ME 341 Engineering Mechanics: Mechanics of Machines PDF
Document Details
Taibah University
Taibah University
Mohamed A Omar
Tags
Summary
These are course notes from Taibah University's Mechanical Engineering department on the topic of Engineering Mechanics: Mechanics of Machines. The notes cover fundamental concepts such as kinematics, dynamics, and rigid body analysis.
Full Transcript
Taibah University Mechanical Engineering Instructor: Mohamed A Omar Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 1.0 Course...
Taibah University Mechanical Engineering Instructor: Mohamed A Omar Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 1.0 Course Objectives Explore kinematics and dynamics of machinery in respect to the synthesis of mechanisms in order to accomplish desired motions or tasks. Implement the mechanism synthesis and design approaches. Explore the mechanism analysis techniques in order to determine their rigid-body dynamic behavior. The main goal is developing your ability to design viable mechanism solutions to real, unstructured engineering problems by using a design process. These topics are fundamental to the broader subject of machine design. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar Mechanics Field Mechanics is the branch of science that deals with forces, time, and motion of matters and systems. Mechanics entails the study of matter and systems behavior under load. Mechanics focus on studying solids in machines & structures. Motion: could be translational, rotational of combination of them. MECHANICS STATICS DYNAMICS KINEMATICS KINETICS Taibah University 3 Course: ME 341 Mechanical Engineering Mohamed A Omar Statics and Dynamics Statics: deals with the analysis of systems under equilibrium without regard to time and inertia forces. Systems under equilibrium could be stationary or moving with constant velocity (Linear or rotational motion). Statics focuses on F =0. Dynamics: deals with the analysis of systems that change configuration with time. Study of motion as a consequence of applied forces and inertia. Concerned with the calculation of displacement, velocity, acceleration, and reaction forces for systems with single or multiple degrees of freedom. Dynamics deals with F =m a. Taibah University 4 Course: ME 341 Mechanical Engineering Mohamed A Omar Kinematics and Kinetics Kinematics: deals with the study of motion without regard to the forces causing it or caused by it. Calculation of the displacement, velocity, and acceleration of the systems with single or multiple degrees of freedom. Deals with the right-hand side of Newton’s second law: F =m a, the acceleration “a” and its integrals. Kinetics: deals with the analysis of forces (causing or caused by) the motion. The terms dynamics and kinetics are sometimes used interchangeably (synonymously). Generally, dynamics is used as an inclusive terms that means both kinematics and kinetics. Taibah University 5 Course: ME 341 Mechanical Engineering Mohamed A Omar 1.2 Kinematics and Kinetics Kinetics: The study of forces and/or torques on systems in motion. According to Newton's 2nd Law: F = m a; the motion and forces are not separable. In design, we consider the desired motion and compute displacements, velocities and accelerations. Then dynamic inertia and reaction forces are computed as function of time. Consequently, the stresses are computed to assure that mechanical components are not failing during operation for specific time. Example: Design of a robot arm based on desired motion trajectory. Design of an excavator linkage based on the desired work envelop. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar Basic Terminologies Particle: An object that has mass and of negligible size or dimensions. Particles move translational motion and the rotation is not important. Body: An object that has mass and the size and dimensions are important. Moves translational motion, rotational motion, or combined motion and the orientation is important. Bodies could be rigid or flexible (deformable). Taibah University 8 Course: ME 341 Mechanical Engineering Mohamed A Omar Basic Terminologies Rigid Body: It is an object with mass and considerable dimensions. It can move translational motion and rotational motion. The term rigid implies that it have no deformation. Two conditions must be satisfied to be considered as rigid body: 1. The distance between any two points on the body is fixed throughout the body motion. 