To those of you who use Spectra line on their bots (i'm looking at

To those of you who use Spectra line on their bots (i’m looking at you, @Tim_Rastall ) - what strength (or diameter) are you using and are the other braided lines, for example Dyneema or Shimano, suitable for our application?

I saw printrbot is using 200lbs hollow stranded line. Forgot the brand.

Firstly Dynema is often branded as spectra but they are both Ultra-High Molecular Weight Polyethylene.
I’m having trouble finding a web link to the specific stuff I’m using but 120lb dynema (from the local tackle shop) is the stuff I’ve had most success with - I’ve tried 80lb spectra from aliexpress but it had too much stretch and fraid quite easilly. the 120Lb stuff is about 1mm wide once compressed on a spool/shaft. It has very little stretch but it helps to pre-stretch it by hanging a heavy weight off it for a day or some before use.
Not had any experience with Shimano but it all boils down to the flexibility of the filament, breaking stress and stretch ratio.
My learnings show that you really can’t have it too tight but that over tightening can damage your mechanics, particularly if you have alignment problems or you don’t support your shafts/spools at both ends (see printrbot simple).
As per the discussion I had with @Whosa_whatsis last week, I’ve got the stuff wrapped 10 times around a 10mm shaft with no means of securing it beyond friction and it just doesn’t move. I’m sure you’re familiar with the capstan equation but it’s worth remembering that for a given coefficient of friction, the holding force deccreases exponentially with increased turns. So, even though UHMWPE has a very low cof of 0.05-0.08, 10 rotations gives (a very pessimistic) holding force reduction of about 23 times the load force.
If I had time to experiment, I’d look for Dynema/nylon weaves as the cof for nylon is so much higher and Dynema is probably stronger than it needs to be. These weaves are used in climbing slings but I’ve never seen ones that are thin enough. Food for thought if you ever wanted to build a really big machine for printing concrete or whatever.

I used 100 lb Spectra from e-bay…there was a LOT of initial stretch, but not so much that I couldn’t take up the slack with a ziptie.

I didn’t realize that you guys were using UHMW. That’s not very well-suited to applications where you want high friction, as you mentioned. Nylon would be better in that respect. Has anyone considered trying Kevlar?

Also, if you want dynema, go to your local kite store.
All good kite lines are made from dynema in various force ratios and thickness

@Whosa_whatsis yeah, hence the interest in composite nylon weaves. Exponential decreases in holding force per turn make it less of an issue as long as you have space available for the wraps. Kevlar might be worth a look, good thinking, off to Aliexpress I go…

Huh. 250lb and 1mm thick. Not sure I need 300m though.

Doing some more research, i’ve come to believe that the main reason why UHMW fiber works so well is its high Young’s modulus (stiffness) of 89-99GPa (Dyneema datasheet), which is at least comparable to the 70-90GPa of the glass fiber strands that typically support toothed belts (of course, one needs to factor in the effective diameter of the fibers). Compare that to the 2.9GPa of Nylon, which does not improve its properties much by being spun into a fiber. Kevlar actually has quite similar properties to UHMW, which is why bulletproof vests and such are starting to use UHMW as a direct replacement for Aramid. It is available in two qualities, one with a Young’s modulus of 59GPa, the other with 127GPa.
Fishing is a huge thing here in lower Bavaria, so getting hold of a specific fishing line shouldn’t be a problem.
Thanks for the input, i’m off to pick up some line!

You’d be surprised…Denver, Colorado has a pretty strong soprtsman’s community, too…Couldn’t find Spectra locally to save my life (and it, too, was in lifetime quantity spools)…I had to resort to ebay.

@Mike_Miller that’s because Colorado is land locked, and you typically don’t need line that’s 60lb+. All you have is piddly little streams and small lakes. Much more likely to find the stuff locally in a place that’s near the ocean.

You’ll get no argument from me there. In Colorado, ALL seafood is a mystery.