FINAL DESIGN
Learn more about the final design of the cradle!
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Requirements:
This project requires the design of a rigid structure that can be built on the Simons Array receiver without interference with the receiver assembly. Then, this structure must provide attachment points to be confidently lifted by human-controlled hoists to place the receiver into its final position. Additionally, the structure must maintain a required structural Factor of Safety of 5 in this process.
Overview:
The final design decided upon by the team is a truss structure that attaches to an existing I-beam on the Simons Array receiver. The structure will be referred to as a “cradle” as agreed upon by the sponsor and the project team. The cradle is split into two halves that are mirrored and built on both sides of the receiver opposite its optical axis. On each half, there are hook attachment points that provide rigid points to lift up the cradle, and as a result, lift up the receiver.
Figure 2.1: Isometric View of most recent design
Figure 2.3a: Telescope clearance measurements
Figure 2.2: Isometric view of PB-2B receiver, cradle, cart, and Simons Array telescope configuration
Figure 2.3b: Telescope clearance measurements
Description:
The structure is built almost entirely from 80/20 T-slot aluminum (Fig. 2.1). This is an extruded 6063-T5 aluminum that has pre-manufactured slots that allow for ease of component placement in assembly. With these slots, the different beams in the truss structure can be easily moved if necessary. This added flexibility will prove beneficial to address the potential discrepancies between CAD models provided by the team and the actual components in Chile, as necessary modifications can be made while assembling (Fig. 2.2). The other necessary components include gusset plates and intermediate plates which require custom hole placement. These will be manufactured from 6061 Aluminum.
The geometry of the cradle is heavily influenced by three major factors: position of the hook attachments to ensure clearances (in the initial and final position when lifting), clearance from protruding components of the receiver itself, and the ability to mount to the I-beam on the receiver. Meeting these three requirements and goal of a high factor of safety (FOS) led to the necessary geometry of the final design solution.
Figure 2.3: Final cradle design with labels
Figure 2.4: Final cradle design resting on cart
Figure 2.5: Nathan with the Cradle
Figure 2.6: Isometric View of completed manufactured cradle
What is the cradle made out of?
Figure 1.10: Example of 80/20 T-slot aluminum bar that is used to compose and assemble the cradle
Cradle Components: 80/20 Aluminum T-slot
The structure is built almost entirely from 80/20 T-slot aluminum (Fig. 1.10). This is an extruded 6063-T5 aluminum that has pre-manufactured slots that allow for ease of component placement in assembly. With these slots, the different beams in the truss structure can be easily moved if necessary. This added flexibility will prove beneficial as there have been concerns by the sponsor of discrepancies between CAD models provided by the team and the actual components in Chile, so necessary modifications can be made when assembly occurs. The only components in the assembly that are not 80/20 are necessary gusset plates or intermediate plates which require custom hole placement. These were manufactured from 6061 Aluminum.
Figure 1.11: McMaster Steel Hoist Ring
Cradle Components: Mounting Hooks
On each half, there are hook attachment points that provide rigid points to lift up the cradle, and as a result, lift up the receiver (Fig. 1.11). This particular hook attachment was chosen and utilized in the design in order to satisfy one of the major requirements of the cradle: to avoid collision with the supporting telescope's truss and to avoid other interferences with the limited space for the cradle. Thus, this steel hoist ring (McMaster part #: 3052T58) was chosen as a small and compact, but also highly-rated for loads, for lifting points for the straps and to support the overall weight of the receiver. The team will also be machining an intermediate plate (pictured in light blue in Fig. X) to attach these rings into the cradle assembly.
Cradle Components: Mounting plate for hoist ring
Because the hoisting ring would be unable to be screw directly onto the 80/20, an intermediate mounting block was necessary to connect the ring onto the cradle. The approach chosen by the team was to use an intermediate mounting plate. The plate served as the adapter between the two components (the ring and the 80/20 bar). The mounting plate would be screwed into the end of the 80/20 stock and the hoist ring is screwed into the mounting plate.
