MSD I Detailed Design
Table of Contents
Team Vision for Detailed Design PhaseThis phase the team completed the initial detailed design of the lifting mechanism for the toilet seat concept. The concept includes a geared rack and pinion mechanism with a VW seat lifting mechanism. There are serious concerns of the feasibility of this concept.
Prototyping, Engineering Analysis, Simulation
All analysis on the toilet seat lifting mechanism was conducted under the theoretical "worst case scenario" in which a 500 lb individual is seated upon the very edge of the toilet seat. As such, the simulations were all conducted as though a 500 lbf point load existed 18 inches from the edge of the toilet seat.
Lifting Mechanism Failure Analysis Areas
Several key areas were defined in the lifter analysis as having a high failure potential. Primarily the main shaft, the linear bearing surfaces, and the ratcheting mechanism.
This is key, because the selected linear bearings only have a one degree misalignment tolerance before locking up. It can be seen above that this will not be an issue, even with over 9000 in-lb of torque resulting from the 500 lbf point load and moment arm interaction.
Not much is currently known about the ratcheting mechanism. The mechanism is a seat lifting and adjustment mechanism from a mid 1990's VW vehicle. The design specifications, loading conditions, and materials used are all unknown. However, there are some assumptions that can be made and investigative work that can be conducted. The two main concerns with the mechanism will be the stress in the gear teeth and the shear stress on the shaft. These two components are what effectively hold the entire system together once raised.
For the purposes of this analysis the Lewis stress was examined in the gear teeth. Here, a few assumptions were made: namely the number of teeth and the Lewis form factor. Both were estimated as a worse case condition.
According to the analysis, if only one gear tooth is handling the stress induced from the user, the tooth is receiving stresses on the order of magnitude of Msi, which would conceivably break the tooth due to the high loads.
There are however, 10 teeth in contact at any given time for the ratcheting system, diving that stress by a factor of 10. However, that stress level is still well above the yield stress of most gear steels. With 20 teeth in contact through two ratcheting mechanisms, the stresses are reduced to more reasonable numbers.
As far as the shaft is concerned, a small hole was drilled through the shaft to determine its design. Through this process it was discovered that the shaft was solid rather than hollow. Its material properties are still unknown, but with two ratcheting mechanisms and two shafts, it is not believed this will be an area of concern.
Cantilevered Seat Failure Analysis Areas
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 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. The resulting values are, however, incorrect. The local coordinate frames for the meshed elements do not coincide with the global coordinate frame governing the loads and boundary conditions. This resulted in unrealistic solutions for the problem. This design and analysis will be completed by December 12th, 2016.
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 have fins on the side and long, bolted toes on the front.
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 base would require that heel to extend over 200 inches beyond the leg. This is obviously not practical, so other approaches were investigated. In order to minimize bulk and "claptrap" on the upper body, it was decided that bolting the device to the floor was the best option. With this decision in place, the heel could also be bolted, removing the need for a long toe. This will be looked at before the semester's end, because the toe in its current state could be a minor hindrance to transfers.
A static stress analysis in Fusion 360 (see below) of the current design shows a maximum stress on the base of about 40 MPa, localized at the point of bolting. This stress is well within the yield strength of aluminum, which would be a likely core material for the base.
Drawings, Schematics, Flow Charts, SimulationsDescribe your design in enough detail for someone else to recreate it. Have you included assembly plans and user manuals? Your team may want to create separate nodes and directories within the Detailed Design Documents directory for CAD, schematics, or software
Bill of Material (BOM)
Lifting Mechanism Purchase Parts
|Name||Quantity||Unit Cost||Total Cost||Supplier/Link|
|Linear Bearing Surface Rod||1||$28.65||$28.65||McMaster|
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.