As for 4-wheeled vehicles, each has a pair of front wheel bearings and a pair of rear wheel bearings. They perform two main jobs, namely: allowing the wheels with minimal friction; and supporting the vehicles' weight. The wheel bearings must be in tip-top shape to perform such tasks. The seals must be leak-free, as well. This would protect the lubricants inside from harmful contaminants outside it.
Front wheel bearings and rear wheel bearings could hold up to 850 lbs depending on the front-to-rear weight distribution of a car. In a 6-ton SUV, each bearing can carry about 1,500 lbs. If wrongly adjusted, a wheel bearing may wear out early. The sealed grease, which is the most vulnerable part of the bearing, will start to leak. Dirt enters the bearing cavity, damaging the wheel bearing.
Another factor to wheel bearing damage is water. This causes rusting of the bearing and contaminates the grease in the bearing cavity. Some bearing seals are not built to keep water out of these bearings. Vehicles, most especially off-roaders (with wheels frequently sunk in the mud or hub-deep water during floods), must have their bearings cleaned thoroughly. In this manner, there will be greater interval before the wheel bearings are replaced.