Relationship between Rope and Block sheeve diameters

[Roberto]

What type of rope are you using ?

In terms of behaviour (decrease in strength vs sheave diameter), there is a *lot* of difference between "normal" rope and high modulus rope, the latter being much more sensitive.
Also, especially with high modulus rope, the *shape* of the groove can be quite relevant, say one or two diameters: I mean a 6:1 sheave diameter with a correct groove can have the same strength reduction as an 8:1 but with a "wrong" groove.
.
In general, the higher the modulus, the more sensitive the rope will be to sheave:rope diameter and groove shape.

 

[Lucky Duck]

Friction is more complicated (more factors that affect the picture)
- The bearing between the sheave and the sheave pin (type of bearing), there is big difference here with associated price difference.
- The clearance between rope and the sides of the block
- The design of the block
- The surface of the rope
I don't think sheave diameter have a significant effect on friction.

For low load lines with a low friction requirement ball bearing blocks is a good choice.
Are you moving a lot of rope in/out or just smaller adjustments?

What type of rope are you using ?

In terms of behaviour (decrease in strength vs sheave diameter), there is a *lot* of difference between "normal" rope and high modulus rope, the latter being much more sensitive.
Also, especially with high modulus rope, the *shape* of the groove can be quite relevant, say one or two diameters: I mean a 6:1 sheave diameter with a correct groove can have the same strength reduction as an 8:1 but with a "wrong" groove.
.
In general, the higher the modulus, the more sensitive the rope will be to sheave:rope diameter and groove shape.

It's for the final tackle of the kicking strap/vang after the rod kicker's own plus an external cascade. There isn't a lot of room so I was thinking of using a 40mm double b/b block with 8mm rope.

Unfortunately I can't go with a smaller line diameter as the line terminates at a clutch rather than a cam cleat.

 

[estarzinger]

I don't think sheave diameter have a significant effect on friction.

well, you might want to rethink that.

Bearing friction is usually calculated (see: SKF ) as 'friction torque', and the sheave diameter is a simple lever arm reducing the friction load on the line (in a linear fashion).

I was recently curious about this and took two different size harken T2 blocks (so the design and bearing materials were the same) - 40mm and 29mm - and measured their friction with a 15kg load hanging on a 180 degree deflection. The smaller block had higher friction almost exactly in proportion to the sheave size.

View attachment 39775

 

[knuterikt]

well, you might want to rethink that.

Bearing friction is usually calculated (see: http://www.skf.com/group/products/b...bushings-rod-ends/general/friction/index.html ) as 'friction torque', and the sheave diameter is a simple lever arm reducing the friction load on the line (in a linear fashion).

I was recently curious about this and took two different size harken T2 blocks (so the design and bearing materials were the same) - 40mm and 29mm - and measured their friction with a 15kg load hanging on a 180 degree deflection. The smaller block had higher friction almost exactly in proportion to the sheave size.

Reading http://www.skf.com/group/products/b...bushings-rod-ends/general/friction/index.html

The friction in a spherical plain bearing or rod end is primarily dependent on the sliding contact surface combination, the load and the sliding velocity.

It seems like the diameter of the bearing have more impact than the diameter of the sheave.
So if the purpose is to have small block (disregarding the reduction in BL), one should choose the block with the larger bearing surface.
I would guess that a block like this (with ball bearing)

Would have less friction than a block with a smaller diameter ball bearing but same sheave diameter?

View attachment 39775

Is this graph correct?
I read this as if with;
40mm ball bearing you need 30kg force to lift a load of 15kg
40mm plain bearing you need 32kg force to lift a load of 15kg
29mm plain bearing you need 35kg force to lift a load of 15kg

 

[estarzinger]

^^ no you are reading the graph incorrectly (my fault for poor labeling)

The Y axis is set to a 100 index = the friction from the Harken T2 block, which happens to be 13.4 Newtons (95% confidence interval = 12.9 - 13.9).

So the 40mm plain bearing is 114% of that.

And the 29mm T2 is 133%. And you will note that 40/29 = 138%.

Increasing the ball bearing race diameter (as in the 'air blocks'), and increasing the sheave diameter both (to the first order effect) linearly decrease friction. Increasing the bearing race does it without increasing the block size, which is a nice benefit to that approach.

 

[knuterikt]

^^ no you are reading the graph incorrectly (my fault for poor labeling)

The Y axis is set to a 100 index = the friction from the Harken T2 block, which happens to be 13.4 Newtons (95% confidence interval = 12.9 - 13.9).

So the 40mm plain bearing is 114% of that.

And the 29mm T2 is 133%. And you will note that 40/29 = 138%.

Increasing the ball bearing race diameter (as in the 'air blocks'), and increasing the sheave diameter both (to the first order effect) linearly decrease friction. Increasing the bearing race does it without increasing the block size, which is a nice benefit to that approach.

Still a little bit confused about your numbers, is this calculation correct? I'm using kg describe the force as this is easier to understand for most
40 mm ball bearing you need 16.4 kg force to lift a load of 15kg
40 mm plain bearing you need 16.6 kg force to lift a load of 15kg
29 mm plain bearing you need 16.9 kg force to lift a load of 15kg

This looks more realistic to me.

What would this look like in the OP's case with 50 kg load on the rope?

 

[estarzinger]

Yes, you have it (with a bit of rounding) . . . my test rig allowed me to measure the friction directly without having to subtract the load.

I did not test with 50kg, but the friction should scale linear, so it 'should be' 54.5, 55.2, 56.0. Not a huge difference. These blocks are decently efficient.

View attachment 39882