Chapter 13   mechanical engineering
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  • 1. ST AST Mechanical Engineering Chapter 13 Observatory pp. 424-454 EST SE
  • 2. 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. 427 Linking in Technical Objects  Linking is the mechanical function performed by any component that connects different parts of a technical object.
  • 4. 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  
  • 5. 431 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. 
  • 6. 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
  • 7. 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.
  • 8. 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. 
  • 9. 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  436
  • 10. 445 Components      Types of Components in a Mechanical System Driver component: receives the force required to activate the system Driven component: receives the motion and transfers it to another part Intermediate component: located between the driver and driven component – not all systems have this.
  • 11. 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.  438
  • 12. 438 Gear Trains The direction of rotation changes from one wheel to the next.  The system can be reversed. 
  • 13. 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. Gear size: the higher the number of teeth, the lower the speed of rotation – or bigger diameter  slower speed.
  • 14. 439 Chain and Sprocket Systems The direction of all sprockets on the same size of the chain is the same.  A sprocket on the other side of the chain moves in the opposite direction.  It can be reversed. 
  • 15. 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. 
  • 16. 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. 
  • 17. 440 Friction Gear Systems The direction alternates from one gear to the next.  It is reversible. 
  • 18. 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.
  • 19. 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). 442
  • 20. Speed Changes in Gear Train/Friction Gear Systems The speed change ratio depends on the ratio of gears/threads/diameter of the driver relative to that of the driven component.  Speed Ratio = Driver Gear Driven Gear E.g. = 100 teeth = 5 times faster 20 teeth  442
  • 21. Speed Changes in Worm and 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, the driven component should have less teeth.  To have no change, both the gear and thread should have the same number of teeth.  442
  • 22. 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.  To keep the same speed, the two pulleys should have the same diameter.  E.g. Driver Diameter = 5cm = 1 Driven Diameter 15 cm 3 So the driven pulley is 3 times SLOWER or 1/3 the speed of the driver.  442
  • 23. 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. Resisting Torque slows or stops the rotation of components in mechanical systems.
  • 24. 445 Torque and Speed Change If the ratio of the torque strength between the driver and the driven is:  =1  No speed change  >1  Increased speed  <1  Decreased speed  Arm wrestling is an example of torque, if done correctly. 
  • 25. 447 TRANSFORMATION Systems  Motion transformation is the mechanical function of relaying a motion from one part to another while altering the nature of the motion.
  • 26. Characteristics of Motion in TRANSFORMATION Systems The most common systems are: 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 Translation Rotation Yes Rotation Translation 446
  • 27. 447 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. 
  • 28. 448 Screw Gear Systems The screw gear uses a threaded bolt  to move another gear or itself.  It is used in wrenches and car jacks. 
  • 29. Cam and Follower Systems and Eccentrics A cam is an off-centered circular wheel and acts as the driver.  The follower is driven.  This is used in the timing mechanism for automobiles for the intake of gas into the piston chamber.  449
  • 30. Eccentrics  An eccentric is a cam whose centre of rotation is off-centre.
  • 31. 449 Slider-crank Mechanisms The slider is the piston.  The crank attaches the piston to another wheel.  Usually the slider is the driven component.  This is used in steam engine and car pistons. 
  • 32. 450 Check Up! Chapter 13 – Page 451-2  Q. 7-11  Q. 12 - 17 