Ball Bearing Terminology
Instead of an alphabetical list, this informal bearing dictionary is divided by the major topics of interest to bearing specifiers and product designers. Click a link below to view the applicable section:
- Physical attributes
- Bearing materials
- Shields and seals
- Lubricants and lubrication methods
- Preload and duplex ball bearings
Raceway, Track Diameter, and Track Radius
The raceway in a ball bearing is the circular groove formed in the outside surface of the inner ring and in the inside surface of the outer ring. When the rings are aligned, these grooves form a circular track that contains the ball set.
The track diameter and track radius are two dimensions that define the configuration of each raceway. Track diameter is the measurement of the diameter of the imaginary circle running around the deepest portion of the raceway, whether it be an inner or outer ring. This measurement is made along a line perpendicular to, and intersecting, the axis of rotation.
Track radius describes the cross section of the arc formed by the raceway groove. It is measured when viewed in a direction perpendicular to the axis of the ring. In the context of ball bearing terminology, track radius has no mathematical relationship to track diameter. The distinction between the two is shown in Figure 1.
Radial and Axial Play
Most ball bearings are assembled in such a way that a slight amount of looseness exists between the balls and the raceways. This looseness takes the form of radial play and axial play. Radial play is the maximum distance that one bearing ring can be displaced with respect to the other, in a direction perpendicular to the bearing axis, when the bearing is in an unmounted state. Axial play, or end play, is the maximum relative displacement between the two rings of an unmounted ball bearing in the direction parallel to the bearing axis. Figure 2 illustrates these concepts.
Since radial play and axial play are both consequences of the same degree of looseness between the components in a ball bearing, they bear a mutual dependence. While this is true, both values are usually quite different in magnitude.
In most ball bearing applications, radial play is functionally more critical than axial play. If axial play is determined to be an essential requirement, control can be obtained through manipulation of the radial play specification. Please consult with Application Engineering if axial play ranges for a particular chassis size are required.
Some general statements about Radial Play:
- The initial contact angle of the bearing is directly related to radial play- the higher the radial play, the higher the contact angle.
- For support of pure radial loads, a low level of radial play is desirable; where thrust loading is predominant, higher radial play levels are recommended.
- Radial play is affected by any interference fit between the shaft and bearing I.D. or between the housing and bearing O.D.. See the Assembly and Fitting Procedure section on page 38 for more details.
In addition, the assembly of bearings into a complete device has an important effect on radial play. This may require the radial play specification for the bearing itself to be modified in accordance with the assembly and fitting procedure discussed in this engineering manual. If the system spring rate is critical, or if extremes of temperature or thermal gradient will be encountered, consult with our Engineering Department prior to design finalization.
|Table Of Contact Angles|
|Ball Size||Radial Play Code|
|1/32 & 0.8 mm||161/2°||22°|
The contact angle is given for the mean radial play of the range shown i.e., for P25 (.0002" to .0005") - contact angle is given for .00035". Contact angle is affected by race curvature. For your specific application, contact IBSCO Engineering.
|Typical radial play ranges are:|
|Description||Radial Play Range||IBSCO Code|
|Tight||.0001" to .0003"||241/2°|
|Normal||.0002" to .0005"||22°|
Raceway curvature is an expression that defines the relationship between the arc of the raceway's track radius and the arc formed by the slightly smaller ball that runs in the raceway. It is simply the track radius of the bearing raceway expressed as a percentage of the ball diameter. This number is a convenient index of "fit" between the raceway and ball. Track curvature values typically range from approximately 52 to 58 percent. The lower percentage, tight fitting curvatures are useful in applications where heavy loads are encountered. The higher percentage, loose curvatures are more suitable for torque sensitive applications. Curvatures less than 52 percent are generally avoided because of excessive rolling friction that is caused by the tight conformity between the ball and raceway. Values above 58 percent are also avoided because of the high stress levels that can result from the small ball-to-raceway conformity at the contact area.
The contact angle is the angle between a plane perpendicular to the ball bearing axis and a line joining the two points where the ball makes contact with the inner and outer raceways. The contact angle of a ball bearing is determined by its free radial play value, as well as its inner and outer track curvatures.
The contact angle of thrust-loaded bearings provides an indication of ball position inside the raceways. When a thrust load is applied to a ball bearing, the balls will move away from the median planes of the raceways and assume positions somewhere between the deepest portions of the raceways and their edges.
