1. Final Project Report
EML 4024C
Engineering Design and Practice
Summer 2015
Authored by:
James Cianciolo
Sidney Feldman
Eduardo Larrazabal
David Shamblin
Due Date: August 7, 2015
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Introduction
The project was to create a functional 3D model of a two stroke weed wacker motor
assembly using SolidWorks. Motion analysis was then performed to report the kinematic and
dynamic quantities present when the mechanism was in operation. The intension behind this
project was to test everything that has been learned over the semester within Solidworks. It also
showed firsthand how many mechanical objects can be created and tested before any physical
pieces are ever needed.
Background
What is a two stroke engine?
It’s an engine that takes two strokes to complete a full cycle (up and down motion of rod for one
revolution of crankshaft).
It follows:
Intake→Compression→Exhaust
As opposed to a 4 stroke engine
Intake→Compression→Combustion→Exhaust
Mechanical Design
Parts
Piston:
The piston head was made by first creating a
circle and extruding it.
Figure 1: Piston Cap Sketch
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Another two circles were created and
extrude cuts were applied to give the piston
head its shell as well as the two holes for the
cylinder.
For the pin, two circles were created on the
same plane and the extrude tool was applied.
For the piston rod, hole diameters and total
length were measured before a sketch was
made.
Figure 2: Finished Piston Cap
Figure 3: Finished Pin
Figure 4: Piston Rod Sketch
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Extrude and extrude cuts were then applied
to create the finished piston rod.
Flywheel:
The initial flywheel was created using a
closed sketch and the revolve feature.
The fin was then sketched on the bottom
face of the flywheel with a circular pattern
and then extruded upward with a chamfer
effect.
Figure 5: Finished Piston Rod
Figure 6: Flywheel Sketch
Figure 7: Fin Sketch
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Another chamfer was applied to each fin to
give them the required angle and a fillet was
used on each fin to produce the rounded
edges.
The keyhole was then made using a circle on
the bottom face with an extrude cut and a
chamfer effect. A drawing was then created,
mirrored, and cut extruded with an offset to
finish the keyhole.
Muffler:
All parts were measured using digital
calipers for precision. To start the muffler, a
square was made 2.37 inches tall by 2.38
wide and later extruded to the desired value.
Figure 8: Flywheel with fins
Figure 9: Finished Flywheel
Figure 10: Muffler Sketch
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A cut extrude was made on the sides to
mimic the shape of the muffler. The cut was
done through all.
For the next part, a plane was created at the
desired distance from the front face and a
square was created and extruded and united
with the existing part.
A similar cut extrude to step 2 was done on
the last part to polish the muffler. After the
cut, two holes were created on the outer box
and cut through. This is the result from the
cut.
Figure 11: Muffler Cut
Figure 12: Extruded Muffler
Figure 13: Muffler Sketch 2
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After filleting the top and bottom faces and
shelling the muffler, this was the final result
for the piece.
Carburetor Adaptor:
The adaptor was a complicated part to make.
The group decided to use the engine block
as a reference, since it had the same
dimensions as the carburetor. The sketch
from the block was copied and later
modified to fit the desired dimensions. It
can be seen the whole sketch is fully
constrained.
A lofted cut was later done on the extruded
piece from the back face to the center and
then another lofted cut was done from the
middle to the front face.
Figure 14: Finished Muffler
Figure 15: Carburetor Sketch
Figure 16: Extruded Carburetor
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A cut extrude was done on the bigger circles
to match the carburetor. Also, another cut
extrusion was done for the smaller circles.
They were cut through all. The rest of the
piece consisted of fillets through the entire
piece.
Engine Block:
The Engine Block was started by creating a
square on the top plane and extruding down,
then filleting the edges to create the top shape.
Next a cylinder was extruded from the bottom
surface and extruded as well.
Next, the fins were created by sketching and
extruding the bottom fin first and then using a
linear pattern to create the rest.
Figure 17: Finished Carburetor
Figure 18: Engline Block Extruded
Figure 19: Engine Block with Fins
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Then the holes were cut into the solid
extrusion for the piston cylinder and the holes
for the screws.
The connection point for the exhaust was
created by extruding a cylinder and then
making a lofted cut to create the cut as can be
seen in the image. Also a few extrusions were
made on the sides which give space for holes
to be cut into later on.
The connection point for the carburetor
adaptor was created by sketching the shape
seen to the right and extruding the surface out.
The lofted cut in the center was made in the
same fashion as the previous one.
Figure 20: Engine Block holes
Figure 21: Engine Block Exhaust
Figure 22: Engine Block Carburetor
Adaptor
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Finally the cylinder was cut out as well as the
channels. Numerous fillets were made in
order to cut down on concentrated loads as
well as for aesthetic purposes. Other cuts,
extrudes, and fillets were used for aesthetic
purposes as well.
Spring:
The spring was made using a helix and a
circle. The circle was extruded along the
helix.
