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Transmission Fluids
Background & Properties
Important Transmission Fluid Properties
July 2017
Getting power from the engine to the drive track or wheels is the job of the transmission. The key to effective
power transmission performance and long component life is proper transmission fluid selection. An understanding
of transmission types, transmission fluids and OEM recommendations is important for maximizing transmission
performance.
Transmissions can be grouped into the following types: automatic, powershift, hydrostatic and manual. Each
transmission requires uniqiue lubricant properties.
Transmission Fluids and Friction
Each type of transmission fluid has specialized formulations to create frictional properties that ensure proper
operation of the automatic, powershift or hydrostatic transmission it is protecting. In some instances, friction
modifying additives are added to a base stock to make a lubricant more slippery. Of these fluids, the most friction-
modified fluid is engine oil. As the evolution of transmission and transmission fluids progresses, some transmissions
lubricated by engine oils may result in relatively poor transmission performance and holding capabilities, as well as
increased wear.
With automatic transmission fluids, TO-4 fluids and tractor hydraulic fluids, there is both dynamic and static friction
measurements. Static friction refers to how well the transmission fluid will hold once lock-up has been achieved.
Dynamic friction refers to the amount of slippage a transmission fluid allows before lock-up occurs.
A balanced base oil and additive package for a transmission fluid is dependent upon the role the finished lubricant
must play in the transmission.
A typical transmission fluid must perform several critical functions:
• Lubricate gears, bushings and bearings;
• Provide anti-wear performance;
• Displace heat efficiently;
• Ensure seal performance; and
• Modify frictional characteristics in transmission clutches and wet brake systems.
Formulating a premium transmission fluid begins with a high quality base oil. The base oils must provide oxidative
resistance and high and low temperature performance in order to ensure transmission fluid performance. The
measure of a base oil's ability to resist oxidation or breakdown due to high temperature and severe service is related
to the amount of saturated hydrocarbon molecules it contains.
Group II and Group III base oils are treated with hydrogen to saturate the molecules and eliminate opportunities for
oxygen to attack and break down these molecules. This process fills spaces in the hydrocarbon molecule that might
otherwise fill with oxygen, with hydrogen atoms. This makes the base oil oxidation resistant, which is important to
transmission fluid performance.
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