oldfart
Participating member
- Location
- Asheville, NC
I have two Playlists of YouTube that have been getting some attention and drawing questions:
"Stumpin' : Mechanical Advantage Rigging
https://www.youtube.com/playlist?list=PLUlSYuIlSMrSoqZxg1orpU6de1fLhr7ws
uRIG Pocket Mechanical Advantage for Tree Work
https://www.youtube.com/playlist?list=PLUlSYuIlSMrSrf1flxTpFo2sOLcg1Be72
My "Stumpin' " rig can lay over 21 tons on a stump. You've seen it rip 3/4" tREX like tissue paper and pull out some big damn stumps ...
By contrast,the uRIG is good for 1 ton swl. Regardless, it's one of the most useful rigs I've ever built.
Occasionally a newbie (remember, we all started somewhere) will get those confused. What you see below is an email response to questions republished here in the hope that it might be useful to somebody as a VERY basic primer ...
Hello xxxxx,
Thanks for the interest. I’d be glad to help …
First let me say that I love the uRIG system. I use it all the time around the place. The Rock Exotica hardware is like wonderful jewelry and the overall design is balanced and useful under many situations.
But I don’t use the uRIG for pulling stumps: it’s not strong enough. You’ll find my stumping rig shown in a number of other videos. My big stumping rig is sized for 17 tons swl in the dual snatch and 21.25T against the stump. (Admittedly, that’s huge. Some would say ridiculous.)
The P51D at the head end of the uRIG has an mbs of 36kN. That’s 8100# breaking strength. With a safety factor of 4:1, that delivers a safe working load of about one (1) ton. Huge difference. I made the “MA vs. straight pull” video to illustrate the power of mechanical advantage but it was a very small stump and the tree had already been pulled over by a windstorm. Please see the “replies” thread under the vid “uRIG build details” to see the limitations.
It’s entirely possible that the uRIG could be the “front end” of a compound system, pulling on another bigger set of pulleys. But even in that application, it wouldn’t be a great choice because you don’t need the prussics to pull a stump. Where it really shines is a hand-drawn application where you need to raise, hold then lower a load. For that it’s a great little system.
As you think through your system, let me give you some basic info that could be useful …
Mbs = maximum breaking strength
Swl = safe working load
Sf = safety factor
swl = mbs/sf
The total load on a pulley is the tension force multiplied by the number of strands. For example, if you have a dual snatch block seeing 1 ton of tension in its rope, the pulley needs to be rated swl = 1T x 4 = 4 tons. That would imply 16 tons mbs.
Hardware (pulleys, shackles, but not Rock Exotica) and rigging slings (endless polyester rigging loops, see TrucknTow.com) are usually speced as swl. Cordage is usually speced as mbs (see Samsonrope.com for different products and mbs at different sizes).
For hardware, the sf is often taken at 4:1; for cordage, sf is typically 10:1. For critical applications (overhead lifting, life support) the sf is much higher. For non-critical application (like stumping), you can invade those safety factors somewhat PROVIDED THAT THE OPERATOR IS PROPERLY PROTECTED FROM COMPONENT FAILURE (see the vid “Proof testing the E/E Sling). NEVER stand unprotected in the line of fire and NEVER “lunge” into a rig with a vehicle.
To build up a system design, start with the pulling device/vehicle that you intend to use. Estimate/calibrate its pulling force. You need to get this right because, in effect, the pulling device will become your “mechanical fuse” protecting your entire rig. You don’t want it capable of delivering more force than your rig can handle.
Then estimate the final force that you want to lay against the load. That will give you the mechanical advantage that you need. Remember that more strength is always good but too much MA can work against you. In my vids, you’ll see systems ranging from 5:1 to 42:1. Sometimes the MA can come from a “simple” system, like a block and tackle or it can come from “compound” systems, one pulling on the other.
Your other questions …
The rigging slings I use are either “endless round polyester rigging slings” or chains.
For these devices, you have three (3) choices when you form the connection: straight, basket, and choked. The “straight” connection is 1 x swl. The “basket” delivers 2 x swl. The “choke” reduces swl by 25%.
I don’t remember where I got that bag and I can’t find any manufactures mark on it. Sorry. I have had some luck with carrying bags shown on eBay for lighting equipment and tripods. There are nice offerings, reasonably priced, that have the right general shape. Pay attention to the size dimensions. The uRIG fits into a bag 20x6x6.
I hope this helps with your project, Rich. Once you get familiar with a few basics, this won’t be anywhere near as complicated as I’ve made this sound … !
