上帝充通大￥ Shanghai Jiao Tong University Mechanical Design and Manufacturing II Power/Energy Transmission ll: Belt Drives(带传动)
1 Mechanical Design and Manufacturing II Power/Energy Transmission II: Belt Drives（带传动）
上淄克通大学 Shanghai Jiao Tong University Lectures Power/Energy Conversion (Electrical Motors) Power/Energy Transmission (Gears,Cams,Linkages, Belt Drives, Power Screws) Transmission Joints (Fasteners, Support (Bearings) Structural Connectors) Support (Frames) Tools Statics,Dynamics, Stress Analysis,etc.. 2
2 Lectures Power/Energy Conversion (Electrical Motors) Power/Energy Transmission (Gears, Cams, Linkages, Belt Drives, Power Screws) Transmission Support (Bearings) Joints (Fasteners, Connectors) Structural Support (Frames) Tools Statics, Dynamics, Stress Analysis, etc
上泽充通大学 Shanghai Jiao Tong University Basic Belt Drives2 Fo .A belt drive is a flexible power transmission element that seats tightly on a set of pulleys(带轮) >Sometimes called sheaves .When used for speed reduction,the smaller sheave is mounted on the high-speed shaft >e.g.shaft of an electric motor .Larger sheave is mounted on driven machine. .Belt is installed by placing it around the two sheaves while center distance between them is reduced. .Then the sheaves are moved apart placing the belt in high initial tension(初拉力). 3
3 Basic Belt Drives2 •A belt drive is a flexible power transmission element that seats tightly on a set of pulleys（带轮） Sometimes called sheaves •When used for speed reduction, the smaller sheave is mounted on the high-speed shaft e.g. shaft of an electric motor •Larger sheave is mounted on driven machine. •Belt is installed by placing it around the two sheaves while center distance between them is reduced. •Then the sheaves are moved apart placing the belt in high initial tension（初拉力）
上泽充通大学 Shanghai Jiao Tong University Basic Belt Drives2 .When transmitting power,the driving sheave rotates,friction causes driving sheave to grip the belt,increasing the tension on one side,called“tight side”(紧边)of drive. Opposite side of belt is still under tension,but at a smaller value,called“slack side”(松边)of drive. .The difference between the tensile forces of two sides in belt exerts a tangential force on the driven sheave and thus a torque is applied to the driven shaft via friction force. F2 Slack side Tight side Fy B2 F
4 Basic Belt Drives2 •When transmitting power, the driving sheave rotates, friction causes driving sheave to grip the belt, increasing the tension on one side, called ―tight side‖ (紧边) of drive. •Opposite side of belt is still under tension, but at a smaller value, called ―slack side‖ (松边) of drive. •The difference between the tensile forces of two sides in belt exerts a tangential force on the driven sheave and thus a torque is applied to the driven shaft via friction force
上帝充通大￥ Shanghai Jiao Tong University Basic Belt Drives2 Slack side D vo=Belt speed Tight side Note:D1.D2 are B2 pitch diameters Groove Belt angle ride-out d101/2=d202/2 Typical belt section 01/o2=d2/d speed ratio and groove geometry Pitch Groove diameter depth power=C so T1@1=T2 @2 Sheave outside diameter 01/02=T2/T1 Figure 13-4 Cross Section of V-belt and Sheave Groove d=Pitch diameter,diameter belt appears to ride without slipping Note:slip and creep present,so not exactly equal to ratio 5
5 Basic Belt Drives2 d1 ω1 /2 = d2 ω 2 /2 ω1 / ω 2 = d2 / d1 = speed ratio power = C so T1 ω1 =T2 ω 2 ω1 / ω 2 = T2 / T1 d =Pitch diameter, diameter belt appears to ride without slipping Note: slip and creep present, so not exactly equal to ratio
上帝充通大￥ Shanghai Jiao Tong University Basic Belt Drives2 Ae! VX<VB<VI Except for timing belts,there is some slip and creep 弹性滑动)，so the angular-.velocity between the driving and driven shaft is neither constant nor exactly equal to the ratio of the pulley diameters. 6
6 Basic Belt Drives2 Except for timing belts, there is some slip and creep（ 弹性滑动）, so the angular-velocity between the driving and driven shaft is neither constant nor exactly equal to the ratio of the pulley diameters. V2<VB<V1
