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ProTutor08
Designing Mechanisms In ProEngineer
D Cheshire
[email protected]
ProTutor01 - ProTutor03, ProTutor07
21
31/07/17
ProEngineer is a three dimensional parametric design system. The word parametric
means that the dimensions that define the size and location of parts of the model can be
varied at any time in the design process. This is clearly very useful as it allows a designer
the flexibility to iterate a design to its optimum. Parametric design is particularly useful
when designing a mechanism as the ability to vary dimensions which define the position
of a part in an assembly allow a mechanism to be flexed to determine if the performance
is satisfactory. If deficiencies in the design are found dimensions can be varied to
improve the performance of the mechanism.
Another important aspect of mechanism design is the calculation of forces and stresses in
parts. In ProEngineer these calculations are only performed if an additional module called
ProMechanica is purchased. This document concentrates on the geometric aspects of
mechanism design and so does not cover ProMechanica at all.
The assembly constraints of ProEngineer are not sufficient on their own to define a
mechanism. To assist the assembly of a mechanism the assembled parts are related to a
sketch that is drawn in the assembly. The sketch can be drawn first and the parts drawn
on to the sketch or existing parts can be related to the sketch.
Since a sketch is restricted to 2D it may at first appear that only two-dimensional
mechanisms can be defined in this manner. Consideration of the majority of mechanisms
will show that, either they are two dimensional or can be defined by a series of linked two
dimensional mechanisms in which case they can be modelled in the manner described
here.
One of the most common mechanisms is known as a four bar link. This is made from four
rigid bars with pin (rotational) joints at their ends. By varying the relative lengths of each
of the four bars different effects can be achieved.
Tutorial
We will design a four bar mechanism by first drawing a sketch which defines the
mechanism and then building parts onto the sketch. This is called top down design since
you work from the assembly down into parts rather than building parts and then
assembling them together.
To start create a new assembly using the command sequence FILE > NEW from the pull
down menu, choose assembly and enter the name FOURBAR. In this assembly create the
default datum planes by using the insert datum icon
to the right of the main
window.. Even if you have assembled parts together before it is unlikely you will have
created datum planes in the assembly. Notice that these datum’s are called ADTM1,
ADTM2 and ADTM3.
Designing Mechanisms In ProEngineer
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At this stage we are ready to define the underlying sketch or framework for the
mechanism. We will use a standard sketch much like you will have drawn when creating
features but now we will draw it in the assembly. Select the insert sketched curve icon
to the right of the main window then pick ADTM3 as the sketch plane OKAY 
TOP and pick ADTM2. You will enter the familiar sketching environment. Accept the
references given and draw and dimension the mechanism sketch. Draw the three lines and
dimension them as shown in Figure 1 below.
Figure 1: Sketch of the mechanism skeleton
Since you have only drawn 3 lines you might not consider this to be a four bar
mechanism. In fact the 350mm dimension that rigidly connects the two open ends
represents the fourth bar. End the sketch and choose OK to complete construction of the
framework.
Now you have a framework sketch for the skeleton of the mechanism you may like to see
the sketch acting as a mechanism. The 65-degree angle controls the mechanism. As this
dimension varies the mechanism can be driven around. This can be achieved by
modifying the dimension with the normal command sequence. From the ASSEMBLY
menu choose MODIFY MOD DIM  VALUE  PICK and click on the sketch line.
The dimensions will appear that define the sketch. Click on the 65-degree value and enter
a new value, say 80. You should see the mechanism move as shown in Figure 2.
Figure 2: Flexing the Mechanism
Designing Mechanisms In ProEngineer
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Try repeating the above command sequence for a value of 120-degrees. When you try to
enter this value the mechanism will try to flex to this position but will then revert back to
its original location. An error has occurred-why? Simply put, the dimensions you have
tried to use are incompatible. If you consider the mechanism, if the left-hand link is to
rotate through 360-degrees then 350+130 must be less than or equal to 210+190 – which
it currently is not. To correct this we could change the 350 dimension to be at most 270
(190+210-130).
With this mechanism it was quite easy to see why the failure occurred. There is a
technique that can be used to further help with diagnosing failures. To use these
techniques you need to re-enter the sketching mode. In this case that can be achieved by
FEATUE  REDEFINE and pick on the datum curve. Now choose SECTION 
DEFINE  SKETCH which will return you to the familiar sketching menus. Choose the
modify icon
and pick the 65-degree dimension. The modify dimensions dialog
shown in Figure 3 will appear. You can use the thumb wheel next to the dimension to
vary its value and see the effect on the sketch (alter the sensitivity to get more or less
movement). The mechanism flexes as though you were rotating the driving arm. Watch
what happens to the sketch as the value nears 103-degrees. The dimension value will only
increase until the mechanism comes to the limit of its travel and fails! If you now modify
the 350mm value to 270mm the mechanism will function correctly as shown by the series
of diagrams in Figure 4. End the sketch and choose OK to complete construction of the
framework.
