Lubricating oil compositions for automatic transmissions

Abstract

A lubricating oil composition for automatic transmissions wherein the mass ratio of phosphorus:calcium:boron:sulfur determined by elemental analysis is 1:(0.1 to 2):(0.06 to 2):(0.2 to 20), the concentration of phosphorus is from 0.01 to 0.06 percent by mass, the concentration of the sulfur derived from a base oil is from 0 to 0.1 percent by mass, and the concentration of the sulfur derived from sulfur-based additives is from 0.01 to 0.15 percent by mass, based on the total amount of the composition. The lubricating oil composition can prevent the occurrence of scratch phenomenon even over a long period of time when used in a metal belt type continuously variable automatic transmission.

Description

[0001] The present invention relates to lubricating oil compositions for automatic transmissions and more particularly to those advantageously applicable to belt type continuously variable transmissions for automobiles, which compositions are capable of preventing the occurrence of scratch noise by making the friction material of such an automatic transmission exhibit the .mu.-V characteristics of a positive gradient and of maintaining the positive gradient .mu.-V characteristics for a longer period of time.[0002] Automatic transmissions widely used for modern automobiles are those comprising a gear-shifting unit which is the combination of a torque converter equipped with a lock-up clutch, a wet clutch, and a planetary gear set, and a hydraulic control unit. However, such automatic transmissions have defects that they are poor in fuel efficiency and make a driver feel uncomfortableness caused by shifting shock due to the stepped gear-shifting. Continuously variable transmissions (b...

AI Technical Summary

Benefits of technology

[0053] One or more compounds selected from the above described sulfur-containing additive, i.e., Components (A) through (E) and derivatives thereof may be used in the lubricating oil composition of the present invention. In the present invention, the content of the sulfur-based additives is in such a range that the total sulfur concentration of the sulfur derived from Components (A) through (E) and derivatives thereof and the sulfur derived from other additives is from 0.01 to 0.15 percent by mass, based on the total amount of the composition and the total phosphorus concentration of the phosphorus derived from Components (A) through (E) and derivatives thereof and the phosphorus derived from the phosphorus-based additives is from 0.01 to 0.06 percent by mass, based on the total amount of the composition. When the sulfur-based additives are contained in an amount of less than 0.01 percent by mass in terms of sulfur or in an amount of less than 0.01 percent by mass in terms of phosphorus, they are less in effect to improve the .mu.-V characteristics. When the sulfur-based additives are contained in an amount exceeding 0.15 percent by mass in terms of sulfur or in an amount exceeding 0.06 percent by mass in terms of phosphorus, they deteriorate the oxidation stability of the resulting lubricating oil composition.

[0078] No particular limitation is imposed on the content of Component (G). However, Component (G) is contained in an amount of preferably 0.01 to 5.0 percent by mass, more preferably 0.05 to 4.0 percent by mass, based on the total amount of the composition. Component (G) of less than 0.01 percent by mass would be less effective in suppressing a wet clutch from being decreased in strength against repeating compression applied thereto, while Component (G) in excess of 5.0 percent by mass would decrease the oxidation stability of the resulting composition.

[0090] In the lubricating oil composition, the blend ratio of one or more Components (I) is arbitrarily selected but is preferably 0.002 percent by mass or more, preferably 0.005 percent by mass or more and 0.1 percent by mass or less, preferably 0.06 percent by mass or less, in terms of boron, based on the total amount of the composition. Component (I) of less than 0.002 percent by mass in terms of boron would make Component (A) less effective in providing the resulting composition with enhanced friction properties between metal parts, while Component (I) in excess of 0.1 percent by mass in terms of boron would decrease the storage stability of the resulting oil composition.

[0112] Among these viscosity index improvers, the use of ethylene-.alpha.-olefin copolymers or hydrides thereof is contributive to the production of a lubricating oil composition which is excellent in particularly shear stability.

[0121] The use of the lubricating oil compositions of the present invention can prevent the occurrence of scratch phenomenon by making the belt and elements in an automatic transmission exhibit the .mu.-V characteristics of a positive gradient and can maintain the positive gradient .mu.-V characteristics for a long period of time. Therefore, the lubricating oil compositions of the present invention are suitable for continuously variable transmissions such as belt type CVTs. The lubricating oil compositions can be also suitably used step type automatic transmission and final reduction drive gears equipped with a limited-slip differential.

Problems solved by technology

However, such automatic transmissions have defects that they are poor in fuel efficiency and make a driver feel uncomfortableness caused by shifting shock due to the stepped gear-shifting.

However, it has been found in recent years that automobiles with certain types of belt type CVTs encounter a phenomenon that noises (scratch noises) are made particularly when the automobiles are put into the garage or started, a so-called "scratch phenomenon".

It has already been found that this scratch phenomenon is caused by tooth hit noise of the gears arranged behind the CVTs and resulting from a fluctuation in the rotation of the driven pulley.

ATFs which have been conventionally used in belt type CVT are not always excellent in .mu.-V characteristics and large in change thereof when used in a long period of time, leading to a failure of complete prevention of the occurrence of scratch noise.

As a result, the scratch phenomenon frequently occurs.

It is now found that a lubricating oil after a certain period of running progresses to oxidation deterioration and particularly a lubricating oil containing a relatively large amount of sulfur is susceptible to a decrease in .mu.-V characteristics.

A sulfur concentration in excess of 0.1 percent by mass makes it difficult to maintain stable .mu.-V characteristics over a long time.

When the sulfur-based additives are contained in an amount exceeding 0.15 percent by mass in terms of sulfur or in an amount exceeding 0.06 percent by mass in terms of phosphorus, they deteriorate the oxidation stability of the resulting lubricating oil composition.

An alkyl or alkenyl group having less than 40 carbon atoms would adversely affect the solubility of the compound in a base oil, while an alkyl or alkenyl group having more than 400 carbon atoms would deteriorate the low-temperature fluidity of the resulting lubricating oil composition.

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