ReChain Frame System

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A method to make large stiff and strong structures from many small segment parts by using a combination of shape locking and the principle known from reinforcement of concrete. The number of elastic energy clip locks is minimized as much as possible.

ReChain frame systems (Re for reinforcement) are a proposed class of frame systems for 3D-Printers, Robots and Structures of all kind and size.

  • A ReChain system should not be confused with just yet another construction kit.
  • A ReChain system is fundamentally different from anything in existence as of date (2017-02..). The prime distinguishing design choice is that friction must not be used to hold things together via frictional self locking. This is the core principle. Beside this one there are several more. E.g. No vitamins including magnets and no naive use of clips.

The distinguishing features

NO FRICTION

Meant is of course the usage and not the unavoidable presence of friction.
As said in the introduction these systems do not allow the use of (unreliable) friction.

As consequence the usage of slightly over-sized pegs in tight holes like found in countless construction systems is strictly banned. So is the conventional use of screws and nuts, wedges, nails, ....

The present friction must not even be used accidentally that is:

  • If the present friction is too high to be ignored it must be overpowered.

On the other extreme:

  • If the present friction is near zero and/or there is a lot of vibration in the system (like e.g. at the nanoscale) conventional fastening systems would come loose (gradually or speedy) and finally fail. In such slippery rattly environments the RepRec frame system works without any changes (except some other nanoscale specific changes — an elaboration on that later).

Pervasive Shape Locking

The basic topology of a ReChain system (what links to what) is preserved by the way the parts are made to interlock. The tolerances should be very generous for "fall in assembly". Shape locking alone does not give the formed structures stiffness the connections are ridiculously sloppy almost like a piece of chainmail cloth. The later-on applied tension does gives the stiffness.

In some cases LIFO shape locking (only the piece added last can be removed) might be useful.
Example: Shape Lock Chain II

Pervasive Tension

The system is held together by (is reinforced by) tension alone.
This is not tensegrity: Most instances of the system won't fall under the class of tensegrity but some may. Although tension is of paramount importance "tensegrity" is not at all what these systems primarily aim at.

Self centering surface contacts

Where it is necessary contacting surfaces are self-centering.

When tension is applied the self centering shape of the parts/components (e.g. the hull segments) lead the whole structure to fall one single precisely (pre)defined arrangement.

As an example for a bad design:
Start with a tensioned rod a stack of bad flat-contact-surface-hull-segments with the tensioning chain running through. Apply relative torsional force to one or more of those hull segments at the center of the tensioned rod. As soon as there is enough torsional force to overpower the friction between the hull segments the hull segments slip and rotate a bit since (as mentioned before) there is generous clearance to the chain running through inside. If something is mounted to that bad hull segment it will become misaligned (well at least it can't come off).

If one looks carefully one sees that this also is a form of friction-locking but here loosening cannot cause disassembly (there's only a bounded freedom) and in cases where it also cannot cause system degradation (that is if it's certain that nothing will ever be mounted to those bad hull segments) this is acceptable. (murphy waits). One good example where misalignments due to the lack of self centering are usually irrelevant and acceptable is the tensioning chain running inside the hull segments.

Conical/pyramidal egg crate like contact surfaces work fine. This naturally leads to the truncated octahedron space-fill as standard surface structure. Given full self centering no matter where functional elements (like diverse machine elements) are mounted on the Frame system they are locked onto defined reference points.

normal acting locking clips

Tension is held by clips (reliable energy barriers)

Often (but not always) clips are used in a way where they carry 100% of the load (or a good fraction thereof) coaxially (or near coaxially) to the direction the clip is operated. We'll refer to these as "naive clips". In a ReChain system naive clips are banned (except temporarily for robotic assembly).

Instead permanent load on the clips in their bending direction is avoided. Crossing forces at right angles (or near right angles) allows high to infinite mechanical advantage for locking tension.

  • The clips are used in such a way that they do not stay in a permanent high level bending strain which potentially could permanently deform 3D-printed clips (especially if out of certain plastic materials and under slightly elevated temperature)

In general clips can fall in one of three classes:

  • class1: They can be pulled open again if enough force is applied
  • class3: They cannot be pulled open again by just pulling back AND they have play.
  • class2: The small sliver in-between. (hard to hit thus unreliable)

Non-naive clips do not open when excessively loaded even when the clips are of class1. Other special purpose elements (like intended breakage points) may provide defined overload safety.

For use at the nanoscale clips should furthermore be operable without making clipping sounds and should have only one stable position (more about that in a following dedicated section).

Clip aggregation

Instead of using many small clips (one clip per part connection in the extreme) just one bigger clip is used to lock many parts together. This reduces the number of potential points of failure. This is beneficial for 3D-Printer-Frames but even more so for nanoscale systems where thermal vibrations can and may knock open small clips.

