Tuesday, January 21, 2014


      A mechanism is a device designed to transform input forces and movement into a desired set of output forces and movement.
Pro-Mechanism is a very useful module to design the mechanism and to know the dynamic path of the parts, interference in an assembly.

Connections of Bodies
ØMost Used Connections


ØAlso, we have

ØRigid & weld

Connection - Pin
The pin connection is used to rotate the part about an axis.

üDoor that swings open
üWheel that turns

Pin - axis alignment and a translational constraint 

Joint Axis settings on Rotational motion
   Joint Axis setting is used for control the rotation of a part with some specified angle like 0 deg to 180 deg.
Drag - Drag is the option is used for check the motion of the rotating part manually.

Servo Motor Animate the part  & Run the assembly.

Connection - Cylinder

The cylinder connection is used to capture a component moving in a direction, and able to rotate about that direction
üCycle air pump
üPiston action on cylinders

Joint Axis settings on Parallel Motion
  Joint Axis setting is used for control the distance  of a moving part within specified distance like 1 to 50 mm.
Reverse Servo Motor – Animate the part
Run the assembly.
Connection - Planer 
A Planar connection is used to ensure that a datum plane or planar surface always touches another plane or planar surface,

üLM Guide Rail & Runner Block
üA box sliding across a floor
üAny object that rests on a flat surface, but can be moved about that surface.

üPlaner  - Plane moves on Plane

Connection - Slot
Slot Connection is used to move a component along a path.

Bevel Gauge
Geneva Mechanism
Slot – Point Moves on Curve
Connection - Ball
ball connection is used to simulate a joint where the translation is completely fixed, but all rotational degrees of freedom are available.

Toggle Switch
Ball joint gas springs
Ball Connection - require two points or vertices to be aligned, and they have no joint settings.
Cam Follower
A Cam-Follower is used to simulate an object moving along the surface of another object.

Example-  All Cam design applications
Gear Connection
Gear pairs are used to simulate two gears in mechanism
Standard – Use this when you want your two gears to rotate in the same or opposite directions, such as a spur-spur or worm and wheel gear.

Rack and Pinion – Use this when you want to be able to translate rotational motion into translational motion.
Interference checking & Create a movie file 
We can capture a playback result set as an MPEG file, or as a series of JPEG, TIFF, or BMP files by using playback option.

· Start Time = 0
· End Time = 10
· Frame Rate = 10
· Minimum Interval = .1
· Frame Count = 101     


Step 1 – Assemble Components
Step 2 – Modify Joint Axis Settings
Step 3 – Create Cams or Gear Pairs
Step 4 - Drag Components
Step 5 – Create Servo Motors
Step 6 – Create and Run Analyses
Step 7 – Check interference & Create a movie file

Connection Type
Translational DOF
Rotational DOF
References Needed
2 Axes or Edges & 2 Planes, Planar Surfaces, Datum Points or Vertices
2 Axes or Edges & 2 Planes or Planar Surfaces
2 Axes or Edges
2 Planes or Planar Surfaces
2 Datum Points or Vertices
1 Datum Point and 1 Axis or Edge
2 Coordinate Systems


Ref- Sharptechdesign web.

Wednesday, March 27, 2013



In addition to being able to assemble components using realistic degrees of freedom, mechanism also does a lot more.  The typical progression to using mechanism is as follows.

Step 1 – Assemble Components

Create your assembly using mechanism connections to capture realistic degrees of freedom.  These connections are covered in great detail in the upcoming lessons.

Step 2 – Modify Joint Axis Settings

Control your connections by modifying the joint axes created by the connection.  This is explained in greater detail in the connection lessons individually.

Step 3 – Create Slots, Cams or Gear Pairs

Slots, Cams and Gear Pairs are special tools in mechanism that capture complex interactions between components.  These will each have their own lesson.

Step 4 - Drag Components and Create Snapshots

Dynamically pull or push on components that have open DOF to see them move in the assembly.  Take snapshots of your assembly at different states of motion to use in drawings or to come back to for reference.

Step 5 – Create Servo Motors or Force Motors

Servo motors and Force Motors are used to drive analysis and move your assembly on their own without using drag tools.  Each of these topics will be covered in great detail in their own lessons.

