1. Mechanical EngineeringST
EST
AST SE
Mechanical Engineering
Chapter 13
Observatory pp

2. What is Mechanical Engineering?
426
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
427
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
428
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. 431
Guiding Controls
A Guiding Component or Control is a component whose mechanical function is to guide the motion of moving parts.

9. 431
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
433
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
435
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. 445
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
436
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
438
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. 438
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
439
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
440
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. 440
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
441
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
442
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
442
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 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
442
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. 444
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
445
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
446
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
447
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
448
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

51. Slider-crank System Converts rotation to translation or vice versa
449
Slider-crank System
Converts rotation to translation or vice versa
Used in car engines
The slider is the piston.
The crank attaches the piston to another wheel.
Usually the slider is the driven component.

52. 450
Check Up!
Q. 1-13
Depending on colour code, do the questions.