<< Click to Display Table of Contents >> Navigation: General Design Information > Mechanism Synthesis > Coupler Curve and Coupler Point Synthesis 
A fourbar mechanism, indicated to the left by (AoABBo), is a closed kinematic chain consisting of 4 links. One of the links is the fixed link (AoBo). The opposite link, is the coupler (AB). It is the base of a couplertriangle (ABK), the vertex of which, the couplerpoint K, describes the couplercurve when the mechanism is moved. It is often of engineering importance to synthesise a fourbar mechanism that: •Guides a coupler point to give a particular coupler curve, or shape  or  •Guides a coupler point between a number of points 
The equation of a couplercurve is of the sixth degree and its properties are 'tricircular sextic'. We do not need to know how to solve this equation.
There are three other ways that MechDesigner can help you find couplercurves to suit a particular application.
Option 1: Summary of Steps Drag a point in the Mechanism graphicarea and watch a Coupler Curve update in real time. This is easy to do. 1.Open the PartEditor to add a Point to the coupler of a mechanism with the Add Point tool Do not add dimensions or constraints to the Point. 2.Close the PartEditor 3.Add a TracePoint to the Point 4.Drag and drop the Point in the MechanismEditor Option 2: Summary of Steps (see more details below). Add an atlas or structured array of Points to a coupler in a mechanism. Watch the complete atlas of coupler curves update as you edit the mechanism 1.Assemble a FourBar Mechanism You can make it a CrankRocker or a CrankSlider. 2.Add a number of Lines to the Coupler to give an array of Points. It is useful to add constraints to the Lines. For example, to make the Lines equal in length, vertical (relative to the Coupler) You can also make the Lines proportional to the length of the Coupler, or a fraction of the Coupler. 3.Add a TracePoint to the Point at the end of each Line. 4.Add a DesignSet so you can edit the lengths of the Parts and the Lines in of the array. If you make the length of each Line equal, then the DesignSet needs only one dimension, other than the length of each Line. Option 2: More Details 

A CrankRocker We are trying to find the shape of a coupler curve not the absolute size it. When we find the shape of the coupler curve, we can scale all the Parts to suit the actual size we need. Therefore, the length of the frame can be fixed. In this example, I have made it 100mm.


A CrankSlider
Make the length of the Frame fixed. In this example I have made it 100mm long. 

Add a number of Lines to the coupler to make an array of Points. We will add TracePoints to each Point at the ends of each Line, to give a number of couplercurves. I have found it best to not add too many lines. In this example, •There are eight pairs of lines, mirrored about the axis of the coupler (AB) •The Lines are equal length, vertical, and equally spaced along the axis of the coupler. •The space between each Line is a fixed ratio of the length of the coupler. In this case, the space is equal to half the length of the coupler. •There is one dimension that gives the length of the coupler. •There is a different dimension that gives the length of one Line. All of the other Lines are made an equal length. 

Hide the Elements that you do not need to see In this example, I have hidden all the elements to show only the Points. You can also make the Points smaller with the Symbol size parameter in the Accessibility Settings 

CrankRocker Mechanism  Add TracePoints to each Point in the array This image shows the mechanism is reduced to four CADLines. When you hide the PartOutlines, you can see the TracePoints, or Coupler Curves, more clearly. 

CrankSlider Mechanism  Add TracePoints to each Point of the Atlas of Points This image shows the mechanism is reduce to four CADLines, and the coupler curves are shown. 

Add a DesignSet In the DesignSet: •Add the Length of Each Part  excluding the Frame as this is fixed •Add the dimension that gives the offset of the Points from the Coupler •Edit the Dimensions while the mechanism is cycling or notcycling. 


Watch the Coupler Curves change as you edit a dimension in the DesignSet This is the CrankRocker Mechanism. A DesignSet is used to adjust the Part lengths and the offset of the Coupler Curve from the Coupler. 


Watch the Coupler Curve Change as you edit the Dimensions This is the CrankSlider Mechanism. A DesignSet is used to adjust the Part lengths and the offset of the Coupler Curve from the Coupler. 
Summary of Steps 1.Add Precision Points* to the BasePart for which we would like a Coupler Curve to move through. These are the Points we would like the Coupler to pass through. 2.Add the three (or four or five) lines, that are equal in length, to represent the Crank, Coupler, Rocker. 3.Make the Points that are at the end of the Lines that represent the Crank and Rocker, and would normally be fixed to the machine frame, to be coincident. Even though they would normally be fixed to the machine frame, we leave these Points floating during the synthesis. 4.Add two further lines to the Couplers to make triangles. Add equal constraints to the sides of the triangles, so that the triangles are congruent. 5.Add Coincident Constraints between the apex of each triangle to one Precision Point. * They are 'Precision Points' because the point on the coupler move through them, exactly. 

Add the Points Edit the BasePart Add the Precision Points 

Add Lines to represent the Mechanism at the First Coupler Precision Point A line for the Crank, the Coupler and the Rocker. Add two lines to the Coupler to create a triangle. Make the apex of the triangle coincident with one of the Precision Points. Do NOT dimension or fix the points that will identify the grounded Points. The 'Crank' and the 'Rocker' are interchangeable at this stage. We do not know which will be which. 

Add a second set of Lines to represent a different Mechanism at the Second Coupler Precision Point The lines for the Crank and Rocker at the second position should join the ends of the first Lines that represent the first Crank and Rocker. Add two lines to the Coupler and make the apex of the triangle coincident with different Precision Points Use the Equal Constraint tool to make the 'links' equal in length and the two sides of the triangle on the coupler equal in lengths so that the triangles are congruent. Do not fix the grounded joints to the frame. Leave them 'floating' so that MechDesigner find the best result. 

Add a second set of Lines to represent a different Mechanism at the Second Coupler Precision Point Make the corresponding links the Equal in Length as for the other Coupler Precision Points You will likely need to drag the 'fixed' Points that are potentially the actual fixed Points of the fourbar Linkage. Be aware that the congruent triangle may 'invert' about the Coupler. You must fix this so that the triangles remain congruent. Continue for the Forth and Fifth Coupler Precision Points If you add more, it is very likely that the model will have very long link lengths unless you happen to be lucky. You will need to drag the 'fixed' Points that are potentially the actual fixed Points of the fourbar Linkage. 

Close the PartEditor and construct the Mechanism. 1.Use the Measurement FB to confirm the lengths of the Lines, 2.Make the Parts the same length as the Measurements 3.Add a sketch in the Coupler to give a Point that is coincident with the Precision Coupler Points I have added a sketchloop and Profile. 4.Add a MotionDimension FB. Select a line in the BasePart so that the Mechanism coincides with one of the Precision Points when the Base Value of the MotionDimension FB is zero 


Add the Motion and Cycle the Mechanism 1.Add a Motion FB 2.Edit the motion so that it rotates the 'Crank' an angle equal to the change in angle of the lines at the MotionDimension FB. 3.Cycle the Mechanism. The video shows the mechanism cycling. 
Instantaneous Kinematics gives particular properties of a Coupler Curve at particular instants. I have prepared a different topic on IK  See About Instantaneous Kinematics. 