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One item of everyday technology that passes entirely
unnoticed to all but the pathologically curious, is the
hydraulic door closer <link>. Unnoticed that is, until they
fail, dribbling hydraulic fluid down the door and causing
a
~100lb fire door to go crashing into the frame every time
someone
passes through it.
These items are simple, essentially a fairly
powerful spring that forces the door closed, a feature
essential in a fire door and a damper. With just a spring,
the system
would have a tendency to accelerate the door toward
the
frame, and amplify any force applied by a person,
resulting
in a loud, room-shaking crash*.
To prevent this, the door's motion is controlled by a
hydraulic damper. Similar to a car shock absorber, the
door's motion is linked to a piston that moves through a
hydraulic fluid. This system provides resistive force
proportional to velocity, it doesn't stop the spring holding
the door closed at all, it specifically resists when the
door
is moving quickly. This is great, you don't want a door
slammed in your face, you especially don't want a spring-
assisted 100lb steel door hitting you.
There is an alternative system used on car hatches,
sometimes hoods/bonnets <link>, and on lighter doors
such
as screen doors <link>. These provide damping by moving
air through a metered orifice and by making the shaft
proportionally large, they can make the unit operate as a
pneumatic spring also.
Ultimately they all fail because they rely on seals**. At
some point the oil/air escapes and you're flinching every
time you hear the door or propping your car open with
your
head. So, a solution.
Eddy-current dampers, also called magnetic dampers
<link> or eddy-current brakes <link> are used in various
applications including high-speed trains and roller
coasters
to provide braking. The effect is named "Lenz's law" and
the simplest demonstration of this is the dropping of a
magnet through a copper tube <link>. The tube doesn't
need to be made of a magnetic material, it just needs to
be electrically conductive. The moving magnet induces
an
electrical field in the conductor, this creates an opposing
electromagnetic field proportional to the velocity of the
magnet.
So, how do we put this on a door? Simple, no need to re
design everything, instead of a piston moving through oil,
we remove the oil and have the piston be a powerful
permanent magnet. The magnet will be close to, but not
touching the walls of the cylinder, so no wear. The seals
are no longer needed, in their place super long life linear
bearings or bushings. To modulate the damping effect,
different sized magnets can be specced at the factory,
changes in the leverage can be used for tuning (this is
already used) and modulating the diameter of the
cylinder
through the stroke.
*Have a guess what I'm hearing every ~20s?
**Sea lions are unwieldy, but otters are used in
lightweight
racing applications.
Hydraulic door closer/damper
https://www.homedep...er-608810/308265607
[bs0u0155, Mar 29 2021]
Pneumatic screen door closer/damper
https://www.amazon....-315239372073&psc=1
[bs0u0155, Mar 29 2021]
Gas spring damper
https://www.explain...com/gassprings.html
[bs0u0155, Mar 29 2021]
Eddy-current brake
https://en.wikipedi.../Eddy_current_brake
[bs0u0155, Mar 29 2021]
Lenz's Law magnet/tube demonstration
https://www.youtube...watch?v=N7tIi71-AjA
[bs0u0155, Mar 29 2021]
Eddies in the space-time continuum.
https://hitchhikers...m/image/69807383581
Eddie's sofa. [whatrock, Mar 29 2021, last modified Mar 30 2021]
Lifting hinge
https://www.hingeou...lygAXxoC0ecQAvD_BwE
[bs0u0155, Apr 05 2021]
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//Have a guess what I'm hearing every ~20s?// |
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A small girl playing Christmas carols on a tuba? |
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Eddie's indoors now, is he? Is he also still in the space-
time continuum? |
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(+) If I may make one suggestion? Design a mechanism which slightly retracts the cylinder in half along its length when the interior bar is pushed or the motion sensor sets it to open which then retracts when reaching the end of the doors swing.