Free Angle and Angle of Misalignment
As a result of the previously described looseness, or play, which is purposely permitted to exist between the components of most ball bearings, the inner ring can be cocked or tilted a small amount with respect to the outer ring. This displacement is called the free angle of the bearing, and corresponds to the case of an unmounted bearing. The size of the free angle in a given ball bearing is determined by its radial play and track curvature values. For the bearing mounted in an application, any misalignment present between the inner and outer rings (housing and shaft) is called the angle of misalignment. The misalignment capability of a bearing can have positive practical significance because it enables a ball bearing to accommodate small dimensional variations which may exist in associated shafts and housings. A maximum angle of misalignment of 1/4Ãƒâ€šÃ‚Â° is recommended before bearing life is reduced. Slightly larger angles can be accommodated, but bearing life will not be optimized.
Bearing steel is used for standard ball bearing applications in environments where corrosion resistance is not a critical factor.
52100 or Equivalent
The most commonly used ball bearing steel in chrome steel applications is SAE 52100 or its equivalent. Due to its structure, this is the material chosen for extreme noise sensitive applications.
Stainless Steel DD400™
0.7% C; 13% Cr
A 400 series Martensitic stainless steel combined with a heat treating process that was exclusively developed by NMB's parent company. Miniature and instrument bearings manufactured from “DD” Martensitic stainless steel, or “DD Bearings”, meet the performance specifications of such bearings using AISI 440C Martensitic stainless steel. It is equal or superior to A 400 in hardness, offers superior low noise characteristics, and is equivalent or better in corrosion resistance. These advantages yield bearings with lower torque, smoother operation and longer life.
1% C, 17% Cr, .5% Mo
A hardened stainless steel suitable for applications which require corrosion resistance at room to mid-hot temperature range, AISI440C is the standard choice for a wide range of military and commercial applications.
Alternate Ball Material Silicon Nitride Cerbec® Bearing Components
An extremely hard non-metallic material suitable for speeds up to 2 million dN with reduced skidding, Cerbec components of silicon nitride are corrosion resistant, 40% lighter than steel and non-magnetic. Silicon nitride has a modulus of elasticity 50% greater than steel, therefore it resists corrosion and galling.
Shields and Seals
Shields and seals are necessary to provide optimum ball bearing life by retaining lubricants and preventing contaminants from reaching central work surfaces. IBSCO can supply ball bearings with several types of protective closures that have been designed to satisfy the requirements of most applications.
Different types of closures can be supplied on the same bearing, and nearly all closures are removable and replaceable. Each is manufactured with the same care and precision that goes into our ball bearings. The following are descriptions of the types of shields and seals IBSCO most commonly supplies. For information on special designs that may be suited to your specific applications, consult a member of the Sales Engineering staff.
Z & H Type Shields
A "Z" and "H" designation denotes a non-contact metal shield. "Z" type shields are the simplest form of closure and, for most bearings, are removable. "H" type shields are similar to "Z" types but are not removable.
Shields offer an advantage over seals in some applications because a shield has no interacting surfaces to create drag. This results in no appreciable increase in torque or speed limitations, yielding a bearing that can be compared in operating characteristics to an open ball bearing.
"D" type seals consist of a molded Buna-N lip seal with an integral steel insert. While this closure type provides excellent sealing characteristics, several factors must be considered for its application. The material normally used on this seal has a maximum continuous operating temperature limit of 250°F. It is also capable of providing a better seal than most other types by increasing the seal lip pressure against the inner ring O.D. This can result in a higher bearing torque than with other type seals, which could cause undesirable seal lip heat build-up in high speed applications.
“S” type seals are constructed in the same fashion as “D” type seals. This closure type has the same temperature limitation of 250°F. While it is also impervious to many oils and greases, the same considerations should be noted on lubrication selection. The “S” type seal is uniquely designed to avoid contact on the inner ring land, resulting in significantly reduced torque compared to the “D” type configuration.
"L" type seals are fabricated from glass reinforced TeflonÃƒâ€šÃ‚Â®. When assembled, a very small gap exists between the seal lip and the inner ring O.D. It is common for some contact to occur between these components, resulting in an operating torque increase. The nature of the seal material serves to keep torque increases to a minimum. In addition, the use of this material and seal configuration allows high operating temperatures.
To answer any questions concerning the performance of IBSCO seals in special environments or high speed applications, contact a member of our Sales Engineering staff.
The retainer, also referred to as the cage or separator, is the component of a ball bearing that separates and positions the balls at approximately equal intervals around the bearing's raceway. The most common cages are shown below. In some cases, such as high-load applications, a full compliment design may be the best choice.
