The automotive drivetrain may be the least understood part of a vehicle. After reading this book, you will have a better appreciation of the complexity, and simplicity, of the components that make up the rear axles, differentials, and driveshafts. With the information in this book, you will be able 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.
This Tech Tip is From the Full Book, HIGH-PERFORMANCE DIFFERENTIALS, AXLES, AND DRIVELINES. For a comprehensive guide on this entire subject you can visit this link:
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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 re-assemble 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 out-put of the transmission to the rear axle. For most vehicles, the drive-shaft 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 drive-shaft 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.
The axle attachment connection is at the rear portion of the drive-shaft. 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 prop shaft 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 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.
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.
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.
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.
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.
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.
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 semifloat 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. There fore, the housing must be strong enough to handle these loads.
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.
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 pin-ion 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.
This pinion to stationary ring gear reaction actually rotates the entire axle housing counterclock wise, as viewed from the right side of the vehicle. The result is inefficient transfer of torque. This is why traction bars and pinion snubber style devices came into existence, and were added to many vehicles. It was recognized that if this upward rotation tendency were not resisted, then the suspension would have a difficult time trying to counteract the event. The issue comes into play when this rotation of the axle is finally resisted, and the axle returns back to the correct state back to its original, near horizontal position. As spring releases this stored energy, the wheels may momentarily lift off the ground with some vehicles. This is more evident on pickup trucks but can happen on cars as well.
There are many other events that can cause wheel hop. Even non-beam style axle vehicles can exhibit wheel hop based on the suspension geometry and overall axle drive system stiffness. The worst thing that you can do during a wheel hop condition is to stay on the throttle. As the wheels lose traction and regain traction quickly, enormous spike loads travel through the drivetrain. These impact loads can cause drivetrain components to fail quickly.
Finding the Right Parts and Service
Now that I have covered the fun damentals that apply to all rear axles, let’s review how to find a good shop for parts and service.
Let’s face it; at some point, you will need to find a place to purchase parts. After reading this text, you may even decide that you do not want to tackle the work yourself. What are some important items to consider when deciding where to buy your parts? Price is very important, and most places are competitive. However, you may want to get some technical assistance, and that costs money too. Larger businesses commonly have free tech lines or email assistance. Some very reputable places may not have the best prices, but can help you make the best-informed decision. You definitely want to get your parts from a place that moves sufficient inventory to actually have the parts on hand. Some of the smaller shops cannot afford to have stock on hand, and you could be waiting quite some time to get your parts. Not to say that the smaller shops are bad, especially if you are working on a longer term project and have the time to wait a few weeks for the right parts. If it is your daily driver and weekend race machine, you probably want, and need, your parts quickly.
Now, with the relative ease of shipping and the Internet, you may want to buy complete parts, a complete axle, or even send yours to a reputable shop to get it rebuilt. Some places just sell parts and do not work on the axles themselves. You want to be careful with these places. They are great to get parts from, but be cautious of any advice that they give, as they do not have daily hands-on experience with the parts and vehicles. A lot of these words of caution pertain to any work that you are having performed.
You may decide that you do not have the time to work on your axle this time around, and just want a competent shop to handle it for you. Unfortunately, there have been, and still are, many automobile repair shops that are corrupt and incompetent. The more specialized the repair becomes, the more difficult it is to find a trustworthy, competent shop and mechanic. Most general repair facilities do not like working on axles. Axles are at times more of an art than a science, and some mechanics get frustrated trying to repair them correctly.
It is not always the mechanic’s fault. They are trying to fix an item that, at times, they have not been properly trained to repair. It is difficult to get good training on axles; shop manuals are not always good sources of information, and even your local dealership may not have an axle expert on hand to correct the problems. Realistically, axle problems are not that common, and therefore the mechanics cannot stay up to speed on all the latest items. There are good reputable shops in about every state, but you may need to drive a little further to get to them. We have found that the hassle of driving further to find a good axle repair facility far outweighs the hassle of dealing with a nearby, marginal shop.
At times, shops that do not work on axles often do not have some of those hard to find parts at their fingertips. Your axle may need a new adjuster nut for the differential case support and the shop does not have one available. If they are in a hurry to get the car back to you, they may improvise. Axle support takes up valuable shop space, but it’s well worth it when when it comes time to repair or replace some of the less common axle internal components.
There are even times when you need a good axle shop to work on your normal daily driver. At DTS, they even have good, lightly used parts that work great for your daily driver. Some of these parts have very low miles, while others are open-box customer returns. With options available, there is no need to explore around the local scrap yards in your quest for good used parts.
Here are some good questions to ask:
- How many axles do they work on per week?
- Do they have experience with your specific axle?
- Do they offer a warranty and stand behind their work? (Granted, some high performance applications cannot be warranted.)
- Where do they get their parts? Directly from the supplier or from a middleman?
- Do they have your parts in stock?
- Can they give you an estimate on cost, and how long it will take to repair?
- Can you see the shop area where the work will be performed?
I don’t mean to scare you or sound paranoid, but we have all had great and bad experiences getting repair work done on our vehicles. OK, enough of the disclaimers.
I have covered the fundamentals of any rear axle. You may not have been aware of what is in a traditional axle and some of the reasons behind the vehicle behavior. Chances are that you have experienced some of the above situations but never really wondered what caused them. The following chapters focus more on the specific details of the different type of axles, differentials, and drive shaft arrangements.
Written by Joe Palazzolo and Posted with Permission of CarTechBooks