When I built the Wade-ScribbleJ filament tensioner, I used four of the stiffest springs available in the Little Box o’ Small Springs. They came without a pedigree, of course, and worked quite well. However, the filament would occasionally stop feeding, usually after an intense series of reversals, and it seemed more pressure on the filament was in order.
The 1.5 inch 4-40 screws limit the available length to no more than 12 mm and the tensioner must have at least 1 mm of free travel to accommodate filament thickness variations. Those springs had fairly dense coils and they were pretty much fully compressed.
They turned out to compress 9 mm with 2.5 pounds applied, for a spring constant of 1.2 N/mm or, for we metric-challenged Yanks, 7 lb/in. Some rummaging turned up my Brownell’s No. 71 Compression Gun Spring assortment and I found a spring that compressed 5 mm with 5 lb applied: 4.4 N/mm or 25 lb/in.
I know you’d love pix of that process, but I was already one hand shy of having enough to push the spring scale against the [4-40 screw + washer + spring + washer] over a metric ruler, then apply enough force to compress the spring while reading the distance between the washers. Use your imagination, OK?
I sliced four 4.5 turn lengths from that spring with a Dremel cutoff wheel, cleaned up the ends a bit to get them all to about 13 mm, reassembled the tensioner, and cranked the screws to compress the springs down to 8 mm. The quartet now apply something like 25 lb = 110 N to the idler bearing. That’s about four times what it was before, so that filament should have no reason to slip, even under cough extreme duress.
Tomorrow: Applying some extreme duress…
The Smell of Molten Projects in the Morning 
You can get the tensioner so tight that the filament will squish like a toothpaste tube–you do not want to go that far :)
It is important to have some decent length and pre-travel to the springs. The filament will deform as it reverses, letting the idler get closer to the driver by a small amount. If that small travel reduces the force, the you will have trouble.
Time to invest some big bucks in a trip to the local hardware store for longer springs and longer bolts?
:)
That’s what I didn’t like about the previous springs: far too much compression and not enough headroom. These have much more oomph and more travel, but don’t crush the filament.
The last time we moved, the estimator eyeballed my shop and said “Damn. This is going to be like moving Home Depot…” [grin]