Introduction

Mechanisms are widely used in industry and society

Many mechanisms will be familiar to you

(Intro continued)

Many industrial processes involve electronic control, mechanisms provide the muscle to do the work

All mechanisms involve:

Some kind of motion Some kind of force Make a job easier to do Need an input to make them work Produce some kind of product

4 Basic Kinds Of Motion

Rotary Turning in a circle

Linear Moving in a straight line

Reciprocating Backwards and forwards movement

Oscillating Swinging back and forwards

Motion Task 1

Identify the type of motion shown by the following activities.

Complete a systems diagram for each

Motion Task 2

Consider the tools and machines you have used/ seen in CDT

List up to three tools or machines for each basic type of motion

Rotary

Linear

Reciprocating

Oscillating

G E A R S

What is a gear? Toothed wheel Transmits rotary motion and power

What do they do? Change the direction of motion Change the output speed

Most common gear? SPUR gear

SIMPLE GEAR TRAINS

What is a simple gear train?

Meshed, (Meshing) Two or more gears in series

Input gear = DRIVER Output gear = DRIVEN

What effect does this have on the output (DRIVEN)

Reverses motion Changes speed/ power

Velocity Ratio

What is this?

Ratio of the speed between the input and output gears

Divide number of teeth on DRIVER by the number on the DRIVEN

Practice!

A simple gear train is shown. The driver gear A has 20 teeth, while gear B has 40 teeth.

Calculate the Velocity Ratio

Solutions

Driver = 20 teethDriven = 40 teeth

V.R. = Driver / Driven = 20/40 = 1/2

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 teethDriven = 40 teeth V.R. = Driver / Driven

= 20/40 = 1/2

If the driver has a speed of 200rpm, what is the driven speed?

Output speed = VR x input speed

= ½ x 200 = 100rpm

Idler Gears

What is an IDLER gear?

A third gear inserted between Driver and Driven Allows Driver and Driven to rotate in same direction No effect on Speed of the system Usually a small gear (takes up less space)

More Gears!! 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 teethB = 5 teethC = 30 teeth

For the simple gear train shown below, find the following. The gear that rotates in the same direction as A. The velocity 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 teeth B = 10 teeth C = 25 teeth D = 100 teeth

Compound Gears

What are compound gears?

A gear system with pairs of gears mounted on the same shaft

Produce large speed changes (100 : 1) Provide multiple outputs with different speeds and

directions

Compound Example VR

The velocity ratio for the first pair of meshing teeth is

The velocity ratio for the second pair of meshing teeth is

The 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

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 speed of rotation of wheel D,

Worm and Wheel

What is a Worm and Wheel?

A worm looks like a screw thread It 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 shaft

Why is it used?

Another way of making large speed reductions 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:

Think of worm as 1 toothed spur gear The velocity ratio between the gears shown is  

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.

Velocity ratio = Driver / Driven

Bevel Gears

What is a Bevel Gear?

Two meshed gears at 90 degrees Gears are angled at 45 degrees Different sized gears give different output rotation speeds

Tasks

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

What is a RATCHET?

A wheel with saw- shaped teeth around its rim

What is a PAWL?

A pawl is a small tooth that engages with a ratchet

Ratchet and Pawl

Together they engage and allow rotation in one direction only

Examples: Ratchet and Pawl

Where would you see a ratchet and pawl?

A wheel with saw- shaped teeth around its rim

Belt and Chain Drives

Belts and chains transmit rotary motion between parts of a mechanism

This is usually combined with a change of speed

Too many gears in a simple gear train results in a low efficiency

Belt Drives

A belt is wrapped around two or more pulleys

Pulleys are grooved wheels

The belt is tensioned by one of the pulleys

Also common to use a jockey pulleyFor tensioning purposes

Belts are also angled for greater grip (vee- belt)

Belt Drives

Changes in direction achieved by crossing the belt over

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

D R IVEN

D R IVER

Velocity Ratio for belt drives

Pulleys can be used to transmit rotary motion over large distances

Input speed is often fixed speed/ torque (motor)

Velocity Ratio (VR) = diameter of driver pulley ------------------------------

diameter of driven pulley

Multiplier Ratio = diameter of driven pulley diameter of driver pulley

Toothed Belts

Slipping belts can be an advantage, why?

Protect against shock loads

Toothed belts are used when non-slip is required

Cars use toothed belts as timing belts

If this slipped the pistons would collide with the valves causing damage

Chain Drives

Used for transmitting large forces with no slip Pulley replaced with sprocket

Require maintenance (oiling) When worn will reduce

accuracy of drive

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

Transforms 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 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 velocity Ratio of the motor to the output shaft and then the output speed. Show all your working.  A = 20 teethB = 60 teethC = 40 teethD = 50 teeth M O TO R

A B

CD

O U TPU T

Gary Plimer 2006

We know that there are four kinds of motion. These comprise:

RotaryLinearReciprocatingOscillating.

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?

CONVERTING MOTION

PIN IO N

R AC K

ANSWER: RACK & PINION

Gary Plimer 2006

CAM & FOLLOWER

EC CEN TRICCAM

FO LLO W ER

R O TARYC AM

R O TARYM O TIO N

FO LLO W ER R EC IPR O C ATIN GM O TIO N

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.

With the pear shaped cam, there is a dwell period when the follower does not move.

Pear Cam

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

C R AN K

SLID ER

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

CAM

VALVE

SPRING

COLLAR

42 mm

20 mm

(b) If the cam on the valve mechanism turns half a revolution from the position shown on the diagram, what distance does the valve move?

(a) What is the input & output motion?

Friction & Effect

Friction between moving parts reduces the efficiency of the system

Ways in which we can reduce friction

These include:• Lubrication, Oil or grease• Use roller bearings