1. MECHANICAL SYSTEMS This unit covers the following topics: zMotion zForces zLevers zMoments zLinkages zFree Body Diagrams zBeams zGears zTorque and Drive Systems zConverting motion

2. Introduction zMechanisms are widely used in industry and society zMany mechanisms will be familiar to you

3. (Intro continued) zMany industrial processes involve electronic control, mechanisms provide the muscle to do the work zAll mechanisms involve: ySome kind of motion ySome kind of force yMake a job easier to do yNeed an input to make them work yProduce some kind of product

4. 4 Basic Kinds Of Motion Rotary yTurning in a circle zLinear yMoving in a straight line zReciprocating yBackwards and forwards movement zOscillating ySwinging back and forwards

5. Motion Task 1 zIdentify the type of motion shown by the following activities. zComplete a systems diagram for each

6. Motion Task 2 zConsider the tools and machines you have used/ seen in CDT zList up to three tools or machines for each basic type of motion yRotary yLinear yReciprocating yOscillating

7. Forces zForce causes acceleration zForce is measured in Newtons (N) zThere are several different types of forces that can be applied to bodies and structures

8. Static Forces zStatic forces do not usually cause motion zConsider a tall building yThe weight of the material it is built from, and the people and furniture inside it are static loads

9. Dynamic Loads zUsually causes a movement zThe value of the force can be variable zAgain consider a tall building yVariable winds add an extra force or load to the structure yThe engineer must allow for this

10. Bending Forces zStructures that carry loads across their length are subject to bending forces zConsider a car driving across a bridge

11. Shear Forces zThese are tearing or cutting forces zScissors are an example of these

12. Torsion Forces zTorque is a turning force which tries to twist a structure

13. Compression Forces zCompression forces try to squash a structure zConsider a column yThe weight down is balanced by the reaction from the ground yThe forces act to try and shorten the column

14. Forces in Tension zTensile forces try to stretch a structure zConsider a crane’s lifting cable yThe weight tries to stretch or pull the cable apart yCables in tension can have small diameters compared to members in compression

15. LEVERS zIn its simplest form, a lever is a stick that is free to pivot or move back and forth at a certain point. zLevers are probably the most common simple machine because just about anything that has a handle on it has a lever attached. zThe 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 zHow do you move a heavy person? zIf 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. zWhere's the trade-off? zWell, 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 yDraw the universal system for a lever yCopy the line diagram of a lever

18. Basic Types Of Lever zLevers can be either force or distance multipliers (not both)

19. Force Multiplier Ratio zConsider the lever shown zThe LOAD is about 3 times more than the EFFORT z LOAD/EFFORT gives force multiplier ratio

20. Movement Multiplier Ratio zSomething for nothing? zApplying less force to move the load must involve a trade off. zThe effort must be moved through a greater distance zIn our example the effort moves much more than the load zMovement multiplier ratio = distance moved by effort distance moved by load

21. Efficiency zThe friction and inertia associated with moving an object means that some of the input energy is lost zSince losses occur, the system is not 100% efficient zEfficiency =  = Force Ratiox 100 Movement Ratio zLosses in a lever could be friction in the fulcrum, strain in the lever as it bends slightly and maybe sound. zComplete the following tasks:

22. Task 1 zDraw a universal system diagram for a lever zComplete the following diagram, indicating clearly the LOAD, EFFORT and FULCRUM

23. Task 2 zCalculate the force- multiplier ratio of the following levers, show all working Load 100N

24. Task 3 zA diagram for a lever system is shown below. yFind the force- multiplier of the lever system yCalculate the movement- multiplier ratio of the lever yCalculate the efficiency of the system yIdentify possible efficiency losses in the system

25. Classes of Levers z Levers can be divided into three distinct types (classes) z Determined by the position of the load, effort and fulcrum. zClass 1 z In class 1 levers the effort is on one side of the fulcrum and the load is on the opposite side. y 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 zClass 2 z 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. z The load must be closer to the fulcrum than the effort z 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. EFFORT LOAD FULCRUM

27. CLASS of LEVER zClass 3 z Class 3 levers are similar to class 2 levers except that now the effort is closer to the fulcrum than the load z 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 zA moment is a turning force zConsider the system shown: yA weight is attached to a metal rod yThe rod is free to rotate around a hinge yWhat happens if the rope is cut? yThe weight exerts a moment of 20Nm (Force x Distance) TURNING EFFECT 2 m HINGE P ROPE WEIGHT

30. Lever Systems zThe lever shown is in equilibrium (a steady state) zThe input force exerts an anticlockwise moment zThe output force exerts a clockwise moment zTo be in equilibrium both moments must be equal

31. The Principle of Moments yThe sum of the moments must equal zero y  CWM =  ACWM yExample: 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 F 1¹  d 1 = F 2  d 2 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 zA car footbrake uses a lever action to amplify force transmitted by the driver to the braking system when the driver’s 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 zThis 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. zThe input tends to make the lever turn clockwise; the braking system is opposing the input and so acts to turn the lever anticlockwise. zThe principle of moments states that:  CWM =  ACWM F 1  d 1 = F 2  d 2 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: zFor the system shown: zIf the handle length is 250mm and the effort to turn it is 15N, what moment would close the tap valve? zWhat is the benefit of this type of tap? zSuggest a situation where this type of tap would be useful

37. Task 2 zCalculate 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 zA diagram for a lever system is shown below. yFind the force- multiplier of the lever system yCalculate the movement- multiplier ratio of the lever yCalculate the efficiency of the system yIdentify possible efficiency losses in the system