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OT: Small diameter magnetic roller between wheels to provide frictionless braking resistance?

Discussion in 'Electronic Design' started by John Doe, Aug 29, 2007.

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  1. John Doe

    John Doe Guest

    I'd like to use a roller between two wheels to provide some
    (practically) frictionless braking resistance while going downhill.
    Would an unconnected DC motor be the only possibility for that
    purpose? Preferably the braking device would be all in one small
    cylinder in between the wheels, but I can't imagine anything that
    would provide a small amount of friction while not wearing out
    components like an attached DC motor would. Being able to fine-tune
    the resistance, like by moving the motor body away from the rotor,
    would be ideal.

    Is there some other device besides the magnetism of a DC motor shell
    that might provide near frictionless roller braking?

    Thank you.

    Here's what it might look like, except with a much smaller motor and
    not connected to a power source.
    Newsgroups: alt.binaries.phish
    Subject: attached DC motor to inline skates - File 5 of 5

    Here's some background, for what it's worth.
    One of the most difficult parts of in-line skating is going downhill.
    Currently the best braking mechanism is a rear wheel brake. So
    alternatively, some sort of roller between the wheels would not be
    useful for emergency situations, but it could be useful for providing
    a variable/preset amount of resistance while going downhill. It
    wouldn't wear out the rubber brake and it wouldn't require lifting the
    front wheels off of the ground. The same could might be usable for a
    skateboard or scooter.
  2. Ned Simmons

    Ned Simmons Guest

  3. PeterD

    PeterD Guest

    Consider the concept of conservation of energy. You have to do
    something with the energy that is gained by going down hill, right?
  4. RLM

    RLM Guest

    Dynamic or regenerative braking.

    Electric forklifts use the generated current of the drive motor to stop a
    coasting truck before reversing direction without use of the brakes. This
    is known as "plugging" the truck. Motor becomes a DC generator when

    Some manufacturers of electric trucks use this force to enhance the
    braking system to the point that brake shoes wear for extreem extended
    times between replacement of the shoes. The brake pedal reacts as if it
    has power brakes, when in reality this is an electrical adjustment to
    increase or decrease the amount of (regenerative) braking as the pedal is
    being pressed.
  5. John Larkin

    John Larkin Guest

    Eddy-current braking would be interesting, but I don't know if the
    speeds/forces would work out. It would involve an electromagnet or a
    permanent magnet in proximity to a spinning aluminum or copper disc or
    cup. But I suspect the braking force might be small, given the speeds
    and geometry.

  6. default

    default Guest

    Check out stepper motors. Spun around and they are alternators. Very
    cheap and easy when you just need an amp or two.

    I played with a few to make cheap alternators for some engines (where
    one needs 24 V DC and only has a rotating shaft)

    The downside is they "cog" (slight "friction or detent" from one pole
    to the next). A skate rider probably wouldn't notice it. Twist it by
    hand and you would definitely notice it - very hard to turn with a
    shorted winding.

    Apply a short to the wheels connected to the windings (via something
    like a mosfet) and you could make an electronic brake.

    They already use this technology in "bait casting" fishing reels.
    Apparently "bait casters" can't master, or can't bother to master the
    subtle thumb pressure on the reel as the line spins out when open reel
    casting. - so they apply a kind of magnetic brake - similar to a
    speedometer mechanism.

    Several magnets spin on the shaft and their close proximity to a
    aluminum ring tends to take the ring along in the same direction as
    the spinning shaft - the ring works against a spring and indicates the

    Take a sheet of aluminum or copper (or non-ferrous metal) and drop a
    magnet (strong magnet - nevoid) so it slides down the nearly 90
    degree sheet - it will slowly drop until it reaches the end of the
    sheet - then Newton takes over.

    Point is . . some spinning very strong magnets on the wheels, would
    make some decent brakes as the aluminum rings got closer to them.

    And you didn't think of it first, so don't try to patent the idea.
  7. John Doe

    John Doe Guest

    Controlling speed doesn't take much force, probably much less than the
    36 V DC motor I connected it to. When the motor speed control trigger
    was released and it did the electronic braking thing, it abruptly
    kicked my leg backwards. Not talking about hysteresis or eddy current
    braking, but in my estimation a small DC motor (think RC car) would be
    enough magnetic power.

    If it only requires the small part, doesn't require power, and is
    adjustable (either resistance or top speed) like at the following link
    provided by Ned Simmons, that would be great.

    Now I can find places to buy the things.

