polishing by hand using emery cloth can
generate lead if
the paper is not held perpendicular to the axis
of the shaft.
Automated polishing can ensure the paper is perpendicular
to the shaft, resulting in an acceptable, lead-free finish.
An example of what a paper-polished surface would
look like if viewed through a high-power microscope
is shown in Figure 116.
shown in close-up in Figure 117, honing can
be problematic because it generates lead in the
form of a crosshatched surface. This crosshatching
creates channels through which fluid can escape.
turning is unsuitable for shaft finishing
because it always creates lead. If used, machine
turning should always be coupled with an additional
finishing operation that will eliminate lead.
A close-up example of a machine-turned surface
is shown in Figure 118.
the preceding finishing methods, glass bead
blasting doesn’t generate shaft lead.
Unfortunately, it doesn’t remove it, either.
Dimpling of the metal masks lead caused by machine
turning without eliminating it. A close-up example
of a glass bead blasted surface is shown in Figure
peening also camouflages shaft lead caused
by machine turning with dimples. Unlike with
glass bead blasting, however, these dimples have
tiny notches on one side. As shown in close up
in Figure 120, these
notched dimples mimic lead.
tumbling results in a uniform (rather than
dimpled) shaft surface
finish. However, stone tumbling still does
not remove lead. A close-up example of a metal
surface that has undergone stone tumbling can
be seen in Figure 121.
burnishing doesn’t generate lead,
but it also doesn’t remove it. Roller burnishing
compresses – rather than eliminates – lead
grooves. A close-up example of a roller-burnished
surface is shown in Figure 122.
illustrated in Figure 123, through-feed
centerless grinding (also known as transverse
grinding) can create shaft lead if the feeding
process is too fast. Through-feed centerless grinding
can produce lobing (out-of-round shafts).
lapping involves the finishing of a shaft
by rotating it between two rollers of varying
speeds. One of the rollers utilizes an abrasive medium
to wear the metal surface. The grit size of this
medium can be chosen to give the proper surface
texture, but machine lead can be created if the
rollers are not properly aligned. In addition,
machine lapping may not remove enough material
to eliminate lead caused by turning. A close-up
example of machine lapping is shown in Figure
blasting is a process wherein abrasive particles
(such as sand) are shot against metal to compress
the surface and to leave behind tiny indentations
capable of holding lubrication. Unlike glass
bead blasting (which can only be used on unhardened
shafts), grit blasting is possible with hardened
surfaces. Applied correctly, grit blasting can
eliminate mild to moderate (though not major)
machine lead. A close-up view of a grit-blasted
metal surface is illustrated in Figure
compressed metal particles in a mold is sometimes
used to produce porous shafts
capable of holding lubrication. Though this lubrication-holding
ability is helpful to the seal’s lip, it
comes at the expense of increased seepage and metal
impurity, as well as reduced shaft strength. A
close-up view of a sintered metal surface is shown
in Figure 126.
stamping involves use of a stamping die
to form a metal sleeve. Many wear
sleeves are produced in this way. Out-of-roundness
is possible, and draw or work lines (such as
are shown in Figure 127)
on the sleeve surfaces can cause leakage. It
is recommended that the surface on which the
seal rides be plunge ground or paper finished
to form the proper surface texture without machine
grinding has proven to be the most reliable
finishing method for removing machine lead on rotating
shafts. This is because plunge grinding eliminates
any axial movement of the grinding wheel relative
to the surface of the shaft. A mixed number (rather
than whole number) RPM ratio
(for example, 9.5 to 1) between the grinding
wheel and the shaft (which should be rotating
in opposite directions) is suggested to help
prevent the introduction of spirals onto the
shaft surface. Plunge grinding using mixed number
ratios also greatly reduces the time required
to achieve sparkout,
the point where sparks are visible during the
grinding operation. You must leave enough material
on the shaft so that you can grind it to remove
all traces of lead. If all of these recommendations
are followed, then the shaft surface should be
free of lead. A grinding wheel with an 80-grit
size will provide a surface finish of 8 to 17 µin Ra per
An example of a plunge-ground surface is shown
in Figure 128.
noted in Table 34, there
are two other interrelated concepts relating to
shaft finishing. These are out-of-roundness and
chatter. Out-of-roundness (OOR) pertains to the
oval geometry of a lobed shaft. OOR can make it
difficult for the sealing lip of a shaft
seal to maintain proper contact with the shaft,
particularly at elevated shaft
speeds. OOR has two causes: deformation during
assembly, and machining inconsistencies. When OOR
becomes excessive (greater than 45 cycles or lobes
as defined by the RMA) it is considered grinding
chatter (also known as waviness).
RMA specifications are that OOR be less than 0.0050
mm (0.0002 in) at a maximum of 2 lobes, and less
than 0.0025 mm (0.0001 in) at a maximum of 7 lobes. Figure
129 shows an example of a lobed,
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