INSTRUCnON MANUAL HTM.63 Slotted link (ScotchYoke) MECHANISMS AND LOCI (1I~\ '{tl;f-J ' HTM.S8 Peaucellier Straight.Une Motion INTRODUCTION Machinesare madeup from a numberof partsconnectedtogetherin variousways to producethe requiredmovements.Two parts of a machine which are in contact and which undergo relative motion are called a pair. The types of relative motion commonly required are sliding, turning (rotation) and screwing. The pairs which pernut thesemotions are known as lower pairs. All other motions,partial turningwith partial sliding,are called higherpairs. Examplesare a pair of teethin contactbetweentwo gearwheels,or a belt driven by a pulley, or a car wheel on the road surface. An assemblyof two pairs on a part (nomla1lyat opposite ends) is a link, which is represented diagramaticaIlyas a straight line joining the pairs. A turning pair is shown by a dot at the intersectionof the straightlines. A rectanglewith a dot inside representscombinedsliding and turningpairs. MECflANISMS and LOCI. Page 1 Issue J. December.1994 The designand use of machinesdemandsa knowledgeof the overall motion and the relative motionsof all the parts. Anotherissueis the conceptof how specifiedmotionscan be generated. In somecasesthis is self evidentas in the abovediagram;but with someingeniousmechanisms only a carefulanalysiswill showthe precisenatureof the movements. Thusit is both helpfulandnecessary to build up a collection of typical mechanisms which perfonn set functionssuchas rotary to linearconversion,quick return motion or intennittent transport. The basisof understanding theseis the ability to constructthe loci of the pairs or other points on the mechanism.Althoughrobotswith self-learning(repeating)electronicshave come into use thereare still manyinstances, in food packagingfor example,where mechanisms fold and sealthe closureson filled containers. APPARATUS A seriesof self-containedmechanisms hasbeenbuilt, completewith restraints,on standardA3 size boards. The principlecomponentparts are made of amber Perspex,pivoted with hollow rivets. Beingtinted,andin sharpcontrastto the white melaminesurfacedboard, the movementof the partscanbe seenacrossthe classroom.The hollow rivet connectionsaccepta Staedtlercollet pencil(HTM. 7Oc),for transferringthe loci of all moving parts to a sheetof paper. If A3 size paperis usedspringclipsareavailablefor holdingit in place The selectedmechanismsfall into groups as follows:Straight Line Motion HTM.55 JamesWatt. HTM.56 Scott-Russell. HTM.57 ThompsonIndicator HTM.58 Peaucellier. Rotary - Linear Conversion HTM.51 Crank and ConnectingRod. HTM.63 Slotted Link (Scotch Yoke) Quick Return Motion HTM.S2 CrankandSlottedLever HTM.S3 WhitwOI1h. Intermittent Transport HTM.65 Carrier slide(with safetydevice) HTM.66 'Claw'mechanism. Operating Gear HTM.54 Toggle with Singleand Double Action. HTM.62 AckermannSteeringGear. HTM.64 Rapson'sSlide(Boat Rudder). Loci HTM.S9 SimplePantograph. HTM.60 Loci Board (Involute,Cycloid, etc. HTM.61 Elliptic Trammel. For easeof use by the lecturer V'?hendemonstrating thesemechanisms a standformed from steelis av~lable. . Other accessoriesare collet pencils and board clips. HTM.70b Set of Four Board Clips. HTM.70a Stand. HTM.70c Collet Pencil. MECH.4NISMSand LOCI. Page 2. Issue /. December./994. SLOTTED LINK (SCOTCH YOKE INTRODUCTION The slottedframeA is fixed and alsocarriesthe fixed axis 0 of the revolving link OP. The slotted link BC containsthe slider pinnedto the revolving link at P. As OP revolvesthe elementBC reciprocatesin the slottedframewith a pure simpleharmonicmotion whoseamplitudeis governed by the radiusOP. Two positionsfor P at 50 and 25 mm radii are provided. This mechanismis alwaysusedwhereit is requiredto produceperfect simpleharmonicmotion in a line. One application is for driving the paddle of a wave making machinein hydrological models. The link OP is commonlypart of a completedisc which then acts as a flywheel to even out variationsin the forcesanddriving torque. PROCEDURE To plot the locus ofQ it will be necessaryto usea baseline relatedto the angularposition ofOP. To assistin this P is on a circular protractor with a centreO. Fold an A3 sheetof paperin half lengthwiseand slide it underthe left hand side of the slotted link with the fold touching (or just under)the edgeof the protractor. Adjust the paperso that the fold is perpendicularto OQ and maketwo guide markson the baseboard. and draw a line through the tips. Also makea mark on the paperwhere the edgeof the protractor comes. ~raw a base line through this last mark parallel to the folded edge and then set out points along this line, at a scale of 1 cm = 10° , providing points at 20° intervalsfrom 0 to 180°and then from 190°to 350° with a final point at 360°. With the protractor 0° on the line OQ (Q at its leftmost position) and P pinnedat 50 mm radius slide the paper under the slotted link (and the edge of the protractor if necessary),line up the HTM.63. Page /. ISSIIeI. January. /995. folded edgewith the guide line and position the 0° on the paperagainstthe 0° on the protractor. Mark the position of Q. Also to assistin advancingthe paperupwardsmark the board against two other pointson the baseline. Slide the folded paper upward 2 crn (20~ and turn the protractor scaleto match. Mark Q. Repeatthis procedureup to the 180°point and then make a suitablecheck mark on the board readyfor the 1 crn (100)advanceto 190°. Carry on as before plotting the locus of Q as the link OP revolves. Repeatthe whole procedurewith P at 25 mm radius. Using a french curve draw in the two loci of P through the plotted points. The loci shouldbe cosinecurves. Add a baseline throughthe 900and 2700pointsandcheckthe symmetryof the loci andwhetherthe amplitudesare proportionalto the radii ofP.