The rear axle also translates the rotation of the propshaft 90 degrees in the vehicle. This allows the propshaft to move front-to-back while the axle shafts move side-to-side. This direction change is accomplished by a special gear arrangement known as a hypoid gear set.
The hypoid gear set handles the direction change required for an axle to function properly through a unique, varying, spiral-tooth geometry. Notice the axis of rotation of the smaller pinion gear is 90 degrees relative to the larger ring gear. (Randall Shafer)
The hypoid gear set also provides the necessary torque multiplication and speed reduction. The driveshaft is rotating faster than the axle shafts by the axle ratio factor. More of the specific details of how the hypoid operates are covered in Chapter 6.
Lube Flow
Proper flow of the gear oil is very important to ensure long life of the bearings and gears. Typical axles utilize a splash system, (pumps are generally not used to accomplish this). The lubricant for axles is very specific, to satisfy the requirements of the extreme load that the hypoid gear mesh is subjected to, as covered later in Chapter 6. The accompanying illustrations show how the oil is distributed throughout a typical axle.
Typically, when the vehicle is at rest, the oil level partially submerges the pinion bearing and the lower portion of the ring gear and differential are submerged.
This illustration of the lube sump shows the gears at rest. The differential has been omitted, so you can see that the oil level partially submerses the ring gear. (GKN Driveline)
The pinion has been omitted from this illustration, so you can clearly see the lube return port, which is located toward the pinion tail bearing at the front portion of the axle housing. The oval-shaped slot in the housing allows oil to drain back to the sump. (GKN Driveline)
The return port is just as important as the feed port to the pinion bearings. As mentioned above, the propshaft is spinning faster than the wheels. Therefore, the pinion is spinning faster than the ring gear. The pinion bearings, specifically the head bearing, are operating at the highest speed and load in the axle housing. If lubrication is insufficient, the pinion bearings are the first to suffer and prone to failure.
Tapered roller bearings are not very good at pumping oil. The pumping action is from the smaller diameter to the larger diameter. Therefore, when the oil arrives in between the pinion bearings, the pinion head bearing pumps it back to the sump while the tail bearing pumps oil toward the front of the axle and the pinion seal.
When the gears begin rotating, oil flow looks like this. The gold color shows the path of oil as it flows through the axle housing. The blue pinion and ring gear transfer torque and drive the rear axle. Notice that the ring gear picks up oil from the sump and directs it to a port, which fills the space between the pinion bearings. (GKN Driveline)
As in the earlier illustration, the pinion has been omitted for clarity. Now you can see that the bottom ledge of the return port at the front of the axle controls the oil level to the pinion tail bearing. (GKN Driveline)
If the return port was not adequately sized, omitted, or blocked, then the oil is pumped and trapped between the tail bearing and the pinion seal. The oil would stay there for the entire vehicle life. There are some production axles that have this poor flow characteristic, and the oil is not adequately flushed out of this area. The oil basically cooks itself in this area. Also, since the ring gear is taking oil from the sump and distributing it to the cavity between the pinion bearings, any debris that is in the oil usually gets deposited in this area between the bearings. It is very important during any rebuild or disassembly procedures to clean this area. Any debris that is in the axle usually ends up in this area. This is similar to the bottom of the bucket in hydraulic valve lifters for engines. When you disassemble used lifters, all of the debris in the engine oil ends up in the lifters. They act like little trash cans for the engine and so does the area between the pinion bearings on the rear axle.
The remaining components that need proper lubrication are the axle shaft bearings and seals. The ring gear and differential case 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