NL9400493A - Use of bismuth joints in high pressure grease lubricant compositions for long life rolling bearing applications. - Google Patents

Description

Use of bismuth joints in high pressure grease lubricant compositions for long life rolling bearing applications.

The invention relates to the use of bismuth compounds in high pressure grease lubricant applications for use in rolling bearings.

In general, in cases where rolling bearings are operated under extreme load conditions and with long maintenance intervals, high pressure grease lubricants are used to ensure that a sufficient amount of lubricant film is always present in the bearing, especially on the raceways and rolling elements thereof.

Such high pressure lubricant compositions generally contain an oil, a soap thickener, one or more high pressure (EP) additives, and optionally further additives. The EP additives form a friction-reducing film on the metal surfaces of the bearing, usually as a result of a chemical reaction of the additives with the surface metals. The function of the added high pressure lubricant additives is to reduce wear and prevent chafing and fusion of contacted surfaces. As such, lead / sulfur containing additives can be used.

However, these lead additives are not acceptable because of their toxicity and from an environmental point of view. Therefore, the lead / sulfur containing additives are now usually replaced by sulfur / phosphorus EP additives. However, it has been found that these sulfur / phosphorus additives shorten the life of the bearings.

High pressure lubricant compositions also contain a soap thickener, such as lithium 12-hydroxystearate, which provides the grease with the desired physical and chemical structure. The grease must be able to maintain this structure in the bearing for as long as possible under high temperature, as well as under vibration and mechanical shearing.

In this regard, it is necessary to maintain the mechanical stability of the soap or the thickener of the fat for a long time. As long as this soap structure can be maintained, the grease is able to hold the oil component in place so that it can provide lubrication properties in a controlled manner.

In cases where the soap structure is damaged, the grease is no longer able to hold the oil in place, which will then drain from the bearing. As a result, the lubrication properties of the grease are lost and the bearing life is significantly reduced if the grease is not replaced at short intervals. Furthermore, the grease must not be chemically aggressive with respect to the metal parts of the bearing, especially with regard to the above-mentioned environment with high temperatures and vibration.

In these respects, the lead / sulfur and sulfur / phosphorus EP additives containing grease lubricant compositions of the prior art are not satisfactory. The object of the invention is therefore to overcome these drawbacks.

It has now been found that the use of bismuth joints in high pressure grease lubricant compositions for rolling bearing applications extends the useful life of rolling bearing applications.

According to the present invention, a bismuth-containing additive and in particular an EP additive, a bismuth-containing soap or both can be used.

Grease containing a bismuth additive is known from NLGI SPOKESMAN, vol. 57. No. 2, May 1993. 0. ROHR "Bismuth, a new metallic but non-toxic replacement for lead as EP additive in greases", biz. 6.50 - 13-57 * This article describes that a bismuth additive promotes film formation on the metal surfaces of a rolling bearing and therefore can serve as a substitute for lead as an additive for high pressures in grease. Indeed, the bismuth additive appears to offer better lubricating properties than a lead additive, especially under conditions of high load, high temperature and high shear rate. The organobismuth compound is also reported to act as a corrosion inhibitor and as an antioxidant.

However, this article is not concerned with the field of the present invention, i.e. providing a long useful life of the greases and thus the life of the bearing. While the bismuth additive has been reported to have a beneficial effect on the lubricity of the oil component of the grease, no mention is made of beneficial effects on the life of the bearing.

The use of bismuth additives in a lubricant is also disclosed in SU-A-1.384.603- Here, bismuth is added to lubricating oil compounds for surfaces that are in sliding contact with each other. Although the bismuth additives are reported to reduce friction in a sliding contact, no mention is made of greases or of obtaining a longer life thereof.

The use of a bismuth-containing soap has not been described in the prior art.

According to a first aspect, the invention therefore relates to the use of bismuth compounds in a high pressure lubricant for rolling bearing applications as an additive to extend the useful life of rolling bearings.

According to a second aspect, the invention relates to the use of a bismuth-containing soap in a high pressure lubricant for rolling bearing applications to extend the useful life of rolling bearings.

According to a further aspect, the invention relates to a process for preparing a high pressure lubricant, wherein a bismuth-containing soap is mixed with an oil and optionally one or more EP or other additives. When a bismuth-containing soap is used, it is not strictly necessary to use Bi-containing or other EP additives.

