tailieunhanh - Clutches and brakes design and selection P4

Linearly acting drum brakes are those fitted with shoes which, when activated, approach the drum by moving parallel to a radius through the center of the shoe. Typical linearly acting drum brakes are illustrated in Figures 1–3. Analysis of linearly acting brakes includes those in which the centrally pivoted shoes are attached to pivoted levers, as in Figure 1. Including brakes of this design within the category of linearly acting brakes is justified if they are designed so that the applied forces on the shoes and linings act along the radii of the shafts that they grip when the brakes. | 4 Linearly Acting External and Internal Drum Brakes Linearly acting drum brakes are those fitted with shoes which when activated approach the drum by moving parallel to a radius through the center of the shoe. Typical linearly acting drum brakes are illustrated in Figures 1-3. Analysis of linearly acting brakes includes those in which the centrally pivoted shoes are attached to pivoted levers as in Figure 1. Including brakes of this design within the category of linearly acting brakes is justified if they are designed so that the applied forces on the shoes and linings act along the radii of the shafts that they grip when the brakes are applied. Brakes of this type may act either upon brake drums or directly upon rotating shafts and are suitable for use in heavy-duty applications such as found in mining and construction equipment and in materials-handling machinery. Internal linearly acting drum brakes such as used on trucks in Europe may be designed as in Figure 2. Either pneumatic or hydraulic cylinders or cams may be used to force the shoes outwardly against the drum. The cylinders or cams also serve as anchors to prevent rotation and react against the vehicle frame. The springs shown are to retract the shoes when the brake is released. A collection of segmented brake pads backing plate plus lining along the entire circumference of the drum may be arranged as in Figure 3 to move outwardly against a drum as in Figure 3a or inwardly against a drum as in Figure 3b. The brake pads or shoes are themselves constrained against Copyright 2004 Marcel Dekker Inc. Figure 1 Linearly acting centrally pivoted shoe brake. Courtesy of the Hindon Corp. Charleston SC rotation by anchor pins that fit into short radial slots between the shoes and are attached to the rim of the circular frame as shown in Figure 3a. Brake actuation is accomplished by using air to expand the normally flat elastomeric-fabric annular tube shown in that figure between the brake pads and the circular .