Operational Manual

Slotted link (ScotchYoke)
Straight.Une Motion
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
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
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.
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
HTM.62 AckermannSteeringGear.
HTM.64 Rapson'sSlide(Boat Rudder).
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
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.
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.
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
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.