Mechanisms

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Kinematics of machines

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Mechanisms:

Mechanisms Hareesha N Gowda Lecturer Dept of Aeronautical Engg Dayananda Sagar College of Engineering 3/22/2012 1 Hareesha N G, Dept of Aero Engg, DSCE, Blore

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UNIT 2: Mechanisms: Quick return motion mechanisms: Drag link mechanism, Whitworth mechanism and Crank and slotted lever Mechanism. Straight line motion mechanisms: Peaucellier’s mechanism and Robert’s mechanism. Intermittent Motion mechanisms: Geneva wheel mechanism and Ratchet and Pawl mechanism. Toggle mechanism, Pantograph, Ackerman steering gear mechanism. 3/22/2012 2 Hareesha N G, Dept of Aero Engg, DSCE, Blore

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Quick return motion mechanisms Quick return mechanisms are used in machine tools such as shapers and power driven saws for the purpose of giving the reciprocating cutting tool, a slow cutting stroke and a quick return stroke with a constant angular velocity of the driving crank. Some of the common types of quick return motion mechanisms are Drag link mechanism Whitworth quick return motion mechanism Crank and slotted lever quick return motion mechanism The ratio of time required for the cutting stroke to the time required for the return stroke is called the time ratio and is greater than unity. 3/22/2012 3 Hareesha N G, Dept of Aero Engg, DSCE, Blore

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Drag link mechanism This is one of the inversions of four bar mechanism, with four turning pairs. Here, link 2 is the input link, moving with constant angular velocity in anti-clockwise direction. Point C of the mechanism is connected to the tool post E of the machine. During cutting stroke, tool post moves from E 1 to E 2 . The corresponding positions of C are C 1 and C 2 as shown in the fig. For the point C to move from C 1 to C 2 , point B moves from B 1 to B 2 , in anti-clockwise direction. i.e , cutting stroke takes place when input link moves through angle B 1 AB 2 in anti-clockwise direction and return stroke takes place when input link moves through angle B 2 AB 1 in anti-clockwise direction. The time ratio is given by the following equation. 3/22/2012 4 Hareesha N G, Dept of Aero Engg, DSCE, Blore

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Crank and slotted lever quick return motion mechanism. This mechanism is mostly used in shaping machines, slotting machines and in rotary internal combustion engines. In this mechanism, the link AC (i.e. link 3) forming the turning pair is fixed, as shown in Fig. The link 3 corresponds to the connecting rod of a reciprocating steam engine. The driving crank CB revolves with uniform angular speed about the fixed centre C. A sliding block attached to the crank pin at B slides along the slotted bar AP and thus causes AP to oscillate about the pivoted point A A short link PR transmits the motion from AP to the ram which carries the tool and reciprocates along the line of stroke R 1 R 2. The line of stroke of the ram ( i.e. R 1 R 2) is perpendicular to AC produced. 3/22/2012 5 Hareesha N G, Dept of Aero Engg, DSCE, Blore

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In the extreme positions, AP 1 and AP 2 are tangential to the circle and the cutting tool is at the end of the stroke. The forward or cutting stroke occurs when the crank rotates from the position CB 1 to CB 2 (or through an angle ) in the clockwise direction. The return stroke occurs when the crank rotates from the position CB 2 to CB 1 (or through angle ) in the clockwise direction. Since the crank has uniform angular speed, therefore, Since the tool travels a distance of R 1 R 2 during cutting and return stroke, therefore travel of the tool or length of stroke 3/22/2012 6 Hareesha N G, Dept of Aero Engg, DSCE, Blore

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3/22/2012 7 Hareesha N G, Dept of Aero Engg, DSCE, Blore

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Whitworth quick return motion mechanism This mechanism is mostly used in shaping and slotting machines. In this mechanism, the link CD (link 2) forming the turning pair is fixed, as shown in Fig. The link 2 corresponds to a crank in a reciprocating steam engine. The driving crank CA (link 3) rotates at a uniform angular speed. The slider (link 4) attached to the crank pin at A slides along the slotted bar PA (link 1) which oscillates at a pivoted point D. The connecting rod PR carries the ram at R to which a cutting tool is fixed. The motion of the tool is constrained along the line RD produced, i.e. along a line passing through D and perpendicular to CD. 3/22/2012 8 Hareesha N G, Dept of Aero Engg, DSCE, Blore

