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Ball Bearing Nomenclature || Dynaroll's Technical Overview of Bearings




Ball bearings are rotational components that contribute to the high performance and long lifetime of mechanical assemblies. In all but the most low-end of products, bearings are used with parts that need to rotate. Different types of bearings exist for different purposes: standard roller ball bearings, shielded bearings, sealed bearings, thrust bearings, collar bearings and flangette collar bearings. Bearings can be found in many household items, from blenders to drawer slides to computer disk drives.

Bearings are specified to handle certain maximum amounts of radial and axial (thrust) loads. Typically, ball bearings transfer their loads from their outer races to the balls, then from the balls to their inner races. Since the balls are spheres, they only make contact with the inner and outer races at very limited points, a characteristic that allows the bearings to spin smoothly. Because of a bearing's limited contact area, it can be easily overloaded leading to deformation (i.e. ball flattening), poor performance and eventual failure, so it is important for the person selecting bearings to check load ratings and other parameters.

Single-row deep groove ball bearings are the most common type of rolling bearing, having a wide range of suitable applications. The raceway grooves on both inner and outer rings have circular arcs of slightly larger radius than that of the balls. In addition to radial loads, axial loads can be applied in either direction. Because of their low torque, they offer high speed operation with low power loss.

Miniature and instrument bearings are made with very high levels of precision, therefore these are generally used in applications where accurate rotational performance and low torque is necessary and where load capacity is a secondary issue.


Click on links below for technical explanations.

Symbol Description
d Ball Bearing bore
D Bearing outer diameter
Df Flange diameter
W Open bearing width
Ws Shielded bearing width
Wi Extended inner race width (open)
Wis Extended inner race width (shielded)
Wf Flange width (open)
Wis Flange width (shielded)
li Land diameter (inner race)
lo Land diameter (outer race)
r Max. mating fillet radius (typical)
H Thrust bearing height
Symbol Description
Dw Ball diameter
N Number of balls per row
Ri Raceway radius (inner race)
Ro Raceway radius (outer race)
fi Raceway curvature ratio (inner race)
fo Raceway curvature ratio (outer race
B Total bearing raceway curvature
r Radial play
a Axial play
Symbol Description
α Ball contact angle
αo Initial ball contact angle
dp Ball pitch diameter
uf Free angle of misalignment
Fr Radial load
Fa Axial or thrust load
d Axial displacement under thrust load
dN Speed factor
fn Lubricant factor
L10 Bearing fatigue life (90% reliability)
Lna Bearing fatigue life after normal adjustments
a1 Material and reliability life correction factor
a2 Operating conditions life correction factor
P Equivalent load
Pr Equivalent radial load
Pa Equivalent axial load
C Dynamic radial load rating
Ct C derated for higher temperature
Co Static radial load rating
Cs Static axial load rating
ft Temperature correction factor
fp Interference factor