The present invention relates to a greased rolling element bearing,
in which elements of the bearing such as the bearing cage or roller ends have been
coated with a solid lubricant coating.
Rolling element bearings usually comprise an outer and an inner ring,
rolling elements in the bearing space between said rings, and a cage for holding
the rolling elements at a predetermined mutual spacing as well as a grease for providing
Despite the presence of said grease, during service of such a bearing,
the cage and the rolling elements are in sliding contact. This results in friction
and wear, which limit the service life of such a sliding contact.
Document GB 826 091 A describes cages with a metallic body coated
with a thin layer of plastic such as polyamide or poly tetrafluorethylene containing
about 3% of MoS2 or graphite.
In order to reduce the friction and wear it is known to apply a manganese
phosphate-coating in the cage pockets which contain the rolling elements. However,
such a manganese phosphate coating is not satisfactory for several reasons. One
reason is that these coatings are only useful for "running in" the bearing, e.g.
only provide an initial reduction of the friction at the beginning of the service
life. After a certain period, the phosphate coating will disappear causing metal
to metal contact, a higher running temperature and/or the deterioration of the grease
which is located in the bearing. In some cases this can locally lead to "burnt"
grease and/or dry running of the bearing. Therefore, some grease lubricated roller
bearings with standard manganese phosphate coated steel cages give diminutive tribological
protection in the contacts between roller and cage pocket. This will generally lead
to the formation of brown bands on rollers and raceways. This is attributed to high
sliding contact stress and poor lubrication conditions between roller and cage contacts.
Other factors which adversely influence the condition of the grease
in the bearing, the running temperature, and the useful service life of the bearing
are the high thermal stressing which occurs between the bearing cage and the rolling
elements, and the lack of supply of grease between the components of the bearing.
As a result of these problems, bearings which comprise phosphate-coated
cages should be relubricated frequently; if such frequent lubrication is omitted,
the useful service life of the bearing is impaired.
The object of the invention is therefore to provide a rolling element
bearing which does not suffer from the above disadvantages. This object is achieved
in that the bearing cage is coated with a coating for lowering the running temperature
of the cage, and thus of the bearing.
The invention therefore relates to a bearing cage for a rolling element
bearing, which cage is coated with a solid lubricant containing coating for lowering
the running temperature of the cage containing a disulphide or diselenide of a Group
V or VI transition metal. According to an especially preferred embodiment, the coating
contains MoS2 and/or WS2, optionally in combination with polytetrafluoroethylene
A number of rolling element bearings with MoS2-coatings
present on the cage and/or as a solid lubricant are known in the art, vide for instance
US-A-3 500 525, JP-A-62 141 314 and JP-A-3 255 223. However, all these references
relate to bearings for use in (high) vacuum and/or at elevated temperatures (250°C
or more), which for obvious reasons cannot be subjected to grease lubrication. Such
non-grease lubricated bearings for use in a high vacuum/temperature environment
are not claimed by the present application.
British patent specification GB 1 515 643 A describes bearings of
the ball and roller type, in which the balls and rollers are retained in position
by a cage, said cage having a covering of a low friction plastics material such
as PTFE in order to reduce contact friction.
However, this reference is silent with respect to solid lubricants
containing coatings, such as MoS2 and/or WS2 containing coatings.
As will be seen from the examples hereinbelow, the presence of such a coating, such
as a MoS2/PTFE containing coating, on the bearing cage has a favourable
influence on the properties of the grease in the bearing, also when compared to
a coating containing PTFE alone.
Although not limited thereto, it is believed that the presence of
solid lubricants according to the invention in the polymeric coating provide for
improved affinity with the grease/oil and an increase in load carrying properties
under poor lubrication conditions, thereby increasing the grease life.
Furthermore, it is well known in the art that the wear rate of PTFE
is very poor, so that a bearing with PTFE alone will have a relatively short lifetime
compared to the solid lubricant coatings of the present application.
The term "solid lubricant" is well known in the art and can generally
be defined as a solid material with low friction and low wear rate. The term "solid
lubricant coating" is therefore herein defined as a coating with good adhesion containing
said solid lubricants.