Step 6 – Create and Run Analyses

Start your animations to calculate the results you are looking for.  With servo motors in your assembly, you will be able to produce motion animations.  With Force motors, you will be able to calculate resultant forces and other measures while the animation is running.

Step 7 – View Results and Take Measurements

Run the animation to create MPEG movies, or to calculate interference along the path of the moving objects.  Create and view graphs that measure certain factors over time, such as position or force.


The table at the top of the next page lists the different connections available through the component placement window at the time you assemble in a component.  In addition, the number of translational and/or rotational degrees of freedom are shown for each connection type.

Connection Type
Translational DOF
Rotational DOF
References Needed
2 Axes or Edges & 2 Planes, Planar Surfaces, Datum Points or Vertices
2 Axes or Edges & 2 Planes or Planar Surfaces
2 Axes or Edges
2 Planes or Planar Surfaces
2 Datum Points or Vertices
1 Datum Point and 1 Axis or Edge
2 Coordinate Systems

There are two additional connection types in the list, but they don’t fit into the categories above.  These are the Rigid and General connections, and they allow you to use standard assembly placement constraints, such as Align, Mate, Insert, Tangent, etc.  We will see its usage in Mechanism later.

Wednesday, January 9, 2013

Top Down Design


The idea behind Top-Down design is to try to build in intelligence between the fit, form and function of parts that reside in an assembly.  You try to capture this fit, form and function into the assembly first, and then pass the appropriate information down to the part level so that a change made at the assembly level or to one component in the assembly can drive updates to the rest of the critical parts.

In this method, many of the components are created in the assembly, instead of being assembled into the assembly.


The best way to capture fit, form and function for the assembly is to create a special kind of component called a Skeleton Model.  A Skeleton model is similar to a regular part, but it is treated specially in the assembly.  For example, a skeleton model is automatically excluded from the Bill of Material, where if you just created a regular part and used it like a skeleton, it would still be reported.

There are also restrictions that can be placed on regular parts that skeleton models are exempt from, or get special rights to deal with.  For example, you can make it so you are not allowed to copy surfaces from a regular part to another regular part, but you can still pass surfaces from the skeleton model to a regular part.  In defining such restrictions, you avoid creating parent-child relationships between individual part files, making the model more robust.

To demonstrate this principal, we will first create a new assembly file called Stacker.  Be sure to use a Design sub-type for this assembly, just as we did for the last lesson.  Also, once you have the assembly started, be sure to turn on the features in the model tree.

Wednesday, December 26, 2012

Bottom Up Design


If you recall, when we first got into the placement window for a new component, the current constraint was listed as Automatic.  It was awaiting entity selection to determine automatically which of the pre-defined constraints made the most sense.  I would recommend starting off with Automatic and then adjust the constraints accordingly.  In this section we will look at the pre-defined constraints that can be assumed or selected directly.


A mate takes two surfaces and points their normals towards each other and lines up both surfaces, as shown in the next figure.

Mate Offset

This is the same as a Mate except that the distance between the surfaces can be less than or greater than zero.


An Align takes two surfaces and points their normals in the same direction and lines up both surfaces, as shown in the next figure.

Align Offset

This is the same as an Align except that the distance between the surfaces can be less than or greater than zero.

Orient (Parallel)

This is similar to an Align offset, except that you don’t specify the distance between the surfaces. 

Insert (Coaxial)

An insert takes two cylindrical surfaces and lines up their axes, as shown in the next figure.


A Tangent constraint takes two cylindrical surfaces, or a planar and cylindrical surface and makes them tangent to each other, as shown in the next figure.

Point on Surface

Places a datum point or vertex on a surface, as shown in the following figure.

Edge on Surface

Places a straight edge on a cylindrical or planar surface, as shown in the next figure.

Tuesday, December 25, 2012

Sheet Metal Best way of modeling

Step 1 - Add first flange
Step 2 - thickness value should be mentioned in relation ex t=1.2
Step 3 - Say inner radius in relation ex IR=1.5
Step4 - For manual Relief cuts, add relation &ad15 = IR+t

If we create a sheet metal model by using relations, we change thickness of the part without failures when iteration during analysis.


