Air Tight Fasteners

A project log for Neucuff: A soft orthotic exoskeleton

Neucuff is a simple, powerful, safe exoskeleton for giving Cerebral Palsy patients control over their arms.

Matthew BorgattiMatthew Borgatti 08/16/2015 at 19:530 Comments

You can't get very far in the world of pneumatics without an air tight fastener on hand. One of the persistent irritating features of soft robotics is how difficult it is to fasten an air supply to a robot without some substantial engineering beforehand. For low pressures, cinching up a silicone tube around a harder pvc one with a zip tie can work, but it won't hold for much over 10psi.

I've found that the best solution is to integrate a feature like a sleeve or flange into your silicone bladder and then engineer a fastener to fit it. My first two solutions (on the right of the above image) used a straight sleeve projecting from the silicone. You can see a version of the assembled bladder below. This failed at pressures past 15psi. The problem was the silicone around the fastener was stretching perpendicular to the bottom flange of the fastener, slowly pulling the sleeve out of its seat in the fastener.

The reengineered solution (below) employs a flange cast into the silicone bladder itself. The flange is fed into the top portion of the fastener with a light coating of silicone caulk. Then, the bottom portion of the fastener is screwed on. This fastener has a slightly wider diameter than the silicone inlet, which effectively clamps the flange in place and prevents it from crawling out of its seat as the pressure increases. So far the new fastener design has held up to 30psi, though it could likely hold more. I haven't burst one with this style fastener yet, and I don't want to take it too much higher without a protective container just in case.

The new fastener also features an external lip for fastening to the exterior restraint layer. This is a trick used in space suits for attaching air hoses and valves. The lip allows the fastener to be seated in a specific position with a few loops of thread before the rest of the stitched elements are assembled. It helps keep everything secure while reducing the finesse it takes to get everything stitched.

You can download the latest fastener design on Thingiverse.