Industrial seal self study guide - SKF.com

fig. 7A
Shaft surfaces must be
smooth and free of nicks
and rough spots
The shaft corners
must be burr-free
.070 - .120 INCH
(1.7B - 3.05mm)
Burr-free radius alternatives
can also be used (fig. 7b).
B
fig. 7B
A dial indicator is used to
measure STBM (fig. 7c).
15-30˚
R.125 INCH
(3.18mm)
Dynamic run-out is also measured
using a dial indicator (fig. 7d).
A
Chapter 7—Shaft
and Bore Conditions
In addition to application parameters and environmental factors, shaft
and bore specifications are important to proper seal selection.
Shaft Conditions
Sealing Surface Requirements
SKF produces thousands of seals in designs, sizes and lip materials to
meet the most severe operating conditions.
For proper seal performance, three factors must be considered:
1. The shaft surface has no machine lead (grooves running
diagonally toward or away from the sealing lip).
2. The entrance edge is chamfered or rounded.
3. The surface finish is 8-17 micro inch Ra (.20 μm to .43μm).
Shaft Requirements
A burr-free chamfer (fig. 7a) or radius (fig. 7b) is required as
illustrated (c = chamfer depth).
Because the seal’s inside diameter is difficult to measure and varies
with seal designs, the shaft diameter for which the seal was designed
is used as the cataloged inside dimension.
Shaft Eccentricity
Two types of shaft eccentricity affect seal performance. They are:
• Shaft-to-bore misalignment (STBM)—The amount by which the
shaft is off center, with respect to the center of the bore. This
somewhat common occurrence is caused by machining and assembly
inaccuracies. To measure, attach a dial indicator to the shaft (between
the shaft and bore), rotate the shaft and read the indicator (fig. 7c).
• Dynamic run-out (DRO)—DRO is the measure of the amount
by which the shaft does not rotate around the true center. The
motion away from the center may be in more than one direction.
Misalignment, in-line boring tolerances, shaft bending, lack of shaft
balance and other manufacturing inaccuracies are common causes.
To measure, slowly rotate the shaft and calculate total movement
of an indicator attached to the bore and held against the shaft’s
side (fig. 7d).
For specific limitations on both shaft-to-bore misalignment and
dynamic run-out and the total eccentricity (combined STBM and DRO
readings) indicator, refer to the SKF Handbook of Seals.
60