Tuesday, November 27, 2012

Thread callout in pro-e


Wednesday, November 7, 2012

White paper - Chord half theory

Chord half theory of Four Bar Mechanism
Application of Surgical Instruments
Arun Soundararajan


In ground offset four bar mechanism, when horizontal movement of the right link and the left link in various swing circles are equal also within the limit of 180 deg angle of rotation. Hence, the upper link and ground link meets parallel to each other. When reverse the surgical instruments, ease of reverse the shape and critical dimensions. But functionally reverse the links are to be challenge through reverse engineering. Because, that links design decides the total output of the mechanism and links are operating by its hole positions. Hence I evaluated relation between the various swing circles of links and fix the angle rotation by equaling the chord length. Also, it’s proved by calculations as well as graphically by using 2D software. It has given a way to perfect design with low time consumption to attain the functional needs.

Key Words: Four Bar Mechanism, link design, surgical instruments, Offset ground holes, Crocodile action

PCD        Pitch Circle Diameter
R.L          Right Link
L.L          Left Link
G.L          Ground Link
RDS        Rotational Distance Method
CHT        Chord Half Theory


Crocodile is a type of surgical instrument used in laparoscopy surgery. It has to work through the small operating hole (<10mm). The working movement of the tool is restricted and small tip part has to open in the required angle. The above said are some of the key requirements for the instrument. It’s available in different sizes. When do the reverse engineering for scale up and scale down sizes, the design team found difficult to achieve the key requirements.

Crocodile – Surgical Instrument Fig.1
Instead of using a systematic mathematical solution, the design team was using iteration methods. The iteration method is very tedious and time taking process.
I approached the problem in a systematic manner. The link hole positions are the driving elements to achieve the key requirements. The four bar mechanism has been used to calculate the angle of rotation, cutting angle (q) and link hole positions. This documents talks about fixing the hole positions of the links with help of the mathematical model.

2. Related Work

In the iteration method, Pro-e /AutoCAD has been used to find the hole positions of links. The method is, fixing the ground holes constantly and moving the upper link holes along the length step by step to find relative position of holes. This method requires the extensive use of modeling software (Pro-e/AutoCAD) and highly experienced designers. It is increasing the design cost.

Exploded view and Part names Fig.2
My study on the design brings up me to solve the problem mathematically will take less time, less cost and importantly accurate.
In this instrument, the left and right link are working with various PCD (øM, øS) with offset ground holes.    (Refer the figure 3)
 I started with the assumption of the rotational distances of right link (a) and left link (b) are equal. And ended up with the relation between angle of rotations of the left and right links as below,  
Angle of Rotation (b), (From Figure.1)

This solution gave the required cutting angle but upper link and ground link is not touching them self all along. There was a gap between them  in end. In the closing/opening position, the ground link and upper link should be flushed together all along is one of the key requirement. So, the above mathematical model is not meeting all the requirements.
On further studies told me that, Horizontal movement of Link Right and Link Left should be same.
Summary of literature review/related work:

·         Iteration method is very tedious method, time taking process, high cost and experienced persons needed.
·         Equaling the rotational distance concept is mathematical solution not fulfilled the requirements. But better the iteration method.
·         Chord length method is a systematic mathematical approach meet the 100% design requirements.


Study Model of Four Bar Mechanism Fig.3

In reverse engineering of a crocodile instrument deals with outer profiles, cutting edges and the functionality. The outer profiles and critical dimensions can be measured through the various 3D scanning options. Even functionality can be obtained, if we do 1:1 Scale of reverse engineering. When it comes to scale up or scale down reverse engineering, it is difficult achieve the functionality for the different length and thickness.  The crocodile instrument is smaller in the size for child and bigger for adult. 
No issues on scaling the outer profile and cutting edges. Maintaining the cutting angle and upper link parallel motion are important requirements and difficult to achieve though reverse engineering. The requirements are driven by the following parameters,

1.        Ground link hole positions (O,P)
2.        Right link Swing Diameter (øM)
3.        Angle of rotation - Right link (a)

4.        Left link Swing Diameter (øS)
5.        Angle of rotation of the Left link (b)

The first four parameters are fixed by the width of the instrument. The width should not exceed 12mm at the hand end. The angle of rotation- Left link is driving the hole positions on the upper link. Hence the angle of rotation – Left Link decides the functionality of the mechanism.

Angle of Rotation – Left Link (b) =   ?


Find the angle of rotation (b) of the left link towards the pivot axis.

