Industrial seal self study guide - SKF.com

Shaft Speed
Maximum speeds for effective seal operation depend on shaft finish,
pressure, temperature, eccentricity, lubricant or fluid being retained,
seal type and other conditions. For example, shaft speeds may be
increased (not to exceed manufacturer upper limit) when shaft finish
is improved or eccentricity (run-out) is reduced.
The surface speed at the contact point between the seal and shaft as
measured in fpm (feet per minute) or mpm (meters per minute) or fps
(feet per second) or M/S (meters per second) is generally a better seal
selection guide than rpm (revolutions per minute).
Shaft Tolerance
It is important to note that machinery built with inch vs. metric
dimensions may use different tolerance standards. In general, the inch
(RMA) system uses a ‘floating’ range based on the nominal shaft diameter
plus or minus some value depending on the base size. With the
metric (ISO) standard, the tolerances for shafts are applied as a plus
zero / minus value (typically ISO h11). While the lip interference
of radial lip seals is relatively less sensitive to shaft diameter variations,
the best recommendation is to specify the correct size for the
tolerancing system used. Refer to the SKF Handbook of Seals for
tolerance tables and other details.
Recommended Shaft Materials
Seals perform best on medium carbon steel (SAE 1035, 1045) or
stainless steel surfaces. Properly finished ceramic-coated and chromeplated
or nickel-plated surfaces are also acceptable. Brass, bronze and
alloys of aluminum, zinc, magnesium or plastics are not recommended.
Surface should be hardened to Rockwell C30 or higher to prevent
handling damage or abrasive wear.
Recommended Shaft Finish
Shaft finish is critical to the proper functioning of a lip-type seal (fig.
7e). It is important to note that not all shaft finishes are good ones.
An improper shaft sealing surface will not allow the seal to function
properly. Generally, the shaft surface should be smooth enough to
maintain effective contact with the seal lip. But it also must be rough
enough to form lubricant holding pockets, without causing excessive
seal wear.
Machine lead is always present after the shaft has been machined to
size on a lathe. It is necessary to dress the shaft surface to remove the
lead introduced during the machining operation.
A new shaft with the proper finish is
shown on the left; normal shaft wear
is shown on the right (fig. 7e).
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