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# Ch. 6 - HYDROSTATIC TRANSMISSIONS Fluid Power Circuits and...

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1 HYDROSTATIC TRANSMISSIONS Fluid Power Circuits and Controls, John S.Cundiff, 2001 Introduction Focus { The characteristics of hydrostatic transmission will be compared with mechanical transmissions.

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2 Mechanical Transmissions Assume that a mechanical transmission has four gear meshes. (Refer Fig 6.1) The meshing of these gears gives an output shaft speed lower than the input shaft speed. Specific gear mesh gives a specific ratio of input and output shaft speeds. Mechanical Transmissions Gear mesh is selected to match required output torque to available input torque. Output torque establishes the force the vehicle can exert as it moves,referred to as drawbar pull. A transmission does not increase power; the power is set by the engine.
3 Mechanical Transmissions An automatic transmission has a series of fixed gear ratios. Depending on the design of transmission, these gear ratios each have their own unique clutch or combination of clutches. When a given clutch is activated, the corresponding gear ratio is placed in the drive. Mechanical Transmissions The clutches or combination of clutches are activated in appropriate sequence to move up through the gears. As engine speed increases, the torque the engine can deliver decreases.

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4 Mechanical Transmissions Shifting from first gear to second gear can produce an output power curve (Refer Fig 6.4) The shift is made when the vehicle speed is 2.1 mph. At this shift output power is 72 hp. Potential power line shows that the potential o/p power is 89 hp. The shaded area is called power hole. Mechanical Transmissions In the presence of enough gears, the power holes can be minimized, and the actual o/p power will closely approximate the potential o/p power line. (Refer Fig 6.5) The cost of an automatic transmission increases as the number of gear meshes and clutches increases.
5 Mechanical Transmissions A torque converter is placed between the engine and the automatic transmission to reduce the number of transmission shifts required. Refer Fig 6.6 Torque converters have been designed to transmit hundreds of horsepower. Mechanical Transmissions Shift Control of Automatic Transmission Performance of a direct-drive transmission with (six gears) is compared to a torque converter / automatic transmission with three shifts. Torque converter “broadens” the power transfer bands with the three shifts such that the power line is equally as smooth as the power line produced with six shifts. (Refer Fig 6.9)

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6 Mechanical Transmissions A machine with a direct-drive transmission is the best choice when { High efficiency is needed to transfer maximum power possible. { The load is relatively constant, thus minimal shifting is needed. { Overloads are rare, thus engine stall protection is not needed.
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