Introduction z Mechanisms are widely used in industry

Introduction z Mechanisms are widely used in industry and society z Many mechanisms will be familiar to you

(Intro continued) z Many industrial processes involve electronic control, mechanisms provide the muscle to do the work z All mechanisms involve: y Some kind of motion y Some kind of force y Make a job easier to do y Need an input to make them work y Produce some kind of product

4 Basic Kinds Of Motion Rotary y Turning in a circle z Linear y Moving in a straight line z Reciprocating y Backwards and forwards movement z Oscillating y Swinging back and forwards

Motion Task 1 z Identify the type of motion shown by the following activities. z Complete a systems diagram for each

Motion Task 2 z Consider the tools and machines you have used/ seen in CDT z List up to three tools or machines for each basic type of motion y Rotary y Linear y Reciprocating y Oscillating

GEARS z What is a gear? y Toothed wheel y Transmits rotary motion and power z What do they do? y Change the direction of motion y Change the output speed z Most common gear? y SPUR gear

SIMPLE GEAR TRAINS z What is a simple gear train? y Meshed, (Meshing) y Two or more gears in series z Input gear = DRIVER z Output gear = DRIVEN z What effect does this have on the output (DRIVEN) y Reverses motion y Changes speed/ power

Velocity Ratio z What is this? y Ratio of the speed between the input and output gears y Divide number of teeth on DRIVER by the number on the DRIVEN z Practice! y A simple gear train is shown. The driver gear A has 20 teeth, while gear B has 40 teeth. y Calculate the Velocity Ratio

Solutions z Driver = 20 teeth Driven = 40 teeth V. R. = Driver / Driven = 20/40 = 1/2 z Gear velocity/speed ratio is 1 : 2

Calculating Output Speed We know from previous work that the VR for the gear train shown is: Driver = 20 teeth Driven = 40 teeth V. R. = Driver / Driven = 20/40 = 1/2 If the driver has a speed of 200 rpm, what is the driven speed? Output speed = VR x input speed = ½ x 200 = 100 rpm

Idler Gears z What is an IDLER gear? y A third gear inserted between Driver and Driven y Allows Driver and Driven to rotate in same direction y No effect on Speed of the system y Usually a small gear (takes up less space)

More Gears!! z Calculate the velocity ratio for the simple gear train below. 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 z For the simple gear train shown below, find the following. y The gear that rotates in the same direction as A. y The velocity ratios of A to B, A to C and A to D. y The speed of B, C and D if A rotates at 500 rpm. x A = 50 teeth x B = 10 teeth x C = 25 teeth x D = 100 teeth

Compound Gears z What are compound gears? y A gear system with pairs of gears mounted on the same shaft y Produce large speed changes (100 : 1) y Provide multiple outputs with different speeds and directions

Compound Example VR z The velocity ratio for the first pair of meshing teeth is Driver/Driven = 20/80 = 1: 4 z The velocity ratio for the second pair of meshing teeth is Driver/Driven = 10/60 = 1: 6 z The total speed ratio is calculated by multiplying both ratios: 1/4 x 1/6 = 1: 24

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 speed of rotation of wheel D, D C A B

Worm and Wheel z What is a Worm and Wheel? y A worm looks like a screw thread y It is attached to a drive shaft (the worm can only drive a worm wheel, not the other way about!) y It meshes with the worm wheel (fixed to driven shaft) y Driven shaft runs at 90 degrees to the driver shaft z Why is it used? y Another 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 velocity ratio between the gears shown is Velocity ratio = Driver / Driven z This would mean that for a motor rotating at 100 rpm, the output driven gear would rotate at only 3. 33 rpm. Try the problems on the white board now.

Bevel Gears z What is a Bevel Gear? y Two meshed gears at 90 degrees y Gears are angled at 45 degrees y Different sized gears give different output rotation speeds

Tasks z Produce the greatest possible speed within a compound gear train using spur gears with 8 t, 16 t, 24 t and 40 t. The driver motor is set at 1 rpm.

Ratchet and Pawl z What is a RATCHET? y A wheel with saw- shaped teeth around its rim z What is a PAWL? y A pawl is a small tooth that engages with a ratchet z Ratchet and Pawl y Together they engage and allow rotation in one direction only

Examples: Ratchet and Pawl z Where would you see a ratchet and pawl? y A wheel with saw- shaped teeth around its rim

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 y Inexpensive to produce (rubber and string) Easy to replace Require little maintenance (no lubrication) Absorb shock loads (can slip to protect engine)

Velocity 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 Velocity 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 reduce accuracy of drive z Tension provided by pair of jockey wheels VR = Teeth on Driver / Teeth on Driven

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)

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 1 m long

Rack and Pinion Solutions Task 1 (A) Rack is 1 m long with 100 T, so each tooth is worth 1000/100 = 10 mm This value is known as the Tooth Pitch of the rack. If the pinion rotates once, then it moves 10 T, so the movement of the rack is 10 x 10 = 100 mm (B) If rack is 1 m 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 velocity Ratio of the motor to the output shaft and then the output speed. Show all your working. A = 20 teeth B = 60 teeth C = 40 teeth D = 50 teeth

CONVERTING MOTION We know that there are four kinds of motion. These comprise: Rotary Linear Reciprocating Oscillating. Many mechanisms involve changing one type of motion into another. For example, the rotary motion of a pillar-drill handle is changed to the linear motion of the chuck and drill bit moving towards the material being drilled. What mechanism can achieve this conversion? ANSWER: RACK & PINION Gary Plimer 2006

CAM & FOLLOWER § With the eccentric cam, the follower is moving constantly. The distance between the highest and lowest point of the follower is known as the STROKE of the cam. § Pear Cam With the pear shaped cam, there is a dwell period when the follower does not move. CAM & FOLLOWERS CONVERT ROTARY MOTION TO RECIPROCATING MOTION Gary Plimer 2006

CAM & FOLLOWER Pear shaped The follower stays at the lowest position for half a turn and then rises and falls steadily Eccentric The follower rises and falls steadily Ratchet The follower will rise steadily and fall suddenly. The cam can only turn in one direction without locking Gary Plimer 2006

CRANK & SLIDER The Crank & Slider can convert ROTARY TO RECIPROCATING or RECIPROCATING TO ROTARY The conversion depends on whether the crank or the slider is being driven. Gary Plimer 2006

Pupil Problem (a) (b) What is the input & output motion? If the cam on the valve mechanism turns half a revolution from the position shown on the diagram, what distance does the valve move? Gary Plimer 2006

Friction & Effect y Friction between moving parts reduces the efficiency of the system y Ways in which we can reduce friction These include: • Lubrication, Oil or grease • Use roller bearings