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Homework Help: Force Requireed to Close the Gate on the Bottom of a Hopper

  1. Feb 28, 2012 #1
    Hi Everyone,

    I am working on an engineering design project for University, and am struggling to calculate the forces some of the mechanical equipment will require.

    *This is a conceptual design. If anyone would like more details on how it is meant function, or has suggestions, feel free to post questions below or PM me. I have left out some details in an attempt to focus on the current problem I am experiencing.*

    The machine being designed is a small scale paving machine. The paver will have a hopper at the rear that will raise and lower between 2-6 inches to accommodate different asphalt thicknesses. On the bottom of the hopper are four doors that slide horizontally. Each of these doors are attached to a hydraulic cylinder. Having four doors that operate independently will allow for variability in asphalt widths. For a visual aid, please see Attachment A (Attachment A does not have the hydraulics attached).

    I believe I have calculated the force required to open the door correctly, however, I am struggling to calculate the amount of force required to close the door.

    When opening the door, the force required to move the door is a force greater than the static friction forces.

    *The static friction force for hot asphalt on steel is unknown. However, I was able to find Wet asphalt on Rubber from the link below. The asphalt on rubber coefficient of friction would be greater than asphalt on steel, so I used it in the calculations for this project. I'm aware it will result in an over design.*


    Please see Attachment B for a Free Body Diagram and calculations.
    Not shown on that attachment are the following variables.
    Weight of Asphalt = 2200 lbs
    Number of Doors = 4
    Weight of Door = 38 lbs
    Asphalt Weight/Door = 550 lbs
    (Asphalt Weight + Weight of Door) = 558 lbs

    The frictional forces involved are: Steel on Steel (SS) and Asphalt on Steel (AS)

    Now to where I am struggling. Once the doors open, the asphalt will fall to the ground. When closing the door, it will have to cut through the asphalt and I am not sure how to account for this additional force. The doors will rest in a C-Bracket, and will push the asphalt through the back of the C bracket as it closes.

    How do I calculate the force required to close the gate and push through this asphalt.

    I know in manufacturing they use shearing stresses to calculate the forces required to cut material, however, hot asphalt is not a solid like the materials used in manufacturing.

    I'm really scratching my head on this one, and worry I may be over thinking it.

    I apologize if this question is not in the right thread. If you know a thread better suited for this question, please let me know.


    - T.C.
  2. jcsd
  3. Feb 29, 2012 #2


    User Avatar

    Staff: Mentor

    Nothing I can contribute here, apart from pointing out that if the machine is in motion the task will be manageable, but if the machine is stalled you will be asking the doors to slice through an unmoving pile of partly-set asphalt. Or say the tar supply is exhausted, then this "asphalt" may be just settled dry crushed rock. In such a situation, I think the operator may need to "judder" the controls back and forth to force the door to make any headway at all. I see potential to buckle or bust a few things here. :grumpy:

    Good luck with your project. :wink:
  4. Mar 1, 2012 #3
    Re: Force Required to Close the Gate on the Bottom of a Hopper

    Apparently I forgot to post my attachments.

    Attached Files:

  5. Mar 4, 2012 #4
    Does anyone know a better thread to submit this post?


    - T.C.
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