#  Toroidal Membrane 

 



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####  Membrane Criteria

Sort       ![Three toroidal membranes](/sites/g/files/omnuum4601/files/styles/hwp_1_1__960x960_scale/public/srtsandbox/files/threemembranes.png?itok=xUX_oZBq) 

 

(Left) Ecoflex 00-30 membrane, (Center) LDPE membrane, (Right) vinyl membrane 

 



 The toroidal membrane is the primary structure and source of friction of the drive system. This component was determined first because the design of both the brackets and contractile rings are affected by the properties of the chosen membrane. Four qualities were considered during membrane selection: rolling resistance, heat resistance, ease of manufacture, and ease of adhesion. Qualities were rated on a scale of 0-3 where 0 is unusable and 3 is exceptional. Three toroidal membrane materials were evaluated: Ecoflex 00-30, low density polyethylene (LDPE), and vinyl. The Ecoflex and LDPE membranes were fabricated in-house and the vinyl FFTs were purchased online under the product name “Water Wigglies”.

####  **Membrane** **Evaluation**

Sort      Ecoflex 00-30

   LDPE

   Vinyl (Water Wigglies)

       Heat Resistance

 

  3

 

  0

 

  2

 

    Roll Resistance

 

  1

 

  3

 

  2

 

    Ease of Manufacture

 

  1

 

  2

 

  3

 

    Ease of Adhesion

 

  1-3

 

  0

 

  3

 

    **Total**

 

  **6**

 

  **5**

 

  **10**

 

 



 **Ecoflex 00-30**: The Ecoflex toroidal membrane was fabricated by inverting a long thin-walled tube onto itself. While this membrane had superior heat resistance, it proved difficult to manufacture and invert. Furthermore, Ecoflex bonds well to itself and other silicon rubbers, but is difficult to bond to other materials. This would have resulted in additional design constraints on the brackets.

 **LDPE**: The LDPE toroidal membrane was fabricated using thermally bonded film. This membrane had the lowest rolling friction of the materials tested; however, the SMA actuators melted through it almost instantly. This fact alone was enough to invalidate LDPE as a viable membrane material.

 **Vinyl**: Vinyl FFTs are commercially available on Amazon.com and are reasonably invertible toroidal membranes. Like LDPE membranes, vinyl can be melted by the SMA actuators. However, these actuators melted in minutes rather than seconds. This was deemed acceptable for prototyping purposes. Rolling friction was significantly reduced by lubricating the FFT with dishwashing soap.

####  Future Work

 Vinyl FFTs are the best of the three membranes evaluated; however, the melting problem proved significantly more detrimental than anticipated. Future designs will focus on the development and implementation of non-thermoplastic membranes. Currently, the viability of the Ecoflex membrane is being reinvestigated. The low scores in “ease of manufacturing” and “rolling friction” are potentially a result of the manufacturing process used to fabricate the membrane and not the material properties of the rubber.