Pully Diameter?

[John Mc]

Is there a rule of thumb for minimum diameter for pulleys or sheaves to use with a given sized cable? Maybe something like ___ X the cable diameter?  Does it vary with the type of cable?

I have a 10mm (3/8+") cable I salvaged from my logging winch when it got a bad kink right in the middle of the length. It's swaged, if that makes a difference. Unfortunately, I don't know the details on the strand count. What is the minimum safe diameter pulley to  use with this without causing excessive "pigtailing".

[LeeB]

Minimum diameter is 18:1. European standard is 24:1.

[John Mc]

Minimum diameter is 18:1. European standard is 24:1.

Thanks, Lee.  That's larger than I thought. That would mean my 10 mm (0.394") cable should use a 7" diameter pulley (or almost 9.5" by European standards). I don't know that I've ever even seen a 7" self-releasing snatch block, let alone one 9+ inches. I own two of them, one is about 3.5" to the bottom of the groove, the other is 4.5".
Is that standard perhaps intended for overhead lifting?

[mike_belben]

the bigger the pulley radius, the more individual strands stay in tension. with a tiny radius, say 2x like a knot around a clevis pin, youll get a bunch of strands not just unslacked, but backed up in compression and jumbled while the fewer strands in tension are overloaded for lack of helpers.  

the bigger the radius, the more strands in tension, the higher the capacity.  a small radius is effectively using a smaller cable. thats why the ratings are radius dependant.  same with end terminations.  put a big thimble if you need the full rating at the termination. 

[LeeB]

That measurement is center of the cable to center of the cable, not bottom of the groove. Having said that, a 6" sheave would probable be just fine. 

[LeeB]

What load rating do you need? For a 5t rating, Mckissick snatch block is 4 dia for 3/8" - 1/2" wire rope. That's only 12:1 for 3/8", less than that for 1/2". So not really sure about the 18:1 figure. 

McKissick® 419 Light Champion Snatch Blocks - Crosby Group

[John Mc]

What load rating do you need? For a 5t rating, Mckissick snatch block is 4 dia for 3/8" - 1/2" wire rope. That's only 12:1 for 3/8", less than that for 1/2". So not really sure about the 18:1 figure.

McKissick® 419 Light Champion Snatch Blocks - Crosby Group

My winch is only rated at 7700#.
I've been reading up on cable and pulley specs. Apparently, the two big factors are the ration of pulley to cable diameters and the type of construction of the cable (cables with more & finer strands can handle smaller diameter pulleys for the same diameter cable).
I'm not so worried about breaking the cable - either through outright pulling or through fatigue. The stuff I'm pulling does not require doubling the line, so we're not talking 180˚ bends. I'm more wondering about putting a permanent set in the cable by pulling it over a smaller pulley under load.

[mike_belben]

i am a destructive testing type of person.  shooting from the hip here, anything over 3" wont kink.  if you jam it into the corner of a cheap chinese fairlead, yeah, thatll leave a mark.   

[LeeB]

If you are running the same section of cable over the sheave all the time it will fatigue and fail even with a larger than required sheave. Same way a band blade will eventually break from running around the wheels.

[John Mc]

If you are running the same section of cable over the sheave all the time it will fatigue and fail even with a larger than required sheave. Same way a band blade will eventually break from running around the wheels.

This piece I salvaged from my old cable is just not seeing enough use for fatigue to be a major concern. It was removed from the winch due to the bad kink 110' in to a 230 ft length. The rest of the cable was still in very good shape - which it why I saved a length of it.
My winch use is far from a full-time thing, and this spare cable gets used even less.