Presentation on theme: "G E A R S What is a gear? What do they do? Most common gear?"— Presentation transcript:
1 G E A R S What is a gear? What do they do? Most common gear? Toothed wheelTransmits rotary motion and powerWhat do they do?Change the direction of motionChange the output speedMost common gear?SPUR gear
2 SIMPLE GEAR TRAINS What is a simple gear train? Meshed (interlocking gears)Two or more gears in seriesInput gear = DRIVEROutput gear = DRIVENWhat effect does this have on the output (DRIVEN)Reverses motionChanges speed/ power
3 Movement- multiplier Ratios (Gear Ratio) What is this?Ratio of the movement between the gearsDivide number of teeth on DRIVEN by the number on the DRIVERPractice!A simple gear train is shown. The driver gear A has 20teeth, while gear B has 40 teeth.What is the movement multiplier ratio of the system?If shaft A rotates anti-clockwise, in which direction does shaft B rotate?
4 Solutions M.R. = Driven / Driver = 40/20 = 2 Driver = 20 teeth Driven = 40 teethM.R. = Driven / Driver= 40/20 = 2Gear movement ratio is 2 : 1A rotates anti- clockwise B rotates clockwise
5 Speed Ratio What is this? Practice! Ratio of the speed between the input and output gearsDivide number of teeth on DRIVER by the number on the DRIVENPractice!A simple gear train is shown. The driver gear A has 20 teeth, while gear B has 40 teeth.Calculate the Speed Ratio
7 Calculating Output Speed We know from previous work that the SR for the gear train shown is:Driver = 20 teethDriven = 40 teeth S.R. = Driver / Driven= 20/40 = 1/2If the driver has a speed of 200rpm, what is the driven speed?Output speed = SR x input speed= ½ x 200 = 100rpm
8 Idler Gears What is an IDLER gear? A third gear inserted between Driver and DrivenAllows Driver and Driven to rotate in same directionNo effect on Speed or Gear Ratio of the systemUsually a small gear (takes up less space)
9 More Gears!!Calculate 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 teethB = 5 teethC = 30 teethFor the simple gear train shown below, find the following.The gear that rotates in the same direction as A.The multiplier ratios of A to B, A to C and A to D.The speed of B, C and D if A rotates at 500 rpm.A = 50 teethB = 10 teethC = 25 teethD = 100 teeth
10 Compound Gears What are compound gears? A gear system with pairs of gears mounted on the same shaftProduce large speed changes (100 : 1)Provide multiple outputs with different speeds and directions
11 Compound Gear Example Gear Ratio/ MR The multiplier ratio (Gear Ratio) for the first pair of meshing teeth isThe multiplier ratio for the second pair of meshing teeth isThe total multiplier ratio is calculated by multiplying both ratios:TotalRatio = 4 : 1 x 6 : 1 = 24 : 1
12 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 driverFrom previous example:Gear Ratio = 80 x 60 = 4800 = 24 = 24 : 120 x
13 Compound Example SR 1/4 x 1/6 = 1:24 Driver/Driven = 20/80 = 1:4 The speed ratio for the first pair of meshing teeth isThe speed ratio for the second pair of meshing teeth isThe total speed ratio is calculated by multiplying both ratios:Driver/Driven = 20/80 = 1:4Driver/Driven = 10/60 = 1:61/4 x 1/6 = 1:24Note: The same short cut can be taken with movement ratio, can be taken with the speed ratio
14 Practice 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,ABCD
15 Worm and Wheel What is a Worm and Wheel? Why is it used? A worm looks like a screw threadIt is attached to a drive shaft (the worm can only drive a worm wheel, not the other way about!)It meshes with the worm wheel (fixed to driven shaft)Driven shaft runs at 90 degrees to the driver shaftWhy is it used?Another way of making large speed reductionsCan be used as a safety device, (the worm can only turn in 1 direction. Thus it will not run back if lifting loads.)
16 Example: Think of worm as 1 toothed spur gear The multiplier ratio between the gears shown isThis would mean that for a motor rotating at 100 rpm, the output driven gear would rotate at only 3.33 rpm.