According to the invention said solid lubricant comprises a disulfide
or diselenide of a transitional metal of group V or VI of the periodic table of
the elements, and/or a combination thereof. Especially preferred solid lubricants
are MoS2 and WS2. Such coatings are well known in the art
and are for instance described in the abovementioned references.
The solid lubricant of the invention is preferably present in admixture
with an organic resin, such as PTFE, polyamide, or other polymeric materials such
as polycarboxindole, or polypyrrole. These resins function as a binder so as to
carry the solid lubricant giving a much better bearing performance.
Commercially available examples of MoS2 containing coatings
are the MoS2/PTFE containing coating produced by the Eeonyx-company,
USA, the PTFE/MoS2 coating produced by Dowty, Great Britain (product
1052) and the MoS2 containing Molykote coating 7409 (Dow Corning).
An example of a commercially available WS2 coating is Dichronite.
Such coatings are well known in the art and were used in the prior
art for corrosion protection, and permanent lubrication of bolts, hinges etc. as
well as the lubrication of engine and gear parts, e.g. pistons.
The present invention is based on the surprising finding that during
service, bearings which are coated with solid lubricant coatings according to the
invention still have an effective amount of grease in place on their surfaces, even
though these coated surfaces show less friction than the metallic surfaces usually
present in non-coated bearings. Due to the lower friction of the coating it was
to be expected that the solid lubricant coatings would be less effective in holding
the lubricant into place on the cage.
During service of the bearing, all elements of the bearing which are
in sliding contact should have lubricant present on their contact surfaces. Especially
in for instance rolling element bearings, there is a constant pressure forcing out
the grease due to the centrifugal forces which act on the grease as the bearing
rotates at high speed. It was therefore to be expected that with cages, coated with
a solid lubricant coating, that the grease would be forced out of the critical contact
surfaces and/or even thrown off the cage entirely, therefore reducing the amount
of lubricant available for lubrication.
However, according to the present invention it has now been found
that the negative effect of the solid lubricant coating according to the invention
on holding the lubricant in place is more than offset by the lower running temperature
which is obtained when such a solid lubricant coating is used on the cages. Because
of this, the solid lubricant coating has an overall positive effect on the grease
life, the service life of the bearing as well as the lubrication intervals required.
In this respect coatings containing said solid lubricants in a plastics
material such as PTFE give better results than coatings containing PTFE alone, such
as described in GB patent specification 1,515,643. Due to their higher surface tension,
the MoS2 coatings give better "wetting" of the surfaces of the cage with
the oil and/or grease, improved grease/oil affinity and adhesion and an increase
in load carrying properties, thereby further improving grease life.
The better adhesion of the solid lubricant containing coatings according
to the invention to the coated surfaces is also an important factor in holding the
grease in place.
Besides giving a lower running temperature and better adhesion of
the lubricant, these coatings also provide better adhesion to the substrate (e.g.
the bearing cage) than the phosphate coatings and/or PTFE coatings of the state
of the art, so that they are effective during a longer period of time.
Another advantage of the PTFE/MoS2 coatings is that they
are less hazardous to the environment than, for instance, lead containing coatings.
The invention is, however, not limited to MoS2/PTFE containing
coatings, and any solid lubricant coating according to the invention which gives
a low running temperature of the cage and/or bearing can be used. Such solid lubricant
coatings can be easily determined by a man skilled in he art by means of the methods
described in the Example, i.e. determining the running temperature of the bearing
in a manner known per se. Preferably, the solid lubricant coatings also show good
adhesion to the substrate.
According to the present invention the entire cage can be coated with
the solid lubricant coating, or just the parts and surfaces of the cage which are
in sliding contact with the rolling elements or guide rings, such as the cage pockets
and/or cage bars, as will be clear to a man skilled in the art. Coating the cage
bars with a solid lubricant coating according to the invention can also be used
to create smaller, more accurate cage pockets with lower friction.
The coating according to the invention will give an improvement in
cage performance in ball and roller bearings; which improves the bearing fatigue
life, as can be seen in bearing life tests. The coating according to the invention
can be used in the bore of, for instance, vibrating screen bearings (SRB).
