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I Magnet clamp repelling in coil

  1. Mar 15, 2017 #1
    I have a practical question. I want to make a rubber (silicone) hose/plug that is pinched closed by two magnets ( very strong magnets such as ones from a hard drive the Neodymium kind). I want to place then into a coil and when I want the liquid to flow, I want to turn on the coil and have the magnets pull apart for a few seconds. (1-5 seconds) then turn the coil off and have them snap closed to stop the flow. This is meant to be repeated 100 times a day with no adverse effect on the magnets original snap together. (note: the plan is to mold the magnets into the rubber to keep them from flipping or falling out or moving)

    My questions:
    1. Is this possible to do?
    2. is there a better method of doing this? (ie- low voltage electromagnet on all the time to keep together then reverse to open? Failures in power seem to be an issue)
    3. will the magnets ever fail to snap back or de-magnetize?
    4. Will a single coil do this or will it require half a coil and the other half coil due to the magnets being +- when snapped together?
    5. will a coil get too hot or require a coil that is so big it isn't practical?
  2. jcsd
  3. Mar 15, 2017 #2


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    Staff: Mentor

    Hi CG! 2d4066413017023.gif

    There are a few rules I try to live by when using magnets:
    ⚫don't let them clang together or hit hard onto metal, each shock weakens them.
    ⚫don't immerse them long term in a strong opposing field, this will eventually weaken them.

    I'm not certain whether I am being over-cautious with Neodymiums, I'll listen to what others say.
  4. Mar 15, 2017 #3
    We’ve used magnetic solenoid valves on special fx projects, and they work great. The design is actually very clever, and super reliable. Have a look at the diagram and description in this Wikipedia page. What you have here is basically a “fluid transistor” – a ferromagnetic pin that’s normally held in the closed position by a spring, is drawn up into an magnetic inductor, which allows a little stream of water to control the bulk flow from your hose via a diaphragm-activated fluid chamber:


    Typically these can remain activated for up to eight hours without overheating. And they work for water pressures up to 100psi or so (of course you can get even more industrial solenoid valves for special applications). You can find the common variety on the market for under $30 – I saw some on Amazon that take a 110VAC current. Depending on your requirements, you could either activate the flow with a momentary switch (which only remains on while you’re applying pressure) or a digital timer if you need more precision (you can program an Arduino to trip your switch for exactly 5 seconds, or any other time you may require).
  5. Mar 16, 2017 #4

    Thank you for the above. What I'm more looking at doing is to not use a solenoid, I don't want the fluid to touch anything besides the rubber for hygienic reasons. I get the solenoid mentioned that process is simple enough, however my question more asks the ability to have the magnets against the rubber tube then pulled apart by the coil around it, and then when power is removed the magnets attract again to squeeze the silicone/rubber tube.
  6. Mar 16, 2017 #5
    What about using one or two external solenoids (not valves, just armature types) with enough spring force to pinch the hose closed, and apply current to pull them in to allow for flow? You could have something rounded at the end of the armature with the right shape to pinch the hose closed.

    Depending whether you expect it to be open more of the time, or closed more of the time determines which way you want the spring force and action to go. I think this would be more reliable than magnets.
  7. Mar 17, 2017 #6
    Yep, if an in-line option is out of the question then an external solenoid to crimp the hose would do the trick. Spring-actuated to squeeze it shut until the power is activated to release the pressure on the hose. But hoses don't like to be crimped back and forth like that. Sooner or later the mechanical strain on the rubber is going to wear it out and it'll pop a leak.
  8. Mar 17, 2017 #7
    Yes, but OP didn't offer any details on pressures, flows, etc. If it is low pressure, silicone hose can be pretty flexible and durable. 100 x a day was mentioned, but maybe replacement at regular intervals is practical?

    I'd look into the kind of tubing they use in peristaltic pumps, that application is very similar.
  9. Mar 17, 2017 #8
    They make solenoid valves out of 300 series stainless steel with food-safe elastomers (probably safer than ordinary garden hose rubber) for soft drinks and pharmaceutical applications - this chart breaks down the options:
    http://www.asco.com/ASCO Asset Library/asco-solenoid-valves-engineering-information.pdf

    It's not the water pressure (I'm assuming something in the ordinary household water pressure range), or even the back-and-forth crimping 100x a day, that really sets off the alarm bell for me - it's the 24/7 crush pressure keeping the hose clamped shut. The tightly pinched spots on either side of the crimp - that's just asking for trouble, now matter how you dice it.

    The peristaltic pump idea is a good way to go - it doesn't get much better than the systems they use to circulate blood in bypass operations.
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