In all these aspects of the invention, the presence of bismuth compounds has a beneficial effect on the useful life of rolling bearing applications.

When the bismuth compounds are used as EP additives, these compounds are generally bismuth carboxylates of the formula (R-CO2) 3Bi, wherein R is a branched, straight or cyclic alkyl group of 1-30 carbon atoms or an aryl, alkaryl or aralkyl group having 5 ~ 20 carbon atoms. Paraffinic bismuth carboxylates with 6-10 carbon atoms or naphthenic bismuth carboxylates are preferred, such as bismuth naphthenate and bismuth octoate. However, the use of bismuth compounds as EP additives according to the invention is not limited to the bismuth compounds mentioned above, and other organobismuth compounds can be used, such as the bismuth containing additives known in the above prior art, or compounds which are analogous with known lead / sulfur EP additives in which the lead has been replaced by bismuth.

The bismuth-containing EP additives are used to partially or completely replace known EP additives, such as lead / sulfur additives or sulfur / phosphorus additives, in high pressure lubricant compositions.

When the bismuth compound is used as an EP additive, it can be added to the soap, the oil or to the already formed mixture of the oil and soap thickener. Usually and preferably the additive is mixed with the oil.

The bismuth containing EP additives are used in the usual amounts, generally the amount of bismuth will be 0.1-5% by weight of the total lubricant composition. Mixtures of one or more bismuth compounds can be used, optionally mixed with one or more other EP additives. Furthermore, if desired, conventional additives for lubricant compositions can be used in conventional amounts.

The bismuth-containing EP additives can be soluble or insoluble in the oil component of the lubricant composition.

The bismuth-containing EP additives can also be used in high-pressure lubricant compositions containing, for example, non-soap thickeners, such as polyurea-based compounds, polytetrafluoroethylene or silicone, instead of a soap as a thickener.

When a bismuth-containing soap according to the invention is used, this soap is usually a bismuth salt of a fatty acid with 10-30 carbon atoms or a derivative thereof. Usually, the bismuth analogues of known soap thickeners are used, in which bismuth replaces the metal, ie barium, aluminum, calcium, lithium, sodium, strontium, etc. Examples are bismuth stearate, bismuth tristearate, bismuth tripalmite, bismuth trioleate and derivatives thereof, such as bismuth -12-hydroxystearate. The bismuth soap can contain the same or different fatty acid groups. Mixtures of bismuth-containing soaps can also be used.

The bismuth-containing soap is used in the usual amounts depending on the desired properties of the final lubricant composition. Generally, this amount will vary between 5-14 wt% of the total composition for a "soft" fat to 15-25 wt% of the total composition for a "stiff" fat.

The bismuth-containing soap can also be used in combination with known soaps containing other metals, such as lithium soaps or calcium soaps in a fat formulation. This reduces the cost of the final high pressure lubricant composition.

The invention further relates to a method of preparing a high pressure lubricant composition in which a bismuth-containing soap or thickener is mixed with an oil and optionally one or more EP or other additives. Mixing of the soap, oil and thickener can be performed in a manner known per se for preparing EP fats of the prior art. Preferably, a bismuth-containing EP additive as mentioned above is used. The bismuth-containing soap, the oil and the EP additives and other additives are used in the usual amounts.

According to the invention, it was found that the beneficial effects of the use of bismuth compounds on the useful life are mainly due to the improved mechanical, physical and chemical stability of the fat (the oil / thickener composition). In contrast to the friction-reducing "surface effects" mentioned above, this "bulk effect" has not been described in the prior art. The exact mechanism thereof is not known, but the advantageous influence of the presence of bismuth compounds is obtained with both bismuth-containing soaps and bismuth-containing EP additives.

The bearing life is further enhanced by the beneficial effect of the bismuth additive on stress corrosion and fatigue.

Furthermore, the replacement of lead by bismuth is an improvement with regard to the non-toxic properties of the latter.

The invention will now be illustrated by the following non-limiting Example 1, in which the impact on the life of roller bearings of three bismuth-containing lubricant compositions is compared to a sulfur / phosphorus-containing lubricant composition according to the prior art. Technic. Non-limiting Example 2 illustrates the preparation of a bismuth-containing soap according to the invention.

