Low Friction Rings. Sizes?

[savageseadog]

Thinking of replacing some blocks with rings as they are cheap, simple and have less moving parts. Ones I've seen are normally sized as 7/10 for example (Antal) the hole is 10mm and the maximum cord size round the circumference is 7mm. Does the hole size of 10mm allow the use of 10mm rope or is 8mm more like it?

 

[BelleSerene]

'Low friction' is relative. Leave a spare 50% of line diameter in the hole to keep surface contact reasonably low. Eg for 12mm rope I use 18 or 20mm rings. I use LFRs extensively, for spinnaker blocks (sheets and tack), 2:1 controls at clew outhaul and Cunningham, 2:1 slab reefing and for a 6:1 genoa line tweaker.

 

[SailSouth]

+1 for what BelleSerene has said.

I use the Barton low frictions rings for spinnaker sheet tweakers and the like, the Barton site gave some useful ring and rope sizing info last time I looked.

 

[johnalison]

Friction is independent of contact area. Double the area and the pressure per unit is halved. The angle of bend is going to be more significant.

 

[BelleSerene]

Friction is independent of contact area. Double the area and the pressure per unit is halved. The angle of bend is going to be more significant.

That's an appealing thought. So you don’t think that the same rope running through the toroidal shape of the LFR incurs more friction than over a bar of the same curvature, and less friction than if the rope were as fat as the inside diameter of the ring, so contacting it all round?

 

[DJE]

Friction is independent of contact area. Double the area and the pressure per unit is halved.

.....for idealised rigid bodies.

I think the deformation of rope as it passes through the ring might have some effect.

 

[johnalison]

I agree that it is the idealised situation that the rule refers to, but there is a common misconception that contact area is roughly proportional to the friction, which is clearly untrue. As for contacting the ring all round, again this would be the same in an ideal situation but in practice energy would be used in distorting the line which would not be exactly circular.