# Viton O ring, 30mm inner diameter, 2mm cross section

## Details
*   **Material:** Viton or Nitrile
*   **Inner diameter:** 30mm
*   **Cross-section:** 2mm
*   **Outer diameter:** 34mm
*   **Supplier website:** [http://www.simplybearings.co.uk/]

The O-rings function as springs, to provide a return force on the nut that is pulled upwards by the screw.  I use Viton O-rings, 30mm ID, as they provide about the right force.  You can substitute for rubber bands, but it's difficult to specify the size directly, and they tend to perish and snap in a few weeks.  Viton O rings are well worth the effort and price!

## Alternatives
If you're unable to get hold of Viton O rings of the correct size, substituting for Nitrile or EPDM is probably also fine - though most of our testing has been done with Viton.  

Rubber bands from a stationary store can work, though in our experience they usually perish after a few weeks.  Unfortunately, that happens whether or not they are used.  It's also usually quite hard to get rubber bands exactly the right size.

It's possible to print elastic bands using TPU filament, as described in Bill Schaller's contribution.  Print this part using .1mm layer height, with randomized perimeter start and end positions if possible. For strength, the part should be printed entirely with perimeters, no gap fill. The part is designed to be printed with a 0.4mm nozzle. Printing with a 1mm nozzle hasn't been tested, and isn't recommended.  The part for this is ``actuator_tension_band.stl`` and should be included in the automated build system.

It's also possible to improvise with bungee cord as described by @MisterBurnley in his [build report](https://gitlab.com/openflexure/gallery-wiki/-/issues/5#note_348619271).


