Presentation on theme: "G E A R S zWhat is a gear? yToothed wheel yTransmits rotary motion and power zWhat do they do? yChange the direction of motion yChange the output speed."— Presentation transcript:
G E A R S zWhat is a gear? yToothed wheel yTransmits rotary motion and power zWhat do they do? yChange the direction of motion yChange the output speed zMost common gear? ySPUR gear
SIMPLE GEAR TRAINS zWhat is a simple gear train? yMeshed (interlocking gears) yTwo or more gears in series zInput gear = DRIVER zOutput gear = DRIVEN zWhat effect does this have on the output (DRIVEN) yReverses motion yChanges speed/ power
Movement- multiplier Ratios (Gear Ratio) zWhat is this? yRatio of the movement between the gears yDivide number of teeth on DRIVEN by the number on the DRIVER zPractice! yA simple gear train is shown. The driver gear A has 20 teeth, while gear B has 40 teeth. yWhat is the movement multiplier ratio of the system? yIf shaft A rotates anti-clockwise, in which direction does shaft B rotate?
Solutions zDriver = 20 teeth Driven = 40 teeth M.R. = Driven / Driver = 40/20 = 2 zGear movement ratio is 2 : 1 zA rotates anti- clockwise B rotates clockwise
Speed Ratio zWhat is this? yRatio of the speed between the input and output gears yDivide number of teeth on DRIVER by the number on the DRIVEN zPractice! yA simple gear train is shown. The driver gear A has 20 teeth, while gear B has 40 teeth. yCalculate the Speed Ratio
Calculating Output Speed We know from previous work that the SR for the gear train shown is: Driver = 20 teeth Driven = 40 teeth S.R. = Driver / Driven = 20/40 = 1/2 If the driver has a speed of 200rpm, what is the driven speed? Output speed = SR x input speed = ½ x 200 = 100rpm
Idler Gears zWhat is an IDLER gear? yA third gear inserted between Driver and Driven yAllows Driver and Driven to rotate in same direction yNo effect on Speed or Gear Ratio of the system yUsually a small gear (takes up less space)
More Gears!! zCalculate the multiplier ratio for the simple gear train below and then find the speed ratio. If gear A rotates at 250 rpm in a clockwise direction, calculate the output speed. Show all your working. A = 20 teeth B = 5 teeth C = 30 teeth zFor the simple gear train shown below, find the following. yThe gear that rotates in the same direction as A. yThe multiplier ratios of A to B, A to C and A to D. yThe speed of B, C and D if A rotates at 500 rpm. xA = 50 teeth xB = 10 teeth xC = 25 teeth xD = 100 teeth
Compound Gears zWhat are compound gears? yA gear system with pairs of gears mounted on the same shaft yProduce large speed changes (100 : 1) yProvide multiple outputs with different speeds and directions
Compound Gear Example Gear Ratio/ MR zThe multiplier ratio (Gear Ratio) for the first pair of meshing teeth is zThe multiplier ratio for the second pair of meshing teeth is zThe total multiplier ratio is calculated by multiplying both ratios: Ratio = 4 : 1 x 6 : 1 = 24 : 1 Total
Short cut For a compound gear train the following is true: Gear movement ratio = product of number of teeth on driven divided by the product of number of teeth on driver From previous example: Gear Ratio = 80 x 60 = 4800 = 24 = 24 : 1 20 x 10 200 1
Compound Example SR zThe speed ratio for the first pair of meshing teeth is zThe speed ratio for the second pair of meshing teeth is zThe total speed ratio is calculated by multiplying both ratios: Driver/Driven = 20/80 = 1:4 Driver/Driven = 10/60 = 1:6 1/4 x 1/6 = 1:24 Note: The same short cut can be taken with movement ratio, can be taken with the speed ratio
Practice A B C D In the compound train shown below wheel A is rotating at 100 rpm. If the numbers of teeth in the gear wheels A, B, C and D are 25, 50, 25, and 50 respectively, determine the SR, the GR and the speed of rotation of wheel D,
Worm and Wheel zWhat is a Worm and Wheel? yA worm looks like a screw thread yIt is attached to a drive shaft (the worm can only drive a worm wheel, not the other way about!) yIt meshes with the worm wheel (fixed to driven shaft) yDriven shaft runs at 90 degrees to the driver shaft zWhy is it used? yAnother way of making large speed reductions y Can be used as a safety device, (the worm can only turn in 1 direction. Thus it will not run back if lifting loads.)
Example: z Think of worm as 1 toothed spur gear z The multiplier ratio between the gears shown is z This would mean that for a motor rotating at 100 rpm, the output driven gear would rotate at only 3.33 rpm.
Bevel Gears zWhat is a Bevel Gear? yTwo meshed gears at 90 degrees yGears are angled at 45 degrees yDifferent sized gears give different output rotation speeds
Tasks z Produce the greatest possible speed within a compound gear train using spur gears with 8t, 16t, 24t and 40t. The driver motor is set at 1 rpm.
