There are two common types of bearing assemblies that you’ll typically encounter when servicing front axles: fixed spindle assemblies and unit bearing assemblies. Traditional spindle assemblies use a set of tapered roller bearings inside a wheel hub that is slipped over a spindle mounted to the axle housing. The components can be cleaned, replaced, and relubed as needed. Unit bearings contain the bearings and spindle sealed in a non-serviceable assembly that bolts as a unit to the axle housing or knuckle.
Unit bearings are generally considered to be weaker than spindle bearing assemblies, and are considerably more expensive to service, as they generally require complete replacement when worn. We’ve seen a number of cases where the unit bearings fail with little or no warning, leaving owners to replace bearing assemblies on the side of the road or at other inopportune times.
Nowadays there are kits on the market to convert some factory unit bearing assemblies over to fixed spindle assemblies with serviceable bearings. Along with these conversions you typically add manual locking hubs, allowing you to unlock your front axle assemblies on the road for increased mileage and reduced drivetrain wear.
Here is what you see on a typical front axle (or full-float rear axle). You have a hollow spindle that bolts to the outer steering knuckle (or axle housing flange on rear floater). The wheel hub then slides over the spindle and is held in place with a large spindle nut.
This is the wheel hub for the Dana 60 shown in the previous image. It rides on inner and outer tapered roller bearings. The bearings and seal can be serviced or replaced individually as needed. The dust seal on the backside of the wheel hub keeps debris from entering the inner bearing area. The spindle nut must be tightened correctly by the installer to set the bearing preload and a second nut or lock plate is used to hold the spindle nut in position.
This image shows the front solid axle on a late 1990s Dodge 3/4-ton truck. You can see the outer knuckle and the splined outer axle shaft protruding. A flanged unit bearing bolts to this knuckle using four bolts inserted from the backside of the knuckle.
Here is the unit bearing assembly for that Dodge truck with brake rotor attached. The unit bearing assembly is sold as a sealed unit and is not meant to be serviced. Replacing these assemblies is quite easy, but you don’t have the choice of replacing bearings or seals individually.