P14006: Bathtub Lift

Problem Definition

Table of Contents

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Project Summary

A bathtub lift is a device that provides assistance to an individual with physical disabilities, including limited balance, coordination, or mobility. The device can greatly reduce movement difficulties by raising and/or lowering an individual into the tub by utilizing a seat (powered by a hydro pump) paired with a simple control module. Additionally, the lift can be used to facilitate tub entry and exit. As a result of accelerating population growth, the number of aged individuals will greatly increase over the coming decades. Among other needs, the demand for water powered assist devices will grow in demand due to the device's compatibility with most existing structures, ease of use, and overall reliability.

This project focuses on improving the design of a previously developed bathtub lift. Theresa Loce is a wheel chair user who relies on this type of device daily. However, the model she is currently using does not meet her needs. The goal of this project is develop a water powered bathtub lift that is, among other requirements, sturdy, comfortable, easy to use and clean, makes minimal noise, and takes into account any physical limitations the user may have. The expected end result is an installed bath lift for daily use that allows Theresa to maintain autonomy and privacy.

--- Questions regarding Water Powered Bathtub Lift ---

Number Question Answer
(1) Do we need FDA approval for the type of material used in this device? No, but no nickel allowed in the construction - client is allergic.
(2) What is the acceptable maximum weight range for this device? The lift should be able to carry at least 150 pounds, just to be safe.
(3) Does this device need to have compatibility with various tub designs? No, it only needs to work with the present bathtub. However, it would likely be beneficial to have a fixture that would work with multiple tubs (in the possible event that the tub ever needs replacing).
(4) Should seat go high enough so that user’s feet go over the ridge of the tub? As long as the seat can rotate above the lip of the tub, it should be fine. Perhaps find a way to facilitate tub entry and exit (lift client's legs).
(5) Possibly have an option on the remote so one could adjust the temperature of the running water before going in. The option to control water from the chair would be nice, if it is at all possible/within constraints (cost, time, etc.)
(6) How large should the chair be? What kind of orientation of the chair will be acceptable? Possibly have seat recline. The new chair should be the same size as the existing chair. It needs to be able to swivel but implementing some kind of locking mechanism would be beneficial (to prevent turning during entry/exit and bathing). The chair does not need to recline.
(7) Controls on the chair or on a remote? The chair presently just has a lever on the right-hand side that redirects water flow to the chair. Client would prefer to keep with this design, but is open to other possibilities.
(8) Will there be interchangeable parts on this device so people could accessorize the chair? Things like adding different seats in terms of shape or material (smooth or gel). Even adding options of having baskets of various sizes for shower supplies. No interchangeable parts. The chair is fine as it is and she can use the tub wall to hold any bathing supplies. However, making a caddy that can hang of the tub rim may be a potential option.
(9) What is the existing model that you have? What do you like about it? What don’t you like? The chair/fixture is made by Tubmate, but the company is now out of business. Jim and Dom have both made modifications to the chair but the main issue is that it is unstable, slow, and inefficient. The left arm rest is also broken. Most suction cups have worn out/been replaced. However, the chair is very comfortable.
(10) What will be the most comfortable way of securing the user in the chair while keeping them safe? There is no need for a seat belt – the chair works just fine by itself.
(11) Are there patents for the device that we well be working on? If so, how closely do we need to stay to them? We will be making a one-off model. This will be the only chair of its kind. It should be easy to fix/swap out parts.
(12) Do you have a preference for how the lift is secured to the tub? Suction cups, clamps, etc? Suction cups are being used right now but they are wearing out. We could get better suction cups or find a way to mount the chair on the tub floor or wall.
(13) How easy are you expecting the setup to be? Are you looking for something that you can simply lift out of the tub when not in use or would you prefer a folding design? The chair will be secured in one place – it will never be removed.
(14) Are you expecting a modification of the current device or a whole new system? The client would like a new device, but still based off the old design.
(15) How does the system work? The user turns on the water and gets it to a comfortable temperature. The water flow is redirected to the shower head, where a tube allows water to travel from the head to the chair. The user then turns a lever on the chair arm to allow pressure to start building up in the chair, allowing it to rise. Once the chair is fully risen, the user rotates the chair to get in then rotates back into the tub. The user then turns the lever the opposite direction to release the built-up water pressure to return the chair to a resting position. Once the user is finished bathing, she raises the chair once more to exit the tub. The lever is turned again to release the built-up pressure, returning the chair to its resting state.
(16) What are the dimensions? The team took measurements of the tub and chair. See the constraints section below.
(17) What is the device made of? The client is not sure, but the seat seams to be made of plastic while the frame is metal (steel/aluminum?). The new device will need to be resistant to corrosion.