Cradle FEA Simulation Results
FEA Simulation Set-up:
In order to assess the structural integrity of the assembly, FEA (Finite Element Analysis) was conducted in Solidworks. In this analysis, one side of the cradle was utilized and a half-version of the telescope was modeled into the assembly as well. The setup is as follows:
Boundary Conditions:
A fixed constraint was applied on each strap attachment
Symmetry constraint at the face “cutting off” half of the telescope
Connections:
Pin connections were used between the Receiver/I Beam component and the cradle
Loads:
A gravity load was applied to simulate the weight of the receiver + cradle
Strap force of 200lb applied on the lower beam of the cradle
FEA Simulation Results:
These results yielded a maximum Von Mises stress of approximately 107MPa (Fig. 2.3 and Fig. 2.4). With an assumed yield strength of the 6105-T5 Aluminum being 270MPa, this led to a Factor of Safety (FOS) of approximately 2.5 in one region. Otherwise, high stress regions sat much lower with most of the stresses being closer to approximately 70MPa, which would lead to a FOS of 3.5.
Although the initial desired FOS was 5, the team deemed the FOS of 2.5-3.5 in high stress areas to be acceptable due to the low amount of loading cycles the cradle will see (will be used once per telescope). A higher FOS would have been beneficial if the cradle was being constantly used, but that is not the case for this component. Any increase in the FOS would have required an increase in weight that hurts other priorities of the system such as manufacturability and ease of assembly. With this conclusion on the static structural integrity of the system, the dynamics of the system became the main worry of testing.
Figure 2.3: Von-Mises Stress plot of the cradle
Figure 2.4: Location of the maximum stress point
Physical Testing at EBU II and Powell Laboratory
EBU II Intron Machine Pull Test:
A pull test was conducted using an Instron Machine in the EBUII testing facility in order to determine the strength of the aluminum threads and to test the bolt performance to ensure that the mounting fixtures would not fail during the lifting procedure.
Results:
9 kN of force was used to pull the mounting fixture
Aluminum threads and bolts did not fail
Each of the mounting plates and fixtures have an FOS of 4
Predicted to withstand much more than 9 kN
Powell Labs Structural Integrity Test:
The structural integrity of the cradle was tested in the Powell Lab. This was done by gradually adding W18 blocks and loading blocks to simulate how a load will affect the performance of the cradle. The blocks were also placed at the center of gravity of the receiver to simulate a back-end heavy load. In the testing set-up, the cradle was mounted onto a strong wall using steel bars and lifting straps, and the two sides were secured with another nylon strap to prevent the cradle from spreading open.
Results:
80/20 stock shifted at one of the joints due to a lack of fasteners securing the cradle at that point
Cradle was able to withstand 875 lbf without deforming or yielding
Conclusion: To reiterate, the initial FOS of the cradle assembly was 5, but this requirement was changed to 2.5-3.5 to account for the more lightweight components of aluminum and also taking into account that this cradle will be considered a “single-use” component and will not be under repeated loads. The team also added additional fasteners after testing to increase the structural rigidity of the assembly for future use (Fig.2.X).
Standard Operating Procedure:
Initial, Middle, and Final Lifting positions
Future Design Recommendations:
Addition of Ratchet Straps during Lifting Procedure
Addition of Ratchet Straps during Lifting Procedure
Considering that the design of the cradle is split into two independent halves and that lifting the assembly is reliant on the tension of the straps at the mounting fixtures, the moment caused by these forces would pull the cradle apart (green arrows in Figure X).
While the bolts connecting the receiver’s I-beam and the cradle’s mounting plate are predicted to withstand and counter these tension forces and the weight of the receiver, the Polar bear team suggests to add two additional ratchet straps to wrap around the bottom of the cradle, with tension connecting both halves (Orange strap in Figure 36). This tension would result in a moment in the opposite direction, pulling the cradle back inwards and creating a more balanced load (Yellow arrow in Figure 36).
Based on the FEA done in Solidworks, the two additional straps would have to support a total tension of 200 lbs per side. To support this force, the Polar Bear team recommends PN 8834T151 from McMaster, which has a load capacity of 330lbf. This would give an FOS of 1.65 for each of the bottom straps.
Alternative Material: Steel
Alternative Material: Steel
Compared to aluminum’s yielding strength of 270 MPa, steel has a higher yielding strength at 350 MPa. Thus, using steel could have increased the FOS of the cradle. However, while steel has a lower likelihood of yielding and deforming, steel is about 2.5 denser than aluminum, meaning that constructing the same design with this material would have resulted in a much heavier cradle.
Overall, the team suggests that if meeting the FOS requirement of 5 holds higher priority than the overall weight of the cradle, steel would be an acceptable choice of material for a future design. Because of steel’s rigidity, it is also possible that the cradle can be more compact than the current design, and the cantilevered components of the cradle may not be as significant and the lengths of the beams can be cut down, releasing some weight.
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