One might consider threading tension through the nodes of spaceframes/spacetrusses to unify even more clips together to one big one but:

  • this seems pretty difficult. Big nodes housing complicated mechanics seesaw differentials ...
  • this makes assembly and disassembly harder (especially if done robotically)
  • in the nanoscale clip-reliability grows exponentially with size so it's probably not worth the trouble.

Tensioned chains locked by their shape against disassembly bridge the gap between the spaced clips.

Robotic assemblability

This is about designing the system in such a way that it can be assembled by one single compact pick and place robot. See the RepRec Pick and Place Robot super-project for details. This design choice is not a necessity but highly desirable. Archiving this seems quite challenging though.

Design decisions for assembly with robotic grippers can be quite different from design decisions for operation with human hands (especially in regards to spanning mechanisms).

Usage at the nanoscale

This section is only relevant for nanoscale systems.

As mentioned in this articles introduction a ReChain Frame System is meant to be usable in a wide range of sizes. And with wide what is meant is sizes ranging from DIY tinkering projects all the way down to the nanoscale level.

Conventional construction systems that heavily rely on friction for self holding would not work at the nanoscale (or at least would not be very strong). This is because:

  • there is near zero friction and
  • there is lots of thermal vibration knocking everything loose

With a ReChain system (for robotic assembly) though only slight changes are necessary to adapt for:

  • the lower material stiffness at the nanoscale
  • the atomistic material discreteness.
  • the omnidirectional Van-der-Waals-force (making assembly easier compared to unidirectional gravity)

Also a ReChain frame system intended for the nanoscale may benefit from two more design choices for improving energy efficiency and improving reliability Those are respectively Non-clipping locking-clips and monostable locking-clips.

Non-clipping locking-clips

Special non-clipping design of clips allow it to prevent the emission of sound waves that carry away the energy that is stored temporarily in the clips elasticity (spring). The energy in the clips can be recuperated. This allows to recompose the system without expending too much energy (more energy efficient) and thus faster with the same amount of cooling. Less waste heat faster operation. "nonclipping-clips"

Monostable locking-clips

If by accident a thermal vibration knocks open a clip it does not stay open for long. So open clips can not accumulate. If several adjacent open clips in a structure are necessary to break it (another desirable property for nanoscale ReChain Systems) this strategy can reduce the likelihood of failure multiplicatively.

Details on the chain reinforcement method

So how to replace giant monolithic structural rods with rods out of mutually interlocked medium sized parts?

Stiffness in a structural framework can be archived through reinforcement very similar to the pre-tensioned rebars in concrete. For ReChain systems this works by pulling on chains whose segments are connected by shape locking alone and are spanned within the tubes that go through a big number of stacked short segments of profile struts. It can be a single chain or more for a bigger profile. At one end the start of the chain-cores are widened such that they are prevented from sliding into the the starting profile-segment. At the other end the last chain-core-segments are pulled out of the last profile-segment by relatively large 3D printed screws (in the size scale of the chain). There are also other methods for tensioning and it can also be done at the struts center. The fully assembled multi-part struts can be used to build up an even more rigid trusswork-frame.

Alternatively to tension the struts right away, If the strut is integrated in a trusswork and runs into a hinge where at least three multi-part-(profile)-struts meet the (two) incoming chain-cores can be merged (one) outgoing tensioning core via a trivial mechanical differentials such as simple seesaws (this may not work but it's not a necessity). This auto-equilibrates the tension and reduces the number of points to be finally tensioned even further. Seen topologically one could call this "tension trees" that are built up starting from the leaves moving to the trunk.

The points of where the frame structure is finally tensioned can include a 3D printed worm-gear to get ultra compact high mechanical gain and finally a 3D printed ratcheting spring locks the worm gear in place since we do not want to rely on self holding friction.

Elements that take tensional loads instead of compressive loads like chains to tension a whole trusswork frame are easier to create.

For RepRec Pick & Place Robots

In RepRecs there may be drive chains that are also made from small printed pieces. For a basic RepRec in parallel SCARA design at least seven drive-chains are needed (3-translate)+(3-rotate)+(1-gripper). Note: for a pick and place robot speed and accuracy does not need to be as high as for 3D-printers so a printed chain should work ok.

One method to tension a drive chain wrapped around two sprockets is to tension a rhombus out of four reinforced multi part struts. This way the pull-tensioned profile-struts push-tension the drive chain.

Example parts

  • A shape locking chain that does not fall apart in free space (tension element) Thingiverse Shapelockchain I There is a version better suitable as drive-chain in the works
  • A reinforcement rod demo is in the works.
  • A trusswork node-point is in the works.