Customer Requirements (Needs)

Customer Need Importance Description
CN01 1 Chair turns 180-360 degrees
CN02 1 Chair has handles/armrests
CN03 1 Requires minimal setup
CN04 1 Comfortable and safe to use
CN05 1 Operates through a water-powered system
CN06 1 Sturdy attachment to tub; cannot move
CN07 1 Non-corrosive
CN08 1 Easy to use
CN09 1 Easy to clean
CN10 1 No use of Nickel - allergic
CN11 1 Able to lift at least 150 lbs
CN12 1 Accessible to 10th percentile women (height)
CN13 1 Controls/handles must be easy to use
CN14 1 Compatible with existing tub/system
CN15 2 Easy to maintain/fix
CN16 1 Takes 0-5 minutes to rise
CN17 2 Minimal noise
CN18 2 Flexible temperature control
CN19 1 Can recline to lay user flat in tub
CN20 2 Can fold down to fit in car
CN21 2 Lightweight
CN22 1 Partially/fully powered by battery (water alternative)
CN23 1 Way to measure temperature

You may also want to include links interview notes here.

Engineering Requirements (Specifications)

Number Source Function Specification (metric) Unit of Measure Marginal Value Ideal Value Comments/Status
S1 CN01/CN11/CN17 System Rotation of chair degrees 180 >= 180
S2 CN02/CN04/CN08/CN12/CN12/CN14 System Width of armrests in 17-19 17 1/2
S3 CN03/CN09/CN15 System Time to get device in/out of tub minutes 0-2 1
S4 CN04/CN12/CN14 System Correctly dimensioned seating area in^2 < 17 x 15 17 x 15 Must fit within tub
S5 CN03/CN04/CN05/CN06/CN08/CN11/CN13/CN14/CN16/CN17 System Uses faucet to raise client psi 30-80 60
S6 CN02/CN07/CN08/CN09/CN14 System Chair secured to tub using suction cups psi secure equivalent of 150 lbs secure equivalent of 150-250 lbs
S7 CN02/CN07 System Long lasting material year 100-120 120-260
S8 CN04/CN15/CN08 System Time to engage hydro lift seconds 1-5 1-3
S9 CN04/CN07/CN09/CN10/CN15 System Material use, fixture design minutes 2-5 3
S10 CN02/CN04/CN10 System Concentration of nickel % 0 0 Client allergy
S11 CN11 System Lifting capabilities lbs 150 >= 150
S12 CN12/CN04 System Height of user in 58 - 62 60 women's 10th percentile
S13 CN13/CN04 System Diameter of lever in 1.5 - 2.5 2
S14 CN14 System Dimensions of device/fixture in 18 - 19 19
S15 CN16 System Rising time minutes 5 <= 5
S16 CN17 System Noise dB 30 <= 30
S17 CN18 System Operates on wide range of temperatures Fahrenheit 70-110 60-130
S18 CN19/CN04/CN08/CN11/CN12/CN14/CN15/CN16/CN17 System Chair reclining ability degrees 40-90 90
S19 CN20/CN03 System Compact transport design ft^3 (LxWxH) 20-27(L) x 16-19(W) x 20-30(H) 25(L) x 17.5(W) x 30(H) Dimensions restricted to tub size
S20 CN21/CN09 System Lightweight system design lbs 20-30 25
S21 CN22/CN01/CN04/CN08/CN13/CN19 System Device powered by battery V 12-15 13.2 Alternative to hydraulic
S22 CN23/CN08 System Thermometer to gauge water temperature F 90-106 104

Engineering Matrix

Image:Engineering Matrix.png


Factors, usually system-level, that limit your design space (e.g., cost, total weight, total footprint, total power available)

Bathtub Lift Dimensions

Part Dimension (inches)
Tub width (at top) 24
Tub width (at bottom with curvature) 20.75
Tub width (at bottom w/o curvature) 19
Tub depth (front) 13
Tub depth (rear) 15
Tub length 32
Chair width (with armrests) 17.50
Chair width (w/o armrests) 13
Chair length 15
Chair Height 15
Armrest diameter 1
Chair base width 13
Chair base length 15
Suction cup diameter 3.50
Hose size 0.50
Tub edge width 6
Toilet to tub distance 13.5
Front bathtub to chair front 21
Bath-side shelf protrusion 2
Toilet Height 19
Toilet to tub distance 14
Tub height (outside) 14.75
Toilet handles (outside edges) 25
Pantry to tub 26
Wall to Toilet Handle 27.5
Wall to Top of Faucet 8.5
Sink edge to wall 23.25
Toilet Handle (inner edges) 21.5

Project Goals and Key Deliverables

Effectively assist the user for bathing purposes
Device is easy to access from starting position
Minimized operating time
Provide a comfortable and supportive seating area
Accessible to 10th-70th percentile for both genders, regardless of age
Reasonably lightweight and portable, able to be transported

Solution Parking Lot

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