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specifiers, systems integrators, quality assurance, maintenance personnel
and more. Each issue, our esteemed industry experts will address the
every-day—and the more complex—making-things-move challenges that never
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The inner race diameter for a rolling
element bearing is a design parameter,
and it is chosen by the bearing producer
based on various criteria such
as rolling element distance, number
and diameter of rolling elements, and
what interference fit is supposed to be
used for a given bearing type. Different
(manufacturers) have different ways
and targets when optimizing bearing
performance, and hence there is no
common way to calculate the diameter
the inner race for a given, existing
bearing.
Without knowing the need for the
inner race diameter, a potential use
of that measure is to calculate the frequencies
related to the different contacts
between rolling elements and
raceways, as well as the rotational frequency
of the cage with rolling element
set. These basic frequencies are typically
used when analyzing vibration
measurements of machines as a way to
trace bearings that become damaged.
Since the calculation of these basic frequencies
requires the actual pitch and
rolling element diameters—as well
as the actual contact angle for a given
bearing—the internet homepages of
the larger bearing producers usually
provide help. One example is the SKF
bearing frequencies calculator, available
online or as freeware via iTunes.

The frequencies delivered by such
“bearing frequency calculators” are
based on basic formulae where the
bearing-specific pitch and rolling element
diameters are used. By tradition,
the four frequencies are often called:
“BPFO” (ball pass frequency–outer
ring); “BPFI” (ball pass frequency–inner
ring); “BSF” (ball spin frequency),
referring to the ball. The formulae are,
however, valid for roller bearings as
well, so the term rolling element diameter
is more (precise). The fourth frequency
derived from the same
data is the “FTF” (fundamental
train frequency), which is
the rotational frequency for
the cage with rolling elements.
With a given and disassembled
bearing available, the
rolling element and the inner
race diameters are, typically,
relatively easy to measure; the
pitch diameter is then the sum
of the inner race diameter and
one rolling element diameter.
If the bearing has a contact angle,
however, this is more difficult
to measure—in particular
for bearings with curved raceways;
e.g.—angular contact
ball bearings.