The figures further illustrate the invention and show: 1: the influence of bismuth additives on the life of the bearing;

Fig. 2: monitoring the condition of the bearing during a DGBB test;

Fig. 3 · * a graph of the total trend value against the duration of the test;

Fig. 4: a diagram of the temperature against the duration of the test.

Example I

Several tests have been conducted to demonstrate the effect of a bismuth additive. The following samples were used in these tests:

Sample 1: Lithium based fat, antioxidant (0.5 wt%) + organobismuth and sulfur additive (0.5 wt # Bi) from Miracema, Brazil.

Sample 2: Lithium based grease: antioxidant (0.1 wt. #), Anti-rust (3.2 wt.%), Organobismuth (0.5 wt.% Bi) from Miracema, Brazil or from Pharmacie Centrale de France .

Sample 3: Mineral based oil + antioxidant + (0.5-2% by weight Bi) bismuth carboxylate, e.g. Liovac 3024, from Miracema, Brazil.

Sample 4: Fully formulated, commercially available high pressure lithium soap (E / P) grease containing a sulfur / phosphorus (S / P) EP additive package.

The fat samples 1, 2 and 4 are further described in Table B.

The above grease samples 1, 2 and 4 were then used for bearing service life tests under high load and high temperature conditions, as shown in Table A below:

Table A

Type of bearing: deep groove ball bearing (DGBB 6206 2RS1)

Speed: 2500 rpm

Radial load: 6000 N.

Test temperature: 120 ± 2 ° C (outer ring)

Filling with fat: 2.4 g

Strategy: group of 2 with a sudden-death factor, in which sets of two bearings are tested simultaneously until one of the bearings fails.

Failure type: bearing noise (due to fatigue) rise in temperature (fat failure) C / P: 3.25

Kappa: 0.75 (calculated according to the basic rheology for oil, assuming completely liquid conditions.)

Relubrication of the grease: none

The test results are shown in Figure 1. It is clear that the statistical values for the L10 and L50 life (which includes both fat life and fat life) of fat samples 1 and 2 according to the invention are significantly better than those for sample 4. (L10 = time to failure of 10% of samples in hours; L50 = time to failure of $ 0% of samples in hours).

Further DGBB tests for a range of fats containing bismuth additives were performed. A sample size of the bearings of 10 bearings / grease was used in 5 subgroups. The test performance of a bearing was compared between SKF LGEP2 and two experimentally formulated greases containing bismuth additives shown in Table B.

Table C shows the test life results of a bearing of SKF LGEP2 (sample 4) and the two Bi greases (samples 1 and 2). This shows that the observed L10 life of both bismuth fats is about twice as long as that of the reference fat LGEP2. Based on the statistical hypothesis test procedures, the comparison of the L10 life between LGEP2 and the bismuth fats is classified as weak significant side, ie the probability of significance is greater than 80% but less than 902 · The test equation of the L50 lifetime between LGEP2 and Sample 3 is statistically significant (ie probability> 9C $).

Table B

Note: * special batch prepared by AB Axel Christiernsson

Table C

In the case of testing a bearing with DGBBs, a complex failure of the bearing is observed. Some bearings collapsed due to grease breakdown and some collapsed due to ball / inner ring / outer ring pitting (see Table C). Figure 2 shows the spectrum diagram of a test bearing (# 22). This shows that the frequency of 205 Hz is due to a bullet defect, the frequency of the outer ring defect is 157 Hz, and the frequency of 230 Hz is attributed to the inner ring defect. After examining the collapsed lower, the spectrum data confirmed. In general, all bearings that have collapsed due to rolling contact fatigue have been detected and recorded by the SKF CoMo monitoring system. Figure 3 shows a plot over time versus total energy of the spectrum for a lower (# 23). This was the criterion used to terminate the test automatically when the total increase in value reached an alarm level (A2), as shown.

Bearings that succumb primarily to "dry running" can be detected with the continuous temperature measurement system. Figure 4 shows a diagram of the temperature recorded versus time. The machine of a test group of bearings (# 3 and # 4) stopped due to a sudden increase in temperature of bearing # 3 after about 680 hours of testing.