Applying a solid lubricant polymer coating, preferably containing
PTFE sintered with defined thickness 10-15 µm giving a cage pocket with defined
pocket play and a defined cage bar shape, which allows the cage for eccentric running,
gives an ideal combination for improving the rolling element guidance. In doing
so, negative aspects, such as an increased cage friction by a wear process, scraping
off of the lubricant and metal to metal contact, high noise from the cage, and a
low life are avoided by using a solid lubricant coating according to the invention
in this kind of application.
According to a preferred embodiment, both the cage pockets and the
cage bars are coated with a solid lubricant coating according to the invention for
lowering the running temperature.
By coating the cage of a rolling element bearing with such a coating,
the following advantages are obtained:
- a lower running temperature of the cage and the bearing as compared to the manganese
phosphate coatings of the state of the art;
- the grease around the contacts remains in good condition for a longer period
of time resulting in an improvement of the overall lubrication;
- local burning of grease does not occur;
- a longer useful service life of the rolling element bearing.
- a better retention of the grease lubricant in the cage, in particular on the
cage bars which are situated between the rolling elements and which slide along
- longer relubrication intervals;
- lower thermal stressing with respect to the rolling elements;
- better retention of the lubricant on the rolling elements themselves, leading
to a less, even no brown band formation on these elements and the rings;
- the presence of a thicker lubricant layer on the rolling elements, which lubricant
layer will not easily be scraped off by the cage bars, thereby also providing a
better lubrication between the rolling elements and the bearing rings;
- the coated surfaces of the invention can be provided with grooves for transporting
the lubricant to the contact surfaces, giving even better lubrication of the critical
surfaces during service.
In service, during overrolling the lubricant film is slowly moved
out of the contact track. In non-vibrating starved lubricated line contact bearings
and other types of lubricated bearings, the grooves in the coated surfaces could
provide an additional active mechanism to replenish/redistribute the lubricant film,
thereby preventing lubricant film break-down and subsequent bearing failure. By
applying a pattern of grooves on the cage wall the sliding motion between roller
and cage bar will induce a lubricant motion towards the centre and thus concentrate
the lubricant in the center of the track, counteracting the lubricant film break-down
and its consequences described above. By applying the same patterns of grooves on
both the cage bars, the lubricant film transport towards the center of the rolling
track occurs irrespective of the bearing rotation direction. Therefore, these grooves
will give a decrease in contact stress and an increase in both grease and bearing
life. The form and number of the grooves will be dependent on, for instance, the
cage and bearing used and the application of suitable grooves will be apparent to
a man skilled in the art.
The solid lubricant coatings according to the invention can be applied
to the cage by any suitable method, such as dipping, spraying etc., which methods
will be obvious to a man skilled in the art, and/or are described in for instance
the abovementioned prior art. Preferably however, the PTFE/MoS2 coating
is applied by means of "shot peening", which is described in US patent application
5,262,241. Cages coated by this method give even better results than the cages coated
by spraying or dipping.
Preferably, the coating has a thickness of 5-10µm. It is, however,
possible to apply a thicker coating in order to obtain a cage pocket that gives
better guidance to the rolling element while maintaining the solid lubricant according
to the invention. The use of solid lubricant coatings according to the invention
can also greatly improve the tribological performance of roller-flange contact of
taper roller bearing lubrication. This application overcomes the poor lubrication
and wear problems of roller-flange contacts in service, in particular under high
applied axial loading condition. The bearing life improvement can be simply done
by applying the said coatings on the roller ends. The contact temperature will be
reduced, and minimal metallic wear is resulted, and thus, longer lubricant life
The invention therefore further relates to the use of MoS2
and/or WS2 containing coatings as defined in claim 7.