For operating speed, please refer to the Nmax/fn values in the product tables and multiplier table in this guide. IBSCO can also supply specially designed cages to meet your specific requirements. If in doubt about the correct choice, contact IBSCO for details on optimum cage selection.
|Description||+||Design||Material||Max. Speed (ref.) dN**||Operating Temp Max.||Comments||Typical Applications|
|Ribbon Two-Piece Stamped, Crimped||R||A.I.S.I. 305 Steel||250,000||900°F||Superior Starting Torque Low Cost||General Purpose|
|Crown One-Piece Stamped||H||A.I.S.I. 410 Steel||250,000||900°F||Higher Speed Capability Than Ribbon Retainer Low Cost||General Purpose|
|Crown One-Piece Stamped||KB
|Phenolic-Paper Base Phenolic-Linen Base||1,200,000||250°F||High Speed Impregnated with Oil||Medical, Machine Tools, High Speed Motors|
|Full Type, One-Piece Machined||M4||Polyamide-imide||2,000,000||500°F||High Speed Capability Requires Lubrication Fully Autoclavable||Medical/Dental High Temperature|
|Crown, One-Piece Machined||M5||Polyamide-imide||1,200,000||500°F||High Speed Capability Requires Lubrication Fully Autoclavable||Medical/Dental High Temperature|
|Full Type, One-Piece Machined||KN
|Phenolic-Paper Base Phenolic-Linen Base||2,000,000||250°F||High Speed, Quiet Running, Angular Contact Bearing Only, Porous Material Impregnated with Oil||Machine Tool Spindles High Speed Motors|
|Crown One-Piece Machined||T1*||PGM High Temp. PGM||Consult with Factory||575°F
|Self-Lubricating||Low-Speed Light Load|
+ Typical Part Number Designation
*Controlled by assigned special design number
**dN is bore (in millimeters) x RPM
Oil is the basic lubricant for ball bearings. At one time most lubricating oil was refined from petroleum. Today synthetic oils with improved properties, including diesters, silicone polymers, and fluorinated compounds have found acceptance. Diesters generally have better low temperature properties than petroleum-based oils, and also exhibit lower volatility and better temperature/viscosity characteristics. Silicones and fluorinated compounds outperform diesters in terms of low volatility and width of temperature/viscosity.
Virtually all petroleum and diester oils contain additives that limit chemical changes, protect the metal from corrosion, and improve physical properties.
Grease is an oil to which a thickener has been added to prevent oil migration from the lubrication site. It is used in situations where frequent replenishment of the lubricant is undesirable or impossible. All of the oil types mentioned in the next section can be used as grease bases with the addition of metallic soaps, synthetic fillers and thickeners. The operative properties of grease depend almost wholly on the base oil. Other factors being equal, the use of grease rather than oil results in higher starting and running torque and can limit the bearing to lower speeds.
Oils and Base Fluids
Petroleum Mineral Lubricants
Petroleum lubricants have excellent load carrying abilities and are naturally corrosion resistant, but are useable only at moderate temperature ranges (-25Ãƒâ€šÃ‚Â° to 250Ãƒâ€šÃ‚Â°F). Greases that use petroleum oils for bases have a high dN (in mm X speed in rpm) capability. Greases of this type would be recommended for use at moderate temperatures, on light to heavy loads at moderate to high speeds.
Super-Refined Petroleum Lubricants
While these lubricants are usable at higher temperatures than petroleum oils (-65Ãƒâ€šÃ‚Â° to 350Ãƒâ€šÃ‚Â°F), they still exhibit the same excellent load capacity. This further refinement eliminates unwanted properties, leaving only the desired chemical chains. Additives are introduced to increase oxidation resistance and other desired attributes.
The esters, diesters and poly-a-olefins are probably the most common synthetic lubricants. They do not have the film strength capacity of a petroleum product, but do have a wide temperature range (-65° to 350°F) and are oxidation resistant.
Synthetic hydrocarbons are finding a greater use in the miniature and instrument ball bearing industry because they have proved to be a superior general purpose lubricant for a variety of speeds, temperatures and environments.
Silicone products are useful over a much wider temperature range (-100° to 400°F), but do not have the load capacity of petroleum lubricants and other synthetics. In recent years, the practice in the instrument and miniature bearing industry has been to derate the dynamic load rating (Cr) of a bearing to 1/3 of the value shown in this catalog if a silicone product is used.
Perfluorinated Polyether (PFPE)
Oils and greases using this specialty lubricant have found wide use in applications that require stability at extremely high temperatures and/or chemical inertness. While PFPE has excellent load carrying capabilities, its inertness makes it less corrosion resistant and less compatible to additives.
