Bearings and Bushings

Ball Bearings: Specifications and Selection Criteria for Engineers

30 July 2015

Ball Bearing. Source: wikipedia.comBall Bearing. Source: wikipedia.comBall bearings are used to provide smooth, low friction motion in rotary applications. Ball bearings are able to provide high performance and long lifetime in order to transfer the load from the balls to the inner races. The balls have minimal contact with the inner and outer races due to their spherical shape and this allows them to spin smoothly.

(Find ball bearings by Specification or see our Directory of suppliers.)

A diThrust bearing. Image Credit: WikipediaThrust bearing. Image Credit: Wikipediasadvantage of ball bearings is that the balls can become "flattened" over time with too much pressure on the outer races leading to eventual failure. Therefore it is very important that the load ratings and other parameters are monitored regularly.

Parts

Ball bearings are composed of four main parts: two rings (or races) the rolling elements (the balls), and the ball separator (retainer).

Parts of a Ball bearing. Credit: Bright Hub EngineeringParts of a Ball bearing. Credit: Bright Hub EngineeringRadial ball bearings and angular contact bearings have an inner and outer ring. Radial ball bearings and angular contact bearings are designed to support loads perpendicular to the axis of rotation, referred to as radial loads. The outer race mounts into a housing and is stationary. This part also aids in the transfer of the radial load from the bearing to the housing. The inner race mounts onto a rotating shaft and supports and guides the shaft during rotation.

The rolling elements carry the load and distribute it throughout the raceways. The rolling elements rotate around the inner race, but not at the same speed as the rotation of the inner race.

The separator acts as a barrier between the balls preventing them from bumping into each other. Thurst bearings are designed to support loads parallel to the axis of ratation, referred to as axial loads. Thrust ball bearings have two rings of equal size.

Types

Ball bearing types include:

Radial ball bearings - Radial ball bearings are designed to carry both radial and (light) axial (thrust) loads, while thrust ball bearings are designed for pure thrust loads only.

  • Deep groove- Radial, or deep groove bearings, can take radial and axial loads to varying degrees but are used when the primary load is in the radial direction. They consist of an inner ring, an outer ring, balls and sometimes a cage to contain and separate the balls.
    Insert Round Bearing
  • Deep Groove Ball Bearing.Deep Groove Ball Bearing.Angular contact - Angular contact ball bearings are designed such that a contact angle between the races and the balls is formed when the bearing is in use. The major design characteristic of this type of ball bearing is that one or both or the ring races have one shoulder higher than the other. In order for these bearings to function properly, they must be assembled with a thrust load. This loading (or preload) creates a line of contact (or contact angle) between the inner race, the ball and the outer race. The preload can be built into the bearing or created when the bearing is inserted into an assembly. The contact angle varies from 15° to 40° and is measured relative to a line running perpendicular to the bearing axis. Angular contact bearings are one-directional thrust bearings that can withstand heavy thrust loads and moderate radial loads.

Thrust ball bearings - Thrust ball bearings are designed for pure thrust loads. These bearings can handle little or no radial loads. The rolling element can be a ball, needle, or roller. Slewing ring or turntable bearings can accommodate axial, radial and moment loads. They are not mounted in a housing or on a shaft, but instead are mounted directly to a seating surface. The inner and outer rings are supplied with mounting holes. The inner ring, outer ring, or both may have integral gears. These bearings are referred to as tabletop bearings, turntable bearings, and slewing rings.

Watch a video from NTN Bearing Corp. on how ball bearings are made.

Specifications

Dimensions

Important dimensions to consider when specifying bearings include:

  • Bore - The bearing industry uses a standard number system for bearings with metric diameter bores. For bore sizes 04 and up multiply by 5 to obtain the bore in millimeters. If the bore is a hex this refers to the dimension across the flats. If the bore is tapered this refers to the smaller diameter.
  • Outside diameter - The outside diameter of the bearing includes the housing if a housed unit, but excludes the flange if a flanged bearing. The outer ring width is the overall width of the outside of the bearing.
  • Overall width - The overall width of the bearing or bearing assembly includes the locking collar, if present.

Operating Specifications

Important operating specifications to consider when searching for bearings include rated speed, dynamic axial or thrust load, and dynamic radial load.

