Blocks and Pulleys

Well Greg.....(just joking).

As I know it (and I've been proven wrong more than right), an arborist rigging block has side plates the are deeper than pulleys. This protects the rope from being pinched agianst a tree if it becomes aligned squarley.
 
My understanding has been that a sheave, when mounted on an axle (usually in some sort of frame) makes a pulley, and a block would be either a pulley with additional hardware allowing the frame to be fastened to something, or more usually, a pair of them used to produce a mechanical advantage (as in block-and-tackle).

That's probably just the sort of answer you were looking for... :)

Glen
 
i always took block to mean large pulley with massive, fixed cheeks. Thus, large like a square block. The rescue pullies open freely, and it takes a carabiner to hold close that doubles as mount, but aren't as rough and tumble as our blocks. Either Bearings or Bushing. The size of the sheave, how much leverage was exerted over the axle with the bearinigs/bushings. The cheeks holding all together.

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On-line encyclopedia determines a pully as a sheave with an axle, a block is a series of pullies retained in a frame (used to be wood). At school we teach that a rigging block is identifed by the manner in which it anchors.....'a SHEAVE with an axle, in a frame by which a SHIV is used to anchor the device to a rope which is subsiquently used to secure an anchor' "A pulley has a sheave and axle however a second metal device is required to provide atatchment to the anchor, NOT TO BE SHOCK LOADED"
 
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CAn anybody tell me why one is called a block and the other a pulley?

Thanks
Greg

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Greg;

If I understand the question, you are not asking what the difference is between a block and a pulley, but why there are different names.

In Arborist Equipment (p. 180), Don Blair offers this:

"The terms are interchangable as far as this book is concerned. Early pulleys consisted of a sheave or grooved wheel mounted on an axel between two blocks of wood. Called pulley blocks, they've been abbreviated in both directions." I think the phrase "...in both directions..." means that sometimes the term 'pulley' is used and sometimes the term 'block' is used.

Although Blair states "The terms are interchangable..." the rest of his text seems to adhere to slightly stricter definitions.

Most, but not all (eg. the Petzl Fixe) pulleys have rotating cheekplates. They are meant to be used for light duty work and require a connector (carabiner, screw link etc.) to attach them to the sling. They are often categorized by the diameter of the sheave.

Blocks may also have rotating or fixed cheekplates. What distinguishes them from pulleys is that they are constructed of thicker, heavier material and do not require a separate piece of hardware ( a connector) to attach them to the sling. The attachment is achieved by placing the sling over what looks like a small sheave on the top of the block. This small sheave is called the bushing, which can be confusing because the (main) sheave rotates on bearings that are either ball or, more often, bushing bearings.

Arborist blocks are manufactured to withstand heavy dynamic loading and have extended cheekplates to prevent the rope from rubbing against the tree.

Blocks are often categorized by the width of the tread of the sheave (i.e. the largest diameter rope that will run over the sheave). Thus a 5/8ths inch block sounds smaller, but is actually heavier than a 4 inch pulley.



Mahk
 
Pulleys need a link (carabiner, shackle, quick link, etc.) to connect them to the sling.
Blocks have an aluminum or steel bushing that the sling goes around. No link needed.
Both can have swiveling cheek plates.
Pulleys were designed for rescue use in caving, mountaineering, building, etc.
Blocks were designed with the Arborist in mind.
 
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Blocks have an aluminum or steel bushing that the sling goes around. No link needed.
Blocks were designed with the Arborist in mind.

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That is incorrect. Farmers and sailors have been using "block" and tackle for a lot longer than arborists have been rigging. Blocks can be seen with hooks or rope loop attachments as well.

Dave

artcon-jun03-2.jpg
 
Although i know the physical differences by look; it was interesting hearing the actual definitions of the pieces. Thanks everyone for getting to the true definitions. Everyone had a great bit of valuable info to share. That's what i love about TreeBuzz.
I wish i could add a little more to that but i'm pretty sure you all just really summed it up completely. /forum/images/graemlins/tongue.gif
 
If you look in Jerry Beranek"s cd (A Tree Story) there is a Story in there about him and his friend Spotting a large block in the top of an old Redwood snag. The block was made somewhere in the turn of the century. ( hand made from the look of it)Very interesting story of climbing this old snag and then the hunt after the block was thrown out from the tree.

Greg
 
Two posts identify the 'bushing' as the point of attatchment to the sling. My understanding was that it was predominantly called a SHIV. This did make sense you have a sheave, a shiv, and you may have bushings or ballbearings for less friction. Any thoughts or do I need to go back to school?
 
You might want to rethink it a little.

I once got told-off by a guy working at a hardware store who disagreed with me saying a sheave was not a pulley. I was drawing on machine design experience but he was drawing on jailhouse terminology. He presented his Buck knife and told me "this is a sheave". At least that's what I thought he said.

I looked up shiv yesterday at your prompting and that scene came immediately to mind.

I would agree with "bushing".

Glen
 
We'd say the point of attatchment is a pin; if it had a piece that moved on the pin (making bigger eye/less leverage bend in sling, and reduce wear from shifting)it was a bushing(but i guess sheave would fit too). Bushings take shock better, and the increased efficeincy (and expense) of bearings not needed, and would get lost etc. when open.

Bearings being more delicate and costly, used for non-shock rigging at higher efficiency. Other position for efficiency gain is size of sheave giving leveraged multiplier into the bushing or bearing efficiency of the 2nd class leverage of sheave over axle. Rope ride is a 1/1 leverage- no increase/decrease; but first class lever cuz it changes the direction of the effort; while the mechanics of the sheave over the axle doesn't change the direction of application of effort, so is non 1st class (in my imagery; there is only 1st and non 1st lever classes).

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Re: Blocks and Pulleys *DELETED*

Post deleted by Mahk_Adams

I deleted this and my previous post because the formatting went wacky. I re-entered it farther down. The content is the same as what was here, even though it is a little out of place in the thread.

Mahk
 
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