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sconisbee
01-26-2008, 01:23 PM
How flexible is acetal? say in an application where a disc of plastic was used between two shafts that can be out of alignment by up to 1.5* typically for this kind of thing polyurethane is used but would acetal do? or would it fatigue? (this isnt for a specific part currently, im just trying to understand plastics a bit more)

Peter Neill
01-26-2008, 01:37 PM
Acetal is pound-for-pound just about the stiffest unfilled polymer you can get. This is what makes it fantastic material for springs in the right section, or semi-structural mouldings in compression with another section.
And cross-sectional shape is the key to it. In a shape something like an oldham coupling to align off-centre shafts it would be a poor choice IMO, as the shape would prevent flex. Polyurethane works well here because it is very flexible but extremely tough and abrasion resistant.

Acetal has a TG of -60C, which means it does not really become rigid in small, thin, sections until it passes below this point, hence it has become the first choice material for items like bindings and buckles on ski boots.

Peter

winchman
01-26-2008, 02:59 PM
Lexan or (another polycarbonate) would probably be a better choice. I've been using 1/8" Lexan as the flexible "pedal" in a footswitches for years with no problems at all. One narrow end of a 2" by 4" piece is firmly attached, and the other end moves about 1" each time the switch is depressed.

Roger

Evan
01-26-2008, 03:48 PM
I use an acetal compression spring to preload the Z axis leadscrew on my mill. It's a short, fat disc bored out in the center and partially slotted several times (like fins) on the outside and the inside so the inside and outside slotting overlaps but doesn't join. I needed a compact, short travel high rate spring and it works perfectly.

sconisbee
01-26-2008, 03:58 PM
Thankyou Peter thats just the information i was looking for, the application im thinking of is very very similar to an oldham style connector. So once i finalise designs ill look at aquiring some HDPE barstock. Evan that spring sounds interesing and brings up further possibilitys for acetal in some of my other designs but for this one wouldnt have enough torsional strength. I'm not intentionally being mysterious:rolleyes: just getting a few things sorted before i start elaborating on some of my work.

thankyou all!

Edit: On a related note. I just stumbled upon one of the plastic tech specs sites I had lost its here, covers quite a few of the common plastics
http://www.plastim.co.uk/dati_tecnici.htm