Simulation

All simulations were performed using ANSYS Workbench 2019.

Frame

Conditions:

• 500 N downward axial load applied on upper frame face (worst-case reaction to sample loading)
• 250 N transverse load applied on upper frame face (worst-case transverse reaction to sample loading)
• Frictionless support applied to bottom .5" of frame

Requirements:

• Axial deflection < .0015"

Note that the above deflection corresponds to axial strain of ≈ .01 %, and so satisfying this requirement also implies a substantial safety factor with regards to stress. Additionally, no formal specification for transverse (bending) deflection was established, but this was considered in simulation results.

Results:

• Convergence: <1%
• Axial deflection: 3.15e-6"
• Transverse deflection: 4.28e-6"

These results far exceed requirements. This is primarily due to the fact that frame was oversized in order to satisfy powertrain mounting requirements. A representative image is included below.

Power Screw & Guide Rods

During simulation, each guiderod was assumed to support the entirety of the applied load, an extremely conservative assumption. As the guiderods and screw are made of the same material, and as the screw is a larger diameter than the rods, satisfying requirements for the rod implies satisfaction of screw requirements. \

Conditions:

• 500 N downward axial load applied highest possible crosshead position
• 250 N reaction forces applied at highest possible crosshead position
• Fixed support applied to rod screw face
• Frictionless support applied to upper .15" of rod

Requirements:

• Axial deflection: < .0015"

Similar to the frame study, satisfaction of strain requirements implies stress is well below yield stress. Additionally, no formal specification for transverse deflection was established, but this was considered in analysis.

Results:

• Convergence: < 1%
• Axial deflection: .0092"
• Transverse deflection: .00019"

Again, these results far exceed expectations, even under worst-case conditions. As such, there is likely significant weight and cost savings that could be achieved here. A representative image is included below.

Gripper

The failure point(s) for the gripper are the screws and guide pins. As such, analysis was restricted to these components. As the loading mode is bending, our simulation was conducted with load applied at the maximum expected extension of the pins and screws. Additionally, in order to simplify our simulation and mitigate the effect of stress singularities, the pins / screws were sliced and a fixed support was applied at the sliced end to approximate the frictionless support that would normally be applied upon the length of the pin that was sliced off. Finally, the applied sample load was assumed to be split evenly between the pins and screws. Because the screw and pins are made from similar classes of steel, if the pin satisfies requirements, the screw is assumed to also satisfy them.

Conditions:

• 84 N transverse load applied at end face
• fixed support applied to opposite end face

Requirements:

• Transverse deflection: < .003"
• Max Von Mises Stress: < 45 ksi

Results:

• Convergence < 2%
• Transverse deflection: .00219"
• Max Von Mises Stress: 46.7 ksi

Note that we did not satisfy the stress requirement. However, these results represent a safety factor of 1.43 with regards to yielding, and so they are likely acceptable for the time being. A representative image is included below.

Extensometer

Note that extensometer design was completed using a previous design, which incorporated 7075-T6 Aluminium. The final design will instead use 6061-T6 Aluminum. While the simulation has not been updated, similar results are expected with the modified design.

Conditions:

• 10N transverse load applied to upper arm face
• Fixed support applied to lower arm face

Requirements:

• Transverse deflection of arm face: 1.52 mm
• Max Von Mises stress: 360 MPa

Results:

• Convergence: < 2%
• Transverse deflection: 1.52 mm
• Max Von Mises stress: 380 MPa

Max stress exceeded our design target. Simulated results suggest a safety factor of ≈1.42 with respect to yield strength, likely acceptable given the focus on minimum-weight design. A representative image is included below.

Sample

Conditions:

• Load: .25*t lbf applied to end face of sample where t is time (seconds)
• Min timestep: .1 s
• Max timestep: 1 s
• Max time: 180 s

Requirements

• Convergence < 1%

Results.

• As expected, stress is concentrated in the gage of the sample. Videos of simulated stress and extensions are included below.