P16006: Tub Lift 2

Systems Design

Table of Contents

Team Vision for System-Level Design Phase

For the System-Level Design Phase, our goal is to clearly define our overall concept for this project while conducting basic analysis to ensure our design is feasible and realistic. Currently, we have two concepts, a primary and a secondary, that are feasible. The secondary project is simpler, but less innovative. Based on elementary static calculations, we found our primary design to be feasible. However, much more complex analysis will be required to determine the specifics of the design including but not limited to: water pressure, torque and power calculations along with a stress analysis.

Functional Decomposition


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Functional Decomposition



The above chart represents commercially available tub lifts on the market available for purchase. All of the lifts portrayed in the table, with the exception of the Portable Bath Lift, represent electrically powered lifts. The Portable Bath Lift is the token hydraulically powered lift. To capture an appropriate average, we will utilize the Bellavita Auto Bath Lift as a reference when comparing against our custom designs. None of the consumer grade lifts have a built-in swiveling function.

Another important comparison that we will need to make is to compare our new custom designs against the final product of the Tub Lift rev 1 project. The previous project had a cost that exceeded $1500 and had a manual swiveling range of motion of 360 degrees. There was no range of recline since the chair was a single solid object. The overall footprint of the design was slightly better than that of the current existing unit in the customer's bathroom. The overall weight of this final product was incredibly heavy, requiring two people to lift and move it. Tub Lift rev 1's final product was battery powered and buckled during the lowering process.

Overall, as far as design concepts are concerned, all units primarily utilize electrical or hydraulic systems. Other types of considerations that could be important may be pneumatic or manual systems.

Concept Development

From our functional decomposition, we can conclude that the main six subsystem functions that the tub lift device must perform include the following:

  1. Raise/Lower Device
  2. Transfer In/Out of Device
  3. Assist Legs Into/Out of Tub
  4. Rotate/Swivel Device
  5. Recline Chair
  6. Powering of Lift

In the case of all subsystems, with the exception of the transfer in/out of device, the main points of consideration would have to be hydraulic, electrical (from battery), manual or pneumatic system controls. These systems would largely be a a product of what the main power source of the lift would be, although hybrid systems are certainly feasible. In the case of the transferring into and out of the device, we considered different options for handle bar implementation. The location of the bars relative to the chair and degree that the handle bars are adjustable are definitely points to consider. Another possible option would be that there would be no handle bars, which would be a case of no solution.

From these considerations, we developed the following blank morphological table:


To view all morphological tables and pugh charts relevant to the next few sections, please click the following to download the file:

Morphological Tables & Pugh Charts

Morphological Chart and Concept Selection







Concept Selection






Overall, the base existing design currently installed in the customer's bathroom performs adequately in regards to the comfort of the seat, unit weight and unit footprint, whilst having a 0-90 degree range of motion for the chair swivel. It has no ability to recline and replacing parts is incredibly costly. The tub lift rev 1 project has a seat that is equal in comfort to that of the base design and has a chair that can swivel from 0-360 degrees, but manually. It has no ability to recline and has a decent unit footprint, superior to that of the existing design. However, it has an inferior unit weight, being incredibly heavy. Furthermore, the cost of developing the unit exceeded $1500, making its price unattractive. The Bellavita Autobath lift, our main benchmark, is incredibly lightweight, at around 20 pounds, and has the ability to recline from 0 to 50 degrees backwards. The unit's footprint is incredibly small, but the seat is in no way comfortable; there is a severe lackk of ergonomic design when looking at the chair design. The Bellavita unit also has no ability to swivel, but only costs $565. Between these three units, the benchmark performs best in terms of unit cost, range of reclining and unit weight. The tub lift rev 1 project performs best in terms of the chair swiveling range of motion, although a 0-360 degrees range of motion is unnecessary for the scope of the customer requirements. Both tub lift rev 1 and the base design perform best when it comes to the comfort of the chair, considering these three options.

When compared to our main three design options, formulated from the morph tables, the base design, tub lift rev 1 design and benchmark design all under-perform. Based on the nature of our designs, we believe the unit weight, unit footprint and unit range of reclining motion will be on par with that of our benchmark. We are also confident that our three design options would be less costly than the benchmark price. We have around $750 in budget money to utilize for the entire project. Some of this money will ultimately be consumed through rework during the prototyping process. As a result, the final cost of all of the components for the final unit we design should not be more than $750; more than likely we believe that the total cost of the final design will be significantly less than the maximum amount allotted for our spending. Considering that our designs will have to swivel 0-90 degrees to meet customer requirements our main three design options will outperform the benchmark in this area, but under-perform when compared to tub lift rev 1, which can swivel from 0-360 degrees. Our custom designs will utilize an ergonomic chair so the custom designs will perform at a level on par with that of both the base design and tub lift rev 1 design, but outperform the benchmark.

The comparison between our main three custom designs took the following into account. All of our designs have the same level of comfort, range of recline and range of swiveling motion. The main differences consist of the unit footprint, unit weight and unit cost. Overall, hydraulic systems will be contribute less to weight and cost than electronic systems. Additionally, a purely electronic system will ultimately have a larger unit footprint than a hybrid or purely hydraulic system. This would be due to the difficulty in separating electronic components from the water.

Systems Architecture


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System Architecture

Feasibility: Prototyping, Analysis, Simulation

Given a hydraulically powered system, the following image describes the different possible cases necessary for consideration for different city standard pressures (P), generated forces (F) and angles at which the force is applied in a static position (theta). These cases were assumed under worst, best and average scenarios. The (L) parameter is the length of the lever arm that lifts the chair. The (R) parameter is the length between the pivot and the point at which the piston applies its pressure. From these parameters, we calculated the Length of the pipe needed as well as the radii required for the pipe in each of the three cases described.


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Feasibility Analysis

Designs and Flowcharts



Risk Assessment


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Risk management Assessment

Design Review Materials

Please download the following file to view our Problem Definition design review presentation:
Systems Phase Design Review

Plans for next phase

As a group we plan on conducting further systems and subsystems analysis of our design. We may need to incrementally improve our design as we go through this process. Once satisfied, we will begin generating a Bill of Materials for the finalized design along with starting the search for possible vendors to purchase parts so we can begin to prototype.


Please refer to the following attachments to view individual plans for the next phase of the project:

Branch - SubSystem Design Plan
Hall - SubSystem Design Plan
Krall - SubSystem Design Plan
Swearingen - SubSystem Design Plan

Template: Individual 3-week Plan Template

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