double braid rope over V-shaped, wire sheave

[david samuelson]

Hi
What are the problems associated with putting rope over an old, cleaned, V-shaped wire sheave?
Thanks, David

[Alan Mighty]

Originally Posted by david samuelson

What are the problems associated with putting rope over an old, cleaned, V-shaped wire sheave?

You can expect no damage to your sheave, but a good (but not precisely known) chance of damage to the rope if it is subject to cyclical loads close to its safe working load.


No theory exists to cover this case. So the damage is up to testing (or your experience if you put the rope in that position).


For that matter, the experts have developed little theory for what happens in a rope sheave or a thimble. More is known about what happens when double braid is put around a circular-section bollard, because that is how most rope testing is done.


A rope sheave or a thimble share the same shape, a more-or-less semi-circular groove to hold the rope. In such cases, very few rope fibres fail due to simple compression. But rope fibres will fail due to what is called 'interface pressures' - the deal being that under loads around the SWL of the rope the rope is changing its shape from being nearly circular in cross-section to adopting the shape of the sheave or thimble groove. The semi-circular section of a sheave or thimble groove is of course kind to the rope. But under higher pressure and repeated cycles, the rope will change shape and spread wider. If the increased width puts the rope in contact with the edge of the thimble or the sheave, then cyclical loading can cause abrasion against the that edge: if the edge of the thimble (for example) is sharp, fibres in the cover get broken and cut.


The case of a polyester double braid around a bollard, with a circular cross-section, and without being constrained in a groove, is perhaps easier to explain. At the sector of highest pressure, the rope changes from circular cross-section to an elliptical cross-section. The pressure profile in the contact zone has a triangular form: a central peak tapering off towards the two edges of the contact zone.


Putting polyester double braid in the more V-shaped groove of a wire rope sheave means that if the tension is close to or above the SWL of the rope then the rope will be forced to adopt the shape of the sheave groove. The interface pressure will involve fiber-to-fibre abrasion as the rope changes shape in addition to fibres in the cover having to move over the surface of the sheave as the rope adopts the new shape. So some breakage of core fibres and some abrasion and breakage of cover fibres. If the sheave surface is rough (as is almost always the case if it has been used with wire rope), then expect that roughness to act like razor blades on the fibres of the cover (and eventually the core).


Let's say you always hoist your halyard such that the same portion of halyard sits in the sheave. And the halyard is subject to cyclical loads around its SWL. Sooner or later, you'll notice increased fuzziness of the cover due to broken cover fibres. And with enough time, I expect you'll find a hollowness to the core in that portion of halyard, due to broken core fibres.


Inspection should always reveal the weakness before the halyard breaks. But if you don't inspect your halyards, then a break might be the first sign.


One of the few accessible (but short) discussions of this can be found in McKenna, Hearle, and O'Hear, _Handbook of Fibre Rope Technology_, Baton Rouge (FL): CRC Press and Cambridge: Woodhouse Press & The Textile Institute, 2004, p. 226.

[Alan Mighty]

Originally Posted by Alan Mighty

The case of a polyester double braid around a bollard, with a circular cross-section, and without being constrained in a groove, is perhaps easier to explain. At the sector of highest pressure, the rope changes from circular cross-section to an elliptical cross-section. The pressure profile in the contact zone has a triangular form: a central peak tapering off towards the two edges of the contact zone.


Putting polyester double braid in the more V-shaped groove of a wire rope sheave means that if the tension is close to or above the SWL of the rope then the rope will be forced to adopt the shape of the sheave groove. The interface pressure will involve fiber-to-fibre abrasion as the rope changes shape in addition to fibres in the cover having to move over the surface of the sheave as the rope adopts the new shape. So some breakage of core fibres and some abrasion and breakage of cover fibres. If the sheave surface is rough (as is almost always the case if it has been used with wire rope), then expect that roughness to act like razor blades on the fibres of the cover (and eventually the core).

Some relevant published research and recommendations also exist for bollards (and similar rope contact points) on naval and merchant vessels and on docks. The research and recommendations are all about rust and the roughness (the same sort of micro-razor blades ready to cut rope fibres) that it presents to rope. You'll find that merchant ships and naval fleets have prescribed routines on keeping those surfaces smooth. Samson Ropes has a Technical Bulletin about it (see: http://samsonrope.com/Documents/Tech...UN2005_WEB.pdf).


If your sheaves have handled wire rope, they are likely too rough for any fibre rope.

[mogulskibum]

Easy to asses problem is total size - wire is generally a smaller diameter, so the rope might not actually really fit, which would cause chaffing and early failure.

The bigger problem is a little more subtle - even if the rope fits properly, the v shape will pinch the rope as a lode is applied. With wire that doesn't really compress all that much, it's not too big a problem. But with rope, that pinching will cause wear on rope and early failure.

Keep in mind that the pinch of the sheave will also be at the highest load point on the rope, which will increase the wear (possibly dramatically). Especially if the strain is consistently applied to the exact same place on the rope (as would be generally be the case with a halyard).

Change the sheave.

[Jim Cate]

AlanM: two very good posts! Thank you for a very clear exposition of the factors.

Jim

[UNCIVILIZED]

Alan_Mighty, excellent posts, thanks! As to avoiding the probem, especially given the cost of cordage, change the sheave. They're cheap, & in the short term you could even make one yourself. The only real issue is changing them, due to where they're located. But with a bit of forethought even this isn't an issue.

Note that depending on what your current ones are made of, & what kind of shape the material is in. In theory, it may be possible to re-profile them/the groove in them. But in the long run, especially on larger boats, it's best to get one which fits the rope well & runs on bearings/a bearing surface. Since hoisting bigger sails can be a beast as it is, so lower friction sheaves do help with this.

[david samuelson]

Thanks very much Alan and Mogul - I couldn't have asked for more. David

[nhschneider]

Wait a second! Is this a trick question? Are you the same David Samuelson, the rigger, working out of East Marine at Boat Lagoon in Phuket, Thailand? The same very knowledgeable guy who re-rigged my main mast? Replaced both the forestay furler and the staysail furler? The guy that takes his pet parrot to lunch every day?

If you're the same guy who I spent a significant amount of time with, it surprises me a little that you'd ask such a question. I've gotten to know several very competent riggers in my life and you ranked right up there. During the re-rigging of my main mast, you pointed out quite a few details that I would have missed and would only have been caught by a professional. I remember specifically one conversation we had that detailed the quality of various rope manufacturers. You brought up issues with rope manufacturing that I had never considered.

Are you okay?

Fair winds and calm seas.