ENGR 1100 Introduction to Mechanical Engineering

Transcription

1 ENGR 1100 Introduction to Mechanical Engineering

2 Mech. Engineering Objectives Newton s Laws of Motion Free Body Diagram Transmissibility Forces and Moments as vectors Parallel Vectors (addition/subtraction) Equilibrium (2D) (Parallel forces only)

3 Mechanical Engineering Mechanical Engineering involves the creation, testing, evaluation, manufacture, and distribution of machines and devices

4 Mechanical Engineering Professional Societies Associated with Mechanical Engineering The American Society of Mechanical Engineers (ASME) The National Academy of Engineers (NAE) United Engineering Foundation (UAE) National Society of Engineering Education (NSEE) Society of Automotive Engineers (SAE)

5 Vectors Quantity that has magnitude AND direction Displacement or position velocity Acceleration Torque Scalar-magnitude only distance Speed mass density

6 Vectors A vector quantity is a quantity which is fully described by both magnitude and direction. Vector diagrams are diagrams which depict the direction and relative magnitude of a vector quantity by a vector arrow direction magnitude

7 Force A "force" is an action that changes, or tends to change, the state of motion of the body upon which it acts. It is a vector quantity that can can be represented either mathematically or graphically A "force" cannot become physically apparent until it meets resistance.

8 Forces What are the horizontal and vertical resultants of the following forces?

9 Vectors Parallel vectors are added algebraically Note subtraction can be considered as addition of a negative vector

10 Transmissibility The point of application of an external force acting on a body (structure) may be transmitted anywhere along the force's line of action without affecting the other external forces (reactions and loads) acting on that body.

11 Newtons Laws Newtons 1 st law: Every object continues in a state of rest or uniform motion unless compelled by an external force to act otherwise It therefore follows that if a body is stationary, the sum of the external forces acting on it has to be zero

12 Newton s Laws Newtons 2 nd Law: The rate of change of momentum of a body is proportional to the applied force and takes place in the direction of the force Newtons 3rd Law: To every action there is an equal and opposite reaction

13 Summing Parallel Forces

14 Reaction Forces Reaction Forces will occur wherever bodies contact one another and a degree of freedom is restrained. The reaction force will be perpendicular to the tangent at the contact point What is meant by degree of freedom?

15 More Reaction Forces

16 Free Body Diagram Engineers will analyze a system of forces actying on a body by using a Free Body Diagram (FBD). This is a representation of an object and a graphical representation of all the forces acting on that object. Body is isolated from any constraints (eg. surfaces) all the forces acting on the given object are represented as arrows

17 Force equilibrium Note: Reaction forces will occur at A and B where there is contact All forces must sum to zero if the body is stationary therefore A+B-40,000lb=0 or A+B=40,000lb

18 Equilibrium example The Free Body Diagram will be as below, let the reaction force be R, then we have R R= =440 units

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20 Hydraulics The basic hydraulic principle utilizes the fact that the pressure exerted at end of an incompressible fluid will be exerted at the other end. Force transmitted are able to be multiplied

21 Hydraulics A small force can be used to generate a large force by increasing the area of the output piston Po P i F A o o F i A i Fo F i A A o i

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23 Inclined Plane With the use of the inclined plane a given resistance can be overcome with a smaller force than if the plane is not used. Using an inclined plane requires a smaller force exerted through a greater distance to do a certain amount of work.

24 Screw Jack One of the most common applications of the principle of the inclined plane is in the screw jack which is used to overcome a heavy pressure or raise a heavy weight of W by a much smaller force F applied at the handle, P-pitch Neglecting friction,

25 Gears A gear, or toothed wheel, when in operation, may be considered as a lever with the additional feature that it can be rotated continuously, instead of rocking back and forth through a short distance. Notice that the larger gear will make only one-half turn while the smaller makes a complete turn. That is, the ratio of speeds (velocity ratio) of the large to the smaller is as 1 to 2.

26 Compound Gears A gear train may have several drivers and several driven gears. Gear A cw. gear B 4x ccw gear C 1x cw. Hence B does not change the speed of C from what it would have been if geared directly to gear A, but it changes its direction from ccw to cw.

27 Compound Gears compound gears in which two gears are on the middle shaft T-teeth Gears B and D rotate at the same speed since they are keyed (fixed) to the same shaft. Gear A rotates at 100 r.p.m. cw, gear B turns 400 r.p.m. (rotations per minute) ccw and gear C turns 1200 r.p.m. cw.

28 Belts and Pulleys Pulleys are gears without teeth, instead of running together directly they are made to drive one another by cords, ropes, cables, or belts. As with gears, the velocities of pulleys are inversely proportional to their diameters.