Hydraulic System

Hydraulic System

A hydraulic system uses a liquid to perform work. In an automatic transmission, this liquid is automatic transmission fluid (ATF). Automatic transmission fluid is one of the most complex fluids produced by the petroleum industry for the automobile.

ATF performs several jobs. It transmits engine torque as in the torque converter, controls valve body operation, and operates planetary controls such as in multiple-disc clutches, band and servo mechanisms. It also lubricates shaft bushings, thrust washers and bearings, and planetary gear train assemblies. ATF smoothly develops the action between the automatic transmission fluid and multiple-disc clutch friction discs and clutch plates, as well as between rotating drums and stationary bands. It acts as a cooling agent and transfers heat at the transmission cooler assembly while trying to operate the automatic transmission within the desired temperature range. Finally, ATF interacts with all the existing chemicals found in the automatic transmission and those developed as a result of operating extremes.

Hydraulic Principles

An automatic transmission uses ATF fluid pressure to control the action of the planetary gearsets. This fluid pressure is regulated and directed to change gears automatically through the use of various pressure regulators and control valves.

More than 300 years ago, a French scientist named Blaise Pascal discovered the basic principles that pertain to all hydraulic systems. His work resulted in Pascal's Law. It states, "Pressure exerted on a confined liquid or fluid is transmitted undiminished and equally in all directions and acts with equal force on all areas."

The piston in cylinder A has an area of 1 square inch. The area of the piston in cylinder B is 10 square inches. The piston in cylinder A is being moved by 1 pound. A pressure of 1 pound is exerted on every square inch of the piston in cylinder B. The area of piston B is 10 square inches. Total pressure exerted on piston B is 10 x 10 pounds per square inch.

This example shows how fluids can be used to increase work force. Fluids work well in increasing force because they are perfect conductors of pressure. Fluids cannot be compressed. Therefore, when a piston in a cylinder moves ahead and displaces fluid, that fluid is distributed equally on the load piston's surface.

Hydraulics are used to engage planetary gear controls. A typical hydraulic application is the servo assembly. The fluid pressure in the servo is 70 psi. The output force exerted by the servo is 880 pounds. Remember this servo output force is further multiplied by the servo mechanical linkage and the natural self-energizing action of the band to stop and hold a planetary member from rotating.

A multiple-disc clutch assembly also makes use of increasing force by using hydraulics. If the fluid pressure in the automatic transmission is 70 psi, the force applying the clutch pack is 1979 pounds.

If the clutch assembly is designed with a belleville spring having a mechanical advantage of 1.25:1, the clutch engagement force would change. The new engagement force would be 1.25 x 1,979 = 2,747 pounds, engaging the clutch pack.

To form a complete, working hydraulic system, the following elements are needed: fluid reservoir, pressure source, control valving, and output devices.

The automatic transmission reservoir is the transmission oil pan. Transmission fluid is drawn from the pan and returned to it. The pressure source in the system is the oil pump. The valve control body contains control valving to regulate or restrict the pressure and flow of fluid within the transmission. Output devices are the servos or clutches operated by hydraulic pressure.

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