食物垃圾粉碎機(jī)設(shè)計與仿真【說明書+CAD+PROE+仿真】,說明書+CAD+PROE+仿真,食物垃圾粉碎機(jī)設(shè)計與仿真【說明書+CAD+PROE+仿真】,食物,食品,垃圾,粉碎機(jī),設(shè)計,仿真,說明書,仿單,cad,proe 1. Screws T hreaded fasteners such as screws, nuts, and bolts are important components of mechanical structures and machines. Screws may be used as removable fasteners or as devices for moving loads. 1.1 Screw Thread The basic arrangement of a helical thread wound around a cylinder is illustrated in Fig. 1.1. The terminology of an external screw threads is (Fig. 1.1): j Pitch, denoted by p, is the distance, parallel to the screw axis, between corresponding points on adjacent thread forms having uniform spacing. j Major diameter, denoted by d, is the largest (outside) diameter of a screw thread. j Minor diameter, denoted by d r or d 1 , is the smallest diameter of a screw thread. j Pitch diameter, denoted by d m or d 2 , is the imaginary diameter for which the widths of the threads and the grooves are equal. Figure 1.1 244 Machine Components Machine ComponentsThe standard geometry of a basic prole of an external thread is shown in Fig. 1.2, and it is basically the same for both Unied (inch series) and ISO (International Standards Organization, metric) threads. The lead, denoted by l, is the distance the nut moves parallel to the screw axis when the nut is given one turn. A screw with two or more threads cut beside each other is called a multiple-threaded screw. The lead is equal to twice the pitch for a double-threaded screw, and to three times the pitch for a triple-threaded screw. The pitch p, lead l, and lead angle l are represented in Fig. 1.3. Figure 1.3a shows a single-threaded, right-hand screw, and Fig. 1.3b shows a double-threaded left-hand screw. All threads are assumed to be right-hand, unless otherwise specied. A standard geometry of an ISO prole, M (metric) prole, with 60 symmetric threads is shown in Fig. 1.4. In Fig. 1.4 D(d) is the basic major diameter of an internal (external) thread, D 1 d 1 is the basic minor diameter of an internal (external) thread, D 2 d 2 is the basic pitch diameter, and H 0:5 3 1=2 p. Metric threads are specied by the letter M preceding the nominal major diameter in millimeters and the pitch in millimeters per thread. For example: M 14 2 M is the SI thread designation, 10 mm is the outside (major) diameter, and the pitch is 2 mm per thread. Figure 1.2 1. Screws 245 Machine ComponentsScrew size in the Unied system is designated by the size number for major diameter, the number of threads per inch, and the thread series, like this: 5 00 8 18 UNF 5 00 8 is the outside (major) diameter, where the double tick marks mean inches, and 18 threads per inch. Some Unied thread series are UNC, Unied National Coarse UNEF, Unied National Extra Fine Figure 1.3 246 Machine Components Machine ComponentsUNF, Unied National Fine UNS, Unied National Special UNR, Unied National Round (round root) The UNR series threads have improved fatigue strengths. 1.2 Power Screws For application that require power transmission, the Acme (Fig. 1.5) and square threads (Fig. 1.6) are used. Power screws are used to convert rotary motion to linear motion of the meshing member along the screw axis. These screws are used to lift weights Figure 1.4 Figure 1.5 1. Screws 247 Machine Components(screw-type jacks) or exert large forces (presses, tensile testing machines). The power screws can also be used to obtain precise positioning of the axial movement. A square-threaded power screw with a single thread having the pitch diameter d m , the pitch p, and the helix angle l is considered in Fig. 1.7. Consider that a single thread of the screw is unrolled for exactly one turn. The edge of the thread is the hypotenuse of a right triangle and the height is the lead. The hypotenuse is the circumference of the pitch diameter circle (Fig. 1.8). The anglel is the helix angle of the thread. The screw is loaded by an axial compressive force F (Figs. 1.7 and 1.8). The force diagram for lifting the load is shown in Fig. 1.8a (the force P r acts to the right). The force diagram for lowering the load is shown in Fig. 1.8b (the force P l acts to the left). The friction force is F f mN; Figure 1.6 248 Machine Components Machine Componentswhere m is the coefcient of dry friction and N is the normal force. The friction force is acting opposite to the motion. The equilibrium of forces for raising the load gives P F x P r N sinlmN cosl 0 1:1 P F y FmN sinl N cosl 0: 1:2 Similarly, for lowering the load one may write the equations P F x P l N sinlmN cosl 0 1:3 P F y FmN sinl N cosl 0: 1:4 Figure 1.7 Figure 1.8 1. Screws 249 Machine ComponentsEliminating N and solving for P r gives P r F sinlm cosl coslm sinl ; 1:5 and for lowering the load, P l F m cosl sinl coslm sinl : 1:6 Using the relation tanl l=pd m and dividing the equations by cosl, one may obtain P r F l=pd m m 1 ml=td m 1:7 P l F m l=pd m 1 ml=pd m : 1:8 The torque required to overcome the thread friction and to raise the load is T r P r d m 2 Fd m 2 lpmd m pd m ml : 1:9 The torque required to lower the load (and to overcome a part of the friction) is T l Fd m 2 pmd m l pd m ml : 1:10 When the lead l is large or the frictionm is low, the load will lower itself. In this case the screw will spin without any external effort, and the torque T l in Eq. (1.10) will be negative or zero. When the torque is positive, T l 0, the screw is said to be self-locking. The condition for self-locking is pmd m l: Dividing both sides of this inequality bypd m and using l=pd m tanl yields m tanl: 1:11 The self-locking is obtained whenever the coefcient of friction is equal to or greater than the tangent of the thread lead angle. The torque, T 0 , required only to raise the load when the friction is zero, m 0, is obtained from Eq. (1.9): T 0 Fl 2p : 1:12 The screw efciency e can be dened as e T 0 T r Fl 2pT r : 1:13 250 Machine Components Machine ComponentsFor square threads the normal thread load, F, is parallel to the axis of the screw (Figs 1.6 and 1.7). The preceding equations can be applied for square threads. For Acme threads (Figs 1.5) or other threads, the normal thread load is inclined to the axis because of the thread angle 2a and the lead anglel.The lead angle can be neglected (is small), and only the effect of the thread angle is considered (Fig. 1.9). The angle a increases the frictional force by the wedging action of the threads. The torque required for raising the load is obtained from Eq. (1.9) where the frictional terms must be divided by cosa: T r Fd m 2 lpmd m seca pd m ml seca : 1:14 Equation (1.14) is an approximation because the effect of the lead angle has been neglected. For power screws the square thread is more efcient than the Acme thread. The Acme thread adds an additional friction due to the wedging action. It is easier to machine an Acme thread than a square thread. In general, when the screw is loaded axially, a thrust bearing or thrust collar may be used between the rotating and stationary links to carry the axial component (Fig. 1.10). The load is concentrated at the mean collar diameter d c . The torque required is T c Fm c d c 2 ; 1:15 wherem c is the coefcient of collar friction. Figure 1.9 1. Screws 251 Machine ComponentsEXAMPLE A double square-thread power screw has the major diameter d 64 mm and the pitch p 8 mm. The coefcient of frictionm is 0.08, and the coefcient of collar frictionm c is 0.08. The mean collar diameter d c is 80 mm. The external load on the screw F is 10 kN. Find: 1. The lead, the pitch (mean) diameter and the minor diameter 2. The torque required to raise the load 3. The torque required to lower the load 4. The efciency Solution 1. From Fig. 1.6a: The minor diameter is d r d p 64 8 56 mm; the pitch (mean) diameter is d m d p=2 64 4 60 mm: The lead is l 2p 2 8 16 mm: Figure 1.10 252 Machine Components Machine Components