Loading only one side of a double pulley?

[pete3d]

Could anyone please comment on using an Rock Exotica Omni-Block double sheave swivel pulley loaded only on one side, that’s to say as a single sheave block.

I have an application where it would be convenient to use a pair of these double blocks, but often only rigging them as singles, and wonder if the misalignment this would create is cause for concern.

Cheers.

 

[TheTreeSpyder]

Would think could only safely push to safe load limit properly balanced and aligned etc.
Also, would look for more rope chafing potential.....

 

[Chaplain242]

I would be less inclined to do this with an double omni than a regular fixed cheek double pulley. The swivel and opening mechanism would be weighted in unconventional manner. Depends on how much you are loading it.

 

[pete3d]

Depends on how much you are loading it.

Considering this - I only intend to load it (an Omni-Block 1.5 double) in this lopsided manner with a CMI Rope Jack, so about 3.5 kN per rope leg max, no shocks and no life support involved.

 

[evo]

I’d say in reality it’s dependent on the loads. If set up as a z rig or similar maybe try loading the left side on one and the right on the other and see how they fair.
For hand pulls I’d venture to guess you are way under 10:1 of the MBS perhaps well below 20:1. Weigh this…

 

[Bart_]

In concept, I'd be more comfortable doing so on a relatively narrow, larger diameter (longer) double pulley with three "side"plates construction. All based on alignment of loads with original intended alignments.

 

[TheTreeSpyder]

'cheek plates'
Been always wondering why not have like a 4-3-1 w/dbl.sheaves, where dead-end/terminate to 1 sheave with Bowline etc.
For more properly balanced pulls on 1 pulley and almost on the other, instead of both off-balance/not aligned.

 

[SeanRuel]

Hmm, sounds like a great reason to buy another pulley or two to me! I'm hesitant/ won't load one Side of a double, just looks wrong

 

[pete3d]

Thanks all for your input to help me consider this issue of misalignment.

I must admit I was hoping (and rather assuming) I would get responses more like, “We do that every morning before breakfast and twice on Tuesdays.”

Alas a man wants to hear (or see) what he wants hear, so I did a little experimentation with my single Omni-Block and the results make confident that the double pulley could serve my modest, non-life support, no shock load, requirements.

 

[evo]

Good to hear? there is a very fine line of ‘I need to buy the right tool for the job’ and ‘I want to buy the right tool for this random light configuration’…. Not unlike ‘I can hook up a 5:1 off a porty, or I can buy a grcs’.
A grcs can do much more than a 5:1 + porty, but a 5:1 and a porty can do (shudder) what a grcs can do (if enough input force). Knowing that line is the challenge. So working with what you got (within reason) is just fine, but it also shouldn’t be an excuse not to get the right tool. The right tool is an investment not only geared for production, but manual labor/wear and tear. Even if used a few times a year, will pay dividends back in time.

 

[Jonny]

I actually did this on Friday to pull over a dead ash. Maybe 80’ tall and 20” DBH. Set the line around 50’ up.
Plan was to use both sheaves 5:1, but was going to need to add more rope so we gave it a test tug with just 3:1 and it was plenty.

Worked fine for pulling by hand. I wouldn’t pull on it with a truck or machine though, I got plenty of single pulleys.

 

[Tree-Taylor]

For anyone else interested in this configuration, RE recommends the use of both sheaves. The pulley is designed to be in-line with the load and single sheave use causes a slight offset towards the outer plate.