2. The angle between any two lines is also fixed. Flexible Body: An object that has mass and the size & dimensions are important It can move translational and rotational motion. The body can deform under the effect of external forces and inertia forces. A distance between any two points on the body could change during the body motion. Shape and geometry depend on external and inertia forces and the body elastic properties. Taibah University 9 Course: ME 341 Mechanical Engineering Mohamed A Omar Basic Terminologies In general: All bodies can deform to certain extent. If the deformation is small compared to the motion and the body dimensions, the deformation could be neglected and the body could be treated as rigid body. In the analysis of dynamic systems, links and bodies are considered rigid bodies to simplify the kinematic and dynamic equations. In this course: all bodies are assumed to be rigid. Advanced courses deal with flexible bodies. Taibah University 10 Course: ME 341 Mechanical Engineering Mohamed A Omar Machines and Mechanisms Mechanism: Is an assemblage of many connected rigid bodies. Its function is to transform/transfer motion/load. e.g. Transform rotary motion into linear motion or the opposite. Transfer motion from one axis or plane to another. Transform forces into torques or the opposite. Transfer effect or power from one location to another. The motion of one part can control those of the remaining parts. The main purpose of a mechanism is to achieve the desired motion. Taibah University 11 Course: ME 341 Mechanical Engineering Mohamed A Omar Machines and Mechanisms Machine: Is an arrangement of parts, components, and/or mechanisms. Its function is to transform energy into work. Its function is to apply/change direction/nature of power. Examples: Internal combustion engine. Car. Mechanical press. Jet Engine. Related terms: Force, torque, work, and power. Taibah University 12 Course: ME 341 Mechanical Engineering Mohamed A Omar According to Reuleaux Mechanism is an “assemblage of resistant bodies, connected by movable joints, to form a closed kinematic chain with one link fixed and having the purpose of transforming motion.” Machine is a “combination of resistant bodies so arranged that by their means the mechanical forces of nature can be compelled to do work accompanied by certain determinate motions”. Structure is an “an assemblage of resistant rigid bodies connected by joints intended to be rigid and do no work or motion and intended to have no internal (relative) motion.” Franz Reuleaux (30 September 1829 – 20 August 1905), was a German mechanical engineer and a lecturer of the Berlin Royal Technical Academy, later appointed as the President of the Academy. Taibah University 13 Course: ME 341 Mechanical Engineering Mohamed A Omar Fundamental Components of Mechanisms A mechanism is made up of two main component types: links and joints. Links: Are rigid bodies connected by kinematic pairs or joints to form the mechanism. Links transmit motion from the input link (driver) to the output link (follower). Links hold constant the distance between the elements of its pairs (joints). The motions between links must be constrained to produce the desired overall mechanism motion specified by the designer. Taibah University 14 Course: ME 341 Mechanical Engineering Mohamed A Omar Fundamental Components of Mechanisms Joints: Mechanical components that connect the links while allowing relative motion. The relative motion could a translational motion, a rotational motion, or combination of them. The mechanism motion requires proper choice of number of links and type of joints used to connect them. Each joint type has shape characteristics that determine the type of relative motion between the connected links. The nature of the mechanism kinematics is controlled by the type, location, and orientation of the joints used. Taibah University 15 Course: ME 341 Mechanical Engineering Mohamed A Omar Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar Machine/System Car Subsystems Suspension system/Steering system/Braking system Mechanism Double control arm, Sturt mechanism Links/Joints Upper/Lower control arm, Spherical joint, revolute joint. Parts Individual components. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 1.10 Units SI System: Absolute - why? Mass, length & time are the basis while force is derived from Newton’s second law. One Newton is the force giving a one kilogram mass an acceleration of 1 m/s2 ---- (N = kg.m/s2 ) US System: Gravitational - why? Force, length & time are the basis while mass is derived from the second law. One slug (32.174 pound-mass) will have an acceleration of one ft/s2 when acted upon by a force of one pound --- (slug = lb.s2/ft) Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar Units SI Units US Units Quantity Dimensional Units Symbol Units Symbol Symbol Mass M kilogram kg slug — Length L meter m foot ft Time T second s second sec Force F Newton N pound lb Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar The Design Process 1. Design: 1. Invention and Creativity 2. Identification of Need 3. Background Research 4. Goal Statement 5. Performance Specifications 6. Ideation and Invention 7. Analysis 8. Selection 2. Detailed Design 3. Prototyping and Testing Production Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar Design, Invention, Creativity Engineering design embodies all three of these activities. Design, Invention, Creativity. Engineering design is defined as "... the process of applying the various techniques and scientific principles for the purpose of defining a device, a process or a system in sufficient detail to permit its realization. Design may be simple or enormously complex, easy or difficult, mathematical or nonmathematical. Design may involve a trivial problem or one of great importance. Design is a universal constituent of engineering practice. