20801970 -
MECHANICS APPLIED TO MACHINES
(objectives)
The course helps the students to increase their capabilities in analyzing the mechanical systems that are commonly employed in industrial and non-industrial applications. The students will be able to understand how the mechanical systems work and how to improve their performances during ordinary working. For this reason, the modeling and the design of the mechanical systems are studied in details, and many fundamental aspects of mechanics are illustrated, such as, topology, kinematic and dynamic of multibody systems, tribology, lubrication, mechanical efficiency, power flows, and mechanical vibrations. These fields are applied to particular systems such as transmissions, gears, brakes and cam-follower mechanisms.
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BELFIORE NICOLA PIO
( syllabus)
Kinematic Structure of Mechanical Systems Classification of kinematic pairs and their degrees of freedom, Lower pairs, Higher pairs, Shape and force closure pairs, Kinematic chains and mechanisms, Classical representation of kinematic chains and mechanisms, Introduction to the analysis and synthesis of mechanisms, Topological formulas for calculating the number of degrees of freedom (DoF), Mechanisms with lower pairs only. Correspondence between mechanism graphs. Enumeration of mechanisms with 1 DOF and 4 members. Geometry of Constrained Systems Introduction to graphical methods, Representation scales, Classical geometric approach to the problem of initial configuration, Slider-Crank mechanism, Grashof's rule for the planar four-bar linkages, four-bar parallelogram, anti-parallelogram, and rhomboid linkages. Analytical methods for determining the initial configuration, Numerical methods for determining the generic configuration of a mechanism, Dependent and independent coordinates, Solution of the problem of the initial configuration using the numerical method of Newton–Raphson, Application of Dini's theorem to the calculation of degrees of freedom: Instantaneous critical forms, Permanent critical forms, Graphical method for determining the configuration of an articulated quadrilateral given the crank angle and the length of the links, Methods for determining the configuration of an articulated quadrilateral given the crank angle and the length of the links: analytical, using complex numbers, and numerical, using constraint equations. Technograph, Pantograph, Use of the oscillating slider crank system, Hart's, Peaucellier's straight-line generator.
Geometry of Masses and Moments of Inertia Static moments or first-order moments, Moments of inertia or second-order moments, Huygens-Steiner theorem, Principal axes and moments of inertia. Moment of inertia of the section about the neutral axis and polar moment of inertia of a shape. Analytical formulation based on matrices and vectors.
Introduction to Kinematic Analysis of Planar Systems Review of kinematics of the material point, Review of kinematics of the rigid body, Motion law for a planar system and first-order centrodes, Fundamental formula of kinematics, Velocity center, First-order centrodes, Acceleration field. Euler-Savary formula in the first and second formulations. Inflexion circle, Curvature of trajectories, Null tangential acceleration circle, Review of the theorem of relative motions, Apparent wind velocity in a sailing boat with close-hauled and running reach, Aronhold-Kennedy theorem, Graphical methods of kinematic analysis. Graphical method based on vectors diagrams, vectors diagrams based on the fundamental formulas of kinematics, vectors diagrams based on the theorem of relative motions, Graphical determination of K, note F, Center points method, Graphical determination of the center of curvature of the trajectory of a point M of the mobile plane, knowing the centers of curvature of the first-order centrodes. Graphical determination of the first-order centrodes, Kinematic analysis of mechanisms, Kinematic analysis of mechanisms with higher pairs, Construction methods of conjugate profiles, The method of the envelope curve, The method of normal lines, The cycloid curve method with a point trajectory, The cycloid curve method with an auxiliary envelope curve, Assortment profiles. Justification of equivalent mechanisms with the cycloid curve method with a point trajectory, Center of curvature of the trajectory of a point, Kinematic analysis with the method of constraint equations, and with complex numbers. General case of a system with rehonomic constraints or time-dependent constraints Kinematic analysis of mechanisms with rolling or sliding bodies, Kinematic analysis of a planar four-bar linkages with numerical method based on constraint equations, Parametric equation of the involute curve, Parametric equation of the cycloid curve.
Recap of Statics Newton-Euler equations of Statics, Elementary cases of equilibrium, Body subjected only to two forces, Body subjected only to three forces, Examples, Body subjected only to four forces in the plane, The principle of fragmentation, Equivalent force systems, The principle of virtual work, Static equilibrium of mechanisms with one degree of freedom.
