In mechanical engineering, a rolling-element bearing, also known as a rolling bearing, is a bearing which carries a load by placing rolling elements (such as balls, cylinders, or cones) between two concentric, grooved rings called races. The relative motion of the races causes the rolling elements to roll with very little rolling resistance and with little sliding. One of the earliest and best-known rolling-element bearings is a set of logs laid on the ground with a large stone block on top. As the stone is pulled, the logs roll along the ground with little sliding friction. As each log comes out the back, it is moved to the front where the block then rolls onto it. It is possible to imitate such a bearing by placing several pens or pencils on a table and placing an item on top of them. See "bearings" for more on the historical development of bearings. A rolling element rotary bearing uses a shaft in a much larger hole, and spheres or cylinders called "rollers" tightly fill the space between the shaft and the hole. As the shaft turns, each roller acts as the logs in the above example. However, since the bearing is round, the rollers never fall out from under the load. Rolling-element bearings have the advantage of a good trade-off between cost, size, weight, carrying capacity, durability, accuracy, friction, and so on. Other bearing designs are often better on one specific attribute, but worse in most other attributes, although fluid bearings can sometimes simultaneously outperform on carrying capacity, durability, accuracy, friction, rotation rate and sometimes cost. Only plain bearings are used as widely as rolling-element bearings. They are commonly used in automotive, industrial, marine, and aerospace applications. They are products of great necessity for modern technology. The rolling element bearing was developed from a firm foundation that was built over thousands of years. The concept emerged in its primitive form in Roman times. After a long inactive period in the Middle Ages, it was revived during the Renaissance by Leonardo da Vinci, and developed steadily in the seventeenth and eighteenth centuries.
History of bearings timeline
Beginning in 2600 BCE - The Ancient Egyptians were the first to notably use the concept behind rolling bearings, they first did this by using logs under these stones with groups of builders on either side to push and pull the weight of the stones. 40 BC - In the remains of a sunken Roman Ship in Lake Nemi. This discovery shows the continual development of the principle. The remains of the ship do not show clear signs of an indication of what these bearings were used for. 17th century - Galileo describes the functionality of a caged bearing 1740 - John Harrison invented the first caged roller bearing for H3 marine timekeeping. 1794 - The first patent for the ball race was given to Philip Vaughan of Carmarthen, Wales. This is the first design seen with a spherical object moving through a groove. 1869 - Jules Suriray receives the first patent for a radial ball bearing, his design was used by James Moore to win the first 80 mi (130 km) bicycle race from Paris to Rouen.
Overall design Design description Bearings, especially rolling element bearings, are designed in a similar fashion across the board consisting of the outer and inner track, a central bore, a retainer to keep the rolling elements from clashing into one another or seizing the bearing movement, and the rolling elements themselves. The internal rolling components may differ in design due to their intended purpose of application of the bearing. The main five types of bearings are ball, cylindrical, tapered, barrel, and needle. Ball - the simplest following the basic principles with minimal design intention. Important to note the ability for more seizures is likely due to the freedom of the track design. Cylindrical - For single axis movement for straight directional movement. The shape allows for more surface area to be in contact adding in moving more weight with less force at a greater distance. Tapered - Primarily focused on the ability to take on axial loading and radial loading. It does this by using a conical structure enabling the elements to roll diagonally. Barrel - Provides assistance to high radial shock loads that cause misalignment and uses its shape and size for compensation. Needle - Varying in size, diameters, and materials these types of bearings are best suited for helping reduce weight as well as smaller cross sections application, typically higher load capacity than ball bearings and rigid shaft applications.
Specific design types
Ball bearing
A particularly common kind of rolling-element bearing is the ball bearing. The bearing has inner and outer races between which balls roll. Each race features a groove usually shaped so the ball fits slightly loose. Thus, in principle, the ball contacts each race across a very narrow area. However, a load on an infinitely small point would cause infinitely high contact pressure. In practice, the ball deforms (flattens) slightly where it contacts each race much as a tire flattens where it contacts the road. The race also yields slightly where each ball presses against it. Thus, the contact between ball and race is of finite size and has finite pressure. The deformed ball and race do not roll entirely smoothly because different parts of the ball are moving at different speeds as it rolls. Thus, there are opposing forces and sliding motions at each ball/race contact. Overall, these cause bearing drag.
Roller bearings
Cylindrical roller
Roller bearings are the earliest known type of rolling-element-bearing, dating back to at least 40 BC. Common roller bearings use cylinders of slightly greater length than diameter. Roller bearings typically have a higher radial load capacity than ball bearings, but a lower capacity and higher friction under axial loads. If the inner and outer races are misaligned, the bearing capacity often drops quickly compared to either a ball bearing or a spherical roller bearing. As in all radial bearings, the outer load is continuously re-distributed among the rollers. Often fewer than half of the total number of rollers carry a significant portion of the load. The animation on the right shows how a static radial load is supported by the bearing rollers as the inner ring rotates.
Spherical roller
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