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar Design, Invention, Creativity Typical textbook problems take the form of "given A, B, C, and D, find E." for educational purposes only. (Structured problem). Unfortunately, real-life engineering problems are almost never so structured. Real design problems more often take the form of "What we need is a framus to stuff this widget into that hole within the time allocated to the transfer of this other gizmo." The new engineering graduate will search in vain among his or her textbooks for much guidance to solve such a problem. This unstructured problem statement usually leads to what is commonly called "blank paper syndrome." Engineers often find themselves staring at a blank sheet of paper pondering how to begin solving such an ill-defined problem. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar Design, Invention, Creativity Engineering education deals with analysis, which means to decompose, to take apart, to resolve into its constituent parts. The engineer must know how to analyze systems of various types: mechanical, electrical, thermal, or fluid. Analysis requires a thorough understanding of both the appropriate mathematical techniques and the fundamental physics of the system's function. But, before any system can be analyzed, it must exist, and a blank sheet of paper provides little substance for analysis. Thus the first step in any engineering design exercise is that of synthesis, which means putting together. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar Design, Invention, Creativity The design engineer continuously faces the challenge of structuring the unstructured, ill-defined incomplete, and design problems. Using an engineering approach define the problem to ensure that any proposed solution will solve the right problem before any analysis. Excellent engineering solutions can end up ultimately rejected because they solved the wrong problem, i.e., a different one than the client really had. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar Design, Invention, Creativity Design processes intended to provide means to structure the unstructured problem and lead to a viable solution. Some processes present dozens of steps, others only a few. In this presentation, 10 steps process is proposed. 1. Identification of Need 2. Background research 3. Goal statement 4. Performance specifications 5. Ideation and Invention 6. Analysis 7. Selection 8. Detailed design 9. Prototyping and testing 10. Production Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 1. Identification of Need This first step is often done for you by someone, boss or client, saying "What we need is... " Typically this statement will be brief and lacking in detail. It will fall far short of providing you with a structured problem statement. For example, the problem statement might be "We need a better lawn mower." Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 2. Background Research This is the most important phase in the process, and is unfortunately often the most neglected. The term research means gathering background information on the relevant physics, chemistry, or other aspects of the problem. Also to find out if this, or a similar problem, has been solved before. (No point in reinventing the wheel) You can find a ready-made solution on the market, it will no doubt be more economical to purchase it than to build your own. You will learn a great deal about the problem to be solved by investigating the existing "art“ associated with similar technologies and products. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 2. Background Research The patent literature and technical publications in the subject area are rich sources of information and are accessible via the internet. If the solution exists and is covered by a patent still in force, you have ethical choices: buy the patentee's existing solution, design something which does not conflict with the patent, or drop the project. Spend sufficient energy and time on this research and preparation phase in order to avoid concocting a great solution to the wrong problem. Most inexperienced (and some experienced) engineers give too little attention to this phase and jump too quickly into the ideation and invention stage of the process. Discipline yourself to not try to solve the problem before thoroughly preparing yourself to do so. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 3. Goal Statement After fully understanding the originally stated problem, recast that problem into a more coherent goal statement. This new problem statement should have three characteristics: concise, general, and uncolored by any terms which predict a solution. It should be couched in terms of functional visualization, meaning to visualize its function, rather than any particular embodiment. Example, if the original statement of need was “Design a Better Lawn Mower,” after research into the ways to cut grass, the goal might be restated as “Design a Means to Shorten Grass.” You should use functional visualization to avoid unnecessarily limiting your creativity! Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 4. Performance Specifications Formulate a set of performance specifications (not design specifications). Performance specifications define what the system must do. Also, it carefully defines and constrain the problem so that it both can be solved and can be shown to have been solved. Design specifications define how it must do it. At this stage of the design process it is unwise to attempt to specify how the goal is to be accomplished. That is left for the ideation phase. It would be inappropriate to require a gasoline engine for specification 1, since other possibilities exist which will provide the desired mobility. Likewise, to demand stainless steel for all components in specification 2 would be unwise, since corrosion resistance can be obtained by other, less-expensive means. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 4. Performance Specifications The specifications constrain the design without overly restricting the engineer's design freedom. The performance specifications serve to: define the problem in as complete and as general a manner as possible. contractual definition of what is to be accomplished. The finished design can be tested for compliance with the specifications. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 5. Ideation and Invention In this stage, the designers exploit their creativity to its utmost extent. Main characteristics of this step: full of both fun and frustration, potentially the most satisfying to most designers, is also the most difficult. Although, creativity is a common human trait, various techniques can enhance your creativity: Creative Process: Many techniques have been developed to enhance or inspire creative problem solving. This creative process can be thought of as a subset of the design process and to exist within it. The ideation and invention step can thus be broken down into these four substeps. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 5. Ideation and Invention 1. Idea Generation: is the most difficult of these steps. The goal here is to obtain as large a quantity of potential designs as possible. Even very creative people have difficulty in inventing "on demand." Many techniques have been suggested to improve the yield of ideas. The most important technique is that of deferred judgment, which means that your criticality should be temporarily suspended. Do not try to judge the quality of your ideas at this stage. That will be taken care of later, in the analysis phase. Even superficially ridiculous suggestions should be welcomed, as they may trigger new insights and suggest other more realistic and practical solutions. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 5. Ideation and Invention 2. Brainstorming: is a technique for generating creative solutions. This technique requires a group, preferably 6 to 15 people, and attempts to generate ideas. Most people, when in a group, will not suggest their real thoughts on a subject, for fear of being laughed at. Brainstorming rule: No one is allowed to make fun of or criticize anyone's suggestions. One participant acts as "scribe“ and his duty bound to record all suggestions, no matter how apparently silly. When done properly, this technique can be fun and can sometimes result in a "feeding frenzy" of ideas which build upon each other. Large quantities of ideas can be generated in a short time. Judgment on their quality is deferred to a later time. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 5. Ideation and Invention When working alone: Analogies and inversion are often useful. Attempt to draw analogies between the problem at hand and other physical contexts. If it is a mechanical problem, convert it by analogy to a fluid or electrical one. Inversion turns the problem inside out. For example, consider what you want moved to be stationary and vice versa. Synonyms; define the action verb in the problem statement, and then list as many synonyms for that verb as possible. For example: Problem statement: Move this object from point A to point B. The action verb is "move." Some synonyms are push, pull, slip, slide, shove, throw, eject. jump, spill. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 5. Ideation and Invention The aim in this ideation step is to generate a large number of ideas without particular regard to quality. At some point, your "mental well" will go dry. You will have then reached the step in the creative process called frustration. It is time to leave the problem and do something else for a time. While your conscious mind is occupied with other concerns, your subconscious mind will still be hard at work on the problem. This is the step called incubation. Suddenly, at a quite unexpected time and place, an idea will pop into your consciousness, and it will seem to be the obvious and "right" solution to the problem Most likely, later analysis will discover some flaw in this solution. If so, back up and iterate! More ideation, perhaps more research, and possibly even a redefinition of the problem may be necessary. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 5. Ideation and Invention In "Unlocking Human Creativity“, Wallen describes three requirements for creative insight: Fascination with a problem: provides the motivation to solve the problem. Saturation with the facts, technical ideas, data, and the background of the problem (research). A period of reorganization (refers to the frustration phase). To enhance your creativity, saturate yourself in the problem and related background material. Then relax and let your subconscious do the hard work! Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 6. Analysis At this stage, you have structured the problem, at least temporarily. Now apply more sophisticated analysis techniques to examine the performance of the design in the analysis phase of the design process. Further iterations will be required as problems are discovered from the analysis. Repetition of as many earlier steps in the design process as necessary must be done to ensure the success of the design. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 7. Selection When the technical analysis indicates that you have some potentially viable designs, the best one available must be selected for detailed design, prototyping, and testing. The selection process usually involves a comparative analysis of the available design solutions. A decision matrix sometimes helps to identify the best solution by forcing you to consider a variety of factors in a systematic way. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 7. Selection A decision matrix for our better grass shortner is shown in Figure 1-2. Each design occupies a row in the matrix. The columns are assigned categories in which the designs are to be judged, such as cost, ease of use, efficiency, performance, reliability, and any others you deem appropriate to the particular problem. Each category is then assigned a weighting factor, which measures its relative importance. For example, reliability may be a more important criterion to the user than cost, or vice versa. You as the design engineer have to exercise your judgment as to the selection and weighting of these categories. The body of the matrix is then filled with numbers which rank each design on a convenient scale, such as 1 to 10, in each of the categories. Note that this is ultimately a subjective ranking on your part. You must examine the designs and decide on a score for each. The scores are then multiplied by the weighting factors (which are usually chosen so as to sum to a convenient number such as 1) and the products summed for each design. The weighted scores then give a ranking of designs Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 7. Selection Be cautious in applying these results. Remember the source and subjectivity of your scores and the weighting factors! There is a temptation to put more faith in these results than is justified. After all, they look impressive! They can even be taken out to several decimal places! (But they shouldn't be.) The real value of a decision matrix is that it breaks the problem into more tractable pieces and forces you to think about the relative value of each design in many categories. You can then make a more informed decision as to the "best" design. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 8. Detailed Design This step usually includes: creation of a complete set of assembly detail drawings or computer-aided design (CAD) part files, for each and every part used in the design. Each detail drawing must specify all the dimensions and the material specifications necessary to make that part. From these drawings (or CAD files) a prototype test model (or models) must be constructed for physical testing. Most likely the tests will discover more flaws, requiring further iteration. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 9. Prototyping and Testing MODELS: One cannot be sure of the correctness or viability of any design until it is built and tested. This usually involves the construction of a prototype physical model. A mathematical model, while very useful, can never be as complete and accurate a representation of the actual physical system as a physical model, due to the need to make simplifying assumptions. Prototypes are often very expensive but may be the most economical way to prove a design, short of building the actual, full-scale device. Prototypes can take many forms, from working scale models to full-size, but simplified, representations of the concept. Scale models introduce their own complications in regard to proper scaling of the physical parameters. For example, volume of material varies as the cube of linear dimensions, but surface area varies as the square. Heat transfer to the environment may be proportional to surface area, while heat generation may be proportional to volume. So linear scaling of a system, either up or down, may lead to behavior different from that of the full- scale system. One must exercise caution in scaling physical models. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 9. Prototyping and Testing MODELS: You will find as you begin to design linkage mechanisms that a simple cardboard model of your chosen link lengths, coupled together with thumbtacks for pivots, will tell you a great deal about the quality and character of the mechanism's motions. You should get into the habit of making such simple articulated models for all your linkage designs. TESTING: Testing of the model or prototype may range from simply actuating it and observing its function to attaching extensive instrumentation to accurately measure displacements, velocities, accelerations, forces, temperatures, and other parameters. Tests may need to be done under controlled environmental conditions such as high or low temperature or humidity. The microcomputer has made it possible to measure many phenomena more accurately and inexpensively than could be done before. