Winning the battle against axle noise and vibration

There are over 8 million trucks and SUVs produced each year in North America. If 50 percent of those vehicles are four-wheel-drive or all-wheel-drive, a total of 12 million axle assemblies are needed to supply those vehicles. Twelve million axles per year equal 32,876 per day, or 1,370 per hour, or 22.83 axles per minute. What are the chances that every axle is set up correctly? The chances are pretty low. I tell my students, “Just because it is new, does not mean it is set up correctly.”

Over the years, I have received many questions related to what has become known as “Chevy Shake” vibrations on later model Chevrolet, GMC, and Cadillac trucks and SUVs. Similar vibrations have occurred on the Ford Mustang the Ford F-150, and Ram 1500 and 2500 trucks. The frustrating thing for the customer is that nobody can diagnose and repair them, or even worse, they are told it is normal.

A tale of two trucks

I have been personally involved in diagnosing two vehicles that were purchased back from the customers for unresolved vibration problems:

1. A $90,000 2015 Cadillac Escalade 4x4 SUV.
2. A $55,000 2014 GMC Denali 1500 Series 4x4 Crew Cab short-bed pickup truck.

Both of these vehicles had the exact same cause of the vibration: A rear axle that was not set up correctly.

2015 Cadillac Escalade 4x4 SUV

In the spring of 2016, I was teaching a manual drivetrain class at the university where I work. As part of that class, we always go through the complete inspection, disassembly, repair and assembly process of a rear axle. We had three vehicles on the hoists and five axles on the workbenches. I love to work on vehicles with real problems, so I gathered my students around the 2015 Cadillac Escalade 4X4 on the hoist and told them about the vibration problem that resulted in the buyback of this vehicle. I told them we would carefully inspect the 9.75” rear axle as we disassembled it to see if the rear axle had any troubles that may have contributed to the vibration.

Following the same diagnostic procedure I have taught for years, one of the very first steps you should perform when diagnosing and disassembling a rear axle is to measure what is known as the “Total Turning Preload” of all the preloaded bearings that hold the ring and pinion gearset in place inside the axle housing. This measurement should be done at the pinion nut with the driveshaft, wheels, and tires, and brake rotors removed. Using a flexible beam type or dial-type inch-pound torque wrench, a technician should rotate the pinion nut and see what the constant rotational torque (effort) is.

Typically, the measurement is at least 1.7 - 2.8 Nm (15 - 25 in*lbf) on an axle that is set up correctly, and we measured zero! I could not believe it! The only time I had ever read anything close to 0 Nm (0 in*lbf) of rotational torque was on a 35-year-old severely worn axle with high mileage on it. This axle is in a 1-year-old Cadillac; how could this be?

2014 GMC Denali 1500 Series 4x4 Crew Cab short-bed pickup truck

Next, we moved to the 2014 GMC Denali 1500 Series 4X4 Crew Cab short-bed pickup truck and performed the same measurements. It also read 0 Nm (0 in*lbf) of rotational torque for the “Total Turning Preload” measurement. The truck had the same rear axle housing, except it had a 9.5” ring gear rather than the 9.75” ring gear in the Cadillac. Having two vehicles with zero bearing preload is unheard of; there must have been a problem with the axle setup, the lubricant, or something was wrong from the factory. My suspicions in 2016 were confirmed earlier this year (2019) when a friend of mine, who is also a GM Field Service Engineer, confirmed that “many of the axles out of an assembly plant in Mexico had problems.”

There is no excuse to have an improper setup axle on a new vehicle anymore. Axles have been mass-produced for over 100 years now. Sure, there have been improvements in materials and machining, but the same setup process has been in use all of that time. A properly set up axle is silent under all operating conditions and temperatures; it outlasts the lifespan of the rest of the vehicle.