上充通大学 Shanghai Jiao Tong University Flexible Transmission Elements2 Belt Drives >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. 7
7 Flexible Transmission Elements2 •Belt Drives 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
上泽充通大学 Shanghai Jiao Tong University Flexible Transmission Elements2 Chain Drives(链传动) >A chain drive is a flexible power transmission element that operates on a set of toothed wheels called sprockets(链轮) >Used at lower speed where high torques are needed >At higher speeds, Noise,impact between chain link and sprocket teeth and lubrication become major problems. Xooo66o06形
8 Flexible Transmission Elements2 •Chain Drives（链传动） A chain drive is a flexible power transmission element that operates on a set of toothed wheels called sprockets （链轮） Used at lower speed where high torques are needed At higher speeds, Noise, impact between chain link and sprocket teeth and lubrication become major problems
上帝充通大￥ Shanghai Jiao Tong University Advantages of Different Transmissions3 Advantages of Belt Drives(high-speed,low-torque) 1.Electrical insulation provided because there is no metal-to- metal contact between driver and driven units. 2.Less noise than with a chain drive 3.Flat belts can be used for extremely long center distances where chain weight would be excessive 4.Flat belts can be used at extremely high-speeds where chain inertia (must be considered as influencing chain fit at sprocket and chain tension. 5.No lubrication required. 6.Shaft center distance variation and shaft alignment is much less critical than for gear drives or chain drives. 9
9 Advantages of Different Transmissions3 Advantages of Belt Drives(high-speed, low-torque) 1. Electrical insulation provided because there is no metal-tometal contact between driver and driven units. 2. Less noise than with a chain drive 3. Flat belts can be used for extremely long center distances where chain weight would be excessive 4. Flat belts can be used at extremely high-speeds where chain inertia（惯性） must be considered as influencing chain fit at sprocket and chain tension. 5. No lubrication required. 6. Shaft center distance variation and shaft alignment is much less critical than for gear drives or chain drives
上充通大学 Shanghai Jiao Tong University Advantages of Different Transmissions3 Advantages of Chain Drives(low-speed,high-torque) 1.Shaft center distance variation may be more easily accommodated than with gear drives 2. Chains are easier to install and replace than belts because the center distance between driven and driver units need not be reduced for installation(splice and link belts overcome this objection at the cost of lower power ratings). 3. Chains require no tension on the slack side(松边)，so that bearing loads are reduced. 4. Chains do not slip or creep as belt drives do (except toothed belts). 5. Chain drives more compact because sprocket diameters are smaller and chains are narrower than sheaves and belts for same power transfer. 6. Chains do not develop static charges 7. Chains do not deteriorate (with age,heat or oil and grease. 8. Chains can operate at higher temperatures than belts. 10
10 Advantages of Different Transmissions3 Advantages of Chain Drives(low-speed, high-torque) 1. Shaft center distance variation may be more easily accommodated than with gear drives 2. Chains are easier to install and replace than belts because the center distance between driven and driver units need not be reduced for installation (splice and link belts overcome this objection at the cost of lower power ratings). 3. Chains require no tension on the slack side（松边）, so that bearing loads are reduced. 4. Chains do not slip or creep as belt drives do (except toothed belts). 5. Chain drives more compact because sprocket diameters are smaller and chains are narrower than sheaves and belts for same power transfer. 6. Chains do not develop static charges 7. Chains do not deteriorate（老化） with age, heat or oil and grease. 8. Chains can operate at higher temperatures than belts