09-07-2012, 10:00 AM
Ball bearing
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For the last four decades, Tedric Harris' Rolling Bearing Analysis has been the "bible" for engineers involved in rolling bearing technology. Why do so many students and practicing engineers rely on this book? The answer is simple: because of its complete coverage from low- to high-speed applications and full derivations of the underlying mathematics from a leader in the field. The fifth edition of this classic reference is divided conveniently into two volumes, each focused on a specialized area of bearing technology. This option allows you to select the coverage that is best suited to your needs. The second of two books, Advanced Concepts of Bearing Technology steps up the level to more dynamic and complex loading, more extreme operating conditions, and higher-speed applications. The authors examine several topics that are unique to the book, including mathematical relationships for internal load distribution under conditions of high speed, combined radial, axial, and moment loading, as well as the effects of raceway and roller profiling. They also delve into the mathematical development of rolling element-raceway lubricant film thickness and contact friction, the stress-life method for calculating bearing fatigue endurance, and the effects of shaft and supporting structure flexure on bearing loading and deflection. Advanced Concepts of Bearing Technology is the perfect aid for analyzing complex performance and fatigue-life phenomena in advanced applications.
ball bearing
n. Abbr. bb
1. A friction-reducing bearing consisting essentially of a ring-shaped track containing freely revolving hard metal balls against which a rotating shaft or other part turns.
2. A hard ball used in such a bearing.
Ball bearing
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For individual balls that are sometimes incorrectly called "ball bearings", see Ball (bearing).
Working principle for a ball bearing
A 4 point angular contact ball bearing
A ball bearing with a semi transparent cage
Wingquist's and SKF's self-aligning ball bearing
Wingquist bearing
A ball bearing is a type of rolling-element bearing that uses balls to maintain the separation between the bearing races.
The purpose of a ball bearing is to reduce rotational friction and support radial and axial loads. It achieves this by using at least two races to contain the balls and transmit the loads through the balls. In most applications, one race is stationary and the other is attached to the rotating assembly (e.g., a hub or shaft). As one of the bearing races rotates it causes the balls to rotate as well. Because the balls are rolling they have a much lower coefficient of friction than if two flat surfaces were rotating on each other.
Ball bearings tend to have lower load capacity for their size than other kinds of rolling-element bearings due to the smaller contact area between the balls and races. However, they can tolerate some misalignment of the inner and outer races.
[edit] Common designs
There are several common designs of ball bearing, each offering various trade-offs. They can be made from many different materials, including: stainless steel, chrome steel, and ceramic (silicon nitride (Si3N4)). A hybrid ball bearing is a bearing with ceramic balls and races of metal.
[edit] Angular contact
An angular contact ball bearing uses axially asymmetric races. An axial load passes in a straight line through the bearing, whereas a radial load takes an oblique path that tends to want to separate the races axially. So the angle of contact on the inner race is the same as that on the outer race. Angular contact bearings better support "combined loads" (loading in both the radial and axial directions) and the contact angle of the bearing should be matched to the relative proportions of each. The larger the contact angle (typically in the range 10 to 45 degrees), the higher the axial load supported, but the lower the radial load. In high speed applications, such as turbines, jet engines, and dentistry equipment, the centrifugal forces generated by the balls changes the contact angle at the inner and outer race. Ceramics such as silicon nitride are now regularly used in such applications due to their low density (40% of steel). These materials significantly reduce centrifugal force and function well in high temperature environments. They also tend to wear in a similar way to bearing steel—rather than cracking or shattering like glass or porcelain.
Most bicycles use angular-contact bearings in the headsets because the forces on these bearings are in both the radial and axial direction.
[edit] Axial
An axial ball bearing uses side-by-side races. An axial load is transmitted directly through the bearing, while a radial load is poorly supported and tends to separate the races, so that a larger radial load is likely to damage the bearing.
[edit] Deep-groove
In a deep-groove radial bearing, the race dimensions are close to the dimensions of the balls that run in it. Deep-groove bearings have higher load
[edit] Construction types
[edit] Conrad
The Conrad style ball bearing is named after its inventor, Robert Conrad, who was awarded British patent 12,206 in 1903 and U.S. patent 822,723 in 1906. A Conrad bearing is assembled by placing the inner race into an eccentric position relative to the outer race, with the two races in contact at one point, resulting in a large gap opposite the point of contact. The balls are inserted via this gap and then evenly distributed around the bearing assembly, causing the races to become concentric. Assembly is completed by fitting a cage to the balls to maintain their positions relative to each other. Without the cage, the balls would eventually drift out of position during operation, causing the bearing to fail. The cage carries no load and serves only to maintain ball position.
Conrad bearings have the advantage that they are able to withstand both radial and axial loads, but have the disadvantage of lower load capacity due to the limited number of balls that can be loaded into the bearing assembly. Probably the most familiar industrial ball bearing is the deep-groove Conrad style. The bearing is used in most of the mechanical industries.
[edit] Slot-fill
In a slot-fill radial bearing, also referred to as a full complement design, the inner and outer races are notched on one face so that when the notches are aligned, balls can be slipped in the resulting slot to assemble the bearing. A slot-fill bearing has the advantage that the entire groove is filled with balls, called a full complement, resulting in a higher radial load capacity than a Conrad bearing of the same dimensions and material type. However, a slot-fill bearing cannot carry a significant axial load on the loading slot side. Also, the slots cause a discontinuity in the races that has a small but adverse effect on strength. Note that an angular contact bearing can be disassembled axially and therefore is a full complement design.
[edit] Split-race
The outer race may be split axially or radially, or a hole drilled in it for filling. These approaches allow a full complement to be used, but also limit the orientation of loads or the amount of misalignment the bearing can tolerate. Thus, these designs find much less use.
[edit] Rows
There are two row designs: single-row bearings and double-row bearings. Most ball bearings are a single-row design, which means there is one row of bearing balls. This design works with radial and thrust loads.[1]
A double-row design has two rows of bearing balls. Their disadvantage is they need better alignment than single-row bearings.
[edit] Flanged
Bearings with a flange on the outer ring simplify axial location. The housing for such bearings can consist of a through-hole of uniform diameter, but the entry face of the housing (which may be either the outer or inner face) must be machined truly normal to the hole axis. However such flanges are very expensive to manufacture. A more cost effective arrangement of the bearing outer ring, with similar benefits, is a snap ring groove at either or both ends of the outside diameter. The snap ring assumes the function of a flange.
[edit] Caged
Cages are typically used to secure the balls in a Conrad-style ball bearing. In other construction types they may decrease the number of balls depending on the specific cage shape, and thus reduce the load capacity. Without cages the tangential position is stabilized by sliding of two convex surfaces on each other. With a cage the tangential position is stabilized by a sliding of a convex surface in a matched concave surface, which avoids dents in the balls and has lower friction. Caged roller bearings were invented by John Harrison in the mid-18th century as part of his work on chronographs.[2] Caged bearings were used more frequently during wartime steel shortages for bicycle wheel bearings married to replaceable cups.
[edit] Ceramic hybrid ball bearings using ceramic balls
Ceramic bearing balls can weigh up to 40% less than steel bearing balls, depending on size and material. This reduces centrifugal loading and skidding, so hybrid ceramic bearings can operate 20% to 40% faster than conventional bearings. This means that the outer race groove exerts less force inward against the ball as the bearing spins. This reduction in force reduces the friction and rolling resistance. The lighter ball allows the bearing to spin faster, and uses less energy to maintain its speed.
Ceramic hybrid ball bearings use these ceramic balls in place of steel balls. They are constructed with steel inner and outer rings, but ceramic balls; hence the hybrid designation.