Causes of axle vibrations and noises

Improper Bearing Preloading

Proper bearing preload on the ring and pinion gear set prevents the gears from moving vertically, horizontally, or diagonally as they rotate. Improper bearing preloading allows the gears to push away from each other as you accelerate your vehicle and pull together as your vehicle decelerates. Any gear movement can cause the gear backlash to become too small and have the gears bind with each other as they try to rotate. These conditions cause noises, vibrations and oil leaks:

• The ring gear rotates at tire speed and can mimic a tire speed-related vibration. 
• The pinion gear rotates at driveshaft speed and can mimic a driveshaft speed-related vibration.
• Improper pinion bearing preload can cause what appears to be a pinion seal leak, but the seal is fine, the pinion gear is moving vertically, horizontally, or diagonally as is rotates. No seal can hold on oil under those conditions.

Improper Gear Tooth Contact Pattern

A properly set up axle utilizes the entire length of each gear tooth to transfer all the torque from the engine/transmission to the axles/wheels. A gear tooth contact pattern check will reveal if the proper contact is being made. If improper tooth contact is made, only part of the gear tooth is transferring all the torque from the engine to the wheels. This can cause noises, uneven tooth loads, and contribute to broken gear teeth and catastrophic axle failure. Improper bearing preloads can contribute to these conditions:

• An axle noise that only occurs on acceleration is an indication of improper gear tooth contact on the drive side of the ring gear teeth.
• An axle noise that only occurs on deceleration is an indication of improper gear tooth contact on the coast side of the ring gear teeth.
• An axle noise that occurs on acceleration and deceleration is an indication of improper gear tooth contact on both the drive and coast sides of the ring gear teeth.

Diagnostic disassembly steps

Do you want to be able to quickly diagnose axle problems as we did with the two trucks listed above? You should never start unbolting parts and disassembling an axle with noises or vibrations. There are many clues you can find by carefully taking measurements and making observations during disassembly. I have my students use the following steps during axle disassembly in my classes:

1. Measure the Total Turning Preload (TTP)— This measurement should be done at the pinion nut with the driveshaft, wheels, and tires, and brake rotors removed. Using a flexible beam type or dial-type inch-pound torque wrench, rotate the pinion gear at the pinion nut to measure the Constant Rotational Torque (CRT) of the pinion bearings and the differential case side bearings combined. Note: Some axles use the “Break Away Torque (BAT)” rather than the CRT. Typically, the CRT measurement is at least 1.7 - 2.8 Nm (15 - 25 in*lbf) on an axle that is set up correctly. Anything less is an indication of a problem. IMPORTANT: while rotating the pinion gear at the pinion nut, watch and feel for any fluctuations in the rotating effort. Any fluctuations can indicate damaged bearings.
2. Inspect the oil/fluid— Utilizing a drain pan to capture and save the fluid, remove the differential cover and inspect the oil for a burnt odor, for contamination from water/rust/corrosion, for metal flakes (many differential covers have a magnet to attract loose metal flakes). Any problems with the oil/fluid can indicate the need for a complete overhaul of the axle assembly.

Ring gear type

Look at the ring gear to determine if it is a face-milled ring gear (with unequal gear tooth depth), or face-hobbed ring gear (with uniform gear tooth depth). Face milled ring gears are more prone to suffer from backlash variation problems. These two ring gear types also have different gear tooth contact patterns.

1. Measure the backlash variation– If the TTP from step one was within specification, measure and record the backlash of every gear tooth on the ring gear, (yes, all 32-48 gear teeth). Any difference between the lowest backlash measurement and the highest backlash measurement greater than 0.051mm (0.002”) indicates a problem. The problem could be with the ring gear itself, or with the differential case to which it is bolted. Further measurements of differential case lateral and radial runout are necessary. Any differential case runout higher than 0.051mm (0.002”) indicates the differential case must be replaced.
2. Measure the Pinion Bearing Preload (PBP)– Important: Mark all parts as left side or right side when removing them. Remove the axle shafts, differential case bearing caps, and the differential case. The differential case should be difficult to remove if it has the proper amount of side bearing preload. You may have to pry on the ring gear bolts to remove the differential case from the differential housing. Using an inch-pound torque wrench, rotate the pinion gear to measure the CRT of the pinion bearings alone. Typically, the CRT measurement is at least 1.1 - 1.7 Nm (10 - 15 in*lbf) on an axle that is set up correctly. Anything less is an indication of a problem.