Dynamics of Planar Mechanisms Classifications of dynamic actions, External and internal forces, Driving and resisting forces, Active and reaction forces. Review of the dynamics of the point mass, d'Alembert's principle, Application of d'Alembert's principle to systems of lumped point masses. Review of the dynamics of the rigid body, Equation of motion of a translating body, Equations of motion of a body rotating about an axis, Equivalent force system, Body subjected to generic planar motion, Newto-Euler equations in space, Dynamics of systems through the Principle of the Virtual Work PLV. Direct and indirect dynamic problems, Application of PLV, Scotch-yoke mechanism, Lifting hoist, Dynamics of the planar four-bar likage, Kinematics and Dynamics of the slider crank mechanism. Analysis of the main dynamic actions, Static and dynamic balancing of rotors, Static balancing of the slider crank mechanism. Examples of direct or inverse dynamic problems.
Analytical Dynamics Dynamic analysis with the method of Lagrange multipliers, the role of reactions in the method of Lagrange multipliers. The setting up of the dynamic problem, Integration of equations using initial conditions, Approach based on minimizing the constraint equations, Approach based on the partitioning of coordinates, Lagrange multipliers and reaction forces.
Friction and Wear Elements of surface mechanics, Ideal and real surfaces, Contact between two real surfaces, Stresses and deformations in surface contacts, Hertz's Formulas. Typology of dissipative phenomena, Types of friction, Types of wear, Models for calculating tangential dissipative forces in direct contacts, Determination of the coefficient of sliding friction in adhesive wear regime. Experimental determination of tribological characteristics of a pair of materials, Models for wear calculation, Models for the case of predominantly abrasive wear, Friction in the revolute pair, axial revolute pair and journal bearings. Friction circle. Classification of friction based on the relative motion between bodies in contact, Rolling friction due to material hysteresis. Rolling friction parameter, Loaded and Unloaded driving wheel. First definition of rolling friction coefficient for material hysteresis, Driven wheel: further definition of rolling friction coefficient for material hysteresis, Rolling friction due to impacts, Rolling bearings, Friction circle for rolling bearings, Design of axial and journal rolling bearings in Static and fatigue conditions.
Basics of Lubrication Theory Viscosity in lubricating fluids, Other characteristics of lubricating oils, Additives for lubricating oils, Classification of lubrication types, friction in lubricated prismatic pair. Expression of the flow rate of a fluid in a fluid film. Pushing rolling pair, Hydrostatic lubrication with constant flow rate or pressure, Circuits with hydrostatic compensation, Hydrodynamic lubrication: need for a film with variable thickness. Film with constant height, Film with constant height in sections: step bearing, Hydrodynamic lubrication in a gap with variable thickness, Film with linearly variable thickness. Effect of lateral leaks, Kingsbury-Michell bearings, Axial revolute pair with lubrication, journal bearing, Half-Somerfield Theory. Geometric meaning of the mediated friction angle in the lubricated rolling pair, Inverse problem. Lubricated flat sliding pair, Direct problem. Michell bearings with self-aligning pads, Direct problem. Michell bearing with fixed orientation.
Work and Energy Kinetic energy, Variation of kinetic energy, Potential energy, Conservation of mechanical energy, Energy balance and efficiency, Instantaneous and average efficiencies, Backward (reverse) motion efficiency, Other expressions of efficiency, efficiency of mechanisms, Force and torque reduction, Mass reduction, Reduction of masses in a crank mechanism, Reduction of stiffness, Design of a Flywheel.
Gears Transmission by higher kinematic pairs, Levers with constant transmission ratio, Friction wheels, Types of gears, Nomenclature of gears. Involute profiles, Line of action and action line, Drawing of the involute profile, Tooth thickness, Calculation of slip between teeth, Specific slips. Interference phenomenon, Methods to overcome interference, Rack-wheel pair with involute profile, gear cutting, Helical gears for parallel axes, characteristics of teeth on normal and front planes, Transmission from rotation to translation using gears, Efficiency of gear pair with involute profiles, Worm wheel pair.
Transmission Joints Analysis of kinematic structure, Homokinetic joints, Oldham joint, Cardanic joints, Cardan joint and double Cardan joint. Overview of other types of joints.
Cam-Follower Mechanisms Classification of cam-follower mechanisms, Classification by absolute motion type of driver. Classification by contact type between driver and follower, Classification by absolute motion type of driver and follower, Classification by combination of operating phase sequences. Classification by member contact mode, Conical or spherical eccentrics, Followers. Cam profiling: graphical method, centric roller follower, centric plate follower, Eccentric roller follower, Standard motion profiles, Constant velocity profile, Constant acceleration profile (parabolic), Harmonic profile, Cycloidal profile, Polynomial profiles.
