Bearings utilize a relatively simple structure: balls with smooth metal surfaces inside and outside that help them roll. The balls themselves carry the weight of the load - the force of the loaded weight is what drives the bearing to rotate.
But not all loads exert force on the bearing in the same way. There are two different kinds of loads: radial and thrust.
Radial loads, such as in a pulley, simply put weight on the bearing, causing it to roll or rotate due to tension. Thrust loads are significantly different and stress the bearing in a completely different way.
If a bearing (think of a tire) flips over on its side (now think of a tire swing) and is subjected to full force at that angle (think of three children sitting on a tire swing), this is called a thrust load. A bearing used to support a high stool is an example of a bearing subjected to thrust loads only.
Many bearings are susceptible to both radial and axial loads. For example, automobile tires are subjected to radial loads when they are driven in a straight line: the tires roll forward in a rotational manner due to tension and the weight they support.
Extended Information:
Bearing Classification:
1. Ball Bearings
Ball bearings are very common because they can withstand radial and axial loads, but can only support a small amount of weight. They are found in a variety of applications, such as roller blades and even hard disk drives, but are prone to deformation if they are overloaded.
2. Roller bearings
Roller bearings are designed to carry heavy loads - the main rollers are cylindrical, which means that the load is distributed over a larger area, allowing the bearing to carry more weight. However, this construction means that the bearings can carry mainly radial loads but are not suitable for thrust loads.
For applications where space is an issue, needle bearings can be used. Needle bearings are suitable for small-diameter cylinders and are therefore easier to mount in smaller applications.
3. Ball thrust bearings
These types of bearings are designed to handle thrust loads almost exclusively in low-speed, low-weight applications. For example, bar stools utilize ball thrust bearings to support the seat.
4. Roller Thrust Bearings
Roller thrust bearings are much like ball thrust bearings in that they can withstand thrust loads. The difference, however, is in the amount of weight the bearings can support:
Roller thrust bearings can support significantly larger amounts of thrust loads, so they can be found in automotive transmissions, where they are used to support helical gears. Gear support is usually a common application for roller thrust bearings.
5. Tapered roller bearings
This type of bearing is designed to handle large radial and axial loads - due to their load versatility, they are found in automotive hubs, where the wheels are expected to be subjected to extreme radial and thrust loads.
Source:Baidu Encyclopedia - Bearings