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MECHANICAL SYSTEMS

MECHANICAL SYSTEMS. This unit covers the following topics: Motion Forces Levers Moments Linkages Free Body Diagrams Beams Gears Torque and Drive Systems Converting motion. Introduction. Mechanisms are widely used in industry and society Many mechanisms will be familiar to you.

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MECHANICAL SYSTEMS

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  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 crane’s 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. EFFORT = 260 N LOAD = 750 N 600 mm 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. EFFORT = 150 N LOAD = 450 N 650 mm 200 mm 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. LOAD EFFORT FULCRUM 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. ROPE WEIGHT HINGE P 2 m TURNING EFFECT 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 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 • 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. EFFORT = 150 N LOAD = 450 N 650 mm 200 mm 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

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