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Thread: Forty foot linear motion, bidirectional

  1. #1
    Associate Engineer
    Join Date
    Jan 2014

    Forty foot linear motion, bidirectional

    My company is interested in building a prototype system for measuring soil stability by running various tires (atv thru semi size) across prepared soil beds. Putting it in a 40 foot shipping container allows particulates to be measured and is wind and precipitation independent; but it limits the space available. My idea is to lay down two tracks that carry a wheeled cart. The tire under test will be mounted to the cart via a pivoting arm; load will be controlled by adding weights to the arm. The assembly will weigh about 1200 pounds. The cart and test tire need to be driven independently, probably using electric motors. At times the tire will be driving, braking, or free wheeling across the soil bed.

    The goal is to accelerate in about 12 feet, hold a constant speed for 12 feet and then decelerate in 12 ft. Top speed is 10 mph. My concerns are what type of guide rails to use so the cart will be stable underway and what type of drive to use on the cart wheels. A friction traction drive would be simplest and 2 or 4 of the wheels can be powered. If I get information on friction coefficients it should indicate if that is practical. Cable could be used to move the cart, but that seems more complicated since it would require one on each end (to accelerate and retard motion). It will require multiple passes so controlled motion in each direction is desireable.

    Has anyone seen anything similar or have any suggestions on rails, cart, and drive type? If possible the design should provide for future growth in length and speed.

  2. #2
    Technical Fellow Kelly_Bramble's Avatar
    Join Date
    Feb 2011
    Bold Springs, GA
    Welcome to EE!

    Not knowing your budget or the number of test configurations... Could you consider the following:

    • Concrete pad/road with guides installed in most any configuration should work as long as the cart tracks properly perpendicular to the soil.
    • Design a retrofit device that can level or compact the soil relative to the cart.
    • Place test system in a building and bring soil into building.
    • Build wood or other walls and roof around test area to block wind and rain.

    Powering the cart with the most readily available power source makes the most sense to me (gas engine, electric or whatever).

  3. #3
    Senior Engineer
    Join Date
    Jan 2014

    My Two Cents

    Sounds like an interesting design problem. A few thoughts you may want to consider. A cable or even chain drive would still be the simplest, just loop it at the ends so both ends are attached to the cart. Kinda like fixing the cart to a reversible conveyor. I would think that you would not want to support the cart on drive wheels which would require tires for friction. You would be introducing unwanted motion from the deflection of those tires when driving or braking the test tire. Instead, think of something like ball bushings. Bolt the tracks to the floor or walls, using the structure of your container. Much less friction than tires and the cart is simpler. They make rails systems that can handle very large loads. Drive it with a suitable servo motor & gear train and close the location feedback loop with a laser range finder. This setup will allow you to very simply control the speed and acceleration profile with a computer.
    How about some more details? Will the prepared beds be inside the container or will you just cut a slot in the floor and then just set it over the test beds?
    Last edited by Timelord; 01-07-2014 at 05:16 PM.

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