Masthead Sheaves--re-cut old aluminum or buy new delrin?

[JohnV]

The 3 pairs of original bronze-bushed aluminum masthead sheaves on my '79 Alajuela 33 cruiser were grooved for wire and rope.
I want all 7/16" rope for simplicity's sake. So, I need rope grooves. Machining the 6 old ones will still leave me with about 3 1/8" diameters (7.1 times rope dia.), and aluminum obviously has greater strength.
Six new Delrin sheaves would be lighter, a little bigger at 3.5" diameters (8 times rope dia.) probably cost not much more than the machine work, and entail less chasing around.
Is there a significant reason to go one way or the other--old aluminum or new Delrin?
Or is it a coin-toss? (painful metaphor, huh.?)
Thanks for your thoughts.
John V.

[Douglas]

Hi John,,,, did you notice any corrosion between the bronze bushing and the aluminum sheeve ?

Was it an interference or pressed fit, between the two ? Or , just a loose fit with a grease coating ?

Brion and most riggers go to great lengths to isolate different metals, and no one that I know would put bronze and aluminum together even in close proximity if that could be avoided.


Douglas

[JohnV]

Douglas,
It looks like a press fit, with no insulating layer that I can see. I'd surmise that the fit is tight enough to exclude water, and I don't see any corrosion. Just a little soft black surface crud, which is probably dry lubricant.
I believe them to be original to the Lefiel mast, and their reputation, from what I gather, seems pretty good.
Plus, they're 30 years old and have lasted this long, so while it's sensible to think corrosion would be a problem, it obviously hasn't been.
John

[Douglas]

Hi John , there were other mast fabricators (Kenyon) that used bronze bushings pressed into aluminum sheeves, not just LeFiel , and on a 30 year old boat here in Singapore, there was no corrosion on those sheeves, either.

I have had good luck ( reduced corrosion, less rusting ) with using SS and bronze together, like using bronze toggles and SS clevis pins, or using bronze bolts and screws on SS lifeline and dodger hardware.

My Forespar sheeves were some sort of white plastic, but the thin part of the cheeks were starting to crack and break away.

I did have new sheeves fabricated, but forgot which plastic it was,,,, I also had bushings made for them, out of "Turcite" plastic .

There must be a guide somewhere that would advise what type of plastic, new sheeves should be made of ??? I got some info on Turcite , from the McMaster-Carr online catalog .

Delrin , turns out to be hydroscopic and absorbs water, then swells ,,,, how much , I don't know.

Douglas

[JohnV]

Douglas,
My knowledge is limited to what I've read about it recently, mostly in N. Calder and D. Casey as well as B. Toss's book. Authors characterized Delrin, a trade name for acetal plastic, as mid-range in quality between nylon and stainless or aluminum, and as a sutiable material for the purpose. None expresse a definitive preference otherwise, though.
New aluminum sheaves are over twice the price of Delrin.
My sheave axles are stainless and don't appear to have any corrosion, either. Maybe the parts dry out enough in their location to that corrosion isn't an issue.
Zephyrwerks, apparently a trusted sheave maker, uses bronze bushings in their Delrin sheaves.
I believe nylon is known to swell when wet--not sure about Delrin. Your Forespar sheaves sound like nylon--a surveyor told me it gets powdery and brittle with exposure to sunlight over time. Some of my white plastic sheaves are.
It's probably not a critical decision. I just tend to obsess about this kind of choice out of habit. I could have worse problems to solve, I guess.
John

[Jack]

If it is only the smaller groove you are trying to eliminate. Why not fill the smaller groove with a thickened epoxy. If you have experience with epoxy, I would think a pretty thick batch of epoxy, and a Popsicles stick (or similar) the same radius as the bigger groove to clean it out. I have seen epoxy used in worse conditions that I picture a sheave being in. Just a thought, but I would sure try it before spending big bucks on a new sheave.....:-)

[JohnV]

That occured to me. I'd think that the epoxy might fracture or break out eventually due to uneven expansion or or something.

[Jack]

Originally Posted by JohnV

That occured to me. I'd think that the epoxy might fracture or break out eventually due to uneven expansion or or something.

Get or make some fiberglass rope. It would not take much. Making rope is easy and cheap. I would make it narrow enough and long enough to make a number of full turns inside the groove. Pre soak the rope in epoxy, put a coat of epoxy iside the groove, lay the rope in there.... around and around till you get the build up you need......sand it down when you are done....It's not like these things are high speed or anything. Would probably be stronger than the original.

Just a thought anyhow.

[JohnV]

An interesting idea. Begs the question of why there aren't fiberglass-reinforced epoxy sheaves. Though I guess this would basically be an aluminum sheave with an epoxy-lined groove.
JV