tailieunhanh - Clutches and brakes design and selection P2

Band brakes are simpler and less expensive than most other braking devices, with shoe brakes, as perhaps their nearest rival. Because of their simplicity, they may be produced easily by most equipment manufacturers without having to purchase special equipment and without having to use foundry or forging facilities. Only the lining must be purchased from outside sources. Band brakes are used in many applications such as in automatic transmissions (Figure 1) and as backstops (Figure 5—devices designed to prevent reversal of rotation), for bucket conveyors, hoists, and similar equipment. They are especially desirable in the last-mentioned application because their action. | 2 Band Brakes Band brakes are simpler and less expensive than most other braking devices with shoe brakes as perhaps their nearest rival. Because of their simplicity they may be produced easily by most equipment manufacturers without having to purchase special equipment and without having to use foundry or forging facilities. Only the lining must be purchased from outside sources. Band brakes are used in many applications such as in automatic transmissions Figure 1 and as backstops Figure 5 devices designed to prevent reversal of rotation for bucket conveyors hoists and similar equipment. They are especially desirable in the last-mentioned application because their action can be made automatic without additional controls. I. DERIVATION OF EQUATIONS Figure 2 shows the quantities involved in the derivation of the force relations used in the design of a band brake. Consistent with the direction of rotation of the drum indicated by n the forces acting on an element of the band are as illustrated in the lower right section of Figure 2. In this figure r is the outer radius of the brake drum and F1 and F2 are the forces applied to the ends of the brake band. Because of the direction of drum rotation F1 is greater than F2. Equilibrium of forces in directions parallel and perpendicular to the tangent to a typical brake-band element at its midpoint requires that F dF cos F cos a pwr dU 0 1-1 Copyright 2004 Marcel Dekker Inc. Figure 1 Band brakes used in an automatic transmission system. Copyright 2004 Marcel Dekker Inc. Figure 2 Quantities and geometry used in the derivation of the band-brake design relations. F dF sin f F sin 2 pwr dU 0 1-2 when the brake lining and the supporting brake band together are assumed to have negligible flexural rigidity where a represents the coefficient of friction between the lining material and the drum p represents the pressure between the drum and the lining and w represents the width of the band. Upon simplifying equations 1-1 and 1-2 and