"Natural" acetals (which is how plastic manufacturers refer to Plain-jane grades) are indeed prone to UV problems, in similar degree to natural nylons.
SnowP asks about using a metal bush to get around the swelling problem: this certainly works, but given that swelling is typically only a problem with nylon, it's easier just to avoid using it, given that it has no unique virtues in the role, and there are plenty of viable alternatives.
As Evans says, opacifiers help acetal to survive (usually carbon black, but sometimes titanium dioxide, which is white) and there are also special grades like Celcon UV90Z which are virtually immune to UV.
Reinforced acetal is
made, typically 20% glass-filled (eg Delrin 570), but it's ridiculously expensive, especially in the UV-resistant grades, and hard to source, at least here down under.
SnowP: when you talk of sheaves splitting, another possibility to consider (although once again, be prepared for price
shock) would be a high-pressure laminate, something along the lines of Micarta.
I have used this in the epoxy/glass grades with considerable success for sheaves which get a particularly hard time, eg for wire) - it uses the concept
of Tufnol, but it's a bit less Victorian.
The G10 grade has become well known because it is used for high performance printed circuit boards. And I think people who make custom knives have taken to using it for handles. FR4 is a similar beast but has flame retardant properties which are not normally required for sheaves !
The best laminate layup
for sheaves is rod or tube form. This seems to run contrary to the usual advice for making gears from phenolic, where the conventional wisdom is to use the sheet form to avoid delamination
, but the reason I say this is that the tensile stresses in a gear
are at right angles to those in a sheave, and you want the tensile stresses to align with the predominant direction of the fibres.
Other grades of Micarta use different resins, including polyester, which is great for making items to be moulded into FRP, like keelcase bushings. As well as the high quality of the documentation
, I've been really impressed with the mechanical qualities of all the grades I've used; some have been in the field for 25 years and I'm not aware of a single