The difference in fat life observed between LGEP2 and Bi fats cannot be attributed solely to the presence of different additive systems, due to many variables that could not be precisely controlled under the test conditions, e.g. lack of lubricant, amount of "active" grease in the bearing after initial greasing, channel loss, etc. It should be noted that thermal and mechanical stress, oxidation and breakdown of the lubricant and separation and migration of oil also significantly increase the life and performance of grease can influence.

Analysis of the collapsed DGBBs reveals that the failure of a bearing is the result of a combination of fat failure (dry running) and contact fatigue. Interestingly, all bearings tested with Bi-grease sample 1 succumbed and the bearings tested with the other Bi-grease (sample 2) succumbed mainly as a result of "running dry". This suggests that the presence of an "active" sulfur additive in grease may cause problems by decreasing thermal / oxidation and mechanical stability, as well as promoting fatigue on rolling contact.

Fat samples 1 and 2 have also been shown to provide better shear stability than fat sample 4 containing an S / P additive. The poor results of sample 4 are due to the softening of the fat structure resulting in excessive oil leakage from the fat.

The mechanical stability of Bi fats was evaluated using the SHELL roll stability tester. The results show that the consistency of the SKF LGEP2 grease changes from 2 to a liquid-like lubricant less than "00" for 50 hours at 80 ° C under test conditions. This poor inherent property of the fat is mainly due to the nature of the soap thickener and to some extent the S / P additive package used in the fat. Bi-EP greases showed a significant improvement in mechanical stability.

To determine flange roll wear prevention of bismuth-containing lubricant compositions, an oil containing an organobismuth additive, sample 3, was tested in an SKF R3 testing machine. The test conditions are shown in Table D.

Table D:

Conditions of the flange-roller contact test:

Type of bearing: tapered roller bearing (580/572);

Speed: 2500 rpm

Test temperature: 75 ± 2eC (outer ring) C / P: 2

Kappa: 1.2

Speed of oil supply: at equilibrium 1 liter / min

Sample 3 was shown to extend the life of a ball bearing of flange roller bearings under these conditions and avoid the possible negative effects of an S / P-EP additive on its life.

Example II

Preparation of bismuth soaps:

Bismuth soap is prepared like other metal soaps, such as lead soap. This preparation can be carried out either in an open vessel or in an autoclave. Normal bismuth based fat can be prepared by: 1. saponification - reaction of bismuth compounds, such as bismuth oxides and bismuth hydroxide, with selected acids, such as fatty acids or glycerides. Typically used metal soaps such as the bismuth soap of 12-hydroxystearic acid, bismuth stearate, bismuth oleate are prepared. The soap at this stage contains approximately. 25_50 percent bismuth; 2. dispersing the soap in oil - this is done by adding an oil or oils to the bismuth concentrate at about 150-16CPC, then cooling (usually stirring for further addition of other additives) to room temperature ; 3- the cooled grease is then passed through a mill and a filter system.

The mixed, complex, bismuth based greases can also be prepared by the manufacturing technology of other metal soaps, such as lead or lithium.

Claims (12)

1. Use of bismuth joints in a high pressure grease lubricant composition for rolling bearing applications to extend the useful life of rolling bearings. Use of bismuth joints according to claim 1 for extending the fatigue life of rolling element bearings. Use of bismuth compounds according to claim 1 for extending the life of rolling bearings upon oxidation at elevated temperatures. Use of bismuth joints according to claim 1 for extending the life of the axial bearings of rollers and flanges of wear under axial load. Use of bismuth joints according to claim 1 for protecting metal components of rolling bearings from stress corrosion cracking. Use of bismuth joints according to claim 1 for protecting metal components of rolling bearings from chemical attack. Use of bismuth compounds according to claim 1 for extending the useful life of a high pressure lubricant composition for rolling bearing applications. Use of bismuth compounds according to claim 1 and / or 7 to improve the high pressure characteristics of the soap or thickener component in the high pressure lubricant composition. Use of bismuth compounds according to claims 1, 7 and / or 8 for increasing the shear stability of the soap or thickener component in the fat. Use of bismuth compounds according to claims 1-9. wherein the bismuth compound is used as an additive, preferably a high pressure additive (EP additive). Use of bismuth compounds according to claims' 1-9, wherein the bismuth compound is used as a soap or thickener. 12. A process for preparing a high pressure lubricant composition in which a bismuth-containing soap or thickener is mixed with an oil and optionally one or more EP or other additives.