The invention will now be illustrated by means of the following example
in which the performance of the solid lubricant coatings according to the invention
for lowering the running temperature is compared to a manganese phosphate coating
according to the state of the art, as well as a PRCA-coating, which does not give
a lower running temperature and a coating containing PTFE alone. The Figures 1-5
are running temperature graphs vs time for the bearings used in the Examples, showing:
1. Bearing test
- Figure 1: Bearing performance of standard phosphate cages
- Figure 2: Bearing performance of PCRA-coated cages (comparative)
- Figure 3: Bearing performance of Eeonyx-coated cages (invention)
- Figure 4a: Bearing performance of Dowty-coated cages (invention)
- Figure 4b: Bearing performance of Molykote-coated cages (invention)
- Figure 5: Bearing performance of selected coatings on cages
The bearing test with standard and coated cages were conducted according
to the SKF R2F "A" test conditions as shown in Table 1.
2. Test materials
22312 E/C3 SRB
R3 (Shell) / LGMT3 (SKF)
Polymer coated SRB cages based on methylacrylate and/or fluoro-polymer,
and polytetrafluoroethylene (PTFE) / MoS2 were used in this study. Both
proprietary and commercial coating processes based on these materials were used
to produce coatings on cages. Details of the coatings and suppliers are listed in
Coating material manufacturer
Main coating compositions
Polymer Research Corp of America (PRCA), USA
"Chemical grafting" (US-A-3,698,931)
Methylacrylate, Fluoro polymer (proprietary formulations)
Eeonyx (Formerly Preemptive technologies), USA
"Shot peening" (US-A-5,262,241)
Spraying (commercially available)
PTFE/MoS2 (Product 1052)
Mavom, The Netherlands
Dipping (commercially available)
MoS2 (Product 7409)
All test cages used were based on the standard E design. At the time
of the project, plain steel cages i.e. untreated cages could not be obtained, all
polymer/anti-friction coated cages were executed based on pre-phosphate cages. The
phosphate coating was removed at the cage factory (SKF Kogellagerindustrie, Veenendaal,
The Netherlands) by rotor finishing. Table 3 summarises the test program conducted.
3. Performance ranking
* Representative of Dow Corning in the Netherlands
Molykote (7409) + phosphating pretreatment
All tested bearings were ranked according to their performance with
respect to the running temperature (ΔT) and visual damage inspection.
The bearing temperature (outer ring) was continuously monitored and
recorded via computer until the end of the test duration. The pre-set temperature
limit for the termination of test is 200°C. The actual bearing running temperature,
ΔT, reported is the difference between the outer ring surface temperature
and the ambient temperature. Interpretation is generally based on that low running
temperature ensures small solid lubricant losses and a smooth plot usually implies
a good performance.
The rolling elements and the cages of all test bearings were investigated
and graded according to the following levels. The higher the score, the better the
cage performed its tribological function. The ranking was conducted per bearing
side. 5: roller almost as new/little or no wear on roller-cage bar contacts 4: roller
with some scratches/slight wear on roller-cage bar contacts 3: small brown or polished
bands (< 30% roller surface)/small wear on roller-cage bar contacts 2: moderate
brown or polished bands (30-50% roller surface)/medium-high wear on roller-cage
contacts 1: large brown/polished bands (> 50% roller surface)/medium-high wear
on roller-cage contacts
4. Test Results
a. Performance of standard manganese phosphate coated cages (prior
Three bearings with standard phosphate coated cages were tested. The
high temperature peaks observed for all tests (see Fig.1) indicate problems of bearing
lubrication under test conditions. The duration of all temperature peaks observed
is relatively short, and the subsequent drop is probably due to lubricant replenishment.
It is suggested that the rapid rise in temperature is mainly due to the high frictional
losses between roller and cage pocket contacts. The average running temperature,
ΔT, was noted to be in the range of 45-50°C (see Fig. 1).
Visual inspections of the test bearings (nos. 2, 4 & 10) were
conducted. The ranking is shown in Table 4. The grading of 3 and below indicate
the formation of brown bands on rollers and raceways. The phosphate coating on cage
contacts was completely removed and medium/high cage contact wear was observed.
The results of these tests are in agreement with many previous observations for
grease lubricated roller bearing tested under extreme conditions.
b. Performance of PRCA coated cages (comparative)
Figure 2 shows the performance of PRCA grafting polymer coating on
cages. The compositions of these coatings were based on fluoro- and acrylate-polymers.
