Lubricants for Improving Fatigue Life

The present invention relates to lubricants for improving fatigue life, and more particularly to lubricants for improving fatigue life which comprises lubricating base materials and specific polar compounds.

For gears used in machine tools, construction machines, automobiles, machines for ships, etc. as well as industrial machines such as rolling mills have heretofore been used lubricants having superior scoring properties so that the gears can withstand high pressure exerted on the tooth faces thereof. Further development of lubricants having more improved scoring properties have been continuously made.

Examination of failures of gears has revealed that scoring occurs only limitedly; rather the fatigue phenomenon such as pitting, is mainly responsible for various troubles. This fatigue phenomenon is also observed for bearings. In connection with the fatigue phenomenon, it is generally known that the fatigue life is prolonged when the viscosity of the lubricant used is increased, whereas the life is reversely shortened when those lubricants containing polar compounds such as extreme pressure additives are used.

Accordingly, as to a procedure for improving the fatigue life of gears, bearings, etc., there have been known no other effective procedures than that in which the amounts of extreme pressure additives, etc. being added are controlled as low as possible, and lubricants having a high viscosity are employed.

As a result of extensive investigations to develop lubricants which overcome the above-described defects of the conventional lubricants, it has been found that the use of lubricants comprising lubricating base materials and specific polar compounds enables to greatly prolong the fatigue life of gears, etc. without increasing the viscosities of the lubricants.

The present invention relates to: (I) a lubricant for improving fatigue life which comprises:

(1) a lubricating base material; and

(2) a compound having alkylthiocarbamoyl groups (hereinafter referred to as "Compound (a)") represented by the general formula (i): ##STR4## (wherein R.sup.1, R.sup.2, R.sup.3, and R.sup.4 are each an alkyl group containing from 1 to 20 carbon atoms, R.sup.1 and R.sup.2, or R.sup.3 and R.sup.4 may be bound to each other to form a ring, and (A) is S, S--S, S--S--S, S--S--S--S, S--S--S--S--S, S--S--S--S--S--S, S--CH.sub.2 --S, S--CH.sub.2 CH.sub.2 --S, or S--CH.sub.2 CH.sub.2 CH.sub.2 --S); and (II) a lubricant for improving fatigue life which comprises:

(1) a lubricating base material;

(2) Compound (a) represented by the general formula (i);

(3) a molybdenum compound (hereinafter referred to as "Compound (b)") represented by the general formula (ii): ##STR5## (wherein R.sup.5 and R.sup.6 each contain from 1 to 30 carbon atoms and are each an alkyl group, a cycloalkyl group, an aryl group, or an alkylaryl group, and m is a real number meeting the requirement of 0.ltoreq.m.ltoreq.4) or the general formula (iii): ##STR6## (wherein R.sup.7 and R.sup.8 each contain from 1 to 30 carbon atoms and are each an alkyl group, a cycloalkyl group, an aryl group, or an alkylaryl group, and n is a real number meeting the requirement of 0.ltoreq.n.ltoreq.4); and/or

(4) an ester compound (hereinafter referred to as "Compound (c)").

The FIGURE is a schematic view of a two-cylinder testing machine used in the Examples and Comparative Examples.

The reference numerals are as follows:

1--Test specimen on the driving cylinder,

2--Test specimen on the cylinder to be driven,

3--Load lever,

4--Weight,

5--Transmission gear,

6--Universal joint,

7--V-shaped belt,

8--Filter,

9--Oil tank,

10--Heater.

The present invention provides:

(I) a lubricant for improving fatigue life (hereinafter referred to as "Lubricant (I)") which comprises (1) a lubricating base material and (2) Compound (a) represented by the general formula (i); and

(II) a lubricant for improving fatigue life (hereinafter referred to as "Lubricant (II)") which comprises (1) a lubricating base material, (2) Compound (a) represented by the general formula (i), (3) Compound (b) represented by the general formula (ii) or (iii), and/or (4) Compound (c).

Various types of lubricants, greases, etc. can be used as lubricating base materials in the present invention. In addition to mineral oils, synthetic oils can be used, and furthermore, their mixtures can be used.

