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

1、 Chapter 13 Belt Drives 13.1 Characteristics of Belt Drives13.2 Potential Failure and Belt Materials 13.3 Flat Belts13.4 V-Belts 13.5 Synchronous Belts Chapter 13 Belt Drives 13.1 Characteristics of Belt DrivesBelt drives are well suited to applications in which the center distance between rotating

2、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

3、 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

4、 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

5、belts have both the advantages of flat belts and V-belts. Round belts are used to transmit low power.Commercially available belts oChapter13-Belt-Drives-机械零件设计英文教案-课件-Design-of-Machine-ElementsChapter13-Belt-Drives-机械零件设计英文教案-课件-Design-of-Machine-Elements Tension cords: prime-quality cotton; polyami

6、de 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. Neopren

7、e, 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 BeltsForce analysisThe power transmitted Centrifugal-force-induced belt tension The friction torque transmitted The basic slip equa

8、tion 13.3 Flat BeltsForcStress analysisThe bending stress Centrifugal-force-induced stress Pulling stresses Stress analysisThe bending strTight 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-BeltsV-belt confi

9、gurations 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-bBasis for the tables and calculations of belt designIn a belt drive system, nominal power is t

10、hat 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 KA, The application factor depends upon the characteristics of the driving unit the driven machine or load,

11、 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 powerBasis for the tables and calcu Choice for belt cross section This chart indicates a suitable cross-sectional size, e .g., A or Z a

12、s 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 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.F

13、or 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 distancesLong center distanc

14、es 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 endu

15、rance testsCatalog 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

16、 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 methodThe value of Kis an experimentally based tension fac

17、tor 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 rate

18、d power =P+P. The corrected rated power The required number of belts The power-rating equation13.5 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 vide a constant angular velocity ratio (no slip or creep);requires minimal belt pretension (only enough to prevent “tooth skipping”);can