FEA Analysis:

OVERVIEW:

The structural components that were not purchased were designed for function and ease of manufacturing. Materials for components that the design team designed were selected based on application and what was readily available in the undergraduate machine shop. The function was the overriding factor in structural design, since position was critical for the control design and electromechanical integration of the machine.

In order to check that the machine wouldn’t fail under operation, a failure assessment was performed. The failure assessment evaluated all the individual parts of the machine. The components that were most critical in the operation of the machine were analyzed in COSMOSworks. The parts selected were the base gear, lead cap, lead screw for the linear actuator, slide of the gripper, spinner, and the top slide. All conclusions that were generated from COSMOS were cross checked with hand calculations, as well as the intuition that the design team accumulated through the past seven semesters of education at Stevens Institute of Technology including work experiences.

SLIDE STATIC ANALYSIS:

One of the parts identified in the failure assessment was the slide part. This part is driven by the base gear into the side wall of the medication container. To run an analytical test on the slide part design, the slide was loaded and restrained at the locations shown in Figure 12.2 of the part. The slide was assigned the material properties for AIS 304. The maximum torque placed on the base gear will be 10 pound-inches and the maximum radius that the slide can be is 1.5 inches, so the maximum load normal to the y-axis of the slide will be 15 pounds. The load was placed over the entire contact area of the gripper and the part was constrained over the entire area where the slide contacts the base gear. After running a static analysis of the slide part for the pre-described parameters, it was concluded that the part would not fail. This conclusion was made after viewing the stress plot, Figure 12.2, and the displacement plot, Figure 12.3. The stress plot shows that the maximum stress in the part is 5.234 kilo-pounds per square inch, which means the part has a minimum factor of safety 5.7. The displacement plot shows that displacement was negligible.

BASE GEAR STATIC ANALYSIS:

The next part identified in the failure assessment was the base gear. The base gear is driven by a pulley around its outer diameter and constrained by a shaft through its inner diameter. The base gear was loaded and restrained at the locations shown in Figure 12.4 for the part. The base gear was assigned the material properties for AIS 304 before running the analytical static test. The load applied was a 10 pound-inch torque around the contact surface of the base gear and pulley. The surface where the base gear meets the shaft was restricted from moving and the static analysis was run. The maximum equivalent stress was found to be 2.505 kilo-pounds per square inch in Figure 12.4, which equates to a factor of safety of about 32. This combined with displacement of the part shown in Figure 12.5 shows that the base gear will not fail, when it is rotated with the maximum possible operating torque.

SPINNER STATIC ANALYSIS:

The spinner part has two different operating conditions that were identified in the failure assessment. The first operating condition occurs, when the spinner is axially loaded, after the slide moves down and engages the medication container’s safety feature/locking screw. To simulate this operating condition, the spinner was loaded with 15 pounds of axially load; given material properties of AIS 304; and restrained as shown is Figure 12.6. Two studies, the buckling and static stress study, were run for this operating condition; the buckling study was performed first. The buckling study illustrated that the spinner had a Buckling Load Factor 1325.7. Figure 12.7 explained that that the Buckling Load Factor of the spinner meant that buckling was not expected for the slenderness ratios present in the spinner.

The second study for this operating condition, the static stress study, was run with the same loads and restraints. The results from the static study’s stress plot showed that the spinner had a maximum equivalent stress of 240.9 pounds per square inch, which meant the spinner part has a minimum factor of safety of 124.49 for this loading condition. These values concluded that the spinner part will not fail in the initial load condition.

The next operating condition occurs when the spinner part is loaded axially and then spun by the rotational motor. Figure 12.10 shows the location of the restraint has not moved from the square extrude face, where the rubber insert (high friction gripper) will be placed. Figure 12.10 also shows the locations of the torque load of 5 pound-inches and the axial load of 15 pounds. After running the static study, it was conclude that the spinner would not fail; since, in addition to the other spinner operating condition study findings, the maximum equivalent stress found was 1.644 kilo-pounds per square inch. This meant that the minimum factor of safety for the spinner is 18.2. After viewing the minimal displacements present in Figure 12.11, it was definitive that the spinner part would not fail.

SLIDE PLATE STATIC ANALYSIS:

The operating condition for this static study is that the four motor mount holes transmit a load or 5 pound-inches of torque to the top slide plate part. The part is restrained in the worst case scenario, contact of the side wall and two opposite vertical edges. The load and the restriction can be viewed in Figure 12.12. The part was given the material properties of HDPE and the static study was run on the part. The stress plot showed that the maximum equivalent stress present was 23.8 pounds per square inch, which meant that the top slide plate has a minimum factor of safety of 134.7. The factor of safety and displacements shown in Figure 12.13 concluded that this part would not fail.

LEAD CAP STATIC ANALYSIS:

The lead cap was loaded axially with 7.5 pounds at the location shown in Figure 12.14 for the part. Also, the figure shows where the lead cap is restrained. This operating condition simulates the effects of hanging the slide subassembly from the part. The lead cap was given the material properties of AIS 304 and then the study was run on the lead cap. The stress plot shows the maximum equivalent stress to be 264.9 pounds per square inch, which correlates to a minimum factor of safety of 113.2. Taking the factor of safety into consideration and the displacements in Figure 12.15, it was concluded that the lead cap will not fail.

LEAD SCREW STATIC ANALYSIS:

The lead screw was the only purchased part that was checked for failure; due to the team’s concerns about the small diameter of the lead screw and its critical importance in the machine’s operation. The lead screw has two operating conditions; since, it is placed in compression when actuating the medication container’s safety mechanism and placed in tension, when pulling up the slider subassembly. The first operating condition, the lead screw in compression, required that a buckling study be performed. A load of 15 pounds was placed at the surface of the lead screw and the restraint was placed at the top cross section neck where the lead screw changes diameter, as shown in Figure 12.16. The lead screw was then assigned the material properties given to us by the manufacturer, Haydon, and the buckling study was then run on the lead screw. The study returned that the Buckling Load Factor was 3.6 for the compression loading condition. After looking at Figure 12.7, it was determined that the lead screw would not buckle.

The tension operating condition was tested by changing the direction of the axial load of 15 pounds, as shown in Figure 12.17. The static study was then run on the lead screw returning a maximum equivalent stress of 1.454 kilo-pounds per square inch. This value corresponds to a minimum factor of safety of 20.6 for the lead screw. The displacements in Figure 12.18 were looked at before concluding, that the lead screw part was not going to fail.