After making all the measurements and inspections listed above, you will have a good idea of the parts required and how much work is needed to repair the axle. Always replace the bearings when the bearing preload is out of specifications. Notice: Replacement bearings can be thicker or thinner than the original bearings. Because of this, it is very rare that the thickness of the original pinion depth shim(s) and differential case side bearing shims will be correct to use with the new bearings.

Critical reassembly steps

Almost anyone in a wrinkly T-shirt can bolt the axle parts together and send the vehicle down the road; It takes a trained technician with a lot of patience and attention to detail to set up an axle correctly. Proper setup of an axle can take four to eight hours depending upon how lucky you might be.

Before reassembling an axle, clean everything thoroughly. Always use the correct thread locking compound and primer before installing bolts or nuts. Only use the correct axle gear lubricant during reassembly to lubricate bearings and gears.

Use the following steps to assemble properly and set up an axle:

1. Avoid bearing Brinelling– When assembling an axle assembly, do not hit bearings or parts with bearings on them with a hammer or impact wrench; bearing Brinelling (small indentations in the bearing surfaces) can occur. Bearing Brinelling causes fluctuations in the rotating effort of the bearing. Any fluctuations can indicate damaged bearings.
2. Set pinion depth– If you have access to the special tools needed to measure the shim(s) required for the pinion depth, use them. If you do not have access to the tools, use the old pinion depth shim(s), but be mentally prepared to disassemble the entire axle again to change the depth shim if the gear tooth contact indicates the shim thickness is incorrect. Changing the pinion depth shim typically requires replacing the pinion seal, front pinion bearing, and the crush sleeve (if equipped).
3. Set pinion bearing preload— Set the pinion bearing preload at the high end of the specifications. Over time the bearing preload will only loosen, so if you start at the high end of the specifications, it will last longer. There are two methods of setting bearing preload:
   1. Method one uses a crush sleeve and a non-reusable pinion nut. Tighten the nut until the proper PBP is obtained. If the PBP is incorrect, change the crush sleeve, front bearing, seal, and nut and measure the PBP again.
   2. Method two uses pinion preload shims and a non-reusable pinion nut. Tighten the pinion nut to a specific torque and then check the PBP. If the PBP is incorrect, change the preload shims, front bearing, seal, and nut and measure the PBP again.
4. Set the differential side bearing preload- Set the differential side bearing preload at the high end of the specifications. Side bearing preload cannot be directly measured; it can only be calculated. The formula: ((TTP) – (PBP)) x Axle Gear Ratio = Side Bearing Preload.Example: ((30 in*lbf) – (20 in*lbf)) x 3.23 = 32.3 in*lbf. If the Side Bearing Preload is incorrect, change the side bearing shims by an equal amount on the left and the right sides to prevent changes in the ring gear backlash. Some axles use threaded side bearing adjusters rather than shims to set bearing preload and backlash.
5. Set the Ring gear backlash— Set the ring gear backlash at the middle of the specifications. If the backlash is incorrect, adjust the side bearing shims by decreasing the shim thickness on one side by the same amount of the shim thickness increase on the other side. This will maintain the side bearing preload.
6. Run a contact pattern check– Using gear marking compound, paint the drive and coast side of every ring gear tooth, not just three or four teeth. Install the axle shafts and brakes. Partially apply the brakes to load the ring gear to the point that it takes about 50 ft*lbf to rotate the pinion flange. Rotate the flange 3 to 4 rotations forward and backward.
7. Interpret the contact pattern- Make all final adjustments to the axle set up based upon the gear tooth contact pattern results. Always recheck the pattern after adjustments are made. There are five possible patterns if the backlash is within specifications.

Summary

Regarding the two trucks at the start of this article. We replaced all the bearings and one ring and pinion gear set and set up the axles correctly upon reassembly. We could not drive the vehicle on the road since they were donation vehicles to my school, but we did test them on the hoists, the vibrations were gone.

There were incorrectly setup/failing parts in these axles, but nobody (including several competent flat-rate technicians and a GM “Field Service Engineer”), detected them. Two unhappy customers were the result. Thousands of dollars were wasted on attempted repairs; many hours of labor were wasted. My students and I diagnosed both of these vehicles in less than 30 minutes by following basic diagnostic steps; you can do the same thing. Best wishes!