4. SYSTEM design
        The right link in rotating in larger pitch circle and left link rotating in smaller pitch circle and it’s pivoted in ground with offset holes. Upper link is connected to the ground link through the two links as shown in Figure 1.
Considering the larger pitch circle (øM) from fig.1
Right link design Fig.5
Initial Assumptions for design:-
·       Chord lengths are equal for RU and LL.
·         When right link start to rotate from R to U makes a chord in larger pitch circle diameter and forms a triangle PRU.
·         Pivot point P divides the side RU of triangle PRU symmetrically and forms two right angle triangles TRP and TUP. Hence, RT=TP.
·         Angle a is equal to both right angle triangles TRP and TUP.
Considering the smaller pitch circle (øS) from fig.1

Left link design Fig.6            
From the right angle triangle TRP,
Sin q = Opposite side / Hypotenuse
Sin a = RT/ PR
RT = Sin a * PR ---------------- Equ.1
Here, PR = Radius of  øM
Assume, Chord lengths (RU&LL) are equal.
Therefore,  RT=NK --------- Equ.2
Considering the smaller pitch circle (øP) from fig.1
Sin b = NK/ON -----------Equ.3
NK = Sin b * ON --------Equ.4
Here, ON = Radius of  øS
Substitute equ.2 in equ.3
Sin b = RT/ON
Hence, the angle of rotation (b) of øS,
b = Sin-1(RT/ON) in radians
b = Sin-1(RT/ON) X (180 ° / p ) in degrees.
By simplification,
Substitute Equ.1 & 4 in Equ.2
Radius of øM               Sin b                180
------------------- = ------------ X  -------- in deg.
Radius of øS               Sin a                  p

Angle of Rotation b in Degrees.

                           Radius of øM                        180
b   = Sin -1 {-------------------------- X Sin a } X -----------
                         Radius of øS                          p

6.       Evaluation

After find the angle of rotation of left link from the calculation, links of the Crocodile product is designed by this above method by equalling the chord length of swilling circle. Motion of the mechanism is tested by pro-mechanism by applying the limit for the angle of rotation by the calculated value. Hence, the upper link works perfectly.
Also, angle of rotation of left link can be calculated by graphical method. By using AutoCAD, When we draw the fig.5 for the requirements we can measure chord length from the diagram. Apply the same chord length in fig.6, directly measure the angle of rotation (b) from the diagram.
Instrument closed condition upper link flushed with ground link Fig.7

Instrument open condition upper link flushed with ground link with required cutting angle Fig.8
Figure 7 & 8 Shows the perfect working conditions of crocodile instrument. Equalling the chord length method is satisfying all the key requirements of the design.
Design validation of model four bar mechanism
 in Pro-E Figure.9
        Design validated for various angles and equalling the chord length of different pitch circles by using pro-e. Every design satisfied the motion of upper flushed with ground in Pro-E Mechanism in open & close condition.
Graphical method:-

Graphical method to find angle of rotation Fig.10
By using AutoCAD
From the fig10,
Ø Draw Swing circle for dia 30 and the inputs angle of rotation 20 deg as shown.
Ø Then measure the chord length = 10.26 mm
Ø Draw the dia 20 swing circle. Then offset the vertical axis by the half of the chord length at the both sides as shown
Ø Connect the intersection points. Then measure the angle of rotation 31.26 deg.
Performance matrix with time taken and cost for related concepts.  Time (hours) & Cost (INR)
Graph Plotted for time and cost for various methods
Ø Angle of rotation of left link should be with in 180 deg when calculated. Hence upper gets parallel motion.
Ø Initial design assumptions values are depend upon only the width of the instrument.
Ø Angle of rotation of right link is always decides the cutting angle of the tip part.



Link design is very significant in four bar mechanism. Relative motion of the links decides the output of the perfect mechanism. Every mechanical needs satisfied by exact relations in a mathematical approach.
Chord Half Theory model – Four bar mechanism
According to my Chord half theory, When axis of circle divide the chord symmetrically, half of the chord length of various swing circle of the right link and the left link are equal and also angle of rotation is less than 180 deg, the upper link and ground link flushed with each other parallel in the four bar mechanism.
                Where the crocodile action needed for operate a human body, hence we can use the chord half theory to design the link of the surgical instruments.
[1]     www.gescoindia.com
[2]     www.google.com