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...

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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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