Rolling bearing

Abstract

The invention aims to provide a rolling bearing that secures a sufficient bearing life economically even when used under such conditions that water from the outside or water formed by moisture condensation may seep into the lubricant or the bearing is affected by the vibrations, and particularly a rolling bearing suited to the electric parts and accessaries of au automobile engine, such as an alternator. To accomplish the object, the hydrogen ion exponent pH of the grease sealed into the inside of the bearing is adjusted in a range of from 7 to 13. For the same purpose, the hydrogen ion exponent pH of the grease is adjusted in a range of from 5 to 13 where a prescribed amount of an organic metal salt or ADTC is added to the grease, where a prescribed amount of an inorganic compound having an average particle size of 2 squarem or smaller is added to the grease, or where a diurea compound containing an aromatic amine or a mixture of the diurea compound is added to the grease as a thickener.

Description

[0001] This application is a continuation of application Ser. No. 09 / 254,172 filed Mar. 2, 1999, the disclosure of which is incorporated herein by reference.[0002] 1. Field of the Invention[0003] This invention relates to a rolling bearing and particularly a rolling bearing which is used under such a condition that water may seep in the lubricant or the bearing is affected by high temperature, high-speed rotation or vibrations and is suitable to electric parts and accessories of an automobile engine such as an alternator.[0004] 2. Description of the Related Art[0005] A rolling bearing has a lubricant sealed in the annular space formed by rolling elements and races thereby to protect the rolling bearing from damage due to seizure and to prevent reduction of bearing life L.[0006] It is generally known that incorporation of water into the lubricant results in great reduction in durability. For example, incorporation of 6% of water into a lubricant reduces the rolling fatigue life of th...

AI Technical Summary

Benefits of technology

[0036] The applicant's experiment in which an alkaline substance is added to a lubricant in increasing amounts revealed that the cathode reaction can be suppressed to improve the rolling fatigue strength when the hydrogen ion exponent pH of the lubricant is in the range of from 7 to 13.

[0038] It is known that an organic metal salt in a lubricant forms a chemical reaction film on the steel surface of a bearing material, which prevents metal-to-metal contact and reduces the coefficient of friction, bringing about improvements in load resistance, seizure resistance, and wear resistance.

[0039] Noting such a function of an organic metal salt, the applicant has further studied extensively and found, as a result, that early flaking of a bearing can be prevented by setting the hydrogen ion exponent pH of a lubricant at 5 or higher where the lubricant contains an organic metal salt. It has also proved that the same effect can be produced when an ash-free dialkyldithiocarbamic acid (ADTC) is added to the lubricant in place of the organic metal salt.

[0041] In addition to the above-mentioned approach relying on cathode reaction control, it is also considered effective for preventing corrosion progress in the bearing site to minimize small crevices that may be formed between non-metallic inclusions and the metal matrix on the bearing material.

[0042] Improvement on oily film formation between the rolling surface and the raceway surface is believed to reduce the tangential force between the rolling surface and the raceway surface thereby to suppress formation of such small crevices.

[0043] Paying attention to this respect, the applicant continued his study and has found that a sufficiently strong oily film can be formed between the rolling surface and the raceway surface by incorporating fine particles of an inorganic compound having an average particle size of 2 .mu.m or smaller into the lubricant and by adjusting the hydrogen ion exponent pH of the lubricant at 5 or higher, whereby the metal-to-metal contact can be prevented to improve the bearing life L under high temperature and high speed conditions.

Problems solved by technology

It is generally known that incorporation of water into the lubricant results in great reduction in durability.

However, in case that the contact rubber seal undergoes deterioration or damage, water can seep into the chock and then into the lubricant of the bearing.

These electric part and accessory bearings are subject to invasion by muddy water or rain water splashed up from a road, and the water pump bearing is subject to invasion by circulating water for engine cooling.

That is, where vibrations are applied during running, the oily film formed between raceways and rolling elements becomes insufficient to impose a tensile stress to the contact area, and where a rotating shaft and the inner race are fitted with a strong interference fit to reduce the rigidity of the bearing housing, a tensile stress always acts on the raceways.

As a result, the bearing may tend to undergo early flaking under the influences of the water content originally present in the lubricant even with no water incorporated from the outside into the lubricant, resulting in a reduced bearing life L.

Therefore, these bearings are very susceptible to the influences of the vibrations and the like.

Bearings for automobile wheels are subject to invasion by water of muddy water or rain water on a road into their lubricant.

Bearings for guide rolls of continuous casting equipment of iron and steel materials and those for back-up rolls of a rolling mill are also subject to invasion by cooling water or rolling water into the lubricant.

Bearings for drier rolls of a paper machine are subject to attack of steam because they are used in a step of drying water-containing wet paper and are therefore liable to early fracture on account of the increased water content in the lubricant (see W. J. Culter, TAPPI Journal, Vol. 79, No. 2, pp.

However, since bearings for drier rolls of a paper machine are usually used under a high temperature condition, it is difficult to apply a contact rubber seal as used in the bearings for the work roll or the bearings for automotive wheels from considerations for heat resistance.

A later survey of actual use of work roll bearings found that seizure accidents reduced drastically, but it turned out that the time until flaking, i.e., the bearing life L was not so improved.

It seems that the reduction of seizure accidents is attributed to reduction of leakage of the lubricant by the contact rubber seal fitted in the inside of the bearings and that the failure to improve the bearing life L is because water seeps in the lubricant to greatly reduce the rolling fatigue strength of the bearing.

That is, the first prior art fails to completely prevent water from seeping into the lubricant.

