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Homework Help: Material choices and Stress Analysis for product im developing for school

  1. Jun 26, 2012 #1
    1. The problem statement, all variables and given/known data
    Hello Im designing a product basically called Transfer Board, and need some guidelines on what I should do to get the most accurate and relevant stress analysis result needed to score well. This might take some time for anyone but any help of yours is really appreciated.

    Heres an example of a transfer board, though this board design is nowhere similar to ours, but the uses are the same
    Basicaally a patient will seat ontop and slide over the board to move their position.

    For our design here something my group project mates had sketched out, how the board would look like.
    http://img859.imageshack.us/img859/6356/boardh.jpg [Broken]

    And then a cushion seat will be mounted on top of it supported by bearings laying on the 3 tracks shown on top. They will roll on the 3 tracks with minimum friction resistance to allow users to slide over the board easily.

    Cushion Concept
    The cushion will be base on 3 rectangular blocks(2 Uses as support and 1 for the main seat) joined by 2 strong rod,
    http://img685.imageshack.us/img685/4558/pic1sh.jpg [Broken]

    At the moment i feel like choosing aluminium alloy for the rods, rectangular blocks bases as we want the board to be as light and portable as it can be and the rods must be strong enough to support the weight of the user.

    Ok back to explanation what we had again. So basically someone seating here will be obstructed to enter if we just simply use the above concept like the image shown below. (Figure 4.1)

    To counter the issue, the seat left support block will be unlifted when the board is at the left side of the board from the screen, as shown in figure 4.2. And when a weight of 180x9.81 is finally detected by the weigh system in the center of the main seat cushion, sliding began as the person slide over the board. Over the first few distance of around 20cm. The seat's left support will slowly lift itself up gradually and stay on when it got upright to be use as the left support, the user continue to slide to the right of the screen and as it finally is about reach the right end of the board. The right support block will finally lift itself down gradually to provide unobstructed path for the user to get off the board as shown in figure 4.5.
    http://img441.imageshack.us/img441/7158/pic3la.png [Broken]

    *Other criteria of the board, it must slide pass a distance of 80cm.
    *It must be safe for a person weighing 180kg.
    *Safety factor of 1.5

    This is basically our design idea.

    2. Relevant equations
    Stress = Force/Area (Pa)

    3. The attempt at a solution
    To obtain an accurate stress analysis so that we can confirm whether the chosen materials and dimension.
    Our group assume the Tensile Strength we must take into consideration given force/area on the cushion. The 2 supports must also be strong enough to support the person weighing 180kg. Meaning the rod joint will also be calculated using force/area .

    Given safety factor of 1.5. Force assumed = 180kg*9.81*1.5 = 2648.7N
    Assume diameter of rod is 3.5cm, Area of top circular face of the rod =14.14cm^2
    Stress exposed by the rod is hence 2648.7/14.14 =187.32N/cm^2 = 18.73MPa
    Assume the stress each rod must be able to withstand is a minimum of 18.73MPa, the yield strength of the material must be 18.73MPa too. To meet the criteria, Aluminium alloy is used with yield strength of 414MPa.

    And we can confirm the rod is safe.

    We also assume the force applied on the seat as the basement of the main seat cushion is base on light weight aluminium as well. So assuming the main seat is a 50x30cm rectangle block.
    http://img703.imageshack.us/img703/6736/pic4l.png [Broken]
    Area is hence = 0.5m * 0.3m = 0.15m^2
    Force = Same the maximum stress the person weighing 180kg can cause.
    Stress = Force/Area
    Stress cause by person weighing 180kg would hence = 2648.7N/0.15 =17658Pa = 1.7658MPa
    Assuming material is made of Aluminium Alloy as well, with yield strength of 414MPa Safety check is passed as well in this area

    The stresses will then be passed on to the 3 roads(the 3 tracks as shown in the first image) through the bearings.
    http://img560.imageshack.us/img560/4139/pic6yf.png [Broken]
    The road calculation are as followed. Assuming the Area on the track exposed to the weight of the person is 0.217cm^2.
    Area = 0.0000217m^2 * 3 (3 tracks will share the stress equally) = 0.0000651m^2
    Total Stress = 2648.7N/0.0000651 = 406.87MPa
    Since Yield strength of aluminium alloy is 414MPa and the tracks are made of Aluminium Alloy, strength criteria is made once again.

    The Stresses in the board. The 3 tracks will then pass out the stress to the board equally as shown in the pic below.
    http://img443.imageshack.us/img443/9158/pic7k.png [Broken]
    So assuming the people is seating on the middle of the board supported by a bed and a wheelchair, there will be bending stress presence which I had forgotten how to calculated as I was taught this 2 years ago. How do I do so? Are there tensile stress present in the board body as well? Would Acetal Polymers be a good choice for the board's main body?

    And for the stresses calculation for the bearings that the seat is mounted onto it, what area do we use? The top surface area of the bearing is there a value we can use? Like Area of bearing exposed to the seat.

    Had to present to my lecturer what I found out in about 10 days. But I hope I can finish it in a day or 2 so I can touch up and improve base on lecturers feedbacks. Hope someone can give me some guide on my questions and tell me something if I done wrong so far if any.

    Here it is.. hope someone can help me thanks
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
    Last edited by a moderator: May 6, 2017
  2. jcsd
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