Pseudo Tensegrity End Table

Held stable with only tension straps.

Most tensegrity projects have a rigidly connected post up from the base and another that projects down from the top.  They are connected by a simple tension cable. An unstable connection.  To stabilize this usually 4 more cables run from top to bottom at the outer corners. The end result is barely stable.  An unexpected side push will usually collapse such a table. 

My project starts with a rigid post that is not attached structurally to either the top or the bottom.  An unstable assembly. Both ends are rounded over and sit in a slippery socket at the top and bottom centers. The center post is threaded.  Right hand threads on one end and left hand threads on the other.   

Bottom and top of the center post rounded over.

Screw holes patched with walnut plugs.

Holes repaired

The ball ends of the post rest in sockets cut in HDPE that inset into the bottom of the table top and top of the base. 

Socket test.

A good fit.  Post spins easily in the slippery socket.

Hub nuts screw onto the post. They have slots around their edge. Webbing straps will slip into the slots and be pinned in place with a 5mm pin though a grommet in their end. 

With both RH and LH threaded hub nuts, a twist of the post will tighten or loosen the straps. I hook the straps to the underside of the top and the hubs in a slot with a pin dropped in a hole or slot.  The pin passes through a grommet in the ends of the straps.  The straps end in a slot perpendicular to the pin. 5mm x 5/8" steel pins.

Strap connection to hub nut.

To cut threads in the hubs I've used a side cutting V bit. Shelf pins pin the strap end in place.  I made a friction fit hole for the shelf pins so they fit snuggly and will not drop out easily.  It took some test holes to find the best size of drill bit to use.  A #9 drill bit provided the closest fit. 

The top has 4 perimeter slots for straps, and an inset HDPE section with a ball socket cut in the middle.  Socket test seen above.   

Underside of the table top.

The key to this project is how the straps are connected to make the project work. 

The non-threaded section of the post is where you grasp the post to twist it.  It might be easier to grasp and twist if it's shape was a hexagon or octagon cross section.  

For detail continuity the base is a scaled down version of the top. The contoured surface is on its top rather than its bottom. 

Base

I made both top and bottom inserts from some 1" thick HDPE I have, re-sawn to just under 3/4" thick, then milled down to 5/8" thick on my CNC. 

Socket inserts for the top and base.

A  test assembly.  Straps were installed with the nut lower on the post.  Once attached the post was twisted to lift the hub nut higher to tension the straps. 

Repeated for the top.

Repeat the straps and hub on the other end to hold the table top securely horizontal.

Making straps for the design I've realize matching strap sets is a hit or miss problem. It is easy to make two or more the same length.  As those in the long axis aren't the same length as those in the short direction, and all have to be a precise length to tighten up at the same time. Finding the length of a set to go with the other set is the challenge.  Making and installing the set for the long direction would be easier if they could be adjusted in length after installing.  Change them from being a fixed length with a grommet in both ends to an adjustable length with a grommet in one end. The other end wrapping around something to come back and attach to itself.  The attachment point should be adjustable to change the total length of the strap.  Something I'll look into should I use straps in future projects. 

The wood parts of this table need some final sanding and a finish applied.  I like tung oil on walnut.  This table does provide an interesting glance down at the base while in use.  More interesting than any typical table. It was an enjoyable exercise to design and build. 

Straps in tension make a stable assembly.

4D