This would allow the door to open easily while not letting it smack you in the face when closing. |
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Hmm, I suppose I do need much more significant damping in
the closing direction rather than the opening. I think the way
to do it, is to have the magnet slide with a relatively modest
spring on the opening side. |
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Also, after rummaging around in my big box of nostalgia, I've
found an RC car, a Tamiya Grasshopper. Now, the rear "shock
absorbers" are just springs with a plastic tube imitating what
would be a hydraulic damper in the full-sized car. Looking at it,
it's going to be trivial to replace that plastic tube with metal,
and glue in a neodymium magnet on top of the shaft. I think I
might have the world's first magnetically damped RC car on my
hands here. |
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To achieve dampening in only one direction, the tube could
have a gap extending the length of the tube. When the
magnet moves in one direction the tube is compressed from
the sides so the gap is closed (thus eddy currents can form);
when the magnet moves in the other direction the tube is no
longer compressed from the sides and the gap opens (thus
eddy currents cannot form). A switch at each end of the
tube
mechanically activates/deactivates the tube compression. |
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Instead of compressing the tube, the gap could be filled with
a member that can switch between conducting and non-
conducting. For example, each side of the gap could have an
alternating series of conductors and non-conductors. The
gap-filling device would also have a series of alternating
conductors and non-conductors. When the gap-filling device
is pulled, it aligns its conductors with the gap conductors,
thus allowing the current to flow around the tube. |
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Perhaps a slightly conical shaped exterior casing instead of
cylindrical? It would progressively slow down when closing but
progressively get easier as you open. |
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Could have a kind of ratchet system. When you open the door, the ratchet would slip making the door as easy to open as you like. As soon as the door started closing, the ratchet would bite and the impeder would start to resist the closing force. |
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//the ratchet would slip making the door as easy to open as
you like.// |
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The problem is, with slip, you don't move the magnet out for
it then to do the damping on the return, you need a way to
get the magnet out to full extension at the start of the
closing. BUT you don't need THE magnet, you could do it
with recirculating ball magnets and then the ratchet system
would be fine. It would look great, but it's not as elegant as
the two spring sliding magnet solution. |
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C'mon - seals are good. It's just that they carry on so when
they get a flipper caught in the door. |
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The ratchet could drive a brake disk from a rack and pinion.
Geared, the speed would be higher, allowing braking force
with slower door movement. In rotary motion, your magnets
don't get lost somewhere. |
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You could use a directional planetary gearbox from the rack-
n-pinion (or at the hinge); one gear in one direction, a
different gear in the other (ie. the input is 2 different
directions; the output is 2 different SPEEDS in the SAME
direction). Magnets spin in their "shell" with eddy currents
doing their magic. |
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Does the eddy field need a closed loop around the magnet? In other words would a magnetic falling through a copper C rail still work? If the field eddies are local wall patches around the magnet, a C track would be very easy to implement. |
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//need a closed loop around the magnet?// |
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No, roller coaster brakes are often just a plate on the car that
runs into a slot with the magnets on either side. The effect
depends on the strength of the magnets and how close they
are. |
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So the door could have a slightly curved rod magnet on an arm and the wall could have the flanged reciprocal curved copper c channel . Easy. |
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Angling the frame slightly and using the door weight would take way any springs or mechanisms but may need slightly upgraded hinges. |
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Then again, nothing is simple. The complexity would be in in the angles and setup alignment. |
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//Angling the frame slightly and using the door weight would take way any springs or mechanisms but may need slightly upgraded hinges. // |
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There's already a type of hinge that lifts the door by rotating on a helix-type arrangement <link> |
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As the piston/magnet moves through the coil, this device
should put the current generated through an LED strip
running around the door frame. So, as the door closes,
there would be a brief, satisfying pulse of light around
the door. If you slam the door really hard, the pulse will
be shorter but brighter. |
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True, that pitch cut on the hinge will make the door slightly harder to open but hopefully the door will slip back down the pitch and close by itself. |
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No matter what happens, to have the door close, the energy has to come from the traverser*. |
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