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Chapter 13 Belt Drives 13.1 Characteristics of Belt Drives13.2 Potential Failure and Belt Materials 13.3 Flat Belts13.4 VBelts 13.5 Synchronous Belts Chapter 13 Belt Drives 1 13.1 Characteristics of Belt DrivesBelt 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 2Commercially available belts of different cross sections Owing to the wedging effect,the force of friction on Vbelt drives is larger than that of flat belts.So Vbelts have higher pulling capacity,and find more application.Vribbed belts have both the advantages of flat belts and Vbelts.Round belts are used to transmit low power.Commercially available belts o3Chapter13-Belt-Drives-机械零件设计英文培训 教案-ppt课件-Design-of-Machine-Elements4Chapter13-Belt-Drives-机械零件设计英文培训 教案-ppt课件-Design-of-Machine-Elements5 Tension cords:primequality cotton;polyamide strips or polyester cords for flat belts and Vribbed(PolyV)belts;polyester,fiberglass,or aramid fibers for toothed timing belts and conventional or high capacity Vbelts.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-qualit6 13.3 Flat BeltsForce analysisThe power transmitted Centrifugalforceinduced belt tension The friction torque transmitted The basic slip equation 13.3 Flat BeltsForc7Stress analysisThe bending stress Centrifugalforceinduced stress Pulling stresses Stress analysisThe bending str8Tight side:Loose side:Maximum stress:Which takes place at the contact place of tight side entering the smaller driving pulley.Tight side:9 13.4 V-BeltsVbelt configurations have become well standardized,and widely tested for reliability and life.Vbelts are specified by the section identifications(see Table13.1 and Table 13.2)with the belt lengths.13.4 V-BeltsV-b10Basis for the tables and calculations of belt designIn a belt drive system,nominal power is that under a steadystate condition,which may be matched by the nameplate rating of the electric motor or other driving unit.Design power is nominal power multiplied by an application factor KA,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 Vbelt applications.Design powerBasis for the tables and calcu11 Choice for belt cross section This chart indicates a suitable crosssectional 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 be12 Sheaves sizeIn 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 sizeI13 Center distancesLong 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 dist14 Belt endurance testsCatalog data originate from laboratory endurance tests on selected belts of average length for each size and construction of crosssection.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 enduranc15 The powerrating equation based design methodThe value of Kis an experimentally based tension factor K1 which is reduced by the subtraction of two of two or three terms,one of which if for a tension K2/equivalent to bending stress and another,a tension K3()2 due to centrifugal forces.When one sheave has a larger diameter,the belts basic power may be increased by an increment P:Belt rated power =P+P.The corrected rated power The required number of belts The power-rating equation1613.5 Synchronous Belts Toothed timing belts(synchronous belts)do not rely upon friction for transmission of torque and power;but by virtue of positive engagement of a toothed belt meshing with toothed sprocket grooves.1)provide a constant angular velocity ratio(no slip or creep);2)requires minimal belt pretension(only enough to prevent“tooth skipping”);3)can be operated at high speeds(up to 80 m/s),and can transmit high torques and high power.13.5 Synchronous Belts Too17 Tooth profilesEach belt tooth can correctly mesh with the corresponding pulley groove.The teeth of standard synchronous belts are trapezoidal in profile.For heavyduty applications,the tooth profile is sometimes modified.Helical tooth arrangements are also used for highperformance synchronous drive applications for smoother,quieter operation.At normal operating speeds toothed timing belts tend to operate smoothly and quietly.Tooth profilesEach belt 18The end of Chapter 13The end of Chapter 1319
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