Special Issue; Products for Industrial Machinery - NTN

The Rust Guard Bearings of Highly Corrosion-resistant Bearings
The initial efforts to realize longer life for roll neck
bearings have included an attempt to achieve a higher
bearing capacity by increasing the number of rollers
through adoption of a pin-type cage and efforts to
prevent failure that starts internally by adopting
cleaner bearing materials. However, despite these
efforts, premature failures still occur frequently
because flaking that starts on the surface is
unavoidable. A starting point for flaking could be a
rusty spot or dent resulting from the ingress of water
or foreign matter into the bearings, and the production
sites in steel mills are usually contaminated with much
water (rolling water and cooling water) and foreign
matter (scale and the like). To address this problem,
the lubricant and chock seal were improved, and
sealed bearings have also been incorporated into
components such as the work rolls in cold rolling
machines. Despite these attempts, ingress of water
and foreign matter has been unavoidable.
NTN has already marketed the EA Bearings, which
are specially heat-treated products boasting limited
loss in life even in contaminated conditions. This
product line has attained a good reputation, and, when
installed on a hot rolling machine, this bearing type
has achieved L10 life three times as long compared
with conventional bearings.
Incidentally, the problem known as “downtimeinduced
rust” (Photo 2) should be noted. This results
from the ingress of water into the bearing and occurs
when the bearing is at a standstill while the rolls are
being polished or inspected. To address this type of
rusting, NTN developed a special phosphating
process (hereafter referred to as the “previous
specification”) and has adopted this technique in
conjunction with EA Bearings on actual steel mill
machinery, verifying its effectiveness.
However, despite our efforts, the need to further
improve the corrosion resistance of bearings has been
mounting because much longer maintenance intervals
are needed at the workplaces of steel manufacturers.
3. Features of RustGuard
The author’s newly developed RustGuard coating is
a unique type of coating with improved corrosion
resistance that was developed through further
improvement of the previous special phosphate
bearing coating (previous specification).
A phosphate coating is a type of a chemical
conversion coating, in which chemical reactions with
the compositions of the steel base material form
crystals and an accumulation of these crystals creates
the phosphate coating. Generally, phosphate coating
is used to provide the initial sliding fit for sliding
sections or to prepare a base for the application of
paint. The purpose of phosphating on the bore surface
of the inner ring of ordinary roll neck bearings is to
provide initial sliding fit for sliding sections.
Capable of retaining lubricating oil between its
crystals due to the capillary effect, the phosphate
coating also provides corrosion resistance. However,
because the base material is somewhat attacked
during the initial coating formation process, the
roughness of the base material surface (boundary with
the coating) will deteriorate and can lead to loss in
rolling life even though the oil retention capability of
the base material will be improved. Therefore, the
ordinary phosphate coating cannot be applied to the
raceway surface of a bearing.
To address this problem, NTN has long been
applying a special phosphate coating that does not
lead to loss in rolling life by employing a special
miniaturization technology. Photo 3 shows rollers of
the previous specification that underwent operation for
a duration equivalent to five times as long as the
calculated life L10 and new non-treated rollers. Both
samples were partially submerged for 24 hours in tap
water. From the state of rusting at the water surface of
each sample, it can be seen that the rollers of the
previous specification maintained corrosion resistance
after use.
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Downtimeinduced
rust
Flaking resulting from
downtime-induced rust
Photo 2 Examples of Downtime-Induced Rust