MENG 372 Mechanical Systems Spring 2011 Dr. Mustafa Arafa American University in Cairo Mechanical Engineering Department mharafa@aucegypt.edu 1 Course Information Course goals: • Analyze & design planar mechanisms • Analyze forces, velocities & accelerations in machines • Use computers for the above Textbook: Design of Machinery, R.Norton, McGraw-Hill, 3rd ed., 2004. Computer usage: Working Model, MATLAB Grading: attendance 5%; homework 10%; quizzes 5%; mid-term exams 30%; projects 25%; final exam 25% Lecture notes: will be posted my website. I will communicate with you on BlackBoard. Additional material will also be covered on the board. Please print out the notes beforehand & bring them to class. 2 MENG 372 Chapter 2 Kinematics Fundamentals All figures taken from Design of Machinery, 3rd ed. Robert Norton 2003 3 2.1 Degrees of Freedom (DOF) or Mobility • DOF: Number of independent parameters (measurements) needed to uniquely define position of a system in space at any instant of time. Rigid body in a plane has 3 DOF: x,y,q Rigid body in space has 6 DOF (3 translations & 3 rotations) 4 2.2 Types of Motion • Pure rotation: the body possesses one point (center of rotation) that has no motion with respect to the “stationary” frame of reference. All other points move in circular arcs. • Pure translation: all points on the body describe parallel (curvilinear or rectilinear) paths. • Complex motion: a simultaneous combination of rotation and translation. 5 Backhoe Excavator 6 Slider-Crank Mechanism 7 2.3 Links, joints, and kinematic chains • Links: rigid member having nodes • Node: attachment points – Binary link: 2 nodes – Ternary link: 3 nodes – Quaternary link: 4 nodes • Joint: connection between two or more links (at their nodes) which allows motion Classified by type of contact, number of DOF, type of physical closure, or number of links joined 8 Joint Classification • Type of contact: line, point, surface • Number of DOF: full joint=1DOF, half joint=2DOF • Form closed (closed by geometry) or Force closed (needs an external force to keep it closed) • Joint order = number of links-1 9 Types of joints 10 Kinematic chains, mechanisms, machines, link classification • • • • Kinematic chain: links joined together for motion Mechanism: grounded kinematic chain Machine: mechanism designed to do work Link classification: Ground: fixed w.r.t. reference frame Crank: pivoted to ground, makes complete revolutions Rocker: pivoted to ground, has oscillatory motion Coupler: link has complex motion, not attached to ground 11 Determining Degrees of Freedom • For simple mechanisms calculating DOF is simple Open Mechanism DOF=3 Closed Mechanism DOF=1 12 Determining Degrees of Freedom Two unconnected links: 6 DOF (each link has 3 DOF) When connected by a full joint: 4 DOF (each full joint eliminates 2 DOF) Gruebler’s equation for planar mechanisms: DOF = 3L-2J-3G Where: L: number of links J: number of full joints G: number of grounded links 13 2.4 Determining DOF’s • Gruebler’s equation for planar mechanisms M=3L-2J-3G • Where M = degree of freedom or mobility L = number of links J = number of full joints (half joints count as 0.5) G = number of grounded links =1 M 3 L 1 2 J 14 Example 15 Example 16 2.5 Mechanisms and Structures • Mechanism: DOF>0 • Structure: DOF=0 • Preloaded Structure – DOF<0, may require force to assemble 17 2.7 Paradoxes • Greubler criterion does not include geometry, so it can give wrong prediction • We must use inspection E-quintet 18 2.10 Intermittent Motion • Series of Motions and Dwells • Dwell: no output motion with input motion • Examples: Geneva Mechanism, Linear Geneva Mechanism, Ratchet and Pawl 19 Geneva Mechanism 20 Linear Geneva Mechanism 21 Ratchet and Pawl 22 Fourbar Mechanism Twobar has -1 degrees of freedom (preloads structure) Threebar has 0 degrees of freedom (structure) Fourbar has 1 degree of freedom The fourbar linkage is the simplest possible pin-jointed mechanism for single degree of freedom controlled motion -1 0 1 23 4-Bar Nomenclature • Ground Link • Links pivoted to ground: – Crank – Rocker • Coupler B Coupler A Rocker Crank Link 1, length d Ground Link Pivot 02 Pivot 04 24 Where would you see 4-bar mechanisms? 25 Sheet Metal Shear (Mechanical Workshop) 26 Sheet Metal Shear (Mechanical Workshop) 27 Door Mechanism (ACMV Lab) 28 Door Mechanism (ACMV Lab) 29 Backhoe Excavator 30 Brake of a Wheelchair Folding sofa 31 Honda Accord trunk Chevy Cobalt Garage door Desk Lamp 32 Inversions • Created by attaching different links to ground • Different behavior for different inversions 33 Inversions of a 4-Bar Mechanism Crank-rocker Crank-rocker Crank-crank Rocker-rocker 34 2.12 The Grashof Condition • Grashof condition predicts behavior of linkage based only on length of links S=length of shortest link L=length of longest link P,Q=length of two remaining links If S+L ≤ P+Q the linkage is Grashof :at least one link is capable of making a complete revolution Otherwise the linkage is non-Grashof : no link is capable of making a complete revolution 35 For S+L<P+Q • Crank-rocker if either link adjacent to shortest is grounded • Double crank if shortest link is grounded • Double rocker if link opposite to shortest is grounded 36 For S+L>P+Q • All inversions will be double rockers • No link can fully rotate 37 For S+L=P+Q (Special case Grashof) • All inversions will be double cranks or crank rockers • Linkage can form parallelogram or antiparallelogram • Often used to keep coupler parallel (drafting machine) Parallelogram form Deltoid form Anti parallelogram form 38 Problems with Special Grashof • All inversions have change points twice per revolution of input crank when all links become collinear • Behavior at change points is indeterminate • If used in continuous machine, must have some mechanism to “carry through” 39 2.13 Linkages of more than 4 bars 5-bar 2DOF Geared 5-bar 1DOF • Provide more complex motion • See Watt’s sixbar and Stephenson’s sixbar mechanisms in the textbook 40 Linkages of more than 4 bars Volvo 740 Hood 41 Volvo 740 Hood 42 Animation using Working Model ® 43 Cabinet Hinge 44 2.15 Compliant Mechanisms • Compliant “link” capable of significant deflection acts like a joint • Also called a “living hinge” • Advantage: simplicity, no assembly, little friction 45 More Examples: Front End Loader 46 Drum Brake 47