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When the driving crank CA moves from the position CA 1 to CA 2 (or the link DP from the position DP 1 to DP 2) through an angle in the clockwise direction, the tool moves from the left hand end of its stroke to the right hand end through a distance 2 PD. Now when the driving crank moves from the position CA 2 to CA 1 (or the link DP from DP 2 to DP 1 ) through an angle in the clockwise direction, the tool moves back from right hand end of its stroke to the left hand end. A little consideration will show that the time taken during the left to right movement of the ram ( i.e. during forward or cutting stroke) will be equal to the time taken by the driving crank to move from CA 1 to CA 2. 3/22/2012 9 Hareesha N G, Dept of Aero Engg, DSCE, Blore

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Similarly, the time taken during the right to left movement of the ram (or during the idle or return stroke) will be equal to the time taken by the driving crank to move from CA 2 to CA 1. Since the crank link CA rotates at uniform angular velocity therefore time taken during the cutting stroke (or forward stroke) is more than the time taken during the return stroke. In other words, the mean speed of the ram during cutting stroke is less than the mean speed during the return stroke. 3/22/2012 10 Hareesha N G, Dept of Aero Engg, DSCE, Blore

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The ratio between the time taken during the cutting and return strokes is given by 3/22/2012 11 Hareesha N G, Dept of Aero Engg, DSCE, Blore

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Straight Line Mechanisms One of the most common forms of the constraint mechanisms is that it permits only relative motion of an oscillatory nature along a straight line. The mechanisms used for this purpose are called straight line mechanisms. These mechanisms are of the following two types: 1. in which only turning pairs are used, and 2. in which one sliding pair is used. These two types of mechanisms may produce exact straight line motion or approximate straight line motion, as discussed in the following articles. 3/22/2012 12 Hareesha N G, Dept of Aero Engg, DSCE, Blore

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Condition for Exact Straight Line Motion Mechanisms The principle adopted for a mathematically correct or exact straight line motion is described in Fig. Let O be a point on the circumference of a circle of diameter OP. Let OA be any chord and B is a point on OA produced, such that OA × OB = constant Then the locus of a point B will be a straight line perpendicular to the diameter OP. 3/22/2012 13 Hareesha N G, Dept of Aero Engg, DSCE, Blore

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Proof: Draw BQ perpendicular to OP produced. Join AP. The triangles OAP and OBQ are similar. But OP is constant as it is the diameter of a circle, therefore, if OA × OB is constant, then OQ will be constant. Hence the point B moves along the straight path BQ which is perpendicular to OP. O A P B O Q Base Base 3/22/2012 14 Hareesha N G, Dept of Aero Engg, DSCE, Blore

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Peaucellier exact straight line motion mechanism: It consists of a fixed link OO 1 and the other straight links O 1 A, OC, OD, AD, DB, BC and CA are connected by turning pairs at their intersections, as shown in Fig. The pin at A is constrained to move along the circumference of a circle with the fixed diameter OP, by means of the link O 1 A. In Fig., AC = CB = BD = DA ; OC = OD ; and OO 1 = O 1 A It may be proved that the product OA × OB remains constant, when the link O 1 A rotates. Join CD to bisect AB at R. Now from right angled triangles ORC and BRC, we have Since OC and BC are of constant length, therefore the product OB × OA remains constant. Hence the point B traces a straight path perpendicular to the diameter OP. 3/22/2012 15 Hareesha N G, Dept of Aero Engg, DSCE, Blore

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Robert’s mechanism This is a four bar mechanism, where, PCD is a single integral link. Also, dimensions AC, BD, CP and PD are all equal. Point P of the mechanism moves very nearly along line AB. 3/22/2012 16 Hareesha N G, Dept of Aero Engg, DSCE, Blore

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Intermittent Motion Mechanism Intermittent motion means that the motion is not continuous, but it ceases after definite intervals. Intermittent rotary motion is required generally in machine tools where work table, hexagonal turret, and spindle are to be indexed. 3/22/2012 17 Hareesha N G, Dept of Aero Engg, DSCE, Blore

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Geneva Mechanism Figure shows a Geneva wheel mechanism which consists of a driving wheel 1. It rotates continuously, and carries a pin P that engages in a slot in die driven member 2. The follower or driven member 2 is turned 1/4th of a revolution for each revolution of plate 1. 3/22/2012 18 Hareesha N G, Dept of Aero Engg, DSCE, Blore