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 10. Production Finally, with enough time, money, and perseverance, the design will be ready for production. This might consist of the manufacture of a single final version of the design or making thousands or even millions. The danger, expense, and embarrassment of finding flaws in your design after making large quantities of defective devices should inspire you to use the greatest care in the earlier steps of the design process to ensure that it is properly engineered. The design process is widely used in engineering. Engineering is as much a method, an approach, a process, a state of mind for problem solving, as it is an activity. The engineering approach is that of thoroughness, attention to detail, and consideration of all the possibilities. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 10. Production You must discipline yourself to suffer the nitty-gritty, nettlesome, tiresome details which are so necessary to the completion of any one phase of the creative design process. For example, to do a creditable job in the design of anything, you must completely define the problem. If you leave out some detail of the problem definition, you will end up solving the wrong problem. You must thoroughly research the background information relevant to the problem. You must exhaustively pursue conceptual potential solutions to your problem. You must then extensively analyze these concepts for validity. Finally, you must detail your chosen design down to the last nut and bolt to be confident it will work. If you wish to be a good designer and engineer, you must discipline yourself to do things thoroughly and in a logical, orderly manner, even while thinking great creative thoughts and iterating to a solution. Both attributes, creativity and attention to detail, are necessary for success in engineering design. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 1.6 Other Approaches To Design: Design Methodology is the study of the process of designing. One goal of this research is to define the design process in sufficient detail to allow it to be encoded in a form amenable to execution in a computer, using "artificial intelligence“ Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 1.6 Other Approaches To Design: Axiomatic Design: There are four domains: customer domain, functional domain, physical domain, and the process domain. These domains represent a range from "what" to "how" i.e From a state of defining what the customer wants through determining the functions required and the needed physical embodiment. To how a process will achieve the desired end. Two axioms that need to be satisfied to accomplish this: 1. Maintain the independence of the functional requirements. 2. Minimize the information content. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 1.7 Multiple Solutions By the nature of the design process, there is not anyone correct answer or solution to any design problem. There are as many potential solutions as there are designers willing to attempt them. Some solutions will be better than others, but many will work. Some will not! There is no "one right answer" in design engineering. The only way to determine the relative merits of various potential design solutions is by thorough analysis, which usually will include physical testing of constructed prototypes. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 1.7 Multiple Solutions (Cont.) Because this is a very expensive process, it is desirable to do as much analysis on paper, or in the computer, as possible before actually building the device. Where feasible, mathematical models of the design, or parts of the design, should be created. In the design of mechanisms and machines it is usually possible to write the equations for the rigid-body dynamics of the system, and solve them in "closed form“ with (or without) a computer. Accounting for the elastic deformations of the members of the mechanism or machine usually requires more complicated approaches using finite difference techniques or the finite element method (FEM). Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 1.8 Human Factors Engineering Is the study of human-machine interaction. The machine designer must design devices to "fit the man" rather than expect the man to adapt to fit the machine. Ergonomic Engineering: “The applied science that coordinates the design of devices, systems, and physical working conditions with the capacities and requirements of the worker.” Examples of Human factors: Dimensions of human body and its distribution with age. Human ability to withstand force, pressure, heat, noise, acceleration, etc… Human ability to generate force by different members in each direction. