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Note for Kinematics of Machinery - KM By Dhanen Thiran

  • Kinematics of Machinery - KM
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SRMTRPEC DEPARTMENT OF MECHANICAL ENGINEERING  Based on the nature of mechanical constraint Closed pair, Unclosed pair Types of constrained motion  Completely constrained motion, incompletely constrained motion, successfully constrained motion. 6 When a linkage becomes mechanism?  If one of the links of a kinematic chain is fixed, then the chain is known as mechanism. If a mechanism uses four links, then it is known as simple mechanism and if the mechanism uses more than four links, then it is known as compound mechanism. 7 Define transmission angle of a four bar mechanism.  The angle between the coupler link and driven link. The worst value of transmission angle is less than 45° 8 Define Grashoff’s law  It states that in a planar four bar mechanism, the sum of shortest link length and longest link length is not greater than the sum of remaining two links length, if there is to be continuous relative motion between two members. (s+l)<=(p+q) s= length of the shorter length, l= length of the longest link 9 What is Mechanical advantage?  It is defined as the ratio of output torque to the input torque also defined as the ratio of load to effort. M.A ideal = TB / TA TB =driven (resisting torque), TA =driving torque 10 Differentiate a kinematic chain from a mechanism. S.No 1 2 3 TWO MARKS Kinematic Chain It is a combination of kinematic pairs. The relative motion between the links is completely or successfully constrained or locked. Types: Four chain, Slider crank chain, Double slider crank chain, etc. Mechanism If any one of the link in the kinematic chain is fixed then the kinematic chain is said to be a mechanism. The relative motion between the links is completely or successfully constrained. Types: Simple mechanism, Compound mechanism. ME8492-KINEMATICS OF MACHINARY

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SRMTRPEC DEPARTMENT OF MECHANICAL ENGINEERING Unit 2- KINEMATICS OF LINKAGE MECHANISMS 1 Define Instantaneous centre  The combined motion of rotation and translation of the link may be assumed to be a motion of pure rotation about some centre.  Total number of Instantaneous centre N=n(n-1) / 2 2 What is Coriolis component of acceleration and When coriolis component of acceleration occurs?  Coriolis component of acceleration occurs when a point on one link is sliding along another rotating link, such as in quick return mechanism.  Whenever a coincident point exists in a mechanism, we have to consider Coriolis component of acceleration. 3 Write the relation between the number of instantaneous centres and the number of links in a mechanism?  First of all determine the number of instantaneous centre.  Locate the fixed and permanent centre by inspection.  On the circle diagram, join the point by solid lines to show that centers are already found. 4 What is a relative pole, with respect to velocity analysis?  A pole of a moving link is the centre of its rotation with respect to a fixed link. However, if the rotation of the link is considered relative to another moving link, the pole is known as relative pole.  The relative pole can be formed by fixing the link of reference and observing the motion of the other link in the reverse direction. 5 What are the different methods used for finding the velocity?  The two important methods for determining the velocity are.  Graphical methods. Instantaneous centre method and Relative velocity method  Analytical method 6 Describe configuration diagram  Configuration diagram is a line sketch of a given mechanism drawn to a suitable scale. The configuration diagram forms the basis for the construction of both velocity and acceleration diagrams. 7 Define rubbing velocity of that point. What will be the expression for rubbing velocity at a pin joint?  The links in a mechanism are mostly connected by means of pin joints. The rubbing velocity is defined as the algebraic sum between the angular velocities of the two links TWO MARKS ME8492-KINEMATICS OF MACHINARY

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SRMTRPEC DEPARTMENT OF MECHANICAL ENGINEERING which are connected by pin joints, multiplied by the radius of the pin  Rubbing velocity of pin = ( ωA + ωB ) * r (links moves in Opposite direction)  Rubbing velocity of pin = ( ωA - ωB ) * r (links moves in Same direction) 8 Define Velocity ratio theorem.  The magnitude of the velocities of the points on a link is directly proportional to their distance from the instantaneous centre. The direction of their velocities is perpendicular to the line joining them with the instantaneous centre 9 Explain the radial component of acceleration  Radial component of acceleration is perpendicular to the velocity of the particle at the given instant.  The magnitude is given by arBA= ω2 . AB = v2BA/AB 10 What is the objective of Kinematic analysis?  The objective of Kinematic analysis is to determine the Kinematic quantities such as displacement, velocity and acceleration of the element in a mechanism TWO MARKS ME8492-KINEMATICS OF MACHINARY

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SRMTRPEC DEPARTMENT OF MECHANICAL ENGINEERING Unit 3- KINEMATICS OF CAM MECHANISMS 1 State the expression for maximum acceleration of a follower moving with cycloidal motion.  Maximum acceleration of a follower moving with cycloidal motion is a = 2πω2s / θ2 where a=acceleration, ω=angular velocity of cam, s= stroke of follower, θ=angle moved 2 Why sometimes the axes of translating roller followers in cam follower mechanisms are offset from the axis of rotation of cam?  When the motion of the follower is required away from the cam axis then it is offset from the axis of cam. Specially used in translating followers where the follower is required to move fast and to very less distances. 3 Define Pressure angle  It is the angle between the direction of the follower motion and a normal to the pitch curve. This angle is very important in designing a cam profile. If the pressure angle is too large, a reciprocating follower will jam in its bearings. 4 What is a tangent cam?  When the flanks of the cam are straight and tangential to the base circle and nose circle, the cam is known as tangent cam. 5 Define undercutting in cam. How it occurs.  The cam profile must be a continuous curve without any loop. If the curvature of the pitch curve is too sharp, then a part of the cam shape would be lost and thereafter the intended cam motion would not be achieved. Such a cam is called an undercut cam.  Undercutting occurs in the cam because of attempting to achieve too great a follower lift with very small cam rotation, with a smaller cam. 6 What are the classifications of follower according to follower shape and different motion of the follower?  According to follower shape Knife edge follower , Roller follower , Mushroom or flat faced follower, Spherical faced or curved shoe follower  Different motion of the follower Uniform velocity, Simple harmonic motion, Uniform acceleration and retardation, Cycloidal motion 7 Why a roller follower preferred to that of a knife-edged follower?  A roller follower is preferred to that of a knife edge follower because the end of the follower is sharp in knife edge follower and roller in roller follower.  The rate of wear is high in knife edge follower to reduce the rate of wear the roller followers are extensively used. TWO MARKS ME8492-KINEMATICS OF MACHINARY

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