The spring was finished by sketching the last
two pieces of the spring and using a swept
extrusion that followed it.
Figure 23: Finished Engine Block
Figure 24: Spring
Figure 25: Finished Spring
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Crankcase:
The crankcase was made by sketching and
then extruding a piece from aluminum. For
the purposes of this project, the most
meaningful parts were those that mated with
other parts. The smaller diameter of the
center hole was extruded to accommodate
the bearings and crank shaft.
The larger diameter in the back was made to
allow rotational movement of the crank shaft
and translation of the piston rod. A hole on
the top was added to let the piston move up
and down. The back of the crank case is left
open, as in regular operation, it is covered
and sealed by the plastic enclosure of the
trimmer.
Gasket:
A rubber gasket was extruded using the
sketch on top of the crankcase. In the
assembly, this would mate between the
crankcase and cylinder.
Figure 26: Crankcase 1
Figure 27: Crankcase 2
Figure 28: Gasket
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Crankshaft:
A crankshaft was created from stainless
steel. The crankshaft contains a keyhole
which allows proper mating with the
flywheel. The rod on the back receives the
combustion force from the piston and
converts that to rotational energy used by
the trimmer.
Figure 29: Crankshaft
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Figure 30: Assembly 1
Figure 31: Assembly 2
Figure 32: Assembly 3
Assembly
For the piston assembly there were only four
mates. To attach the pin to the piston head a
tangent and concentric mate were used.
Then to complete the assembly for the
piston, the piston rod was mated to both the
pin and a plane created using concentric and
distance mates.
The crankcase was the first piece added to
the assembly, as it would mate with the most
pieces.
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Figure 33: Assembly 4
Figure 34: Assembly 5
Figure 35: Assembly 6
An assembly was made with the crankshaft
and bearings. The bearings were mated
concentrically with the shaft. A coincident
mate was added with the face of one bearing
and the front face of the rear element of the
crankshaft. The other bearing was mated
coincidentally with the groove toward the
front of the shaft.
The flywheel was added concentrically with
the shaft and groves on the flywheel were
mated coincidentally with a key slot on the
shaft.
The gasket was added coincidentally with
the face of the crankcase and concentrically
with the screw holes.
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Figure 36: Assembly 7
Figure 38: Assembly 8
Figure 37: Assembly 9
The engine block was mated coincidentally
with the gasket and concentrically with the
screw holes.
The piston assembly was added by mating it
concentrically with the rod on the rear of the
crankshaft and concentrically mating the
head of the piston with the center of the
engine block.
Finally, the peripheral parts were added with
concentric and coincident mates.
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Figure 40: Table of Material and Mass Properties
Mechanism Model Analysis
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Figure 41: Displacement Graph
Figure 42: Velocity Graph
Figure 43: Acceleration Graph
Mechanism Kinematics and Dynamics
Mechanism was applied by attaching a rotary motor to the crank shaft and having it rotate
counterclockwise about the z-axis. The rotation of the crankshaft transferred to linear motion in
the piston head. For the kinematics/dynamic analysis a point on the piston head was chosen, and
the displacement, velocity, and acceleration was found. By choosing a point on the top of the
piston head, the full stroke of the motor can be found. The top dead center of the engine is
80mm below the origin of the assembly, and the bottom dead center is 55mm, thus the full
distance traveled by the piston is 25mm. Using a diameter of approximately 34.3mm, it was
found the engine that was modeled was 23cc.
The linear velocity of the motor, as can be seen, is maximum when the piston is halfway through
its motion and minimum at either TDC or BDC.
The acceleration of the piston head is maximum at TDC and BDC. Which makes sense, because
as the piston slows down and speeds up at those positions is where the greatest amount of force
will be felt.
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Figure 44: Strucutal Analysis 1
Figure 45: Structural Assembly 2
Structural Analysis
A structural stress analysis was performed on the rod of the piston, as it would be taking a large
amount of the loads in various directions from the axis of the rod.
The results showed a maximum stress of 146.3 kN/m^2 that developed on the sharp corners of
the rod and along the center axis. This was to be expected as the greatest shear and moments
would develop at the thinnest parts toward the center, given its general symmetry.
An additional structural analysis was performed on the deformation experienced by the
crankcase and showed maximum deformations at the most extreme corners of the structure. The
value for the displacement was .2805 mm. This made sense as the vibrations from the engine
operation would produce the largest displacements on the crank case at the furthest distances.
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Conclusion
The creation and assembly for the two-stroke weed wacker motor was an excellent way
to test students’ skills with Solidworks. The results obtained for the weight of the motor were
very close to that of the actual model. Using a diameter of approximately 34.3mm, it was found
the engine that was modeled was 23cc. Motion analysis produced accurate results from the single
rotary motor used. Stress and deformation analysis produced reasonable results with maximum
and minimum values where expected.