Figure 3: Modify Dimension Dialog
The skeleton for the mechanism is now designed and all that needs to be done is to add
the parts representing the three-dimensional components. The best way to achieve this
and maintain the relationship to the skeleton is to design the parts in the assembly. Before
doing this it is good to position the mechanism so that none of the links are close to
horizontal or vertical else it is possible that ProEngineer will make an incorrect
assumption. To achieve this a good value for the angular dimension is 120 degrees. Make
it so!
Designing Mechanisms In ProEngineer
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Figure 4: Correctly Functioning Mechanism
To create a part in the assembly use the commands COMPONENT  CREATE. Choose
part solid and type in a name such as Link1 OK. Now choose to create first feature OK.
SOLID  PROTRUSION  EXTRUDE | SOLID | DONE  ONE SIDE | DONE
then pick ADTM3. If required FLIP until the arrow drawn onto ADTM3 is towards you
then choose TOP and pick ADTM2. You will now see the normal sketching environment.
You can now sketch the outside profile of the link but be careful that any alignments etc
are made to the first line of the mechanism and NOT to the datum planes. The easiest
way to achieve this not to add any references. Instead use a command to offset the
existing line. Choose
and pick the line for link1 and type in a distance of 20. Repeat
this same command sequence but enter –20 (negative) as the distance. Complete the two
ends of the link by adding arcs using
use
. Make sure these arcs are tangent to the lines –
to add the tangency constraints if needed. No further dimensions will be needed.
Figure 5: The Sketch for Link1
Designing Mechanisms In ProEngineer
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Leave the sketcher and choose BLIND | DONE and enter 10 as the thickness. The part
will be created after selecting OK in the PROTRUSION: Extrude Dialog box. Of course
there are two more links to be created. These are made in the same manner being careful
to ensure that in the case of the middle link the protrusion direction (as indicated by the
arrow drawn on ADTM3 which can be FLIPped!) is opposite to the other two. You
should end up with a model as shown in Figure 6.
Figure 6: A Solid Model of a Mechanism
Not only do you now have an assembly model but you also have three links as separate
parts. These can be opened and edited independently. You might like to do this now to
add 10mm diameter holes for pins to go through at each of the joints.
It would also be good to prove to yourself that you can still flex the mechanism with the
associated solid parts. The procedure for this is identical to what you did before using
MODIFY. Just take care to pick the skeleton sketch (use QUERY SELECT) for
modifying.
Figure 7: The Flexible Mechanism
Since this sort of rotary mechanism is often modelled facilities have been added to the
config.pro file (The ProEngineer configuration file) at Staffordshire University to
automate the flexing of such a model. Basically this file assigns commands to the F11
and F12 keys that increase and decrease a variable called angle by 10. To achieve this
automation, a little extra work is required on your assembly model. Simply follow the
command sequences below.
Designing Mechanisms In ProEngineer
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From the ASSEMBLY menu choose RELATIONS  ADD PARAM  REAL
NUMBER and type angle <return> 60 <return>
Whilst still at the RELATIONS menu click on the original datum curve and the
dimensions used to create the curve will be shown with the names of the dimensions (not
the values). Make a mental note of the name of the driving angular dimension of your
mechanism e.g. d3:1 as you will need this name very soon.
Now from the RELATIONS menu choose ADD and type the following but replace d3:1
with the name of your driving angular dimensions noted earlier…
d3:1=angle <return> (replace d3:1with the name of the driving angular dimension of your mechanism)
<return>
Having done this task successfully the two keys F11 and F12 will increase/decrease the
value of angle thereby rotating the mechanism. Try it and see! The mechanism should
now flex within the physical limits of the mechanism and so long as the angular
dimension stays within the range 0 to 360. To avoid this last problem you can add further
relations. From the RELATIONS menu choose EDIT REL and edit it so that it looks like
this. (There will be a few lines at the beginning of the file that start with /*. Do not edit
these. Also remember to replace d3:1 with the name of your driving angular dimensions
noted earlier.).
IF ANGLE < 0
ANGLE = ANGLE+360
ENDIF
IF ANGLE >=360
ANGLE = ANGLE-360
ENDIF
d3:1=angle
Choose FILE > EXIT to leave the editor. The mechanism should now flex through the
full range within the physical limits of the mechanism.
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