to select the best driveline equipment for your vehicle and application as well as be able to install it, set it up, and maintain it.
Let’s cover some of the fundamental items that pertain to just about all axle and differential repair work. If you are unfamiliar with what you are working on, it is alright to take pictures, to mark parts as they are taken off, and to mark the orientation of one part to another to make certain that they are put back together correctly, especially if there is going to be considerable time before you reassemble the parts. This is just good common sense to make the job easier. Also, it typically takes one and a half times as long to re-assemble than to disassemble. So, if it takes two hours to dismantle, it will typically take three hours to put back together. Be patient and take your time.
The driveshaft connects the output of the transmission to the rear axle. For most vehicles, the driveshaft is a single piece with a universal joint at each end. The universal joints allow the shaft connections to articulate at an angle. If you are standing at the back of the vehicle looking at the rear bumper, the driveshaft rotation is counter-clockwise. This may sound like a trivial detail, but this is an important aspect of driveline operation that I will discuss later in this chapter.
The axle housing assembly also interfaces with other vehicle systems, mainly the rear suspension and brakes. The rear suspension components, such as control arms, springs, and shocks, typically are attached to the axle housing. The rear brake components, including the calipers, parking brake cables, and hydraulic lines, also attach to the axle housing. Keep these other systems in mind as you work on, and make changes to, the axle so you make certain to not compromise their integrity.
This is a typical driveshaft. The transmission slip yoke is at the right side of the picture while the axle connection is at the left. This is the link between the transmission and the differential. The driveshaft delivers the engine’s torque to the rear axle.
The axle attachment connection is at the rear portion of the driveshaft. The axle has a single input from the driveshaft and two outputs, one to each wheel. The device that splits this torque is called a differential. A typical open differential normally balances the torque evenly between the two rear wheels. This balancing act can get disturbed when one tire is on ice and one is on concrete. The differential is trying to delicately balance torque, but if the torque is drastically reduced on one side, the balance is thrown off. To counteract this, the differential reduces the torque to the tire with good traction. This is the inherent nature of an open differential. I will discuss this at length in Chapter 5.
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
