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Thread: How to apply a constant, gradually increasing load to a machine.

  1. #1
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    How to apply a constant, gradually increasing load to a machine.

    Hello. I'm an amateur engineer that has dabbled in just about everything. Electronics, computer engineering, wood, metal, plastic, the list goes on and on. But I'm having trouble figuring out how to design my newest project. Sure, I do have ideas, but they're not elegant.

    The machine is a piece of equipment that will deliver a lateral linear force against a person to test their resistance. It is being designed for use with the martial art Wing Chun to test the player's stance. A proper stance will allow the practitioner to withstand hundreds of pounds of force incoming from in front of them without moving backward.

    I have absolutely no problem transforming a downward force into a lateral force to push a platform against the player's stance. The point where I encounter difficulty is that I don't just want a constant force, I would like to develop the machine as a sort of strain test; I would like to find a way to gradually increase the force applied to the player in such a way that it can still be resisted, but that it will increase until the player fails the test.

    My original idea was to connect a reservoir to a pulley, and, as you fill the reservoir, it increases the load, which is turned into lateral force against the player. The problem with this is that I want the machine to be easily reset, so that the test can be run repeatedly. I'm having trouble figuring out a way to increase the load gradually in a way that would be smooth and still be easy enough to reset to ready position.

    If anyone has any ideas, I'm open to absolutely any suggestions, even a redesign from the ground up.

  2. #2
    Technical Fellow jboggs's Avatar
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    You considered one source of power: gravity on water. What sources of power might you have available? Electricity? Compressed air? How about gravity on a movable weight? Use the moment created by a weight on a lever and fulcrum. The farther the weight is from the fulcrum, the greater the moment imposed on the other arm. The weight could be supported by rollers on a horizontal arm extending from a fulcrum (pivot) point. The opposing arm of the lever is vertical and resisted by the player. So, the player is supporting the weight by pushing horizontally. The machine operator moves the weight away from the pivot point thus increasing the force the player resists.

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    Quote Originally Posted by jboggs View Post
    You considered one source of power: gravity on water. What sources of power might you have available? Electricity? Compressed air? How about gravity on a movable weight? Use the moment created by a weight on a lever and fulcrum. The farther the weight is from the fulcrum, the greater the moment imposed on the other arm. The weight could be supported by rollers on a horizontal arm extending from a fulcrum (pivot) point. The opposing arm of the lever is vertical and resisted by the player. So, the player is supporting the weight by pushing horizontally. The machine operator moves the weight away from the pivot point thus increasing the force the player resists.
    My good God. I wasn't able to think of a way other than using gravity as a driver. If I used a small motor to draw the weight along the lever, it would be able to increase the work done regardless of the resistance of the player. Reset would be as easy as returning it to the start point. Increasing the load would be a simple matter. Thank you so much. I don't think I would have thought of that, as hung up as I was on pulleys.

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    If you use the above lever method, just remember to include a restriction on the distance that the lever will travel once the machine exceeds the person's resistance point.

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    That's a good point. My first thought was to just have it set so that it runs to the end of the lever and then stops, or, in a perfect world, reverses and resets itself. I suppose it wouldn't be too impossible to build a kill switch into the machine that would activate the motor only when the load surface was met with resistance. Basically, as long as the pressure plate was not in the default position, the motor would work.

    I am met with a new question, though. What would be the best way to operate what I assume would be a trolley that would hold the weights on the lever? Would there be an inexpensive product for it, or would I be better off building it as a robot and just programming a smart motor to roll a chain? I would almost like a garage door opener sort of construction, I think, but all of those that I can find would cost as much just to buy as the entire rest of the build would cost to make, metal, welding, fabrication and all. Any ideas?

    Thank you guys so much for your help. I've gotten more work done with two of your posts than I have for the last week XD

  6. #6
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    Using a pressure switch to stop the motor is a good start but even if the motor stops the lever will still have the same pushing power so you should limit the lever travel travel as well. For what you are doing, a maximum lever travel of an inch or so should satisfy your requirements.

    As for the mechanism to move the weights, a motor driven screw and traveling nut arrangement would probably be the simplest mechanical driving assembly.

  7. #7
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    Thanks for the post, Very informative.

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