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Design Project: Walnut Cracker

  1. Feb 28, 2009 #1
    1. The problem statement, all variables and given/known data

    Walnuts are an important cash crop in Yolo County and are popular to consume around the holidays. To give the market a boost, it would be useful to have a device which would facilitate walnut cracking and also be fun to use. Recognizing this need, the objective of this design project is to design an impact walnut cracker which meets the following design criteria:

    1. The device must operate in the horizontal position.

    2. Impact must be provided by a mass sliding on a track which is accelerated by a linear spring(s).

    3. The mass must strike a ‘firing pin’ which slides in a bushing and applies the impact load to the walnut.

    4. The nut must be temporarily encased during impact to prevent the nut shell fragments from scattering.

    5. The device must be designed to crack an infinite number of walnuts.

    Some of the primary design considerations include the slider mass, the sliding distance, and the spring rate. Specifying these quantitatively depends on the energy required to crack a walnut shell. To determine this energy, you will need to devise a simple test to determine the average and standard deviation of the energy required to crack 10 nuts. This data will then be used for the design.

    Your assignment is to conceive a device which meets the above criteria, perform the necessary calculations to select both material and geometry so that none of the structural components experiences fatigue failure, generate the working drawings to manufacture the design, and finally to communicate your design through the preparation of a design project report. The contents and format of the report are described in a separate handout.

    2. Relevant equations

    To get started, I need to determine the energy needed with an experiment. Anyone have any ideas for how I should do this?
    Also, any suggestions on how else I should proceed would be extremely helpful

    3. The attempt at a solution

    As a simple experiment, I thought of simply using a hammer, but I dont know how to get the energy of each swing.

    I am also going to use an S-N diagram and the idea of endurance limit for the spring (so that it can have an infinite life) using modifying factors and fluctuating loads and the goodman diagram:

    S_e = (K_a)*(K_b)*(K_c)*(K_d)*(K_e)*(K_f)*S_e'

    S_e = endurance limit at the critical location
    S_e' = rotary-beam test specimen endurance limit (Morce specimen) = 0.5*S_ut for
    S_ut < 200psi
    K_a = surface condition factor = a(S_ut)^b
    K_b = size modification factor
    K_c = load modification factor = 1,0.85, or 0.59 (bending, axial, torsion)
    K_d = K_e = 1 (for my class)
    K_f = misc. effects factor



    I suppose i also have to check for static failure for the spring and the mass attached to it. I'm thinking about using the Distortion energy thm
     
  2. jcsd
  3. Mar 6, 2009 #2
    Pivot the hammer on a (frictionless) pin and see how far you have to pull it back to consistently crack walnuts. From the starting angle you can easily compute the initial energy of the system.
     
  4. Mar 6, 2009 #3
    Thx for the reply. I think I pretty much got it down now. What I did was drop a 5lb mass on the walnut and measured the height at which it would break. Then, I calculated the potential energy.
     
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