It can be seen that high running temperature (>50 °C) and high temperature peaks
were recorded for all bearings tested with PRCA coated cages. It is suggested that
the very high running temperature, in particular during the first few days resulted
from a thicker coating compared with the standard coating thickness. The high temperature
peaks are primarily due to the lack of lubricant supply to rolling/sliding contacts
and the coatings applied could not prevent high friction forces generated in contacts.
The ranking of these bearings (nos. 1, 3, 5 & 6) were unsatisfactory as shown
in Table 4. Interestingly, bearing 1 showed no heavy brown bands on rolling elements,
but circumferential wear scratches on rollers. The cage contacts revealed high wear
on coatings and slight to medium wear on the metal of the cage bar. Also, the PRCA-coatings
showed poor adhesion to the metal substrates making them unsuitable for moderate/high
stress sliding applications.
c. Performance of Eeonyx coated cages
A number of experiments were conducted to observe the performance
of the Eeonyx polymer coating on SRB cages. Figure 3 shows the running temperature
of 5 bearings tested. Although few temperature peaks were observed for three bearing
runs, it can be seen that the overall running temperature is low compared with the
standard phosphate coating or the PRCA coatings. The low running temperature implies
solid lubricant between roller and cage contacts, and the temperature peak observed
for the bearings tested is attributed to inconsistent quality of the coating process.
Bearings No.7 and No.12 performed really well under test conditions.
Table 4 shows the performance ranking of this coating type. No brown
bands occurred on the bearing elements of nos. 7 and 12. The other 3 bearings tested,
however, revealed some discoloured bands on only one side of bearing rollers. This
observation is in a very good agreement with the results of bearing running temperature
as discussed above. Bearings performed with ranking 5 showed minimal cage wear,
and a slight wear on steel of the cages is observed for bearing ranking 3. It must
be noted that the thickness of the coating deposited is about 5µm and it is envisaged
that a slightly thicker coating would improve the performance.
d. Performance of other anti-friction coatings
Commercially available coating processes based on PTFE/Mo2S
or MoS2 were also investigated. Figures 4A and 4B show the running temperature
for 4 bearings tested. It can be seen that there is no abnormal or rapid temperature
rise and the overall temperature recorded was in the region of 40°C. This means
that the average running temperature is about 5-10°C lower than the standard phosphate
coating. The cages of bearing 16 were deliberately tested with a residual phosphate
+ MoS2 coating. The test result suggests that the bearing performance
is not significantly affected whether the cages are phosphated prior to MoS2
coating or not. The bearing performance ranking of all bearings tested with these
commercial anti-friction coatings on cages were excellent as shown in Table 4. Minimal
cage wear is observed for all bearing tested.
e. Summary: comparison between selected coated cages
Test Bearing no.
Visual Ranking (bearing conditions
4 & 3
4 & 3
4 & 3
4 & 3
4 & 3
4 & 3
Figure 5 shows a comparison between the bearing performance of non-coated
steel cages, phosphate cages, PTFE/MoS2 cages (Eeonyx, Dowty) and MoS2
(Molykote 7409) coated cages. It can be generally concluded that the MoS2
containing solid lubricant coatings performed better than the standard cages with
or without phosphate coating. The results clearly conclude that a solid lubricant
coating can improve the bearing performance by eliminating the formation of brown
bands on rollers and raceways.
Based on the above results it can be concluded that the problem associated
with the formation of brown bands on rollers and raceways of grease lubricated roller
bearings can be eliminated by means of a solid lubricant coating for lowering the
running temperature, such as a PTFE/MoS2 or MoS2 (Molykote)
coating on bearing cages. Lower running temperature by 5-10°C is expected compared
with manganese phosphate coated cages.
Three processes have been used to deposit the chemicals on to metal
surfaces and bearing cages. The advantage of Eeonyx's process is a fast mechanical
method, it can tailor coating thickness down to 3-5µm and involves no other chemical
carriers. The commercial spraying and dipping processes utilise other chemical binders
and require high temperature curing.