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 1.9 The Engineering Report Engineers spend time doing technical calculations and technical tasks. Engineers spend most of their times in communication: Writing proposals, technical reports, giving presentations, and interacting with support personnel. No one will know your capabilities if you cannot communicate your ideas clearly and concisely. Writing professional technical report is a fundamental skill for every successful engineer. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar That is all for today Questions Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar Taibah University Mechanical Engineering Instructor: Mohamed A Omar Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar Chapter 2 – Kinematics Fundamentals 1. Introduction Grashof-type Rotatability Criteria for 2. Degrees of Freedom Higher-order Linkages 3. Types of Motion 15. Springs as Links 4. Links, Joints, and Kinematic Chains 16. Practical Considerations 5. Determining Degree of Freedom Pin Joints versus Sliders and Half Joints Degree of Freedom in Planar Mechanisms Cantilever versus Straddle Mount Degree of Freedom in Spatial Short Links Mechanisms Bearing Ratio 6. Mechanisms and Structures Linkages versus Cams 7. Number Synthesis 17. Motor and Drives 8. Paradoxes Electric Motors 9. Isomers Air and Hydraulic Motors 10. Linkage Transformation Air and Hydraulic Cylinders Solenoids 11. Intermittent Motion 12. Inversion 13. The Grashof Condition Classification of the Four-bar Linkage 14. Linkages of More Than Four Bars Geared Five-bar Linkages Six-bar Linkages Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar Mechanism- Machine- Structure Mechanism: Is an assemblage of many connected rigid bodies, formed and connected so that they move with definite relative motions with respect to one another Its function is to transform/transfer motion/load. The motion of one part controls those of the remaining parts. The main purpose of a mechanism is to achieve the desired motion. Machine: An arrangement of parts and/or mechanisms that transmit forces, motion and energy in a predetermined manner Its function is to transform energy into work. Terms: Force, torque, work, and power. Structure: is an “an assemblage of resistant rigid bodies connected by joints intended to be rigid and do no work or motion and intended to have no internal (relative) motion.” Its function is to support loads. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar Reference Frame and Inertial Frame: Frame of Reference: Consists of an abstract coordinate system and the set of physical reference points that uniquely fix the coordinate system (location and orientation) and standardize the measurements within that frame. The frame of reference can be represented by three orthogonal axes that are rigidly connected at a point called the origin of this reference. Inertial Frame of Reference: In the dynamic analysis of bodies and mechanisms, two types of coordinate systems are required: The first frame: is referred to as global coordinate system , or inertial frame of reference. It is fixed in time and represents a unique standard for all bodies in the system. The inertial frame of reference may be called: inertial frame, Galilean reference frame, or inertial space. The second frame: is called body reference frame which attached to every moving body in the system. It translates and rotates with the body. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 2.1 Degree of Freedom or Mobility Degrees of Freedom (DOF): the number of independent parameters that are needed to uniquely define the position in space at any instant of time (w.r.t frame of reference) Particle: In plane has 2 DOF. (2 translation: Rx, Ry) In space has 3 DOF. (3 translation: Rx, Ry, Rz) Rigid body: In plane has 3 DOF. (x, y, θ) In space has 6 DOF (3 translation, 3 rotation) Try to identify these DOF of an object in space. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar Types of Displacement Translational Displacement. Rotational Displacement. Translation and Rotational displacements are both a vector quantities. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 2.2 Types of Motion Pure Rotation: The body possesses one point (center of rotation) that has no translational motion with respect to the “stationary” frame of reference. All other points on the body move in circular arcs. The center of the arc is the center of rotation. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 2.2 Types of Motion Pure Translation: The body remain parallel to its original configuration. All points on the body describe parallel (curvilinear or rectilinear) paths. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 2.2 Types of Motion Complex Motion: A simultaneous combination of rotation and translation Translation and rotation represent independent motions of the body. Each can exist without the other. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 2.3 Links Node: A point of attachment from a link to another link. Links: Are rigid bodies connected by kinematic pairs or joints to form the mechanism. The link has at least two nodes and considered the building blocks of the mechanism. Links transmit motion from the input link (driver) to the output link (follower) by holding constant the distance between the nodes. The motions between links must be constrained to produce the desired overall mechanism motion specified by the designer. