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# exam2review - A mechanism is a collection of links...

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A mechanism is a collection of links connected together and restrained in such a way that they carry out a desired motion. They can be thought of as under constrained or unstable trusses Unlike a truss that is designed to be rigid, a mechanism is designed to move in a specified manner. Input motion is where the motor is attached Motor moves an input of rotation, while the wheelchair lift has an output motion of Translation. (thus it’s built to convert rotation to translation) The fundamental concept is that if we take a truss and remove a link, you will get a mechanism! geometrical parameters in trusses: They are all CONSTANT (i.e., just numbers that don’t change) kinematical parameters change with time Degree of mobility or degree of freedom (DoF): # motors needed to move it For the link mechanism shown: Parameters: L0, L1, Ө Geometrical Parameters: L0, L1(remain fixed) Kinematical Parameters: (Change with time) Ө Fundamental kinematics question: (t) supposed to be known function of time. Ө 0 L1 L2 3 A B C D θ ( ) t θ L X(t) Y(t) π = = rev rad 1 min ( ) 300 2 sec 10 rad min rev 60 sec t t t If motor rotates at constant speed of n rpm, 2 n then  (t)= 60 t

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For example, if motor spins at 300 rpm, then: find the relationship between the position of the wheelchair and the angle of the arm. How much should the arm turn if the wheelchair has to be raised by 4-in? Assume that L= 8-in Calculating average rotational speed: (same problem) Assume that the wheel chair must take at most 6 secs to travel 4 inches. Individually, (assuming that L=8 in) what should the average rotational speed of the motor (in rpm) be in order to meet this requirement? (hint: the motor needs to turn 30 degrees for the wheelchair to be lifted by 4 in)
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exam2review - A mechanism is a collection of links...

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