Lubricating oil composition

Abstract

A lubricating oil composition for a metal belt-type continuously variable transmission with a high metal-to-metal friction coefficient and excellent anti-fatigue and anti-seizure properties is provided. The composition contains a lubricating base oil and (A) a high overbased alkaline earth metal sulfonate having a base number of 300 to 500 mgKOH/g in an amount of 0.01 to 0.03 percent by mass as alkaline earth metal, (B) a low overbased alkaline earth metal sulfonate with a base number of 0 to 40 mgKOH/g in an amount of 0.005 to 0.015 percent by mass as alkaline earth metal, and (C) a sulfur-containing phosphite ester in an amount of 0.02 to 0.06 percent by mass as phosphorus. The ratio ([M]/[P]) of the content of (B) as alkaline earth metal ([M]) to the content of (C) as phosphorus ([P]) is 0.15 or greater.

1 . A lubricating oil composition for a metal belt-type continuously variable transmission comprising a lubricating base oil and (A) an high overbased alkaline earth metal sulfonate having a base number determined by a perchloric acid method of 300 to 500 mgKOH/g in an amount of 0.01 to 0.03 percent by mass as alkaline earth metal, (B) a low overbased alkaline earth metal sulfonate with a base number determined by a perchloric acid method of 0 to 40 mgKOH/g in an amount of 0.005 to 0.015 percent by mass as alkaline earth metal, and (C) a sulfur-containing phosphite ester represented by structural formula (I) below in an amount of 0.02 to 0.06 percent by mass as phosphorus so that the ratio ([M]/[P]) of the content as alkaline earth metal ([M]) of (B) the low overbased alkaline earth metal sulfonate to the content as phosphorus ([P]) of (C) the sulfur-containing phosphite ester is 0.15 or greater: 
       
         
           
           
               
               
           
         
         wherein R is a sulfur-containing hydrocarbyl group having 4 to 20 carbon atoms, R 1  is hydrogen, a hydrocarbyl group having 4 to 20 carbon atoms or a sulfur-containing hydrocarbyl group having 4 to 20 carbo atoms. 
       
     
     
         2 . The lubricating oil composition for a metal belt-type continuously variable transmission according to  claim 1  further comprising (D) a phosphite ester and/or an orthophosphate ester in such an amount that the amount of phosphorous in the lubricating oil composition is the range of up to 0.08 percent by mass or less. 
     
     
         3 . The lubricating oil composition for a metal belt-type continuously variable transmission according to  claim 1  further comprising at least one type selected from the group consisting of friction modifiers, metallic detergents, viscosity index improvers, pour point depressants, anti-oxidants, corrosion inhibitors, and anti-foamers. 
     
     
         4 . The lubricating oil composition for a metal belt-type continuously variable transmission according to  claim 2  further comprising at least one type selected from the group consisting of friction modifiers, metallic detergents, viscosity index improvers, pour point depressants, anti-oxidants, corrosion inhibitors, and anti-foamers.