MECHANICAL SYSTEMS

1
MECHANICAL SYSTEMS

• This unit covers the following topics
• Motion
• Forces
• Levers
• Moments
• Linkages
• Free Body Diagrams
• Beams
• Gears
• Torque and Drive Systems
• Converting motion

2
Introduction

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

3
(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
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

5
Motion Task 1

• Identify the type of motion shown by the
  following activities.
• Complete a systems diagram for each

6
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

7
Forces

• Force causes acceleration
• Force is measured in Newtons (N)
• There are several different types of forces that
  can be applied to bodies and structures

8
Static Forces

• Static forces do not usually cause motion
• Consider a tall building
• The weight of the material
• it is built from, and the
• people and furniture inside
• it are static loads

9
Dynamic Loads

• Usually causes a movement
• The value of the force can be variable
• Again consider a tall building
• Variable winds add an extra
• force or load to the structure
• The engineer must allow for this

10
Bending Forces

• Structures that carry loads across their length
  are subject to bending forces
• Consider a car driving across a bridge

11
Shear Forces

• These are tearing or cutting forces
• Scissors are an example of these

12
Torsion Forces

• Torque is a turning force which tries to twist a
  structure

13
Compression Forces

• Compression forces try to squash a structure
• Consider a column
• The weight down is balanced
• by the reaction from the ground
• The forces act to try and shorten
• the column

14
Forces in Tension

• Tensile forces try to stretch a structure
• Consider a cranes lifting cable
• The weight tries to stretch or pull the cable
  apart
• Cables in tension can have small diameters
  compared to members in compression

15
LEVERS

• In its simplest form, a lever is a stick that is
  free to pivot or move back and forth at a certain
  point.
• Levers are probably the most common simple
  machine because just about anything that has a
  handle on it has a lever attached.
• The point on which the lever moves is called the
  fulcrum.
• By changing the position of the fulcrum, you can
  gain extra power with less effort.

16
LEVERS

• How do you move a heavy person?
• If you put the fulcrum in the middle, you won't
  have a chance. But if you slide the fulcrum
  closer to the heavy person, it will be easier to
  lift.
• Where's the trade-off?
• Well, to get this helping hand, your side of the
  see-saw is much longer (and higher off the
  ground), so you have to move it a much greater
  distance to get the lift

17
LEVERS

• Draw the universal system for a lever
• Copy the line diagram of a lever

18
Basic Types Of Lever

• Levers can be either force or distance
  multipliers (not both)

19
Force Multiplier Ratio

• Consider the lever shown
• The LOAD is about 3 times more than the EFFORT
• LOAD/EFFORT gives force multiplier ratio

20
Movement Multiplier Ratio

• Something for nothing?
• Applying less force to move the load must involve
  a trade off.
• The effort must be moved through a greater
  distance
• In our example the effort moves much more than
  the load
• Movement multiplier ratio distance moved by
  effort
• distance moved by load

21
Efficiency

• The friction and inertia associated with moving
  an object means that some of the input energy is
  lost
• Since losses occur, the system is not 100
  efficient
• Efficiency ? Force Ratio x 100
• Movement Ratio
• Losses in a lever could be friction in the
  fulcrum, strain in the lever as it bends slightly
  and maybe sound.
• Complete the following tasks

22
Task 1

• Draw a universal system diagram for a lever
• Complete the following diagram, indicating
  clearly the LOAD, EFFORT and FULCRUM

23
Task 2

• Calculate the force- multiplier ratio of the
  following levers, show all working

Load 100N
24
Task 3

• A diagram for a lever system is shown below.
• Find the force- multiplier of the lever system
• Calculate the movement- multiplier ratio of the
  lever
• Calculate the efficiency of the system
• Identify possible efficiency losses in the system

25
Classes of Levers

•  Levers can be divided into three distinct types
  (classes)
• Determined by the position of the load, effort
  and fulcrum.  
• Class 1
• In class 1 levers the effort is on one side of
  the fulcrum and the load is on the opposite side.
• Class 1 levers are the simplest to understand
  the longer the crowbar the easier it is to prise
  open the lid.
•  
•  
•  
•  

26
CLASS of LEVER

• Class 2
• In class 2 levers the fulcrum is at one end of
  the lever and the load and the effort are spaced
  out on the other end of the bar.
• The load must be closer to the fulcrum than the
  effort
• A wheelbarrow is a good example of a class 2
  lever. The wheel is the fulcrum, the load is in
  the container area and the effort is applied to
  the handles.

27
CLASS of LEVER

• Class 3
• Class 3 levers are similar to class 2 levers
  except that now the effort is closer to the
  fulcrum than the load
• This means that more effort has to be applied to
  move the load. This type of lever is used when
  mechanisms require a large output movement for a
  small input movement.

28
Task 4
For each of the following tools, state the class
of lever
29
M O M E N T S

• A moment is a turning force
• Consider the system shown
• A weight is attached to a metal rod
• The rod is free to rotate around a hinge
• What happens if the rope is cut?
• The weight exerts a moment of 20Nm (Force x
  Distance)

30
Lever Systems

• The lever shown is in
• equilibrium (a steady state)
• The input force exerts an anticlockwise moment
• The output force exerts a clockwise moment
• To be in equilibrium both moments must be equal

31
The Principle of Moments

• The sum of the moments must equal zero
• ?CWM ?ACWM
• Example Prove that the following system is in
  equilibrium

32
Solution

• For equilibrium, the ?CWM ?ACWM.
• A moment is a force multiplied by a distance
• ?CWM ?ACWM
• F1¹? d1 F2 ? d2
• The load exerts a clockwise moment (tends to make
  the lever turn clockwise)
• Clockwise moment 200 N ? 2 m 400 Nm
• The effort exerts a anticlockwise moment.
• Anticlockwise moment 400 N ? 1 m 400 Nm
• ? ?CWM ?ACWM
• Therefore the lever is in a state of
  equilibrium.

33
Task One

• A car footbrake uses a lever action to amplify
  force transmitted by the driver to the braking
  system when the drivers foot presses the
  foot-pedal. If the drivers foot can exert a force
  of 5000N, what force will be transmitted to the
  braking system?

34
Solution

• This is a class 2 lever. Take moments about the
  fulcrum to find the force on the braking system.
  Notice the distance from the fulcrum to the input
  is 600 mm.
• The input tends to make the lever turn clockwise
  the braking system is opposing the input and so
  acts to turn the lever anticlockwise.
• The principle of moments states that ?CWM
  ?ACWM
• F1 ? d1 F2 ? d2
• 5000 N ? 0.6 m braking force ? 0.1 m
• braking force 5000 N ? 0.6 m
• 0.1 m
• braking force 30,000 N or 30 kN

35
Practice
36
Questions

• For the system shown
• If the handle length is 250mm
• and the effort to turn it is 15N,
• what moment would close the tap valve?
• What is the benefit of this type of tap?
• Suggest a situation where this type of tap would
  be useful

37
Task 2

• Calculate the force- multiplier ratio of the
  following levers, show all working. Calculate a
  suitable distance between effort and load to
  produce equilibrium.

Load 600N
38
Task 3

• A diagram for a lever system is shown below.
• Find the force- multiplier of the lever system
• Calculate the movement- multiplier ratio of the
  lever
• Calculate the efficiency of the system
• Identify possible efficiency losses in the system