    Thanks to the replies.
  8. Guest

    What about a pony brake...
    Use a small water pump and restrict the flow...
  9. Carl McIver

    Carl McIver Guest

    The first thing that came to my mind is an eddy current clutch or brake,
    using permanent magnets. You have a few permanent magnets mounted to some
    reasonably sturdy plate (has to be able to resist the opposing torque) and
    you move it close to an aluminum or copper (or any conductive metal,
    depending on braking power needed) disc that you want to slow down. Keep in
    mind that the disc might get really hot, and could possibly come apart from
    the eddy current losses melting it down, so mind that for extended braking
    I asked a fellow who has worked with the North American Eagle on some
    design issues about a related question awhile back (poke around until you find a video of the
    brake bringing a full sized lathe to a full stop by eddy current braking--I
    couldn't find it when I went looking last couple times, though) with the
    question about how much axial force one had to push the magnets, which he
    didn't think was much, but couldn't quantify it.
    The faster the parts move, the more of a difference it makes, especially
    when there's a large speed differential between them. I'm not the
    physicist, so I couldn't offer much more on the topic, other than it's
    fairly easy to implement, but at low speeds the brake makes little
    difference, so a light duty final brake is required.
  10. John Doe

    John Doe Guest

    That's good.
    Yeah, I think a small non-powered device that increases resistance as
    RPM increases (but is adjustable) would be great, even if it requires
    heat sinking.

    I'm still looking for places to buy.
  11. Robbo

    Robbo Guest

    This is a little off what you asked for, but may be useful anyway. This guy
    makes brakes for skateboards / longboards:
  12. Guest

    Not sure what they call it but some smaller class B and K dieslel
    trucks use a magnetic braking system on the drive shaft to help
    control downhill accelleration.

  13. Robert Baer

    Robert Baer Guest

    1) You need to absorb energy (equivalent to friction) to do any braking,
    so reducing friction appears to be a poor choice.
    2) Adding a motor wheel and using it as a generator is a start in the
    right direction; use of a variable resistor across that generator to
    dissipate energy is an improvement.
  14. John Doe

    John Doe Guest

    Some trains do too, according to a History Channel (USA cable TV)
    show, and they dissipate lots of heat.

    I was thinking somehow I wouldn't need a heat sink since a DC motor
    requires force to turn even with the leads disconnected. I'm not
    disputing any of the expert opinions here, I know things aren't always
    simple as they seem.

  15. I like the eddy current idea best. Aluminum disk, strong little
    magnets; wonder if there isn't a location for a pendulum actuator on
    the boots that closes the gap *only* when coasting downhill... now that
    would be sweet! [My damn inlines are gathering dust simply because I
    can't control them tired of launching myself into
    someone's lawn or bushes just to avoid an impending intersection.

  16. John Doe

    John Doe Guest

    Thanks, I can grasp that idea, the force that generates heat is the
    force pushing back against the vehicle rolling down a hill.
    I'm sure you're correct, but I'm not sure what you're saying. I am
    sure that a heat sink would last a lot longer than a rubber friction
    brake, and the braking should produce little heat anyway.
    Not that this is what you mean, but for what it's worth.
    For this application at this time, just using a ready built device
    that can be rolled against the wheel is probably better then trying to
    turn a wheel into the device itself. And using a smaller scale device
    would probably produce smoother rolling (I suppose that has to do with
    the magnet configuration), and smooth rolling matters a whole lot.
    When standing on a skate/board, unnecessary jerking motions play
    havoc with my reflexes.

    Someone else with the same hobby might consider adding flashlight
    bulbs as a side benefit. Personally, I'd really rather just buy an
    eddy current brake, like the ones in the links, if I can find
    them for sale. Hopefully they aren't just OEM parts.
  17. If you look at the bottom of the page you will find a link to "sales
    Gerry :)}
    London, Canada
  18. nick hull

    nick hull Guest

    So they will probably send you a free sample if you ask ;)

    Free men own guns - www(dot)geocities(dot)com/CapitolHill/5357/
  19. John O'F

    John O'F Guest

    That depends what you mean by "little heat". Suppose you are skating
    down a 10% grade of length 50 ft (15.2 m). If you are going 10 mph
    (4.47 m/s), and you weigh 160 lb (73 kg), and your speed is held
    constant, you are generating about 320 W for 3.4 seconds.

    length of hill = 15.2 m, drop = 10% * 15.2 m = 1.52 m
    energy = 9.8 m/s^2 * 73 kg * 1.52 m = 1087 J.
    speed = 4.47 m/s, time = 15.2 m / 4.47 m/s = 3.4 s
    power = 1087 J / 3.4 s = 320 W.
  20. Leon Fisk

    Leon Fisk Guest

    Have you looked at what they use on "Mag-trainers" for
    bicycles. They use a magnet system and dial settings to
    increase resistance. I have one and it will take quite a bit
    of heat. They are made to spin pretty fast too. If you
    aren't familiar with what I'm talking about see this page
    for several different flavors: Trainers and Rollers

    Probably a bit big, but maybe a starting point. You might be
    able to pick up a used unit cheap to dissect...
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