Brakes The dynamic problem of braking, Reye's hypotheses, Determination of braking force, Mechanical clutch engagements, Cone clutch engagements. Brake shoe with rigid and free approach motion. Jamming and partialization.
Introduction to Mechanical Vibrations Preliminary concepts and definitions. Free vibrations of linear systems with one degree of freedom. Equation of motion: Newtonian deduction and energy criterion. Study of undamped free oscillations using the method of complex numbers. Rayleigh's method. Damped free vibrations of linear systems with one degree of freedom. Viscous damping, Logarithmic decrement. Forced vibrations of linear systems with one degree of freedom. Resonance and dynamic amplification, applications. Forced and damped vibrations of linear systems with one degree of freedom, analytical, vector and complex number-based approaches, dynamic amplification coefficient and phase, Influence of the ratio n, quasi-static, resonance, and seismographic regions. Vibration isolation. Forced and damped vibrations, vector approach, transmissibility coefficient. Introduction to systems with multiple degrees of freedom.
Flexural Vibrations and Torsional Pulsations Flexural vibrations. Self-centering of a rotating shaft with a keyed flywheel in the middle. Torsional pulsations, torsional pendulum. Elements of rotordynamics: Jeffcott rotor.
Hoists Lifting organs. Direct and reverse hoist. Applications. Operation in the ideal case. Static analysis of forces. Optional: Witt hoist. Differential hoist. Efficiency of the single pulley.
Practical Exercises 1. Kinematic analysis of a planar four-bar linkage with graphical method based on the properties of points belonging to the same rigid system. 2. The centrodes of the coupler link with respect to the frame in a centered slider-crank mechanism using the graphical method. 3. Kinematic analysis of mechanisms having glyphs, higher pairs, and links; Divided into two parts: Fairbairn's guide and Cam-follower mechanism. 4. Kinematic analysis with graphical method based on the determination of the centers of the first and second orders, First Part Planar four-bar linkage, Second Part Sider Crank mechanism. 5. Involute and cycloid. Study of the trajectory of a point tracking attached to a body for assigned centrodes of the motion (graphical method). Point tracking a cycloid curve. Point tracking an involute curve. 6. Study of the static force balance of mechanisms using graphical methods in the ideal case. First part. Graphical method of the free body for calculating ideal reactions in a mechanism in static/stationary equilibrium. Second part. Ideal constraints. Absence of weight force. Application of the principle of virtual work for calculating driving actions (forces/torques) acting on a mechanism in static/stationary equilibrium under the simultaneous action of a load Q. 7. Practical method for calculating the instantaneous efficiency of a mechanism. 8. Inverse Dynamic Problem. Free body method. 9. Geometry of gears. First Part. Gear geometry. Second Part. Interference in gears.
Exercises based on computer Lab Activities Any programming environment can be used, for example, Python, MatLab (recommended), Mathematica, etc. However, during the exam, you must be able to explain the functioning of the code from a methodological (mechanical) point of view. 1. Elementary example of direct dynamic problem: rectilinear motion of an accelerated mass (with assigned function for force) First Part (analytical method) Second Part (numerical integration methods) 2. Application of the Newton Raphson numerical method for determining the configuration of a planar articulated four-bar linkage and an ordinary centered slider-crank mechanism. First Part - Planar four-bar linkage Second Part – Slider Crank mechanism Third Part (optional) – Slider Crank mechanism with secondary slider-coupler rods. 3. Involute and cycloid - Study of the trajectory of a point tracking solid with a body for assigned centrodes of the motion; Analytical method. Point tracking a cycloid curve. Point tracking an involute curve. Development with analytical approach. 4. Numerical method based on constraint equations for kinematic analysis of mechanisms Second part (optional) – Sider Crank mechanism and Slider Crank mechanism with secondary slider-coupler rods. 5. Study of hydrodynamic lubrication. First Part. Inverse Problem. Second Part. Direct Problem. 6. Multibody Systems Dynamics (MBS Dynamics). Direct Dynamic Problem. 7. Free mass-spring-damper system (use the provided MatLab code PCLAB7.m). Direct Dynamic Problem. Analytical and numerical solution.
( reference books)
Nicola Pio Belfiore, Augusto Di Benedetto, Ettore Pennestrì, Fondamenti di meccanica applicata alle macchine, Terza Edizione, Editore CEA (distribuzione Zanichelli), Pagine 912, 2024, ISBN 9788808220158 Materiale pubblicato sulla piattaforma Moodle e Teams.
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9
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ING-IND/13
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72
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Core compulsory activities
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