Solid Film Lubricants
The term Solid Film Lubricant is used to describe any non-fluids used to prevent wear and reduce friction. They can range from simple sacrificial cages to graphite powder and ion sputtering. Each type must be engineered for the specific application.
Solid film lubricants have definite advantages. They are very useful in areas of temperature extremes, vacuum, radiation, pressure or harsh environments where conventional lubricants would fail. In addition, these lubricants do not deteriorate in storage.
Centrifuging an oil-lubricated bearing removes excess oil, leaving only a very thin film on all surfaces. This method is used on low torque bearings and can be specified for low torque applications.
Vacuum Impregnation of Cages
Vacuum impregnation of a ball bearings with a porous cage forces lubricant into the pores, thus using the cage as an oil reservoir. By using this method with a greased bearing, the cage material is prevented from leaching oil from the grease. The base oil of the grease is normally used in the cage to assure compatibility.
Packing grease into approximately 1/4 to 1/3 of a ball bearing's internal free volume is one of the most common methods of lubrication. Grease quantities are controlled by the use of special lubrication equipment. IBSCO is able to regulate the amount of lubricant to a tolerance of 0.5mg if specified.
The process of grease plating involves lubricating a bearing with a mixture of grease and solvent, then evaporating the solvent at a moderate temperature. The process deposits a thin film of grease on the raceways, balls and cage. Grease plating is used to lower the torque values of grease packed bearings.
Oil plating follows a process similar to grease plating, using oil rather than grease to leave a thin film of oil on raceways, balls and cage. Oil plating greatly lowers torque values of oil lubricated bearings and can be specified for extremely low torque applications.
The maximum usable operating speed of a grease lubricant is dependent on the type of base oil. The speed factor is a function of the bore of the bearing (d) in millimeters (mm) and the speed of the bearing (N) in revolutions per minute (RPM) where: dN = d (bearing bore, mm) x N (RPM).
|Table of fn vs Cage, Lubricant Types and Ring Rotating|
|Metal Cage||Phenolic or Polyimide|
|2-Piece or Crown Type||Crown Type||Full Section Type|
|Lubricant Ring Rotating||Inner||Outer||Inner||Outer||Inner||Outer|
To determine whether a particular bearing will operate satisfactorily at the required speed, multiply the bearing's value (Nmax/fn) by the proper factor taken from the fn vs Cage table shown. Note that the table takes lubricant and cage type into account., The maximum speed Nmax when petroleum or synthetic ester oils are used is dictated by the ball cage material, design, or centrifugal ball loads rather than the lubricant.
For full ball complement types, the listed Nmax values apply regardless of lubricant type or rotation characteristics of the inner or outer ring. For speed limit values Nmax, the Nmax/fn values shown in the product listings must be multiplied by the fn values tabulated above.
|Type||dN||Temperature Range °F (°C)|
|Petroleum||600,000||-25 to +250 (-32 to +121)|
|Diester||400,000||-65 to +350 (-54 to +177)|
|Silicone||200,000||-100 to +400 (-73 to +204)|
|Perfluorinated Polyether||200,000||-112 to +400 (-80 to +204)|
Preload and Duplex Ball Bearings
Ball Bearings are preloaded for a variety of reasons:
- To eliminate radial and axial looseness
- To reduce operating noise
- To improve positioning accuracy
- To reduce repetitive runout
- To reduce the possibility of damage from vibratory loading
- To increase life and axial capacity
- To increase stiffness
A ball bearing may be preloaded through the use of a spring, or through a solid stack of parts. Spring preloading may be accomplished by adding a coil spring or a wavy washer to the assembly, which applies a force against the inner or outer ring of the non-interference fitted bearing. Because the load of a spring the load is fairly consistent over a wide range of compressed length, preloading with a spring eliminates the need for holding tight tolerances on machined parts. In one common application, retaining rings are used in the spindle assembly to save the cost of a locating shoulder, shims or threaded members. A spring would normally not be used when the assembly must withstand reversing thrust loads.
A solid stack method may be used if precise location control is required. In a precision motor, for example, the use of built-in preload is suggested. Ball bearing with built-in preload are often referred to as duplex ball bearings. When the set of bearings is assembled, the thrust load necessary to bring the adjacent faces of the rings into contact becomes the desired preload. Built-in preload helps satisfy design requirements for deflection control and increases in axial and radial stiffness.
There are three methods of mounting preloaded duplex bearings: back-to-back, face-to-face and tandem.
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