  • The rated speed for a bearing running with grease lubrication is lower than a bearing with oil lubrication.
  • The dynamic axial or thrust load is the calculated constant axial load, which a group of identical bearings with stationary outer rings can theoretically endure for a rating life of 1 million revolutions of the inner ring.
  • The dynamic radial loadis the calculated constant radial load, which a group of identical bearings with stationary outer rings can theoretically endure for a rating life of 1 million revolutions of the inner ring.

Bearing Load, Fatigue and Lifespan

Optimum bearing life is obtained when the balls and raceways have absolute minimal surface contact supplemented with proper lubrication. Loads for ball bearings are subject to static or dynamics loads, as well as axial or radial loads. This means four variables need to be accounted for to determine working loads for a bearing. Ball bearings can handle substantially more radial and dynamic loads than axial and static loads. The first sign of non-elastic deformation will be flattened spots on the balls, which will hinder rotation.

Calculated life of a bearing is based on its load, operating speeds, and environmental factors. Industry standards typically require that 90% of bearings are still serviceable after 1 million rotations, and 50% of bearings still be serviceable after 5 million rotations. This is known as bearing fatigue life. An underestimate (for safety) of a bearing's lifespan, as well as the applicable variables to calculate such, is offered with Engineer's Edge online ball bearing fatigue life calculator.

It can also be accomplished with the following formula:

Materials

Special materials and plating options to consider for ball bearings include stainless steel, plastic, ceramic balls, ceramic races, and platings. Ball bearings are made of largely through-hardened materials with a minimum Rockwell rating of 58 Rc. 440C stainless steel and SAE 52100 steel are the most common materials for raceway and ball designs, but these alloys are not suitable for operating or friction temperatures above 350° F. Forms of molybdenum steels are excellent for temperature resistance even over 1000° F.

The shields and seals of a bearing do not carry a radial load and only light axial loads, if any. Metal shields are usually of the same material as the bearing raceways, but other common bearing face materials and their attributes are as follows:

Table credit: Bearing Specialist AssociatesTable credit: Bearing Specialist Associates

Stamped steel is the most common retainer material; stamped or machined bronze or brass and plastic (such as nylon) are also commonly available. These materials feature good temperature resistance in high-speed applications. Plastic retainers have a higher speed capability and are limited by temperatures, but plastic retainers for high-speed applications made of phenolic materials (270° F), PTFE (450° F), or polyimide (500° F) are available.

Applications

Special applications for bearings include airframe or aircraft control, aerospace, agriculture, automotive, ball screw support, instrument or miniature, medical or dental, military, pump, spindle or super precision, and sporting and consumer goods.

The simple design of ball bearings makes them ideal for many weight bearing applications. Specific applications include hard drives, roller skates, rotary actuators, and rail cars just to name a few. Ball bearings come in different sizes, are inexpensive, and require little to no maintenance.

Standards

Quality radial ball bearings are subject to standards, which indicate their precision and efficiency. Bearing quality is rated by ABEC (Annular Bearing Engineers' Committee). These ratings classify different accuracy and tolerance ranges for ball bearings. The higher the ABEC number the tighter the bearing tolerances. Very high-speed applications will see the greatest benefit from a more precise bearing.

A manufacturer does not have to follow these industrial guidelines. North American radial ball bearings are under edict of the ABEC scale, while other ball bearings adhere to ISO 492 or its regional equivalent (DIN, KS, etc.) There are five accepted levels of the ABEC/ISO 492 scale and the level is not related to the size of the bearing. Boca Bearings Inc. has a tolerance chart on the dimensional variances acceptable for each ABEC/ISO standard, organized by bearing dimension.

Some manufacturers may quote an ABEC rating not listed above. This is an inaccuracy; there are only five ABEC ratings.

Ball bearings can be manufactured and tested using standards developed by a diverse range of societies and associations. ANSI -American National Standards Institute, AFBMA - Anti-friction Bearing Manufacturers Association specifies other standards and ISO - International Organization for Standardization also specify standards.

MIL-STD-206 - Friction torque testing for bearings.

ABMA-STD-9 - Load ratings and fatigue life for ball bearings.

ASTM D3336 - Standard testing method for life of lubricating greases in ball bearings at elevated temperatures.



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