tailieunhanh - Clutches and Brakes Design and Selection Part 10

Tham khảo tài liệu 'clutches and brakes design and selection part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 10 Friction Drives with Clutch Capability Friction drives that also have clutch capabilities are attractive because they are relatively simple and inexpensive. However they have been inherently limited to relatively low-power applications because of their dependence upon a coefficient of friction that is usually less than between the contacting materials. A friction drive was used in an early automobile but it was discontinued because of its power limitation. Friction drives recently have been given new life with the development of elastohydrodynamic fluids that become solid under pressure and can change from solid to liquid and back within microseconds. The fluids provide an effective friction coefficient that may be or greater as long as the tangential forces impose a shear stress that is less than the ultimate shear stress of the solid-state form of the elastohydrodynamic fluid. Hence these drives which feature metal-to-fluid solid-to-metal contact can transmit sufficient power to find industrial and automotive applications that benefit from their ability to easily and simply provide continuously variable speeds. At this time they are relatively expensive because of the structure needed to support the large contact forces that induce the fluid-to-solid transformation. They are presently known as traction drives. At this time however no known traction drives in production include a clutch capability consequently they will not be included in this chapter. Several formulas presented in this chapter may be written in nondimen-sional form for three reasons 1 the nondimensional form indicates the relative significance of the ratios selected 2 it allows drive designs to easily be scaled up or down for various applications and 3 it allows any consistent Copyright 2004 Marcel Dekker Inc. set of units to be used for each ratio and the resulting ratios are independent of the units used. Relatively broad curves are shown in the following computer-generated graphs .