tailieunhanh - Clutches and brakes design and selection P9

These are special-purpose clutches that are used in automatic transmissions, in devices for bringing high-speed machinery up to speed, in chain saws, in conveyor drives, and in similar industrial, vehicular, and large- and smallequipment applications. The centrifugal clutches provide a speed-dependent torque which acts only when the rotational speed exceeds a particular value; the one-way, or overrunning, clutches provide a torque that is not speed dependent once they are engaged, but is dependent on the direction of rotation; and the detent clutches provide a torque that cannot exceed a prescribed value. . | 9 Centrifugal One-Way and Detent Clutches These are special-purpose clutches that are used in automatic transmissions in devices for bringing high-speed machinery up to speed in chain saws in conveyor drives and in similar industrial vehicular and large- and smallequipment applications. The centrifugal clutches provide a speed-dependent torque which acts only when the rotational speed exceeds a particular value the one-way or overrunning clutches provide a torque that is not speed dependent once they are engaged but is dependent on the direction of rotation and the detent clutches provide a torque that cannot exceed a prescribed value. I. CENTRIFUGAL CLUTCHES A centrifugal clutch may be described as consisting of an inner cylinder that is attached to the input shaft and an outer housing that is attached to the output shaft as in Figure 1. Sectors of the inner cylinder are cut out to allow it to be fitted with weights that can slide radially outward as the inner cylinder rotates so that the weights are forced against the outer housing by centrifugal force and thereby transmit torque to the outer housing. Centrifugal clutches designed for lower power transfer may use simpler designs. In some chain saws for example it is the weights themselves that are recessed to accept radial guides from the central shaft. Copyright 2004 Marcel Dekker Inc. Figure 1 Centrifugal clutch. Courtesy Dana Corp. Inc. Toledo OH. Because of the variety of centrifugal clutch designs their analysis will be described in general terms. Let A denote the cross-sectional area of each weight in a plane perpendicular to the axis of rotation written in the form of an annular sector of angle f o as A cforo2 1 - h2 1-1 where h rt ro. Parameters h and c are factors that may be used to express other cross-sectional areas in thisform of equation 1-1 . When h 0 c 1 2 and f o 2k area A in equation 1-1 becomes that of a disc of radius ro. Let w denote the width of each weight measured in a direction parallel .