Mast Sheave Material

[Andrew Troup]

Originally Posted by Nicholson58

Alro Plastics NYLATRON®

Up to 2% growth for fully wetted Nylatron. This is a much over-blown problem. ... 2% means multiply 0.5 inch diameter by 0.002 to get the minimum dry clearance. So, your hole would have to be 0.501 to avoid a compressive interference.... .

You've got the decimal point in the wrong place, Nicholson58, which understates the problem by a factor of ten.

I'll freely grant it's a problem of more significance on larger diameters, and anyone contemplating a voyage to the tropics with nylon rudder-shaft bearings would be well advised to note that the problem is also quite temperature dependent.

Steering which is perfectly satisfactory in temperate latitudes can become quite borderline in warmer waters.

I favour Nylatron for applications requiring the exceptional qualities of (particularly) toughness for which it's notable, for instance anchor rollers ... but for a mast sheave, unless say a loaded Talurit is going to be passing over it (not a good idea...) it's hard to see any advantage over the more usual choice in the mast-building trade, which is acetal (eg Delrin)

[Andrew Troup]

Jim

Unconstrained Nylon expands (because of water absorption) in all directions: the ID and OD will both grow.

It's worse in the unconstrained direction when constrained elsewhere, for sure.

If it were pure volumetric expansion (eg pure thermal expansion) your instinct would be correct, but the water is absorbed preferentially in the superficial zones, so the non-expanding deeper material has a constraining influence.

The 2% figure quoted by N58 takes this into account in a crude way: the actual water absorption by weight for typical engineering nylons is more like 7%

[Jim Cate]

Originally Posted by Andrew Troup

Jim

Unconstrained Nylon expands (because of water absorption) in all directions: the ID and OD will both grow.

It's worse in the unconstrained direction when constrained elsewhere, for sure.

If it were pure volumetric expansion (eg pure thermal expansion) your instinct would be correct, but the water is absorbed preferentially in the superficial zones, so the non-expanding deeper material has a constraining influence.

The 2% figure quoted by N58 takes this into account in a crude way: the actual water absorption by weight for typical engineering nylons is more like 7%

Ahah! good point, Andrew. I hadn't considered non-uniform expansion, and I believe that you are correct here.

Stupid plastic stuff... !

Cheers,

Jim

[Snowpetrel]

Ive had a few to many white acetal sheaves split from UV damage to be entirely happy about them, have they improved the UV stabilisation in the more modern ones? What color is best and can you get reinforced acetal? Granted they dont get much UV inside the mast head fitting, but the consequences of a failure are a very badly jambed halyard. Nasty...

I have seen bronze bushed sheaves in one of the tougher plastics (like nylatron). Would this reduce any expansion proplems with the plastic?

[estarzinger]

Originally Posted by Snowpetrel

Ive had a few to many white acetal sheaves split from UV damage to be entirely happy about them, have they improved the UV stabilisation in the more modern ones? What color is best and can you get reinforced acetal? Granted they dont get much UV inside the mast head fitting, but the consequences of a failure are a very badly jambed halyard. Nasty...

I have seen bronze bushed sheaves in one of the tougher plastics (like nylatron). Would this reduce any expansion proplems with the plastic?

Black is the color of choice for Delrin. Rondal switched to all black about a decade ago. Their prior white UV exposed deck hardward sheaves lasted about 15 years before (but the mast sheaves were typically still just fine due to a lot less UV) suffering some amount of damage (chipping and crazing). They claim the black ones are good 'forever' but obviously they don't have enough real world experience yet to know that for absolutely sure.

Harken uses black Torlon for their UV exposed bearings. They (and I) had trouble with their green Torlon brearings (I crushed several their strength was apparently greatly reduced by UV).

I have UHMWPE sheaves. They are 15 years old now and look absolutely new. They don't have (or need) bushings. They also don't have the water absorption issue. There is some temperature expansion, but about the same as teh rest of these plastics.

Aluminum obviously avoids water and UV issues all together. But the anodizing does wear off.

Machined Ti sheaves anyone?

[Andrew Troup]

"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 issue.

[akswimdog]

I sure appreciate everyone contributing- Thanks much and best!!