In automotive terms, ball and roller bearing development coincided roughly with the creation of the horseless carriage, just prior to the dawn of the 20th century.
In the latter years of the 20th century the wheel bearing, while still recognizable as such, has sprouted a host of features that would have been unheard of in Herrs Daimler and Benz’ day.
Bearing manufacturers refer to products succeeding the earliest designs in terms of their generation, any of which may still be found in use today.
Generation I: Compact unit using ball bearings or tapered rollers with optimal load bearing contact angle. Compared to a pair of single row tapered bearings, Generation I bearings allow easier and more reliable pre-load setting and assembly. The bearing doesn’t require spacer adjustment, grease packing or seal press fitting.
Generation II bearings integrate the Generation I bearing into a unit with an integral flange with orbital formed shoulder. You’ll recognize these as wheel hub units found on the driven wheels of many passenger cars. They’re also referred to as Pre-Clamped Units, or PCUs, by virtue of the fact that the formed shoulder retains the bearing in the carrier in such a way as to eliminate the need to consider bearing pre-load.
Generation III bearings are a fully integrated wheel bearing system. There are flanges for both inner and outer ring, plus a formed shoulder. These units provide attachment for the brake disc, wheel and knuckle. They also differ from Generation II units in that they provide higher rigidity and can include integrated wheel speed sensors.
The key driver of these technological changes is the vehicle manufacturer, says Ken Edwards, sales manager for FAG Automotive Vehicle Aftermarket North America, Mississauga, Ont. They’re looking to make assembly easier at the manufacturing stage. If this means integrating various components into one, then so be it, as they’d have to buy all those parts anyway.
“The number one goal as with a lot of other parts is to reduce the overall weight and to have parts reduction.” Of course, he adds, they’re also asking the bearing to do more, like provide wheel speed data input for ABS and other systems.
“Those sensors have various designs, but typically the sensor is built into the seal. It comes down to an overall weight reduction situation. Probably that’s the biggest advantage, but it’s protected from the elements too.”
SKF, while also supplying bearings with integrated wheel speed sensors for cars like the Jaguar X-type, and the Volvo Sport Utility Vehicle set to arrive soon, has also been working with tire maker Michelin. Together, they have been developing a system that will allow a control system in the vehicle to sense when a tire has lost air pressure and, through a system of airway galleries in the hub bearing unit, allow that tire to be re-inflated.
But in the meantime, the existing integration is bound to have an effect on service.
“The more integration you build in, the price goes up. But if you combined the cost of all of the components you used to have to replace, there probably hasn’t been much change,” says Edwards.
The impact on parts life is already being seen. Designs of a decade ago had a theoretical life in the range of 160,000 km, current designs have nearly double this life. But, in the real world, failures of even these durable units happen as early as three years into their life–with much lower mileage–with the peak in replacements coming at about the seven year mark, according to Edwards.
“There have been advantages to the integrated design,” he continues, “but if you are a driver that likes to hit a pothole or a curb once in a while, it is going to cost. They are made better, but they’re also made as light as possible and with as little material as possible.”
On the plus side, integrated hub-bearing systems reduce variables.
“There are fewer returns for incorrect parts, installation is easier, and there is less installation damage. You used to have to have the cup and the outer ring and you needed the right drift, the right hammer, (and the skill). Then there were the seals and greasing. There are a lot of factors that the integrated units remove. You pay more, but there are fewer hassles in installation and installation times are reduced, too. It’s just endless.”