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Friday, 18 May 2012

Important basic concepts in Theory of Machines/Dynamics of Machines (Mechanical Engineering) for GATE 2013

This article deals with common basic concepts to kept in mind for theory of machine/Dynamics of Machine subject.

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  • A kinematics chain needs a minimum of 4 link and 4 turning pairs.
  • If the frequency of oscillation on earth is n then the frequency of oscillation on the moon will be n/√6.
  • The biflar suspension method is used to find out moment of inertia.
  • The total number of instantaneous centers for a mechanism of n link is equal to n(n-1)/2.
  • In case of an elliptical trammel two pairs turn and two pairs slide.
  • A shaft revolving in a bearing forms a lower pair.
  • For kinematic chains formed by lower pair the relation between the number of link L and the number of joint J is given by L = 2/3 (J+2).
  • The mechanism used to enlarge or reduce moments for reproducing drawings to different scales is called pantograph.
  • The number of links in a pantograph is to 4.
  • If a practical of a link has a velocity that changes both in magnitude and direction at any instant, then it must have two components of acceleration e.g., centripetal. And tangential.
  • The Klein’s method of construction for reciprocating engine mechanism utilities a quadrilateral similar to the diagram of mechanism for reciprocating engine.
  • In elliptical trammels have two pairs turning and two pairs sliding.
  • When a ship travel in a sea, the effect of the pitching is more dangerous.
  • If ф is angle of friction, the limiting value of tan ф will be µ.
  • Multiple collars are provided on a flat collar pivot bearing to distribute the axial load due to limiting bearing pressure on a collar.
  • Slip in belt drive is different between the linear speed of the rim of pulley and the belt on it.
  • In case of flat belt drive the length of belt in case of gross belts depends on the sum of the diameters of pulleys.
  • A pantograph is a mechanism having lower pairs.
  • Kinematic pairs are those pairs that have two elements that allow relative motion.
  • A cam with a roller follower would constitute higher pair.
  • Peaucellier mechanism has eight links.
  • Hart mechanism has six links.
  • For a kinematic chain to be considered as mechanism one link should be fixed.
  • The cam angle is the angle of rotation of the cam for a definite displacement of the follower.
  • Klein’s construction is used to find out acceleration of various parts.
  • For a stable governor there is one radius of the governor weight for each equilibrium speed.
  • In case of a governor isochronous is only one equilibrium which is constant for all radii of rotation of the bells within the working range.
  • Module can be expressed in mm.
  • The bipolar suspension method I used to find out moment of inertia.
  • Instantaneous centre of rotation of a link in a four bar mechanism lies on a point obtained by intersection of extending adjoining links.
  • A reverted gear train is one in which the first and last gear are on the same shaft.
  • In case of spur gears the flank of the tooth is the part of the tooth surface lying below the pitch surface.
  • In case of cross helical worm the axes of two shafts are non parallel-non-interesting.
  • In case of gears the addendum is given by one module.
  • Gears for watches are generally manufactured by stamping.
  • A cam where in the follower reciprocates or oscillates in plane parallel to the axis of the cam is called cylindrical cam.
  • Rope break Dynamometers uses no lubricant.
  • Tesla fluid friction dynameters used for power measurement when the speed is high and the viscous force is small.
  • Hydraulic Dynamometers is widely used for absorption of wide range power at wide range of speed.
  • The secondary disturbing force because of inertia of reciprocating mass is given by W/g ω2 r/n cos θ.
  • In a balancing machine the units for unbalance measurements are gm-cm.
  • In a hartnell governor if the stiffness of spring is increased the governor will become less sensitive.
  • The supply of working fluid to the engine to suit the load conditions can be controlled by governor.
  • The governor used in gramophone is of pickening type.
  • A porter governor could be classified as dead weight type governor.
  • The function of a governor is to adjust variation of speed by varying the input to the engine.
  • If D and T be the pitch circle diameter and no. of teeth of a gear, then its circular pitch p = π D/T.
  • If D1 and T1 be the diameter and no. of teeth of gear 1 and D2 and T2 the corresponding value of other gear in mesh, then ratio N1/N2, will be equal to D2/D1.
  • A scott-russel mechanism is made up of sliding and turning pairs.
  • Ackerman steering gear is made up of turning pairs.
  • A sliding crank chain is made up of three turning and one sliding pairs.
  • A motor car takes a bend of certain radius in right hand direction when moving at some speed. If engine rotates in a clockwise direct when viewed from front, then because of centrifugal force reaction on outside wheel is increased and on inside wheels decreased.
  • If δ be the static deflection of shaft under the gravity load, then natural frequency of the system in case of the longitudinal and transverse vibration is equal to 1/2 π   √g/δ.
  • Cold working increased the fatigue strength.
  • Coaxing is the procedure of increasing fatigue limit by overstressing the metal by successively increasing loadings.
  • The notch angle of the Izod impact test specimen is 450.
  • In vicker’s hardness testing, the pyramid indentor apex is 400.
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