The rolling fatigue strength of the bearing material reduces as a result, failing to secure durability with a desired bearing life L.

The problem of the second prior art, however, lies in the difficulty in achieving prevention of water's seeping to such an almost perfect degree that the water concentration in the lubricant be kept at 100 ppm or lower.

Accordingly, it meets difficulty in controlling the water concentration in the lubricant at or below 100 ppm as stated above, thus failing to obtain desired durability.

It follows that the grease according to the fourth prior art softens to reduce its vibration damping capacity, resulting in a failure to prevent early flaking of the bearing.

The early flaking combined with water incorporated into the lubricant can cause reduction in bearing life, failing to obtain desired durability.

Size reduction of ancillary engine parts is unavoidably accompanied with a reduction in output.

Further, the increased tension of the driving belt has been increasing the load on the bearings.

In cases where bearings for electric parts and accessaries are used under such high temperature and high speed conditions, softening of grease is accelerated to reduce seizure resistance of the grease, and the damping capacity of the grease also decreases to incur early flaking of the bearings, resulting in a failure to obtain desired durability.

The fifth prior art is in principle similar to the first prior art in respect of use of a contact rubber seal and therefore has the difficulty in perfectly preventing water's seeping.

Stainless steel has lower heat conductivity than low alloy steel and is therefore more liable to seizure fracture.

Therefore, it is difficult to apply stainless steel to those bearings which are used under the above-described poor lubricating conditions that may allow water to enter the lubricant.

Furthermore, since stainless steel is hardened at a high temperature from 1010 to 1070.degree. C. in the production of bearings, a salt bath furnace should be used as a heating furnace, which may incur an increase of cost of production facilities (see Nihon Tekko Kyokai (ed

Additionally the grinding speed of stainless steel is lower on account of the lower heat conductivity as described above, which increases the grinding cost.

The stainless steel itself, being high alloy steel, increases the material cost.

Although it is known, as previously described, that the state of the lubricant's containing water causes reduction of rolling fatigue strength of the bearing material, there is no established theory about this phenomenon.

Where water enters a lubricant, even a trace amount of water can make oily film formation difficult, and rolling elements and races undergo metal-to-metal contact between the rolling surface and the raceway surface.

The surfaces of the rolling elements and races are not homogeneous, and non-metallic inclusions such as oxides and sulfides are unavoidably formed on the rolling surface or raceway surface.

This cause corrosion reaction to occur in that small crevice.

The water content in the lubricant tends to enter these small crevices to cause corrosion reaction.

Corrosion of this kind is apt to develop particularly on the raceway surface of the outer race.

The water content originally present in the lubricant may induce corrosion reaction even if outside water does not seep in.

The corrosion reaction product clogs the inlets to the crevices so that oxygen is hardly supplied from the surface to the crevices.

The bearing material thus undergoes hydrogen embrittlement and reduces its rolling fatigue strength, which causes flaking and reduction of the bearing life L.

If the hydrogen ion exponent pH exceeds 13 on the other hand, caustic corrosion develops to wear the raceway surface and rolling surface, and vibrations during running of the bearing gradually becomes noticeable.

However, where the inner race and the rotating shaft are an interference fit, a tensile stress is always exerted on the raceway of the rotating race.

It is not only technically difficult but economically disadvantageous to control corrosion reaction by using such high alloy steel as stainless steel (SUS 440C) as a bearing material as previously mentioned.

It may be seen as unnecessary to define the upper limit of the content, but the compounds used as a reaction film forming agent are relatively expensive, and excessive addition of the reaction film forming agent may tend to accelerate reaction with the bearing material abnormally to induce corrosion or abnormal wear.

However, mere addition of the reaction film forming agent is insufficient.

If the hydrogen ion concentration of the lubricant is high, that is, if the hydrogen ion exponent pH is low, the rate of production of corrosion products becomes high so that the reaction film is not sufficiently formed in the crevice, resulting in a failure to sufficiently suppress hydrogen absorption inside the bearing material.

On the other hand, even where a polyol ester oil, which exhibits the most excellent lubrication characteristics of the synthetic base oils, is used, if water seeps into the lubricant and the hydrogen ion exponent pH exceeds 13, there is a possibility that the base oil undergoes deterioration by hydrolysis.

If the average particle size is greater than 2 .mu.m, such particles as act as foreign matter increase to accelerate wear of the rolling surface or the raceway surface, tending to cause early damage of the bearing or to deteriorate acoustic characteristics of the bearing.

As mentioned above, addition of particles comprising an inorganic compound to the lubricant contributes to improvement in bearing durability, but the effect as expected cannot be obtained sufficiently if the amount added is less than 0.001 wt %.

If, on the other hand, the amount exceeds 3 wt %, the inorganic compound particles increase in number to accelerate wear, which adversely affects the seizure resistance.

However, similarly to the second mode, mere addition of the particles is not enough.

If the hydrogen ion concentration of the lubricant is high, that is, if the hydrogen ion exponent pH is low, the rate of production of corrosion products becomes high so that a strong reaction film is not sufficiently formed in the crevice, resulting in a failure to sufficiently suppress hydrogen absorption inside the bearing material.

If the aromatic ring molar ratio Z is less than 0.5, the lubricant is liable to leak outside.

That is, leakproofness of the lubricant is not secured.

If the thickener content in the lubricant is less than 8 wt %, the gelling ability is insufficient for obtaining sufficient hardness, and lubricant leakage increases.

If the content exceeds 35 wt %, on the other hand, the durability under high temperature and high speed conditions is deteriorated noticeably.

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