Wheel bearings are a primary friction reducing element in your car’s suspension. Imagine for a moment a wooden wagon wheel rolling on a fixed solid wooden axle. The contact surface between the wheel’s center (“hub”) and the axle is subject to considerable friction as the two continuously rub against each other. This friction was/is mitigated somewhat through the use of heavy grease between the two.
Simple grease, though, doesn’t lend itself well either to long-term maintenance-free friction reduction (it requires very frequent regreasing) and – because of the amount of “play” between the hub and axle, doesn’t permit very high precision between the hub and axle; the hub/wheel can wobble considerably in any direction.
A set of wheel bearings cures both issues rather well. Most generally, wheel bearings are roller bearings, built as a set of small rolling pins wrapped around the axle (or spindle, which I’ll touch upon in a moment), such that the outer surfaces of the pins contact the hub and their inner surfaces contact the axle/spindle. The pins roll freely within their highly constrained enviroment, providing precision, and they’re almost always sealed with rubber seals to hold grease for addition friction reduction.
In non-driving wheels, as one might see on a wagon wheel or wheelbarrow wheel or on the non-driving pair of wheels on a car, the axle is usually modified to a short stub called a spindle that protrudes from the suspension. The wheel bearings wrap around this spindle, and the wheel’s hub wraps snugly around the wheel bearings. This arrangement also holds true for certain driving wheels, particularly heavier-duty front & rear axles in four-wheel-drive vehicles or work trucks. The vehicle weight is borne by the fixed spindle.
In modern lighter-duty driving wheels, on the other hand, the arrangement is a little different; the axle (which provides driving force to the wheels) is expected to rotate along with the wheel, so there is no fixed spindle; instead, the suspension elements (generally front “steering knuckles”) are hollow, and the wheel bearings wrap around the axle, then are fitted to the inside of the hollow steering knuckles. In this case, the weight of the vehicle is borne at those driving wheels entirely by the rotating-driving axle shaft.
Wheel bearings may fail in several different ways. They may simply wear away, which is very uncommon but possible, resulting in excessive “wobble”, or “slop” in the wheel. They may become overheated, destroying the integrity of their grease, resulting in increasing wear and additional heat until the rollers get hot enough to lose hardness. They may be run with too little grease, too old grease (it hardens with age, and loses lubricity), or grease contaminated by dust or water leaking past the bearing’s end seals. They may – if subjected to considerable impact – become “brinnelled”, meaning that the precision-ground surfaces upon which they roll develop little dents or divots at certain points, which makes the bearings noisy and increases their in-service stress; as each roller passes a dent, it’s subjected to two shock forces – once entering a dent and once leaving a dent. Last, one or more rollers may – also if subjected to considerable impact – shatter in service, leaving tiny bits of steel floating around inside the bearing, scoring those precision-ground surfaces and increasing wear by rubbing directly against a roller or its rolling surface (generally referred to as a “race”).
Any wheel bearing showing any sign of failure through any of these failure modes should be promptly replaced. There generally isn’t much danger of wheel loss, since bearing retaining nuts and other components are nearly always built large enough to prevent “catastrophic disassembly”, but a failing wheel bearing will generally get much worse quickly, and may damage other parts as it continues to fail – axle shafts, spindles, hubs, steering knuckles, all of which are expensive to replace. Certain rear-wheel-drive axles, even, may be destroyed en toto if a wheel bearing is permitted to operated after preliminary failure – the inner bearing’s outer race may spin in the axle housing, enlarging the cavity into which that bearing race must fit tightly, and thereafter making a good bearing fit entirely impossible. This requires replacing the entire axle assembly.