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Ratchet and Pawl Mechanism This mechanism is used to produce intermittent circular motion from an oscillating or reciprocating member. Figure shows the details of a pawl and ratchet mechanism. Wheel 4 is given intermittent circular motion by means of arm 2 and driving pawl 3. A second pawl 5 prevents 4 from turning backward when 2 is rotated clockwise in preparation for another stroke. The line of action PN of the driving pawl and tooth must pass between centres O and A in order to have pawl 3 remain in contact with the tooth. 3/22/2012 19 Hareesha N G, Dept of Aero Engg, DSCE, Blore

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Toggle Mechanism Toggle mechanisms are used, where large resistances are to be overcome through short distances. Here, effort applied will be small but acts over large distance. In the mechanism shown in fig, 2 is the input link, to which, power is supplied and 6 is the output link, which has to overcome external resistance. Links 4 and 5 are of equal length. Considering the equilibrium condition of slider 6, For small angles of α , F (effort) is much smaller than P(resistance). This mechanism is used in rock crushers, presses, riveting machines etc. 3/22/2012 20 Hareesha N G, Dept of Aero Engg, DSCE, Blore

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3/22/2012 21 Hareesha N G, Dept of Aero Engg, DSCE, Blore

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Pantograph Pantographs are used for reducing or enlarging drawings and maps. They are also used for guiding cutting tools or torches to fabricate complicated shapes. In the mechanism shown in fig. path traced by point A will be magnified by point E to scale, as discussed below. In the mechanism shown, AB = CD; AD =BC and OAE lie on a straight line. When point A moves to A’ , E moves to E’ and OA’E’ also lies on a straight line. 3/22/2012 22 Hareesha N G, Dept of Aero Engg, DSCE, Blore

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Steering Gear Mechanism The steering gear mechanism is used for changing the direction of two or more of the wheel axles with reference to the chassis, so as to move the automobile in any desired path. Usually the two back wheels have a common axis, which is fixed in direction with reference to the chassis and the steering is done by means of the front wheels. In automobiles, the front wheels are placed over the front axles, which are pivoted at the points A and B, as shown in Fig. These points are fixed to the chassis. The back wheels are placed over the back axle, at the two ends of the differential tube. When the vehicle takes a turn, the front wheels along with the respective axles turn about the respective pivoted points. The back wheels remain straight and do not turn. Therefore, the steering is done by means of front wheels only. 3/22/2012 23 Hareesha N G, Dept of Aero Engg, DSCE, Blore

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Condition for perfect steering In order to avoid skidding ( i.e. slipping of the wheels sideways), the two front wheels must turn about the same instantaneous centre I which lies on the axis of the back wheels. If the instantaneous centre of the two front wheels do not coincide with the instantaneous centre of the back wheels, the skidding on the front or back wheels will definitely take place, which will cause more wear and tear of the tyres. Thus, the condition for correct steering is that all the four wheels must turn about the same instantaneous centre. The axis of the inner wheel makes a larger turning angle than the angle subtended by the axis of outer wheel. 3/22/2012 24 Hareesha N G, Dept of Aero Engg, DSCE, Blore

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Condition for perfect steering This is the fundamental equation for correct steering. If this condition is satisfied, there will be no skidding of the wheels, when the vehicle takes a turn. 3/22/2012 25 Hareesha N G, Dept of Aero Engg, DSCE, Blore

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Ackermann steering gear mechanism In Ackerman steering gear, the mechanism ABCD is a four bar crank chain, as shown in Fig . The shorter links BC and AD are of equal length and are connected by hinge joints with front wheel axles. The longer links AB and CD are of unequal length. The following are the only three positions for correct steering. When the vehicle moves along a straight path, the longer links AB and CD are parallel and the shorter links BC and AD are equally inclined to the longitudinal axis of the vehicle, as shown by firm lines in Fig.. When the vehicle is steering to the left, the position of the gear is shown by dotted lines in Fig. In this position, the lines of the front wheel axle intersect on the back wheel axle at I, for correct steering. When the vehicle is steering to the right, the similar position may be obtained. 3/22/2012 26 Hareesha N G, Dept of Aero Engg, DSCE, Blore