17 Bevel Gears What is a Bevel Gear? Two meshed gears at 90 degrees Gears are angled at 45 degreesDifferent sized gears give different output rotation speeds
18 TasksProduce 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.
19 Ratchet and Pawl What is a RATCHET? What is a PAWL? Ratchet and Pawl A wheel with saw- shaped teeth around its rimWhat is a PAWL?A pawl is a small tooth that engages with a ratchetRatchet and PawlTogether they engage and allowrotation in one direction only
20 Examples: Ratchet and Pawl Where would you see a ratchet and pawl?A wheel with saw- shaped teeth around its rim
21 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)
22 TorqueTorque 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.
23 Torque and Drive Systems Torque is the amount of turning produced by a forceTorque T = Force x Radius (Units are Nm)ExampleHow much torque is required to tighten a nut if the force needed is 45N and the tool radius is 200mm?T = F x rT = 45N x 200mmT = 45N x 0.2mT = 9Nm
24 Torque and Drive Systems The gearing system shown, can operate with either a 50t or an 80tDriven gear. The Driver gear has 200t. Calculate the output torquefor the gear system if the input torque is 20Nm.Multiplier Ratio 1 = Driven/Driver = 50/200 = ¼Multiplier Ratio 2 = Driven/Driver = 80/200 = 2/5Output Torque 1 = MR x Input Torque = ¼ x 20 = 5NmOutput Torque 2 = MR x Input Torque = 2/5 x 20 = 8Nm
25 Belt and Chain DrivesBelts and chains transmit rotary motion between parts of a mechanismThis is usually combined with a change of speedToo many gears in a simple gear train results in a low efficiency
26 Belt Drives A belt is wrapped around two or more pulleys Pulleys are grooved wheelsThe belt is tensioned by one of the pulleysAlso common to use a jockey pulleyFor tensioning purposesBelts are also angled forgreater grip (vee- belt)
27 Belt Drives Changes in direction achieved by crossing the belt over Inexpensive to produce (rubber and string)Easy to replaceRequire little maintenance (no lubrication)Absorb shock loads (can slip to protect engine)
28 Multiplier Ratio for belt drives Pulleys can be used to transmit rotary motion over large distancesInput speed is often fixed speed/ torque (motor)Speed Ratio (VR) = diameter of driver pulleydiameter of driven pulleyMultiplier Ratio = diameter of driven pulleydiameter of driver pulley
29 Toothed Belts Slipping belts can be an advantage, why? Protect against shock loadsToothed belts are used when non-slip is requiredCars use toothed belts as timing beltsIf this slipped the pistons would collidewith the valves causing damage
30 Chain Drives Used for transmitting large forces with no slip Pulley replaced with sprocketRequire maintenance (oiling)When worn will slipTension provided by pair ofjockey wheelsM.R = # teeth on driven/ # teeth on driver
31 Chain DrivesThe 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)
32 Chain Drives Questions: 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 20NmFind MR of small sprocket = #teeth on driven = 50 = 1:4# teeth on driver 200Find MR of large sprocket = #teeth on driven = 80 = 1:2.5Now find o/p torques:T(small) = Input torque x MR = 20Nm x 1:4 = 5NmT(large) = Input torque x MR = 20Nm x 1:2.5 = 8Nm
33 Rack and PinionTransforms rotary motion into linear motion (or vice versa)Spur gear meshes with a ‘rack’Task 1:A rack with 100 teeth per metreis meshed to a pinion with 10 teeth.1. If the pinion rotates once how fardoes the rack move?2. How many revolutions does it take tomove the rack from one end to the other?The rack is 1m long
34 Rack and Pinion Solutions Task 1 (A)Rack is 1m long with 100T, so each tooth is worth 1000/100 = 10mmThis 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.
35 QuestionsThe 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 teethB = 60 teethC = 40 teethD = 50 teeth
36 Friction & EffectFriction between moving parts reduces the efficiency of the systemWays in which we can reduce frictionThese include:Lubrication, Oil or greaseUse roller bearings