Ratchet and Pawl zWhat is a RATCHET? yA wheel with saw- shaped teeth around its rim zWhat is a PAWL? yA pawl is a small tooth that engages with a ratchet zRatchet and Pawl yTogether they engage and allow rotation in one direction only
Examples: Ratchet and Pawl zWhere would you see a ratchet and pawl? yA wheel with saw- shaped teeth around its rim
Torque Torque (turning force) The turning force of a lever, e.g. spanner, is larger when the effort is further away from the fulcrum. You can get more torque from a spanner with a long handle, than one with a short one. Torque = Effort (newtons) x Distance (metres)
Torque Torque can be increased in a gear or pulley system by changing the size or output speed of the output gears. If we have a Driver of 40t and a Driven of 80T, this will have a lower output torque than say a Driver of 40T and Driven of 100T.
Torque and Drive Systems z Torque is the amount of turning produced by a force z Torque T = Force x Radius (Units are Nm) z Example y How much torque is required to tighten a nut if the force needed is 45N and the tool radius is 200mm? y T = F x r y T = 45N x 200mm y T = 45N x 0.2m y T = 9Nm
Torque and Drive Systems The gearing system shown, can operate with either a 50t or an 80t Driven gear. The Driver gear has 200t. Calculate the output torque for the gear system if the input torque is 20Nm. Multiplier Ratio 1 = Driven/Driver = 50/200 = ¼ Multiplier Ratio 2 = Driven/Driver = 80/200 = 2/5 Output Torque 1 = MR x Input Torque = ¼ x 20 = 5Nm Output Torque 2 = MR x Input Torque = 2/5 x 20 = 8Nm
Belt and Chain Drives z Belts and chains transmit rotary motion between parts of a mechanism z This is usually combined with a change of speed z Too many gears in a simple gear train results in a low efficiency
Belt Drives z A belt is wrapped around two or more pulleys z Pulleys are grooved wheels z The belt is tensioned by one of the pulleys y Also common to use a jockey pulley For tensioning purposes z Belts are also angled for greater grip (vee- belt)
Belt Drives z Changes in direction achieved by crossing the belt over y Inexpensive to produce (rubber and string) y Easy to replace y Require little maintenance (no lubrication) y Absorb shock loads (can slip to protect engine)
Multiplier Ratio for belt drives z Pulleys can be used to transmit rotary motion over large distances z Input speed is often fixed speed/ torque (motor) z Speed Ratio (VR) = diameter of driver pulley ------------------------------ diameter of driven pulley z Multiplier Ratio = diameter of driven pulley diameter of driver pulley
Toothed Belts z Slipping belts can be an advantage, why? y Protect against shock loads z Toothed belts are used when non-slip is required y Cars use toothed belts as timing belts y If this slipped the pistons would collide with the valves causing damage
Chain Drives z Used for transmitting large forces with no slip z Pulley replaced with sprocket y Require maintenance (oiling) y When worn will slip z Tension provided by pair of jockey wheels M.R = # teeth on driven/ # teeth on driver
Chain Drives The chain and sprocket is really a form of pulley system that does not allow slippage. (the sprocket is a pulley with teeth, the chain is a metal belt)
Chain Drives Questions: z The bicycle shown has two rear sprockets (50t and 80t). The driver sprocket has 200t. Calculate the output torque for the rear sprockets if the input is 20Nm y Find MR of small sprocket = #teeth on driven = 50 = 1:4 # teeth on driver 200 y Find MR of large sprocket = #teeth on driven = 80 = 1:2.5 # teeth on driver 200 y Now find o/p torques: x T(small) = Input torque x MR = 20Nm x 1:4 = 5Nm x T(large) = Input torque x MR = 20Nm x 1:2.5 = 8Nm
Rack and Pinion z Transforms rotary motion into linear motion (or vice versa) z Spur gear meshes with a rack z Task 1: y A rack with 100 teeth per metre is meshed to a pinion with 10 teeth. 1. If the pinion rotates once how far does the rack move? 2. How many revolutions does it take to move the rack from one end to the other? The rack is 1m long
Rack and Pinion Solutions Task 1 (A) Rack is 1m long with 100T, so each tooth is worth 1000/100 = 10mm This value is known as the Tooth Pitch of the rack. If the pinion rotates once, then it moves 10T, so the movement of the rack is 10 x 10 = 100mm (B)If rack is 1m long then it will take 1000/100 = 10 revolutions to move from one end to the other.
Questions The compound gear train shown below is driven by a motor that runs at 1000 rpm. Calculate the multiplier ratio of the motor to the output shaft, the speed ratio and then the output speed. Show all your working. A = 20 teeth B = 60 teeth C = 40 teeth D = 50 teeth
Friction & Effect y Friction between moving parts reduces the efficiency of the system y Ways in which we can reduce friction y These include: Lubrication, Oil or grease Use roller bearings