Friction Hinge Mechanism

(This mechanism is used in the following designs: https://www.printables.com/model/658410-folding-pocket-phone-stand )

/**** Update: I've found that a square shaft works better.  You can use an 8mm diagonal square shaft, and print it separate so that it is lengthwise on the bed, and then have a socket on both caps.  This seems to work a lot more consistently, as there's no layer lines to catch on.

****/

This small hinge mechanism provides resistance to motion not a function of angle or hinge diameter.   The force can be changed by either increaseing the length of the hinge's internal mechanism (and thus overall length) or adjusting the tolerance of the sliding shaft internal to the hinge (explained below).

The mechanism consists of a threaded barrel, 2 threaded caps, one of which contains a hexagonal shaft, and an externally threaded slider which will slide up and down the shaft when a rotational force is applied to the external barrel.  The hinge's resistance to move ultimately comes from the friction of the slider moving a short distance across the shaft. 

The shaft size offset used in my phone stand design was 0.15mm per face.  This, along with a a 14mm slider length provided enough force to hold a phone while being easy to move by hand.   Note, it is important to provide enough size difference between the external barrel and the slider so that a full rotation won't jam the slider into the cap.

As seen below, the threads that work pretty well are ANSI Metric M profile.  This is not strictly necessary, and you can provide your own so long as the threads per inch is accounted for with the internal sliding space as just discussed (one rotation will cause a certain displacement of the slider, and the external barrel needs to be at least that much taller than the slider).  The threads in were then offset by 0.1mm on the top, bottom, and side on both the external and internal threads to allow for easy screwing.

Some mechanism is required to prevent the separation of the caps, or else the action of the screw will force the caps apart.  In this example, the parts in which the hinge is embedded provides that force.  Squeezing the caps inward is not necessary.  All that is required is the prevention fo the screw's sliding pushing the caps outward via the motion on the shaft.

The overall size of this example is 30mm long by 16mm diameter.  As stated above, this is suffecient force to hold up a phone (the photos below show an iPhone 13 mini in a case being supported only by the hinge friction).  Please adjust for filament slickness as well when determining length and shaft size offset.

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This design requires no supports, 3d prints well, and is, quite frankly, one of my favorite designs.  If you use this in your design, i'd appreciate a shout out if you learned about it here.  I don't think mechanical mechanisms like this count (or should count) as IP, and I'm certain that I'm not the first to come up with something like this, but still, it was fun to develop.