Specific examples of Compound (a) represented by the general formula (i) are

methylenebis(dibutyldithiocarbamate) represented by ##STR7## ethylenebis(dibutyldithiocarbamate) represented by ##STR8## trimethylenebis(dibutyldithiocarbamate) represented by ##STR9## bis(dimethylthiocarbamoyl)monosulfide represented by ##STR10## bis(dimethylthiocarbamoyl)disulfide represented by ##STR11## bis(dimethylthiocarbamoyl)trisulfide represented by ##STR12## bis(dibutylthiocarbamoyl)disulfide represented by ##STR13## bis(diamylthiocarbamoyl)disulfide represented by ##STR14## bis(dioctylthiocarbamoyl)disulfide represented by ##STR15## bis(didecylthiocarbamoyl)disulfide represented by ##STR16## bis(dioctadecylthiocarbamoyl)disulfide represented by ##STR17## methylenebis(butyloctyldithiocarbamate) represented by ##STR18## diethylthiocarbamoyl diamylthiocarbamoyl disulfide represented by ##STR19## dibutylthiocarbamoyl dinonylthiocarbamoyl disulfide represented by ##STR20## and di(pentamethylenethiocarbamoyl)hexasulfide represented by ##STR21## The amount of Compound (a) being added is not critical; it is usually from 0.01 to 10% by weight, preferably from 0.05 to 5% by weight, based on the total amount of the lubricant.

Lubricant (I) of the invention is prepared by adding Compound (a) to the lubricating base material. By using Lubricant (I) of the invention as a lubricant for gears, etc., the fatigue life of the gears, etc. can be prolonged.

Lubricant (II) of the invention is prepared by adding Compound (b) and/or Compound (c) as well as Compound (a) to the lubricating base material. Compound (b) as used herein refers to molybdenum compounds represented by the general formula (ii) or (iii).

Typical examples of the molybdenum compounds represented by the general formula (ii) include thiomolybdenum diethyldithiocarbamic acid sulfide represented by ##STR22## thiomolybdenum dipropyldithiocarbamic acid sulfide represented by ##STR23## thiomolybdenum diamyldithiocarbamic acid sulfide represented by ##STR24## thiomolybdenum di(2-ethylhexyl)dithiocarbamic acid sulfide represented by ##STR25## wherein EH is 2-ethylhexyl group), thiomolybdenum dilauryldithiocarbamic acid sulfide represented by ##STR26## thiomolybdenum diethyldithiocarbamic acid oxide represented by ##STR27## thiomolybdenum oxymolybdenum diethyldithiocarbamic acid oxide represented by ##STR28## oxymolybdenum diethyldithiocarbamic acid oxide represented by ##STR29## thiomolybdenum di(nonylphenyl)carbamic acid sulfide represented by ##STR30## (wherein NP is nonylphenyl group), thiomolybdenum propyloctyldithiocarbamic acid sulfide represented by ##STR31## thiomolybdenum dioctacosyldithiocarbamic acid sulfide represented by ##STR32## and thiomolybdenum di(cyclohexyl)dithiocarbamic acid sulfide represented by ##STR33##

Typical examples of the molybdenum compounds represented by the general formula (iii) include oxymolybdenum diisopropylphosphoro dithioate sulfide represented by ##STR34## (wherein i-Pr is isopropyl group), oxymolybdenum diisobutylphosphoro dithioate sulfide represented by ##STR35## (wherein i-Bu is isobutyl group), oxymolybdenum di(2-ethylhexyl)phosphoro dithioate sulfide represented by ##STR36## (wherein EH is 2-ethylhexyl group), oxymolybdenum di(p-tert.-butylphenyl)phosphoro dithioate sulfide represented by ##STR37## (wherein BP is p-tert.-butylphenyl group), oxymolybdenum di(nonylphenyl)phosphoro dithioate sulfide represented by ##STR38## (wherein NP is nonylphenyl group), and oxymolybdenum ditetracosylphosphoro dithioate sulfide represented by ##STR39##

