Anatomy of a Rolling Element Bearing
A rolling element bearing (which is a type of anti-friction bearing) involves the motion of two surfaces separated either by rollers or balls (the rolling elements) whose primary purpose is to reduce friction. The typical rolling element bearing will consist of
(a) an inner race,
(b) an outer race (which together form the raceway),
(c) the rolling elements between them, and
(d) usually some type of cage (also known as a separator or retainer) to hold the rolling elements in place and prevent them from contacting each other.
The only kind of rolling element bearing that does not have a cage is a called a full complement bearing. The inner and outer race will have a groove in them, in which the rolling elements rest.
Here are some additional excellent posts on bearings and bushings from Advanced EMC Technologies:
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- Ultimate Qualitative Cheat Sheet for Polymer Bushing Life Expectancy
The bore of a bearing is its inner diameter, which will be directly related to the size of the shaft. It is not uncommon, to achieve a more economical bearing design, for engineers to design the bearing around the size of the shaft. The outer diameter of the bearing is initially a function of the amount of radial space available for bearing. The outside diameter of ball and roller bearings is usually between 1.5 and 3 times the bore of the bearing. The axial dimension, or width, of a ball or roller bearing is usually chosen to be 0.2 to 0.5 times the shaft diameter. If the bearings are going to be lubricated, these dimensions will be larger to accommodate the lubricant.
The outer race remains stationary is mounted in a housing. The outer race assists in transferring the radial component of the bearing load from the bearing to the housing. The inner race is mounted on the shaft and rotates while supporting and guiding the shaft. The rolling elements resting inside the groove rotate around the inner race, but they do not move at the same speed as the inner race. Their task is to carry the load and evenly distribute it to the raceways.
Bearing Rolling Elements
There are different types of rolling elements: balls, cylindrical rollers, spherical rollers, tapered rollers, needle rollers.
– match their name: they are shaped like balls, and due to this shape they provide a minimal area of contact between the ball and the race surface. This is primarily what limits their load capacity: too much weight, and they risk deformation and reduced performance. Benefits of ball bearings include lower friction, low heat generation, and a higher tolerance for misalignment. There are also ball thrust bearings for applications with lower speeds and lower weights. Standard ball bearings are often used with electrical motors.
– are probably the oldest type of rolling element. They are characterized by having their length significantly greater than their diameter. They can carry heavy loads, but are more adapted to radial loads than thrust loads. If their races are not properly aligned, they will fail more quickly than an equivalent ball bearing.
– are barrel-shaped, with more similarity to a cylindrical roller than a spherical ball. Their name actually comes not from the shape of the rollers but from the cross-section of the outer raceway. They are usually arranged in two rows. They carry radial and axial loads very well. You will often see them used in applications with heavy loads but moderate speeds. They can also tolerate some degree of misalignment (3° to 5°), but have a higher coefficient of friction than cylindrical rollers. A good example of where they are used would is rolling mills, where they are subject to extremely large radial loads. They are also used in gearboxes, vibrating screens, and crushers. The major drawback of spherical roller bearings is their cost, which is related to how difficult it is to manufacture them.
– have a conical shape, as do the grooves in their raceways. They work very well as thrust bearings. When heavily loaded, however, they rollers exhibit a wedge behavior such that the load may try to force then out of the bearing. For this reason they require a guide flange to hold the rollers in place. This in turn increases both friction and heat generation. However, they can handle much larger loads than equivalent spherical rollers, and are usually much more inexpensive. Note that sometimes the inner race is called the cone and the outer race is called the cup. Tapered rollers are usually installed back to back in pairs, especially when they will be experiencing simultaneous radial and axial loads. They can be seen in railcar axles, gearboxes, and cone crushers.
– differ from cylindrical rollers based on the ratio of roller diameter to roller length. Obviously, a needle roller will have a much smaller ratio, usually on the order of 0.1 to 0.4. Put another way, the length of a needle roller will be at least four times its diameter. They provide much more contact surface area than roller bearings. Needle rollers are an excellent option when cylindrical bearings are needed, but there just isnt enough radial space available. You see them used in transmissions, planetary gear boxes, drive shafts, and alternators, for example.
Cages serve several purposes; the first of which is to keep the rolling elements in place so that the load is optimally distributed and operation is quiet. The second purpose is to reduce the frictional moment by keeping the rolling elements separated. This also serves to reduce the amount of heat generated by friction. The cage also guides the rolling elements into the unloaded zone. This helps avoid sliding movements that could damage the bearing, and also improve rolling conditions. Finally, it helps to keep the elements in place when a bearing ring is removed.
Polymer Bearings VS. Metal Bearings
While its true that polymer rolling element bearings are not meant to carry the same type of load as metal bearings, they can solve problems that metal bearings cannot. Their primary strength is, pun intended, not necessarily strength, but the other characteristics that can make them so different from metals, such as chemical resistivity, low density, self-lubrication, and electrical insulation.
For more information on Polymer Bearings download Advanced EMC Technologies, “Polymer Bearings Materials Comparison Charts slide deck.“