tailieunhanh - Clutches and brakes design and selection P11

Fluid clutches and brakes may be divided into two groups: those containing a fluid only and those containing a mixture of fluids and solids. Those containing only a fluid rely primarily upon the mass of the fluid and secondarily upon its viscosity to transmit torque. Units containing both a fluid and a solid in a particulate form rely upon the suspended solids to provide the major bond between the components that either transmit or resist torque when under the influence of an external electromagnetic field. The advantage of fluid clutches and brakes is that there is no lining to wear. | 11 Fluid Clutches and Brakes Fluid clutches and brakes may be divided into two groups those containing a fluid only and those containing a mixture of fluids and solids. Those containing only a fluid rely primarily upon the mass of the fluid and secondarily upon its viscosity to transmit torque. Units containing both a fluid and a solid in a particulate form rely upon the suspended solids to provide the major bond between the components that either transmit or resist torque when under the influence of an external electromagnetic field. The advantage of fluid clutches and brakes is that there is no lining to wear and replace. This however is obtained at the expense of some power loss in the transmission of torque and the distinct need for some sort of fluid cooling for both fluid clutches and fluid brakes. Moreover occasional fluid seal replacement may also be required. I. FLUID COUPLINGS AS CLUTCHES Fluid couplings may serve as soft start clutches and as torque limiting clutches. A typical fluid coupling consists of an input shaft attached to an impeller and an output shaft attached to a runner with both encased within a closed housing and oriented as shown in Figure 1. An impeller may differ from a runner in the shape of the radial vanes of the sort shown in Figure 2 and may be attached to and rotate with the housing that contains both the impeller and the runner. As indicated in Figure 1 the shafts are supported by bearings at the housing and by bearings at the far ends of each shaft that in turn are supported by an enclosure as shown in Figure 3. Each impeller and Copyright 2004 Marcel Dekker Inc. Input Impeller . . _ r Output Runner Figure 1 Cross section of a semitoroidal impeller and runner and their enclosure or housing. Courtesy TRI Transmission Bearing Corp. Lionville PA. runner consists of half of a torus as shown in cross section in Figure 1 that is fitted with radial vanes that extend radially inward across the torus as is evident in Figure 2. The .