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Spindle design: simplified and
corrected calculation method
Equivalent static load
Should a bearing be subject to combined static loads, the equivalent static load needs to be calculated
to compare it with the bearing's static load capacity.
- Calculating the equivalent static load:
Po = Xo Fr + Yo Fa
Coefficients Xo and Yo are given in the table
opposite. To define them, the ratio Fa/Fr.
A bearing's static load capacity is given as a reference value rather
than an accurate limit that should not be exceeded. It is useful to take
it into account, for instance, in assessing the influence of punctual
loads such as those generated by tool release or bar advance systems.
15°
25°
Fa/Fr Xo Yo
1.09 1 0
>1.09 0.50 0.46
1.31 1 0
>1.31 0.50 0.38
- Basic static capacity for a bearing Co: This is
defined in ISO 76 standard as the radial load that
generates a Hertz pressure of 4,200 MPa at the
most highly loaded point of contact (rotating body
and raceway).
Safety factor: f s = i Co / Po
i: Number of bearings
Co: Basic static load of bearing
Po: Equivalent static load
In principle, the minimum values
for the safety factor f s :
• 2.5 to 3 for spindles in general
• 1 to 1.5 for a short-term axial load.
Corrected calculation method
The ISO 281 standard gives a corrected nominal service life formula L na which is expressed as a
function of the basic nominal service life L 10 : L na = a 1 .a 2 .a 3 .L 10
- Coefficient a 1
Coefficient used to correct a calculation for a
reliability value other than 90 %. This factor is
given in the table below:
Life Reliability Probability a 1
of failure
L 10 90% 10 1.00
L 5 95% 5 0.62
L 4 96% 4 0.53
L 3 97% 3 0.44
L 2 98% 2 0.33
L 1 99% 1 0.21
- Coefficient a 2
Coefficient for correcting calculation according
to material and internal geometry.
For certain applications, a bearing may be manufactured
from a special steel other than conventional
steel, or have a non-standard internal geometry.
These selections can give a much greater service
life than that of a standard bearing.
In this case, a coefficient a 2 which is greater than
1 is applied. This coefficient is calculated according
to experimental results Material a 2
obtained in SNR's
100Cr6 1
research and testing
XD15N 2.8
centers.
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