Chapter13-Belt-Drives-机械零件设计英文教案-课件--Design-of-Machine-Elements

,单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,Chapter 13 Belt Drives,13.1 Characteristics of Belt Drives,13.2 Potential Failure and Belt Materials,13.3 Flat Belts,13.4 V-Belts,13.5 Synchronous Belts,Chapter 13 Belt Drives,13.1 Characteristics of Belt Drives,Belt drives are well suited to applications in which the center distance between rotating shafts is large.,With proper design insight,belts are usually quiet,easily replaced,and in many cases,because of their flexibility and damping capacity,they reduce the transmission of unwanted shock and vibration between shafts.,Simplicity of installation,minimum maintenance requirements,high reliability,and adaptability to a variety of applications are also characteristics of belt drives.,Because of slip and/or creep,the angular velocity ratio between the two rotating shafts may be inexact,and the power and torque capacities are limited by the coefficient of friction and interfacial pressure between belt and pulley.,13.1 Characteristics of Belt,Commercially available belts of different cross sections,Owing to the wedging effect,the force of friction on V-belt drives is larger than that of flat belts.So V-belts have higher pulling capacity,and find more application.,V-ribbed belts have both the advantages of flat belts and V-belts.,Round belts are used to transmit low power.,Commercially available belts o,Chapter13-Belt-Drives-机械零件设计英文教案-课件-Design-of-Machine-Elements,Chapter13-Belt-Drives-机械零件设计英文教案-课件-Design-of-Machine-Elements,Tension cords:,prime-quality cotton;polyamide strips or polyester cords for flat belts and V-ribbed(Poly-V)belts;,polyester,fiberglass,or aramid fibers for toothed timing belts and conventional or high capacity V-belts.,The cords were embedded in a,matrix of rubber synthetic rubber compounds,to increase flexibility and friction.,Neoprene,to enhance resistance to oil,heat.,Belt cover material,:cotton or nylon cloth impregnated with synthetic rubber,Belt materials,Tension cords:prime-qualit,13.3 Flat Belts,Force analysis,The power transmitted,Centrifugal-force-induced belt tension,The friction torque transmitted,The basic slip equation,13.3 Flat BeltsForc,Stress analysis,The bending stress,Centrifugal-force-induced stress,Pulling stresses,Stress analysisThe bending str,Tight side:,Loose side:,Maximum stress:,Which takes place at the contact place of tight side entering the smaller driving pulley.,Tight side:,13.4 V-Belts,V-belt configurations have become well standardized,and widely tested for reliability and life.,V-belts are specified by the section identifications(see Table13.1 and Table 13.2)with the belt lengths.,13.4 V-BeltsV-b,Basis for the tables and calculations of belt design,In a belt drive system,nominal power is that under a steady-state condition,which may be matched by the name-plate rating of the electric motor or other driving unit.,Design power is nominal power multiplied by an application factor,K,A,The application factor depends upon the characteristics of the driving unit the driven machine or load,and on the frequency of operation.,Typical application factors are shown in Table 13.3 to obtain the design value for required power in V-belt applications.,Design power,Basis for the tables and calcu,Choice for belt cross section,This chart indicates a suitable cross-sectional size,e.g.,A or Z as well as suggested small sheave diameter range.,Fig.13.4 Recommended belt section as,a function of design power and speed,Choice for be,Sheaves size,In order to reduce bending stress in the belt section,the chosen sheave diameter should be larger than the minimum datum diameter given in Table13.4.,For an exact speed ratio,one sheave may require a nonstandard diameter.Larger sheaves result in fewer belts and less bearing load,but larger belt velocities.,When possible,the sheaves should be sized for a belt speed in the neighborhood of 20 m/s.,Sheaves sizeI,Center distances,Long center distances are not recommended for V belts because the excessive vibration of the slack side of the belt will shorten the belt life.,Commonly,center distance is chosen in the range:,The relationships between diameters,center distance,wrap angles,and length may be found in Fig.13.2.,Center dist,Belt endurance tests,Catalog data originate from laboratory endurance tests on selected belts of average length for each size and construction of cross-section.,The tests are run in a drive with two sheaves of equal diameter.,From a necessarily limited number of tests,a formula is used to determine the power capacity of each belt in drives at other speeds,n,and other than equal sheave diameters,and for a service life of three to three to five years.,P=Kv=K(),is the basic power for the belt.,Belt enduranc,The power-rating equation based design method,The value of,K,is an experimentally based tension factor,K,1,which is reduced by the subtraction of two of two or three,terms,one of which if for a tension,K,2,/equivalent to,bending stress and another,a tension,K,3,(),2,due