Adapted from Andrew Markel’s article in Brake and Front End Magazine
Today, the average life span of a sealed wheel bearing or hub assembly is about 85,000 to 100,000 miles. But they can fail sooner than expected without a whisper.
The final signs of failure are noise and play. These two symptoms are the last phases of a failure that started a long time ago. But, many bad bearings may never make a peep or grumble, but they can cause vibrations and problems with the brake system.
The old-school test of pulling on the wheel at 12- and 6-o’clock still applies, but you will be catching only the really bad bearings. New techniques can help you spot bad bearings and work smarter, not harder.
Loads at a vehicle’s corners are concentrated on the relatively small bearing surfaces. All of the weight on some vehicles may rest on an area no larger than the size of a business card. Loads that are put on the bearing are called thrust and radial loads.
When a vehicle hits a curb, pothole or other object in its path, the force is transferred to the small surface area of the bearing. The impact may cause damage to the races and the rollers/balls. This damage is called “Brinelling.”
Brinelling is a material surface failure/defect caused by contact stress/impact that exceeds the material’s hardness limit. The result is a permanent dent or “Brinell” mark.
Brinell marks may, or may not, cause the bearing to make noise immediately. But, as the marks continue rotating, it could be damaging the entire bearing. If the impact is great enough, the pre-load on the bearing can change, leading to more damage and noise.
The health of the wheel bearing’s flange is a direct indicator of the overall health of the bearings inside. If a wheel has endured an impact strong enough to cause damage to the bearings and races, chances are the flange is distorted.
A distorted flange will have runout, and this runout can cause vibration and, eventually, disc thickness variation in the brake rotors. Every manufacturer has its own specification for wheel flange runout. Most specifications fall between zero and 0.0015 in. Most manufacturers are moving to a zero runout and endplay specification in the flange. This “perfect” specification is because the rotor will magnify any runout in the flange.
If a flange has 0.001 in. of runout, a 10-in. rotor may have 0.003 in. of total runout when mounted. If this vehicle is riding on 205/55R16 tires, in one mile, the high spot with 0.003 in. of runout goes past the caliper approximately 836 times. Over 6,000 miles, the spot on the rotor will go past the pads more than 5 million times! Every time this spot passes the pads, a little bit of the rotor’s material is removed. Over 5 million revolutions, enough material is removed to create a thickness variation that can be felt by the driver.
Some amount of runout can be corrected by using an on-the-car brake lathe. But if the runout is too great, removing too much material to correct the runout can compromise the life of the rotor. Also, runout in the flange can be corrected with plates that fit between the rotor and flange. But, if the runout is excessive and cannot be brought within specifications, the only option is to replace the flange and/or bearing.
Most vehicles with hub units are using tone rings mounted inside the wheel bearing hub unit. Most unitized hub units place the tone ring between the inner bearing races.
The air gap on these sensors is very precise. If there is play in the inner or outer bearing, it will be detected by the sensor and ABS/ESC computer. The computer will typically set a chassis trouble code for the erratic signal for each corner of the vehicle.
If you have a scan tool that can access the PIDs and data for the wheel speed sensors, it’s possible to observe play in the bearings as the car corners and brakes. Speeds may drop dramatically when compared to the inputs from the other wheels. If the ABS/ESC system detects this, it will disable the system and illuminate the ABS light on the dash.
If a vehicle has these symptoms, it’s a no-brainer to replace the bearing or hub unit, even if it’s not making noise and no play is present.
Replacing the Bearing
A high-quality bearing is key to performing a comeback-free job. High-quality hub units or bearings typically use higher-quality materials and heat-treating processes that make for harder surfaces. The harder surfaces will not “Brinell” under hard impacts.
When replacing a hub unit, the replacement unit might feel stiff and difficult to turn. Do not return the bearing. Some hub units and sealed bearings are being filled with a special grease that prevents damage while the bearing is being shipped. The grease is designed to have a high viscosity during shipping so the balls or rollers do not destroy the surfaces of races. After the bearing is installed, certain elements in the grease break down and the bearing turns normally.
While it may appear to be easier to use an impact wrench, it’s not recommended. OEMs and bearing manufacturers always recommend using a torque wrench for installation. During removal, an impact wrench can damage the axle nut threads and shock the CV joints. It can also create a false sense of security when adjusting a nut or bolt that may be under- or over-torqued. This can leave a hub assembly susceptible to failure. Also, in almost all cases, use a new axle nut. Some axle nuts are designed to be used only once, and cannot be adjusted.
1. Why are more vehicles using hub assemblies today?
Hub assemblies are unitized, maintenance free and non-serviceable units that are preset, pre-greased and pre-sealed, making installation easier and increasing product reliability for enhanced performance. These hubs require no maintenance or handling, which eliminates the need for preventive maintenance, grease and/or future adjustments.
2. Can I use an impact wrench to remove or install a hub assembly?
While it may appear to be easier to use an impact wrench, it is not recommended. We recommend the use of a certified, calibrated torque wrench. Impact wrenches can damage the axle nut, threads and components. It can also create a false sense of security when adjusting a nut or bolt, which may be under- or over-torqued. This can leave a hub assembly susceptible to failure.
3. Do I need to torque the axle nut and/or the lug nut?
Yes. It is imperative that you follow the manufacturer’s specifications and/or instruction manual to ensure the hub is installed correctly. Failure to follow the manufacturer’s installation instructions and follow the proper installation procedures can cause equipment failure, creating a risk of serious bodily harm.
4. Can I replace the bearings or seals in my hub assembly?
No, because the hub assemblies are unitized, maintenance free and non-serviceable. Most hub assemblies are designed with a unitized bearing or flange that is intricate to the hub and/or bearing housing, which are not replaceable.
5. Do wheel studs come with all hub assemblies?
No. While most hub assemblies do include the wheel studs, there are some that do not. Timken does not recommend reusing the old studs with the new hub assembly.
Courtesy of The Timken Company.
For more information regarding Timken automotive products and services, visit www.timken.com.
How to Measure Bearing Play
Raise the vehicle so the wheel is off the ground. Then grab tire at the 12- and 6-o’clock positions and rock the tire back and forth.
As a rule, you should not feel any play or looseness if the vehicle has sealed wheel bearing cartridges or hubs with sealed wheel bearing assemblies. On older vehicles with serviceable wheel bearings, a little play is normal, but a lot of play is not. Refer to the vehicle’s service specifications for the maximum amount of acceptable play.
Wheel bearing play can be measured with a dial indicator by placing the dial indicator against the hub and rocking the wheel in and out by hand. As a rule, you should see no more than .005 inches of play in the bearings if the be arings are good.
You can also rotate the tire by hand while measuring play, but be careful not to confuse rim or hub runout with looseness in the bearings. As a rule, hub runout should be .0015 inches or less, and no more than .002 to .003 inches of runout at the lip of the rim.
If runout exceeds this amount, try reindexing the wheel on the hub, or remove the wheel and measure runout on the rotor or hub itself.
If there is still too much runout, cutting the rotor with an on-car lathe can eliminate or reduce runout at the rotor.
But it can’t correct runout if the cause is too much play or looseness in the wheel bearings. The only fix for that is to replace the wheel bearings or hub unit.