Lubricating oil composition for transmission

Abstract

Lubricating oil compositions for transmissions comprises (A) a lubricating base oil with a kinematic viscosity at 100° C. adjusted to 1.5 to 6 mm 2 /s, composed of (A1) a lubricating base oil with a kinematic viscosity at 100° C. of 1.5 mm 2 /s or higher and lower than 7 mm 2 /s or (A1) the lubricating oil and (A2) a lubricating base oil with a kinematic viscosity at 100° C. of 7 to 50 mm 2 /s, blended with (B) a poly(meth)acrylate-based additive, so that the composition has a kinematic viscosity at 100° C. of 3 to 8 mm 2 /s and a viscosity index of 95 to 200, (A) and (B) fulfill a specific requirement. The compositions achieve long fatigue life though having low viscosity.

We claim: 
     
         1 . A lubricating oil composition for transmissions, comprising (A) a lubricating base oil with a kinematic viscosity at 100° C. adjusted to 1.5 to 6 mm 2 /s, composed of (A1) a lubricating base oil with a kinematic viscosity at 100° C. of 1.5 mm 2 /s or higher and lower than 7 mm 2 /s wherein Component (A1) is a lubricating base oil comprising a mixture of one or more mineral base oils selected from (A1a) a mineral base oil with a kinematic viscosity at 100° C. of 1.5 mm 2 /s or higher and lower than 3.5 mm 2 /s and (A1b) a mineral base oil with a kinematic viscosity at 100° C. of 3.5 mm 2 /s or higher and lower than 7 mm 2 /s, and
 (A2) a lubricating base oil with a kinematic viscosity at 100° C. of 7 to 50 mm 2 /s, wherein Component (A2) is a lubricating base oil comprising a mixture of one or more mineral base oils selected from (A2b) a lubricating base oil with a kinematic viscosity at 100° C. of 21.5 mm 2 /s or higher and lower than 25 mm 2 /s and (A2c) a lubricating base oil with a kinematic viscosity at 100° C. of 25 to 50 mm 2 /s, blended with (B) a poly(meth)acrylate-based additive containing a structural unit represented by formula (1) below, so that the composition has a kinematic viscosity at 100° C. of 3 to 8 mm 2 /s and a viscosity index of 95 to 200, the composition fulfilling requirement [I]: 
 [I] Component (A) is a lubricating base oil having a kinematic viscosity at 100° C. adjusted to 1.5 to 4.5 mm 2 /s and Component (B) is (B1) a poly(meth)acrylate-based additive containing a structural unit of formula (1) below: 
 
       
         
           
           
               
               
           
         
       
       wherein R 1  is hydrogen or methyl and R 2  is a branched alkyl group having 20 to 30 carbon atoms. 
     
     
         2 . The lubricating oil composition according to  claim 1 , wherein Component (A1) further comprises (A1c) a poly-α-olefin base oil with a kinematic viscosity at 100° C. of 1.5 mm 2 /s or higher and lower than 7 mm 2 /s, and/or Component (A2) further comprises (A2a) a lubricating base oil with a kinematic viscosity at 100° C. of 7 mm 2 /s or higher and lower than 15 mm 2 /s. 
     
     
         3 . The lubricating oil composition according to  claim 1 , wherein Components (B1) and (B2) are each a poly(meth)acrylate based additive having a weight-average molecular weight of 10,000 to 60,000. 
     
     
         4 . The lubricating oil composition according to  claim 1 , further comprising a poly(meth)acrylate containing a structural unit of formula (1) wherein R 1  is a hydrogen or methyl and R 2  is methyl. 
     
     
         5 . The lubricating oil composition according to  claim 1 , further comprising at least one type of additive selected from metallic detergents, dispersants, friction modifiers, extreme pressure additives, seal swelling agents, anti-oxidants, and pour point depressents. 
     
     
         6 . The lubricating oil composition according to  claim 1 , further comprising, on the basis of the total amount of the composition, (C) an imide-based friction modifier having 8 to 30 carbon atoms in an amount of 1 to 5 percent by mass and (D) a sulfer-free phosphorus-based extreme pressure additive in an amount of 0.015 to 0.05 percent by mass in terms of phosphorus. 
     
     
         7 . The lubricating oil composition according to  claim 1 , further comprising (B3) a poly(meth)acrylate based additive consisting of a structural unit represented by formula (1) below and having a weight-average molecular weight of 50,000 to 300,000 as Component (B): 
       
         
           
           
               
               
           
         
       
       wherein R 1  is hydrogen or methyl, R 2  is a hydrocarbon group having 5 to 20 carbon atoms or a group represented by —(R) a -E wherein R is an alkylene group having 5 to 20 carbon atoms, E is an amine residue or a heterocyclic residue, each having 1 or 2 nitrogen atoms and 0 to 2 oxygen atoms, and a is an integer of 0 or 1.