Mechanical Engineering

1. MechanicalEngineering Chapter 13 Observatory pp. 424-454

2. What is Mechanical Engineering? • Mechanical Engineering is the branch of engineering that focuses on the design, production, analysis, working and improvement of technical objects with moving parts. • Main Topics • Links • Guiding • Transmission • Transformation

3. Linking in Technical Objects • Linking is the mechanical function performed by any component that connects different parts of a technical object.

4. Characteristics of Links • You must know the main characteristics of links • Chart on p. 428 in textbook.

5. Links • Direct OR Indirect • Rigid OR Flexible • Removable OR non-removable • Complete OR Partial

6. Degrees of Freedom of Movement • The Degrees of Freedom are the set of independent movements that are possible for a given part in a technical object. • E.g. A door can only rotate around the hinges – 1 degree freedom • E.g. A manual transmission gear shift moves forward/back and left/right – 2 degrees of freedom

7. Degrees of Freedom • Chart p. 429

8. Guiding Controls • A Guiding Component or Control is a component whose mechanical function is to guide the motion of moving parts.

9. Types of Guiding • Translational Guiding ensures the straight translational motion of a moving part. E.g. a vertical window groove • Rotational Guiding ensures the rotational motion of a moving part.E.g. a bicycle wheel hub • Helical Guiding ensures the translation motion of a moving part while it rotates about the same axis. E.g. threaded shank in a vice

10. Symbols for Motion • You must know the symbols for the 6 types of motion provided as a handout.

11. Adhesion and Friction of Parts • Adhesion is the phenomenon by which two surfaces tend to remain in contact with each other without slipping. • In mechanics, Friction is a force that resists the slipping of one part over another. • Lubrication is the mechanical function performed by any component that reduces friction between two parts.

12. 5 Factors affecting adhesion • Nature of materials (eg: steel on asphalt vs. rubber on asphalt) • Presence of a lubricant like oil or wax • Temperature: the colder the temperature the less the adhesion • Surfaces (rough vs. smooth) • Mass of the object (a heavy object will have better adhesion, more friction)

13. Motion Transmission Systems • Motion Transmission is the mechanical function of relaying a motion from one part to another without altering the nature of the motion. • A Motion Transmission System is a set of components that perform the function of transmitting motion.

14. Some definitions: • Driver component: receives the force required to activate the system eg: crankset on a bike • Driven component: receives the motion and transfers it to another part Eg: rear gears on a bike • Intermediate component: located between the driver and driven component – not all systems have this. Eg: the chain on a bike

15. Characteristics of Motion in Transmission Systems • The most common rotational transmission systems are: • Gear Trains • Chain and Sprocket Systems • Worm and Worm Gear Systems • Friction Gear Systems • Belt and Pulley Systems

16. Motion Transmission Systems • See Figure on p. 437 for the main types

17. Construction Considerations for Motion TRANSMISSION Systems • Depending on the need, a system may only rotate in one direction. • That will affect the choice of system.

18. Gear Trains • The direction of rotation changes from one wheel to the next. • The system can be reversed.

19. Gear Trains

20. Gear Train Factors • Gear teeth: all the gear teeth in a system must be identical – same shape, direction, size and be equally spaced. E.g. Straight or helical • Gear type: the rotational axis of the gears can be positioned different ways (eg: car differentials) • Gear size: the higher the number of teeth, the lower the speed of rotation – or bigger diameter  slower speed.

21. Helical Gears

22. Car Differentials

23. Chain and Sprocket Systems • The direction of rotation of all sprockets on the same side of the chain is the same. • A sprocket on the other side of the chain rotates in the opposite direction. • It can be reversed. • The smaller the sprocket the faster it turns • Requires frequent lubrication

24. Worm and Worm Gear Systems • The direction of rotation depends on the direction of the threads on the worm screw shaft. • It is not reversible. • Worm must be the driver

25. Friction Gear Systems • The direction alternates from one gear to the next. • It is reversible. • The smaller the diameter of the gear, the faster its rotation

26. Friction Gear Systems • Friction gear systems are similar to gear trains except that motion is transferred by FRICTION and not by the GEAR TEETH. • They are less efficient because of slippage. • Factors that affect friction gear systems are: gear type (straight, bevel or spherical), gear size and choice of material.

27. Belt and Pulley Systems • Similar to the chain and sprocket system. • The chain is replaced by a belt. • The sprocket is replaced by a pulley. • The choice of the belt material and the tightness of the belt affect the friction and hence the efficiency of the system. • The direction is the same for any pulley on the same side of the belt. • It is reversible. • The smaller the pulley, the faster its rotation

28. Belt and Pulley systems

29. Speed Changes in Motion Transmission Systems • A Speed Change occurs in a motion transmission system when the driver does not turn at the same speed as the driven component(s). • The speed change depends on the ratio of gears/threads of the driver compared to that of the driven component.

30. Speed Changes in a Pulley or Sprocket Transmission System • To increase the speed, the driven component should have a smaller diameter. • To decrease the speed, the driven component should have a larger diameter. • To keep the same speed, the two pulleys should have the same diameter. • E.g. Driver Diameter = 15cm Driven Diameter 5 cm So the driven pulley is 3 times FASTER

31. Calculating Gear Ratios • See p. 443

32. Speed Changes in Worm and Worm Gear Systems • The larger the diameter of the worm gear, the greater the decrease in speed. • To increase the speed, the driven component should have a smaller diameter.

33. Torque

34. Torque • Torque involves two forces of equal strength but applied in opposite directions which cause a component to rotate about an axis. • Engine Torque increases the rotational speed of components in mechanical systems; provided by the engine • Resisting Torque slows or stops the rotation of components in mechanical systems; caused by friction, air resistance, gravitational force.

35. Torque and Speed Change • If engine torque = resisting torque, No speed change • If engine torque > resisting torque, speed increases • If engine torque < resisting torque, speed decreases

36. Motion Transformation Systems • The mechanical function of relaying motion from one part to another while CHANGING the type of motion • Eg: Rotational motion to translational motion

37. Types of MOTION TRANSFORMATION Systems • The most common systems are: • Rack and Pinions • Screw Gear Type 1 • Screw Gear Type 2 • Cam and Followers • Slider-Crank systems

38. Rack and Pinion

39. Rack and Pinion Systems • Converts rotational motion to translational motion or vice versa • The rack is the straight bar with teeth. • The pinion is the gear part. • It is used in many steering systems. • The greater the number the teeth on the pinion, the slower its rotation

40. Rack and Pinion

41. Screw Gear Systems • Converts rotation to translation • The screw gear uses a threaded bolt to move another gear or itself. • It is used in wrenches and car jacks. • Not reversible

42. Screw Gear Type 1 • Car Jack

43. Screw Gear Type 2 • Pipe wrench

44. Cam and Followers • Converts rotational to reciprocating (back and forth) translational motion • Eg. A sewing machine

46. Cams and followers

47. Eccentric Cams • Rotational axis of an eccentric cam is off center, p. 449

48. Slider Crank System

49. Slider Crank System

50. Car engine