Good luck.
Tom Hoffmann
"Stumpin' : Mechanical Advantage Rigging
https://www.youtube.com/playlist?list=PLUlSYuIlSMrSoqZxg1orpU6de1fLhr7ws
uRIG Pocket Mechanical Advantage for Tree Work
https://www.youtube.com/playlist?list=PLUlSYuIlSMrSrf1flxTpFo2sOLcg1Be72
My "Stumpin' " rig can lay over 21 tons on a stump. You've seen it rip 3/4" tREX like tissue paper and pull out some big damn stumps ...
By contrast,the uRIG is good for 1 ton swl. Regardless, it's one of the most useful rigs I've ever built.
Occasionally a newbie (remember, we all started somewhere) will get those confused. What you see below is an email response to questions republished here in the hope that it might be useful to somebody as a VERY basic primer ...
Hello xxxxx,
Thanks for the interest. I’d be glad to help …
First let me say that I love the uRIG system. I use it all the time around the place. The Rock Exotica hardware is like wonderful jewelry and the overall design is balanced and useful under many situations.
But I don’t use the uRIG for pulling stumps: it’s not strong enough. You’ll find my stumping rig shown in a number of other videos. My big stumping rig is sized for 17 tons swl in the dual snatch and 21.25T against the stump. (Admittedly, that’s huge. Some would say ridiculous.)
The P51D at the head end of the uRIG has an mbs of 36kN. That’s 8100# breaking strength. With a safety factor of 4:1, that delivers a safe working load of about one (1) ton. Huge difference. I made the “MA vs. straight pull” video to illustrate the power of mechanical advantage but it was a very small stump and the tree had already been pulled over by a windstorm. Please see the “replies” thread under the vid “uRIG build details” to see the limitations.
It’s entirely possible that the uRIG could be the “front end” of a compound system, pulling on another bigger set of pulleys. But even in that application, it wouldn’t be a great choice because you don’t need the prussics to pull a stump. Where it really shines is a hand-drawn application where you need to raise, hold then lower a load. For that it’s a great little system.
As you think through your system, let me give you some basic info that could be useful …
Mbs = maximum breaking strength
Swl = safe working load
Sf = safety factor
swl = mbs/sf
The total load on a pulley is the tension force multiplied by the number of strands. For example, if you have a dual snatch block seeing 1 ton of tension in its rope, the pulley needs to be rated swl = 1T x 4 = 4 tons. That would imply 16 tons mbs.
Hardware (pulleys, shackles, but not Rock Exotica) and rigging slings (endless polyester rigging loops, see TrucknTow.com) are usually speced as swl. Cordage is usually speced as mbs (see Samsonrope.com for different products and mbs at different sizes).
For hardware, the sf is often taken at 4:1; for cordage, sf is typically 10:1. For critical applications (overhead lifting, life support) the sf is much higher. For non-critical application (like stumping), you can invade those safety factors somewhat PROVIDED THAT THE OPERATOR IS PROPERLY PROTECTED FROM COMPONENT FAILURE (see the vid “Proof testing the E/E Sling). NEVER stand unprotected in the line of fire and NEVER “lunge” into a rig with a vehicle.
To build up a system design, start with the pulling device/vehicle that you intend to use. Estimate/calibrate its pulling force. You need to get this right because, in effect, the pulling device will become your “mechanical fuse” protecting your entire rig. You don’t want it capable of delivering more force than your rig can handle.
Then estimate the final force that you want to lay against the load. That will give you the mechanical advantage that you need. Remember that more strength is always good but too much MA can work against you. In my vids, you’ll see systems ranging from 5:1 to 42:1. Sometimes the MA can come from a “simple” system, like a block and tackle or it can come from “compound” systems, one pulling on the other.
Your other questions …
The rigging slings I use are either “endless round polyester rigging slings” or chains.
For these devices, you have three (3) choices when you form the connection: straight, basket, and choked. The “straight” connection is 1 x swl. The “basket” delivers 2 x swl. The “choke” reduces swl by 25%.
I don’t remember where I got that bag and I can’t find any manufactures mark on it. Sorry. I have had some luck with carrying bags shown on eBay for lighting equipment and tripods. There are nice offerings, reasonably priced, that have the right general shape. Pay attention to the size dimensions. The uRIG fits into a bag 20x6x6.
I hope this helps with your project, Rich. Once you get familiar with a few basics, this won’t be anywhere near as complicated as I’ve made this sound … !
Good luck.
Tom Hoffmann
