INSTITUT FOR

M

EKANIK

,

E

NERGI

OG

K

ONSTRUKTION

Sektionen for Konstruktion og Produktudvikling

DANMARKS TEKNISKE UNIVERSITET

MEK

Georg K. Christensen

Design of a Pro/Engineer motion skeleton model

for a Briggs & Stratton petrol engine

(ProEngineer Wildfire 2.0)

The purpose of this exercise is to introduce the concept of skeleton models used in
ProEngineer. The skeleton models are useful for the creation of assemblies without too
many assembly interdependencies e.g. constraints. In addition - as will be shown by this
exercise, the skeletons are useful in creating simple forms of ”animation” of the models. A
skeleton in ProEngineer is a frame for the rest of the model. This skeleton can be used for
creation of motion, but also as a means for control of space-allocation during product
design. Examples are products with mechanically integrated, but distinct subsystems. In
a mobile phone the visual appearance or shell can be divided in subspaces for display,
keyboard, circuit board, batteries etc.
During the exercise an assembly consisting of a skeleton model and a crankshaft
mechanism is created. The mechanism is animated and a first element the mechanism ,
the piston is assembled. The remainder of the assembly process is left to the user of this
note.

Step 1:

Creation of the

Skeleton part

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A skeleton part must be created as an entity
within an assembly. Therefore start creating a
new assembly KRANK_W.ASM. Accept to insert
the default coordinate system.
The skeleton part is now created within the
assembly sing: Insert/Component/Create/Skele –
ton-Model/Name: KRANK_W_SKEL.PRT (default)

Accept the coordinate system

*. In the model tree redefine

however the names of the coordinate planes to:

SKEL

right

,SKEL

top, and

SKEL

front. Following

this, the screen should look like fig. 1.
Notice that the

Krank_w_skel.prt

now has taken

the top position in the model tree. It is only
possible to have one skeleton part in each

assembly (subassembly) and its position is at the top.

Fig. 1

In order to achieve correctness during the modelling of the skeleton part we must be sure
not to use assembly datum planes as references. This will be achieved here by opening the
skeleton model just as any other part. The features created subsequently will only refer
to items of the

KRANK_W_SKEL.PRT

. Do as follows: Open KRANK_W_SKEL.PRT by right

klicking on the part in the model tree - then select Open. In fig. 2 and fig. 3 the results of
the operations on the model tree is shown. Fig. 2 shows the tree before opening

KRANK_W_SKEL.PRT

and fig. 3 show the tree following the Open command.

*Unix: Browse to exercise1/ and select: mmns_part_solid.prt

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STEP 2: Building the skeleton model

Fig. 4

Fig. 2

Fig. 3

Step 2.1 Insert a datum axis along the
intersection of

SKELtop

and

SKEL

front.

Step 2.2. A datum circle corresponding to the
movement of the crankshaft bearing is created
using the Sketched Datum Curve tool:

Use

SKELri

ght

as the sketching plane and the newly

created datum axis as the centre.

Step 2.3 Create a datum curve corresponding to
the crankshaft arm and a curve corresponding
to the connecting rod

. (select

). As

referen-ces use only the axis, the datum circle
and the piston movement plane (normally:

SKELfront

).

The model should now look like fig. 4. Arrange that
dimensions corresponding to the measures d1
and d2 are present. The angle d2 is used in the
following to create the movement corresponding
to the turning of the crankshaft. Try if the
skeleton can move by modifying the value d2
(regenerate). Save the model.

Step 3: Creation of motion using Relations

Programming a ProEngineer Relation containing the parameter d2 now creates a
controlled movement. D2 represents the crank angle. Before the relations can be progra-

med a new variable crank_angle is defined. Use Tools/Relations/Local Parameters/

Type: crank_angle. See fig. 5.
The Relations are typed in the text area above. Insert the program lines as shown in fig. 5.

The variable d2:0 used here is an
internal variable name given to the
crankshaft angle. The actual variable
name for the crankshaft angle in your
exercise may vary from this.

Make

ProEngineer show the variable names by typing:

followed by a click at the connecting rod in the
Graphics window.

Finally click at the actual variable for

the crankshaft angle

* (in this note: d2:0).

Fig. 5

The relations will be recalculated each
time the model is regenerated.
Therefore the crankshaft angle is
incremented by 30° per regeneration.
Try to regenerate in order to examine if
the programmed relations are making
the crankshaft turn as expected.

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STEP 4: Finishing the
skeleton

4.1 Create datum points at
the ends of the connecting
rod (on the vertex using:

).

4.2 Create datum axis
through these points

using:

and the datum axis toolbox (fig. 6b)

Fig. 6a

Fig. 6b

*UNIX: to find the internal variable name you have to select
the “connection” sketch2 from the model tree, RMB and
EDIT. Remember the variable name and type it into
relations.

In this way a couple of movable datum axis are created. In fig. 6a
the axis are A2, and A3

.

SKELfront is not moving but used as

reference for the piston movement. Once again check if the
relations work properly. The skeleton model is now ready for use.

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Keystrokes and mouse-clicks that are used often can be ”automated” by the use
of Mapkeys. This is done as follows: Utilities /Mapkeys/New (delete: KeySequence).
Define a new KeySequence, preferably by selecting a new one. Here ra could be used
as an abbreviation of: regenerate automatic. Then type Record and execute the
number of mouse-clicks or keyboard actions you want to record. Type:
Stop/OK/Save to end recording. Test the function.

If required at a later state the skeleton model can be modified or
extended in the same manner as parts in an assembly can be
modified e.g. by opening the part. Save and close the window. With
KRANK_W.ASM open notice that KRANK_W_SKEL.PRT now has
become an extended branch of the model tree .The Model tree
should look like fig. 7.

STEP 5: The assembly process continues by assembling
the mechanism parts

The assembly KRANK_W.ASM is now ready to accept the

mechanism parts. The parts are assembled using:

/Add

component to the assembly. During this process only references
that are part of the skeleton model should be used. Examples are
PN1!,PNT2, A1, A2, A3 and SKELRIGHT. Following the above

mentioned procedure the assembled parts will only refer to the movable skeleton and they

can therefore be easily replaced at a later stage. The animation is here done by:

Regenerate/ Automatic.

Fig. 7

SKETCH 1
SKETCH 2

In figure 8 the result of the assembly process for the piston is
shown. By following the outlined procedure none of the assembled
components will have references to each other. Exceptions here are
the two bolts for assembling the connection rod. All others refer
only to the skeleton. It is easy now to replace an old connection rod
with a new, because the rest of the assembly is not directly involved.

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The drawback of the method is that no immediate verification of
the fitting of the involved parts is done. As an example: does the
piston pin actually fit the holes in the piston ? And in the
connecting rod ?
Notice that in this way we can actually “assemble” a too thick pin in
a too small hole. It is therefore necessary to make a number of
assembly drawings in order to check for interference and other

types of mismatch. ProEngineer offers here some good help in:

Fig. 8

Analyse/Model analysis/Global Interference/Compute and related functions.

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