圈上泽支大峰 Flat or Round Belt Simplest type Often made from leather or rubber-coated fabric. Crowned pulleys are used for flat belts,and grooved sheaves for round belts. Driving force is limited by pure friction between belt and sheave. Some designers prefer flat belts for delicate machinery because belt will slip if torque tends to rise to a level high enough to damage the machine. 布平带 物边平帝 圈上泽夫道大学 V-Belt V-shape causes belt to wedge tightly into groove to increases friction and allows high torques to be transmitted before slipping occurs. Most belts have high-strength cords positioned at pitch diameter of belt cross section to increase tensile strength Cords made from natural fibers,synthetic strands or steel Embedded in firm rubber compound to provide flexibility needed to allow belt to pass around sheave. Often outer fabric cover is added to give belt good durability. 包布y 勿题V州 6
6 Flat or Round Belt Simplest type Often made from leather or rubber-coated fabric. Crowned pulleys are used for flat belts, and grooved sheaves for round belts. Driving force is limited by pure friction between belt and sheave. Some designers prefer flat belts for delicate machinery because belt will slip if torque tends to rise to a level high enough to damage the machine. V-shape causes belt to wedge tightly into groove to increases friction and allows high torques to be transmitted before slipping occurs. Most belts have high-strength cords positioned at pitch diameter of belt cross section to increase tensile strength Cords made from natural fibers, synthetic strands or steel Embedded in firm rubber compound to provide flexibility needed to allow belt to pass around sheave. Often outer fabric cover is added to give belt good durability. V-Belt
圈上海文大华 V-Belt Example of V-Belt Construction 普通V带 齿性V带 同步带 年◆单 受 m b)Die cut.cog Timing belt )Wrapped constru Double angle V-bel id)Poly-rib bel e,Vee-band联组V带 双面V带 Dayton,Ohio) 圈上泽夫道大学 Grooved or Timing Belt Requires toothed wheels,or sprockets.Sheaves have mating grooves into which the cogs on belts seat. Can transmit the high forces at low speeds,compete with chain drives. Limited only by tensile strength of belt and shear strength of cogs. 7
7 Example of V-Belt Construction 双面V带 齿性V带 多楔带 同步带 联组V带 普通V带 V-Belt Requires toothed wheels, or sprockets. Sheaves have mating grooves into which the cogs on belts seat. Can transmit the high forces at low speeds, compete with chain drives. Limited only by tensile strength of belt and shear strength of cogs. Grooved or Timing Belt
圆上活支大学 Types of Pulleys and Sheaves Crowned pulleys(Table17-3,5) ·Used for flat belts Grooved Pulleys or Sheaves Used for round or V-belts Toothed wheels or Sprockets Required for timing belts. Dus de ca-eret itcnc a-e:s ey groove 圆上泽夫通大学 Types of Pulleys and Sheaves ▣ Variable Pitch Sheaves Used to change the speed ratio. >As tension increases,spring loaded side of flange moves outward.this causes: Pitch diameter to decrease Speed ratio to reduce. 8
8 Crowned pulleys (Table17-3,5) • Used for flat belts Grooved Pulleys or Sheaves • Used for round or V-belts. Toothed wheels or Sprockets • Required for timing belts. Types of Pulleys and Sheaves Table 17-5 Variable Pitch Sheaves Used to change the speed ratio. As tension increases, spring loaded side of flange moves outward . this causes : • Pitch diameter to decrease • Speed ratio to reduce. Types of Pulleys and Sheaves
圈上海文通大华 Belt Drive Characteristics They may be used for long center distances. Except for timing belts,there is some slip and creep,and so the angular-velocity ratio between the driving and driven shafts is neither constant nor exactly equal to the ratio of the pulley diameters. In some cases an idler or tension pulley can be used to avoid adjustments in center distance that are ordinarily necessitated by age or the installation of new belts. 圈上泽夫通大学 Belt Drive Characteristics Used where rotational speeds are relatively high,Linear speed of belt about 2,500 to 7,000 ft/min,ideal around 4,000 ft/min. At lower speeds,tension in the belt becomes too large for typical belt cross sections At higher speeds,dynamic effects such as centrifugal forces(离心)，belt whip,vibration reduce effectiveness of drive and its life. 9
9 They may be used for long center distances. Except for timing belts, there is some slip and creep, and so the angular-velocity ratio between the driving and driven shafts is neither constant nor exactly equal to the ratio of the pulley diameters. In some cases an idler or tension pulley can be used to avoid adjustments in center distance that are ordinarily necessitated by age or the installation of new belts. Belt Drive Characteristics Used where rotational speeds are relatively high, Linear speed of belt about 2,500 to 7,000 ft/min , ideal around 4,000 ft/min. At lower speeds, tension in the belt becomes too large for typical belt cross sections At higher speeds, dynamic effects such as centrifugal forces(离心力), belt whip, vibration reduce effectiveness of drive and its life. Belt Drive Characteristics