tailieunhanh - Clutches and brakes design and selection P3

Typical externally and internally pivoted shoe brakes are shown in Figures 1 and 2. In all but extremely rare designs, equal forces act upon both shoes to produce equal applied moments about their pivots. External shoe brake control is usually through a lever system that may be driven by electromechanical, pneumatic, or hydraulic means. Internal shoe brake control is usually by means of a double-ended cylinder or a symmetrical cam. Calculation of the moments and shoe lengths to achieve a specified braking torque cannot be carried out directly when the two shoes are pivoted as shown in either of these. | 3 Externally and Internally Pivoted Shoe Brakes Typical externally and internally pivoted shoe brakes are shown in Figures 1 and 2. In all but extremely rare designs equal forces act upon both shoes to produce equal applied moments about their pivots. External shoe brake control is usually through a lever system that may be driven by electromechanical pneumatic or hydraulic means. Internal shoe brake control is usually by means of a double-ended cylinder or a symmetrical cam. Calculation of the moments and shoe lengths to achieve a specified braking torque cannot be carried out directly when the two shoes are pivoted as shown in either of these figures and when the opposing shoes are subjected to equal moments. The tedious task of manually iterating these formulas to get a satisfactory design under these conditions may be eliminated with the use of computer programs such as those mentioned in the following sections that can quickly produce either graphical or numerical design solutions. I. PIVOTED EXTERNAL DRUM BRAKES A. Long Shoe Brakes Externally pivoted long shoe brakes similar to that shown in Figure 1 are often used as holding brakes. As its name implies a holding brake is to hold a shaft stationary until the brake is released. The compression spring on the lefthand side of the brake in Figure 1 applies a clamping force to the brake shoes Copyright 2004 Marcel Dekker Inc. Figure 1 Externally pivoted shoe brake. Courtesy Automation Process Technology Div. Ametek Paoli PA. on either side of the brake drum to hold it stationary without the need for external power. Electrical current through the solenoid on the left side of the assembly releases the brake and holds it open for as long as voltage is applied to the solenoid. Other holding brakes may use slightly different mechanical arrangements and may use either a hydraulic or a pneumatic cylinder to release the brake. One of the applications of a holding brake is in the design of an overhead crane. The value of a