MSD II Revised Detailed Design
Table of Contents
Team Vision for MSDII: Reviewing Detailed DesignAt the end of the MSDI Detailed Design Phase, Team P17011 had achieved a preliminary detailed design involving a geared lifting mechanism. At the conclusion of the final design review for the fall semester, there was a great deal of uncertainty about the final output for the project as well as the current direction of the design work. A lot of this concern was centered around the lifting mechanism reliability and manufacturability as well as the appearance and scaling of the final design.
Prior to the beginning of MSD II in the spring semester, Team P17011 held a meeting to discuss each of our positions and the scope of the project. The result of that meeting was a simplified lifting system involving an initial concept of bottle jacks instead of the geared lifting mechanism. The plan for the first four weeks of MSD II was to complete the detailed design work to reflect the changes made to the overall system design. With a new destination in sight and the same set of deliverables to be met, MSDII began with the goal of having a detailed design along with a bill of materials by the middle of week five.
Prototyping, Engineering Analysis, Simulation
Lifting MechanismOne of, if not the most important components of the toilet seat lifter is the actual lifting mechanism. The preliminary design for the lifting mechanism involved a rack and pinion gear design which was generated in KISSsoft (Gear Modeling Software). The functional breakdown of the geared system with the worst case loading condition can be seen below.
The component of equal criticality was the ratcheting mechanism, a device removed from the drivers seat of a 1990s VW automobile. However, this component brought several unknowns with it.
The idea was to attach this ratcheting mechanism to the end of the pinion shaft allowing for the seat to be raised and lowered. However, this is where this preliminary designed stalled out. Aft several weeks of design and analysis, DFM constraints took over. The system feasability came into question with the amount of time and budget available for this project. At the conclusion of the MSDII kick off meeting, it was decided to alter and simplify the design to a more commercial off the shelf solution, a bottle jack lifter.
MSD II DDR Lifting Mechanism Revision II
The ratcheting mechanism was subsequently replaced with two bottle jacks. The new concept is that the bottle jacks will replace the legs of the seat lifting assembly. Further work was done to reduce the amount of inputs or "pumps" by the user
The full product specifications can be found here: http://www.harborfreight.com/3-ton-heavy-duty-long-ram-hydraulic-flat-bottom-jack-60393.html
Bottle Jack Pump Design
Since two bottle jacks will be utilized, the goal of the bottle jack pump is to tie both jacks together with a single input from the user. Further work was done to optimize the size of the pump cylinders to reduce the shear number of inputs from the user.
Using the above M-files and the hand calculations shown, the pump cylinders were sized accordingly. The key step in this process was understanding the relationships between the amount of travel allowed by the handle, the force input from the user and the area of the piston head. Scaling each of these factors greatly affected the other variables in the problem. After discussing the problem with the Industrial Design portion of the team, a handle length of 18 [inches] was agreed upon with a user input of 15 [lbf]. This resulted in the prescribed dimensions that were used for the Fusion 360 model.
Piston DesignThe cylinder piston was designed using the following resources:
|O-Ring Sizing Resources|
With the information gathered from the various referrences, it was possible to size the o-ring as an AS568-014 class o-ring with an ID of 0.489+/-.005 [in[ and a cross section of 0.07+/-.003 [in] (~1/16 in). Each piston will employ two o-rings for redundancy, the o-rings will be seated in grooves machined into the piston heads. (SEe below hand sketch.)
Composite Seat Design
Due to the shear loads and deflection limitations of the seat concept, it is believed that a sandwich composite consisting of a core fill material and composited laminates is the path forward. The seat was modeled such that (4x) 3/4 [in] bolts could be used to fix the seat to the base assembly.
The design and analysis of the cantilevered seat is currently ongoing. The above values for safety factor and deflection were calculated using E-Glass/Epoxy laminate with a PVC Foam core fill. After slight alterations to the problem boundary conditions, all layers, including the core fill, have positive factors of safety for the applied loading condition.
The Base Assembly is an assembly of components whose sole purpose is to transfer the load from the cantilevered toilet seat to the bottle jacks. The interfacing components under the adapter plates are the fixed points where the bottle jacks are pinned to the base assembly. The assembly is comprised purely of bolted connections of 8-32 bolts and 10-32 bolts. The assembly was simplified, treating every bolted connection as a pinned constraint and simulated with the applied moment and distributed pressure load of a 500 [lb] individual using the seat. The resulting plots of Von Mises Stress and Deflection can be seen below. There is not absolute confidence at this time in the solutions of these simulations, however, it is believed that a positive margin is maintained.
Like the lifting mechanism, the device base was designed under the "worst case" of a 500lb load on the end of the armrest or the end of the seat. Design is still ongoing, especially as engineering and ID go back and forth to find a design that suits both disciplines as well as possible. The current model can be found in the image below.
The base is intended to prevent tipping and mitigate movement of the device in all directions. This is why the feet are placed wider than the toilet and extend forwards, and why the back of the device has beams connecting it to the wall.
The main challenge for this design was the risk of the device being tipped over backwards by a leg spasm from the user. To address this tipping force by only extending the heel of the feet would require that heel to extend over 200 inches beyond the leg. This is obviously not practical, so other approaches were investigated. To keep the base out of the user's way, it was decided that the best option would be to use the wall as a support through those beams mentioned above.
A static stress analysis in Fusion 360 (see below) of the current design shows a maximum stress on the base of just below 4 ksi, which is about the limit of a standard 2x4. This makes material choice much more flexible.
Drawings, Schematics, Flow Charts, Simulations
The detailed design assembly can be viewed below. The entire assembly consists of bolted features. Overall the goal of simplifying the design from the geared rack and pinion mechanism to the bottle jack lifting mechanism has been met. There are still a few open action items, such as the bottle jack relief valve arm which will be modeled by the Industrial Design students. At this time, there are no drawing made for any of the device components. Upon completion of this phase and ordering of parts, drawings will be created and inserted into the build and test portion of this Edge page.
Bill of Materials (BOM)
Lifting Mechanism Assembly BOM-Lifter BOM
Cantilevered Toilet Seat BOM-Composite Seat BOM
ID Testing Criteria and Method
- Physical Testing
- Determining comfort with full size mock up
- Having user sit to test ergonomics
- Appearance Testing
- Is it intimidating or confusing?
- Medical vs. Mechanical
Engineering Preliminary Testing Plan
- Anterior/Posterior Support
- Maximum Load and Moment
- Verify Adaptability
- Lateral arm rest
- Maximum Load and Moment
- Adjustable Seat Unit
- Cyclic Testing
- Maximum Load and Moment
- User Testing
- Identify contact points when transferring using normal toilet and our device
- Identify overall hygiene of these points
- Compare work on muscles on the user when using both toilets
- Completion of survey comparing device to original toilet seat
- Ideal Height Test
- Define ideal heights to lift toilet seat
- Compare work of muscles on user at ranged heights
The FMEA has had one minor change. One of the risks was failure of the pneumatic lifting mechanism, but this mechanism is no longer a component in our design, so its risk has been set to zero.
Plans for next phase
- As a team, what do you need to accomplish between now and the end of the semester?
- As a team, what do you need to do to prepare for MSD II?
- As an individual on the team, what are you doing to help your team achieve these goals? (Use the individual 3-week plan template for this)