Mechanical Engineering

1. Mechanical
Engineering
Chapter 13
Observatory
pp. 424-454
ST EST
AST SE

2. What is Mechanical Engineering?
 Mechanical Engineering focuses on:
• design,
• production,
• analysis,
• improvement of technical objects
Main Topics
• Links
• Guiding
• Transmission
• Transformation

426

3. Linking in Technical Objects
 Linking is the mechanical function
performed by any component that
connects different parts of a
technical object.

427

4. Guiding & Controls
 Guiding is the mechanical function
performed by any component that
controls the motion of moving parts.
 A Guiding Component or Control is a
component whose mechanical
function is to guide the motion of
moving parts.

431

5. Types of Guiding
 Translational Guiding ensures the straight
translational motion of a moving part.
Control: a vertical window groove
 Rotational Guiding ensures the rotational
motion of a moving part.
Control: a bicycle wheel hub
 Helical Guiding ensures the translation
motion of a moving part while it rotates
about the same axis.
Control: threaded shank in a vice

431

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

428

7. Adhesion and Friction of Parts
 Adhesion is the phenomenon by which two
surfaces tend to remain in contact with
each other without slipping.
 Friction is the 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.

433

8. Adhesion Factors
 Adhesion is affected by the following factors:
• Nature of materials – rubber/ice vs
rubber/asphalt
• Presence of lubricant – decreases adhesion
• Temperature – cold decreases adhesion
• State of the surfaces in contact – rough vs
smooth
• Perpendicular force (weight) on surface –
more weight  more adhesion

9. Motion Transmission Systems
 Motion Transmission is the
mechanical function of relaying one
type of motion from one part to
another part.
 A Motion Transmission System is a
set of components that perform the
function of transmitting motion.

435

10. 5 Common Transmission Systems
 Gear Trains
 Chain and Sprocket Systems
 Wheel and Worm Gear Systems
 Friction Gear Systems
 Belt and Pulley Systems

436

11. Characteristics of Motion Systems
 Driver component: activates the motion
and transfers it to another part - pusher
 Driven component: receives the motion
from another part - pushee
 Intermediate component: located
between the driver and driven component
 Reversible – if both the driver and driven
components can initiate motion with each
other.
445

12. Gear Train Factors
 The direction of rotation changes from one
wheel to the next.
 The system is reversible
 Gear teeth: all meshing teeth must be
identical – same size, spacing, shape,
type, direction.
 Gear position: the rotational axes of the
gears can be changed.
 Gear size: the greater the number of
teeth, the slower the rotation
 a/k/a big & slow, small & fast

13. Chain and Sprocket Systems
 The direction of all sprockets on the
same side of the chain is the same.
 A sprocket on the other side of the
chain moves in the opposite
direction.
 It is reversible.

439

14. Wheel and Worm Gear Systems
 The direction of rotation depends on
the direction of the threads on the
worm screw shaft.
 It is not reversible.

440

15. Friction Gear Systems
 Motion transmitted from one disc to
another by friction – NO GEARS.
 Efficiency depends on slippage.
 System is reversible.
 Friction Gear Factors:
• gear type
 straight, beveled, spherical
• gear size
• choice of material.

16. Belt and Pulley Systems
 Similar to the chain and sprocket system.
 System is reversible.
 Efficiency depends on belt material and
belt tightness.
 The direction is the same for any pulley on
the same side of the belt.
 Speed of pulley depends on pulley
diameter.

441

17. Speed Changes in Motion
Transmission Systems
 A Speed Change occurs in a motion
transmission system when the driver
turns at a different speed from the
driven component.

442

18. Speed Changes in Gear
Train/Friction Gear Systems
 The Speed or Gear Ratio depends on the #
gear/threads/diameter of the driver/driven gear.
 Gear/Speed Ratio = Driver Gear
Driven Gear
Calculate speed of driven gear if the driver gear
has 100 teeth and turns at 50 rpm and the driven
gear has 20 teeth. Gear Ratio = 100 teeth = 5
20 teeth
Therefore the smaller (faster) gear’s speed is
50 rpm x 5 = 250 rpm
442

19. Speed Changes in
Wheel & Worm Gear Systems
 Relative to the worm, the more teeth
a worm gear has, the slower its
speed.
 To increase the speed of the worm
gear, it should have less teeth.

442

20. Speed Changes in a Pulley
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.
 Gear Ratio = Driver Diameter = 5 cm = 1
Driven Diameter 15 cm 3
 The driven pulley is 3 times SLOWER
 E.g. If the driver turns at 30 rpm, then the
driven turns at 30 /3 = 10 rpm.
442

21. Torque
 Torque involves two forces of equal
strength but applied in opposite directions
which cause a component to rotate about
an axis.
 E.g. Arm wrestling , if done correctly.
444

22. Torque and Speed Change
 Engine Torque increases the
rotational speed of components in
mechanical systems.
 Resisting Torque slows or stops the
rotation of components in mechanical
systems.
 If the ratio of the torque strength
between the driver and the driven is:
• =1  No speed change
• >1  Increased speed
• <1  Decreased speed
445

23. TRANSFORMATION Systems
 Motion transformation is the
mechanical function of relaying a
motion from one part to another and
CHANGING the type of motion.
447

24. Characteristics of Motion in
TRANSFORMATION Systems
The most common systems are:
446
Type Driver Motion Driven Motion Reversible
Rack & Pinion Rotation
Translation
Translation
Rotation
Yes
Screw Gear,
Type 1
Rotation Translation No
Screw Gear,
Type 2
Rotation Translation No
Cam & Follower Rotation Translation No
Slider Crank Rotation
Translation
Translation
Rotation
Yes

25. Rack and Pinion Systems

 The rack is the straight bar with
teeth.
 The pinion is the gear part.
 It is used in many steering systems.
447

26. Screw Gear Systems
 The screw gear uses a threaded bolt
to move another gear or itself.
 Type 1 Type 2____
Car Jack Pipe Wrench


448

27. Cam and Follower Systems
 A cam is an non-circular wheel
and acts as the driver.
 The follower is the driven gear.
 This is used in car timing
mechanisms for the intake of gas
into the piston chamber.
449

28. Eccentrics
 An eccentric is a cam whose centre
of rotation is off-centre.

29. Slider-crank Mechanisms
 The slider is the piston.
 The crank is the arm attached to the
wheel.
449

30. Check Up!
450
 Chapter 13 – Page 451-2
 Q. 7-11
 Q. 12 - 17