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 2.3 Links Links could be classified based on the number of nodes as follows: Binary link – has two nodes. Ternary link – has three nodes. Quaternary link – has four nodes. Etc… Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar Joints Joint: A connection between two or more links (at their nodes), which allows relative motion or potential motion between the connected links. Joints: are called kinematic pairs and can be classified: by the type of contact between the elements, line, point, or surface. by the number of degrees of freedom allowed at the joint. by the type of physical closure of the joint: either force or form closed. by the number of links joined (order of the joint). Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 1. By Type of Contact Reuleaux’s classification of joints: Lower Pair Higher Pair Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 1. By Type of Contact Reuleaux’s classification of joints: Lower Pair and Higher Pair Lower Pair Joint: the contact in the joint pair is surface contact e.g. pin surrounded by a hole or a block on a surface. Example: Revolute (R), Prismatic (P), Cylindrical (C), Screw (H), etc…. In Planar Motion: Lower Pair joint is called Full Joint and has single degree of freedom. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar Six Types of Lower Pair Planar Joints: Revolute joint and prismatic (translational joint) Spatial Joints: Cylindrical, spherical, planar, and screw joints Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 1. By Type of Contact Higher Pair: the contact in the joint pair is Slippage Occurs line or point contact. Example: Gear joint, Cam etc In Planar Motion: Higher Pair joint is called Half Joint – and has two degrees of freedom. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar Classification based on Number of DOF Full Joint: 1-DOF or lower pair Rotating revolute (R) Translating Prismatic (P) Threaded nut Half Joint: 2 DOF, Higher Pair Slippage Occurs Paradoxically called as Half Joint Roll slide joint Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar Exercise: Joint Classification Identify Full Joint & Half Joint Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 3. Formed and Forced Joints Joint could be formed or forced Formed Joints: The joint is designed to maintain contact between the joint pair. Nor force is needed to maintain contact. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 3. Formed and Forced Joints Forced Joints: The joint require forces to maintain contact between the joint pair. Separation between the joint pair can happen if the force is eliminated or decreased. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 4. Joint Order Joint order = number of links - 1 Order 2 joint Link 2 Link 1 Link 3 Joint Order = 3 – 1 = 2 Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar Summary: Kinematic Chains, Mechanisms, Machines Kinematic Chain: “An assemblage of links and joints, interconnected in a way to provide a controlled output motion in response to a supplied input motion. Mechanism: “A kinematic chain in which at least one link has been "grounded," or attached, to the frame of reference (which itself may be in motion)”. Machine: “A combination of resistant bodies arranged to compel the mechanical forces of nature to do work accompanied by determinate motions.” Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar Closed & Open Kinematic Chain If every link in the chain is connected to two or more links then the chain form one or more closed loops If the link form a closed loops, it is called closed kinematic chain If NOT, the chain is said to be open kinematics chain Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar Kinematics Chain Links joined together for motion: Study the Mechanism Motion: Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar Sample Mechanisms: Crank-Slider - Engine Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar Exercise: Select Proper Joints. Understand the mechanism function. Understand the link relative motions. Select the proper joint. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar Planer and Spatial Mechanisms In a planar mechanisms, all of the relative motions of the rigid bodies are in one plane or in parallel planes Motion of such mechanism is called Coplanar If there is any relative motion that is not in the same plane or in parallel planes, the mechanism is called the spatial mechanism. Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar 2.4 Determining Degree of Freedom The definition of the degrees of freedom of a mechanism is: ------------------------------------------------ The number of degrees of freedom of a mechanism is also called the mobility of the device Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar Degrees of Freedom Cont. Different values of DOF mean different things. DOF = 0 Structure DOF < 0 Statically indeterminate structure DOF 1 Mobility is allowed Taibah University Course: ME 341 Mechanical Engineering Mohamed A Omar
