After a minor panic I’ve succeeded in moving substrate up to a height of 180 cm. The last two days have been drawing mechanisms, cutting machine parts, google’ing and wikipedia’ing like crazy, walking in the hills pondering what I know and returning to the workshop to try out ideas. A lot of effort and imagination for an unremarkable (but effective) solution.
Several months ago I built a plunger device from a metal tube, re-purposed inner tube, and a few bits of wood. It didnt work because the substrate was to thick from its high sand content – basically the internal friction was too high and nothing moved although on the drawing board it looked fine.
Today – after trying several alternative pump ideas (I wont list the numerous alternative designs I’ve been considering since this design’s simplicity would put that entire collection to shame!) in lego, wood, and sketch – I returned to the original device and changed it a bit. It works.
The main differences between the first device and the current one are:
- no starch content – this inevitably means re-testing with real substrate.
- very low sand content – perhaps 10%
- the handle of the plunger is the exit tube
- high water content – perhaps 30%
- mixing the components vigorously before pumping
- hacking together a one way valve with a metal washer several sizes smaller than the plunger handle
I think the best way to imagine the substrate moving from the source to the extrusion point is through a concertina tube.
Prior to this I’ve been thinking of using a standard smooth-surfaced tube, but I think that it will cause problems in the early stages when 2m worth of substrate needs to be pumped around a spiral at the base of the tower to make the tower rings.
A concertina tube has the benefit of being extendable, so its length need not mirror the height of the tower until it is fully extended in the final stage of growth.
The problem with a concertina tube is the massive increase in surface area, and therefore s potentially massive jump up in the friction experienced by a viscous liquid being pumped inside the tube.
To avoid this potential problem of friction, it might be good to return to the idea of telescoping tubes to move the substrate upwards – the spine support already uses this technique. Using segments who’s bases are narrower than their tops, the substrate can move up easily.