Principles of bearing selection and application Precision When dealing with rolling bearings, precision is described by tolerance classes for running accuracy and dimensional accuracy. Table 1 shows a comparison of the tolerance classes used by SKF and different standards organisations. Most SKF super-precision bearings are manufactured to P4A, P4C or SP tolerance classes. Standard and optional tolerance
classes for SKF super-precision bearings are listed in table 2. Each product chapter provides information about the tolerance classes to which the bearings are manufactured.
Table 1 Comparison of the tolerance classes SKF tolerance class
Standard tolerance classes in accordance with different standards Running accuracy Dimensional accuracy ISO 1) ANSI/ABMA 2) DIN 3) ISO 1) ANSI/ABMA 2)
DIN 3)
P4A P4
24) 4
ABEC 94) ABEC 7
P24) P4
4 4
ABEC 7 ABEC 7
P4 P4
P5 P2
5 2
ABEC 5 ABEC 9
P5 P2
5 2
ABEC 5 ABEC 9
P5 P2
PA9A P4C
2 4
ABEC 9 ABEC 7
P2 P4
2 4
ABEC 9 ABEC 7
P2 P4
SP UP 5)
4 2
ABEC 7 ABEC 9
P4 P2
5 4
ABEC 5 ABEC 7
P5 P4
1) ISO 492 or ISO 199 2) ANSI/ABMA Std. 20 3) DIN 620-2 or DIN 620-3 4) d > 120 mm † ISO 4 or better, ABEC 7 or better, DIN P4 or better 5) Depending on bearing size, running accuracy might be even better.
Table 2 Standard and optional tolerance classes for SKF super-precision bearings Bearing type
Standard t olerance class
Optional t olerance class
Angular contact ball bearings
P4A or P41)
PA9A or P2 1)
Cylindrical roller bearings
SP
UP
Double direction angular contact thrust ball bearings in the BTW series
SP
UP
Double direction angular contact thrust ball bearings in the BTM series
P4C
–
Angular contact thrust ball bearings for screw drives
P4A
–
–
–
Axial-radial cylindrical roller
1) Only for 718 D series 2) Radial run-out equal to
24
bearings 2)
or better than P4, axial run-out close to P4. Reduced axial and radial run-out on request.