Compound (c) as used herein refers to ester compounds. Typical examples of the ester compounds are phosphoric acid esters or their amine salts, esters of higher aliphatic acids and alcohols, and dibasic acid esters. Specific examples of the phosphoric acid esters include monobutyl acid phosphate represented by ##STR40## mono-2-ethylhexyl acid phosphate represented by ##STR41## monolauryl acid phosphate represented by ##STR42## dibutyl hydrogen phosphite represented by ##STR43## monooleyl acid phosphate represented by ##STR44## dilauryl hydrogen phosphite represented by ##STR45## distearyl hydrogen phosphite represented by ##STR46## dioleyl hydrogen phosphite represented by ##STR47## diphenyl hydrogen phosphite represented by ##STR48## dibutyl acid phosphate represented by ##STR49## di-2-ethylhexyl acid phosphate represented by ##STR50## dilauryl acid phosphate represented by ##STR51## dioleyl acid phosphate represented by ##STR52## diethylamine salt of dibutyl acid phosphate represented by ##STR53## and mono tertiary C.sub.12 -C.sub.14 alkyl amine salt of a mixture of monooleyl acid phosphate and dioleyl acid phosphate. The higher aliphatic acids used preferably in the esters of higher aliphatic acids and alcohols are higher aliphatic acids which are in the solid state at room temperatures and have more than 9 carbon atoms. More preferably, they have from 12 to 22 carbon atoms. The alcohols used preferably in the esters of higher aliphatic acids and alcohols have less than 19 carbon atoms. Specific examples include isopropyl palmitate, butyl palmitate, octyl palmitate, myristyl palmitate, isopropyl myristate, myristyl myristate, methyl stearate, butyl stearate, octyl sterate, methyl oleate, lauryl oleate, oleyl oleate, and oleyl behenate. Specific examples of dibasic acid esters include dibutyl sebacate, di-2-ethylhexyl sebacate, dioctyl sebacate, di-2-ethylhexyl adipate, and dioctyl adipate.

In Lubricant (II) of the invention, any one or both of Compounds (b) and (c) are added. The amount of each of Compounds (b) and (c) being added is not critical and can be determined appropriately. For example, the amount of Compound (b) added is from 0.01 to 20% by weight, preferably from 0.05 to 5% by weight based on the total amount of the lubricant, and the amount of Compound (c) added is from 0.05 to 50% by weight, preferably from 0.1 to 20% by weight based on the total amount of the lubricant.

In the lubricants of the invention, in addition to the above-described components, if necessary, extreme pressure additives, detergent dispersants, antioxidants, corrosion inhibitors, etc. can be added appropriately.

When Lubricant (I) or (II) of the invention is applied on the surface of each mechanical part, e.g., gears and bearings, by techniques such as coating, spraying, and dipping, the fatigue life of the part can be greatly prolonged. In respect of the effect of prolonging the fatigue life, Lubricant (II) is superior to Lubricant (I).

The lubricants of the invention can be used widely and effectively as lubricants such as gear oil, bearing oil, sliding surface lubricating oil, chain lubricating oil, and engine oil, or as greases to be used in various applications.

The present invention is described in greater detail by reference to the following Examples and Comparative Examples.

A two-cylinder testing machine as shown in the FIG. 1 was used. The two cylinders (one specimen on the driving side and the other on the side to be driven) were repeatedly brought into rolling contact and sliding contact with each other, and the differences in fatigue life between the lubricants having the compositions shown in the Table (viscosity: 100.+-.2 centistokes (40.degree. C.) for all lubricants) were determined. The results are shown in the Table.

The test specimens, experimental conditions, and method of determination of fatigue life were as follows:

Material from which the test specimens were prepared:

SCM-3 (both of the driving side and the side to be driven)

Diameter.times.Length: 60 millimeters.times.20 millimeters (both of the driving side and the side to be driven)

Hardness (HB): 350 (the driving side), 300 (the side to be driven)

Surface Roughness (Rmax): 3 micrometers (.mu.m) (both of the driving side and the side to be driven)

Number of Revolutions: 1,500 revolutions per minute (r.p.m.)

Specific Sliding: -18%

Sliding Speed: 0.7 meter per second

Contact Pressure (Hmax): 75 kilograms per square millimeter (kg/mm.sup.2)

Every three hours after the start of the operation, the testing machine was stopped and the test specimens were examined with the naked eye. The number of contacts (number of revolutions.times.time) counted until ten damages of at least 0.5 millimeter were formed in the surface of the test specimen mounted on the side to be driven was determined as a fatigue life. On basis of the value of fatigue life, a life, L.sub.50, at a damage-formation rate of 50% was determined according to the Weibull plot. In the case of paraffin-based base material, the life (L.sub.50) was 4.7.times.10.sup.6. In the same manner as above, the life of each lubricant was determined as a relative value in relation to the foregoing base material.