to stay at rest. When we apply a torque to the pinion, it is trying to rotate the ring gear. The ring gear is attached to the wheels through the differential and axle shafts, all of which want to remain sitting still. So the pinion ends up applying a rotational load to the axle housing which mimics the pinion trying to point upward in the vehicle. The pinion is said to be climbing the ring gear.