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In a ball bearing, load is carried or transferred
from one ring to another through the balls, making these
points of contact critical in bearing design. The size
and shape of these points of contact are controlled
based on numerous factors including raceway curvature,
contact angle, radial play, and preloads.
Figure 1: Raceway Radius and Ball
Diameter Relationship
Raceway curvature is expressed as the relationship between
the radius of the ball track to the ball diameter, in percentage
terms (see Figure 1).
Modifying this relationship can offer larger or smaller points of
contact between the balls and the raceway as the needs of each
application warrant. This value is typically between 52% and
58%. A larger value is applicable where lower torque values
are desired (smaller area of contact) whereas smaller values
permit greater load carrying capability. Please consult our
Application Support
personnel for assistance with this feature.
Radial Play results from a certain amount of intentional
looseness between the rings and the balls of a properly
designed and assembled bearing. It is measured as the
maximum possible displacement of the inner ring in
relation to the outer ring in a direction perpendicular
to the bearing axis. It is sometimes also referred to as
radial internal clearance. It is one of the most
critical but overlooked design decisions affecting
bearing performance.
Radial play specifications must accommodate such
critical issues as the effect of interference fits of
shaft and housing on the inner and outer rings, the
effect of thermal expansion and contraction from ambient
or operating conditions, housing and shaft misalignment,
the degree of radial loading, and the need to create
contact angle within the bearing to accommodate axial
loads. Further, too much radial play can contribute to
excessive noise and vibration in an application.
Figure 2: Contact Angle for a Bearing
under an Axial Load
Contact Angle is the angle of a line drawn through the
points of contact of a ball and each raceway, in
relation to a plane perpendicular to the axis of the
bearing (see Figure 2).
Contact angle is controlled by manipulating the radial
play, the curvature of the raceways, the diameter of the
balls and the axial load. It is created through the
axial movement of the bearing rings in opposing
directions to take up any existing clearance. The larger
the amount of starting clearance there is, generally,
the larger the contact angle and the greater the degree
of axial load capacity and rigidity.
Even in deep groove miniature bearings - intended for
mostly radial loads - the play that exists between the
bearing components allows for some contact angle to
exist, if desired. Increasing the radial play values
permits a greater contact angle, assuming the radial
play is not taken up with improper fits or clearances in
assembly. Typical contact angles for standard clearances
are 10 degrees to 18 degrees. Please consult an
Application Support
specialist for more details on
required contact angles for your particular application.
Preloading is used to remove the internal clearance of a
bearing and is achieved by applying a permanent thrust
load in an axial direction. Preloading is used to
eliminate radial and axial play, increase system
rigidity, reduce run out, increase the assembly’s
tolerance for vibration, and reduce operating noise.
The amount of preload is measured in the amount of force
applied in an axial direction, past the point where
internal clearance is eliminated. This causes the point
of contact between the balls and the raceways to broaden
into an elliptical shape, increasing the area of
load-bearing contact. While this serves many positive
purposes, it can also cause excessive heat generation
and early failure if the preload is not optimized.
Standard offerings of RotoPrecision miniature and
instrument bearings handle preloads of between 0.2 lbs
and 10 lbs. The three common ways to achieve preloading
in bearing assemblies are with springs, with axial
adjustment, and with duplex bearings preloaded from the
manufacturer.
Springs are one of the simplest methods of applying a
preload. They are fitted against the inner or outer ring
of one of the bearings in the assembly; normally the
non-rotating ring to avoid balancing issues. Springs in
combination with retaining rings can be designed to
maintain consistent force over their compression range,
avoiding the need for tight tolerances on other
components.
Where a tight tolerance on the shaft and housing
location is required, a solid stack preload – or axial
adjustment method -can be used. This can vary from
tightening methods such as threaded screws to the use of
solid shims and/or spacers. Solid stacking is a
challenge when dealing with the delicate components of
miniature bearings, so you are encouraged to consult
with our Application Support personnel for assistance
with assembly methods.
Finally, duplexed pairs of matched bearings can be
specified that have the desired amount of preload
built-in. The inner or outer ring faces of the bearings
have been selectively relieved a precise amount called a
preload offset. When the bearings are assembled and
properly tightened into position, the rings position
themselves to create the specified preload. Several
advantages of duplex bearings include the ability to
withstand bi-directional thrust loads, to greatly
increase radial and axial rigidity and to provide
minimum run out while also being simpler to assemble.
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