4:1 or 5:1 MA - Sherrill's Turn

[chris_girard]

4:1 or 5:1 MA - Sherrill\'s Turn

I haven't received my new 2006 Sherrill catalog yet, but I was looking through the 05 one and noticed on pg. 45 that they say their Tree Pulling Kit will give you a 4:1 mechanical advantage.

Well, the way that they have the lines reeved shows a 5:1 MA, NOT 4:1. The setup is just what ArborMaster Training teaches and shows in their workbook The Art and Science of Practical Rigging on pg. 86.

Just as Wesspur did in their 06 catalog (with the 2:1 and 3:1 MA), I think Sherrill did the same and made a mistake. Do any of you guys with the new 06 catalog know if they still say and show the same setup?

Chris /forum/images/graemlins/confused.gif

 

[Kevin]

Re: 4:1 or 5:1 MA - Sherrill\'s Turn

Chris, that's 4:1.
In order for that to be 5:1 the rope would need to be anchored to the bottom of the roving lead pulley.

 

[chris_girard]

Re: 4:1 or 5:1 MA - Sherrill\'s Turn

Kevin, what about what ArborMaster shows on page 86 of their workbook. They show 5 moving parts at the lead block (including the pull line) which is attached with a Vt hitch. This to me means that you have a 5:1 MA and the reaction force at the anchor point is 4 times the input force of the pull line.

 

[Kevin]

Re: 4:1 or 5:1 MA - Sherrill\'s Turn

I would agree that it's 5:1ma due to five parts of the line but if you look at page 87 you'll see where it's only four times the reaction force where you can get five times with the same amount of pulleys.
The reaction force is more important than the mechanical advantage when it comes to what the line pull actually is.

 

[Kevin]

Re: 4:1 or 5:1 MA - Sherrill\'s Turn

Here is an attachment with 5:1 ma which applies five times the reaction force.

 

[glens]

Re: 4:1 or 5:1 MA - Sherrill\'s Turn

Kevin, Paint now (finally) has the ability to "save as" PNG format. If you'd do that, the results would be both smaller in terms of file size and (more importantly) much cleaner/clearer. Try it next time you create such a sketch.

 

[chris_girard]

Re: 4:1 or 5:1 MA - Sherrill\'s Turn

Thanks for the sketch Kevin showing the 5 times reaction force. I know what you mean about the reaction force being more important than the MA. Alot of times as you know people will actually reave a MA system to disadvantage and put more force on their anchor point which could compromise the setup.

Glens, thanks for the tip about Paint having the ability to save as PNG format. I did not know that. Your computer tips are always helpful.

Chris

 

[TheTreeSpyder]

Re: 4:1 or 5:1 MA - Sherrill\'s Turn

i told them before that it should say that the pulleys added 4x pull. The Prussik Tending Pulleys are cool, they actually tend the prussik safety without getting it into pulley, and theya re bearing pulleys, so conserve more force to recycle back into work on load. This would give less effort neede for same work, easier to pull apart to reset, and less anchor laoding for same work load.

 

[Mahk_Adams]

Re: 4:1 or 5:1 MA - Sherrill\'s Turn

Chris;

You are right, the catalogue ('05 and '06) is wrong. The catalogue (p. 45 in the '05 catalogue, and p. 41 in the '06 catalogue) says that the kit produces four times the tension that can be created by hand. In the drawing, however, the MA is actually 5, and the reaction force is 4. The drawing shows the correct setup, the text states the wrong MA.

 

[TheTreeSpyder]

Re: 4:1 or 5:1 MA - Sherrill\'s Turn

Usually, we are looking at compression rigs piggybacked on a line, where the mainline below the prussick is slack/ no tension. Then this would be a 4/1.

But, because this rig maintains the mainline as part of the pull, by reeving thru the pulleys/ not hanging slackened, this is a 5/1. That is why i tried to explain it to them, that it would be more proper to say that this rig added 4/1. For the mainline pull is maintained (1/1) then the 4/1 added to it's pull (not replacing the 1/1 pull) for a total of 5/1. As shown, this loads the anchor more.

With Porty, and using compression jig to grab more purchase of line tension, we will piggyback jig to mainline; so as to remove jig before lowering. In pulling over tree, we can piggy back (especially if compression jig is stored pre-strung) or, do as shown and use the tail of mainline to reeve thru a jig you make on spot.

Either jig can be 2 handed for 5/1 of bodyweight or leg lift used on one hand + 8/1 of opposite hand. Anchor loading is 4x (bodyweight or leg force) + 8x other hand on piggy back (main tail slack) or 5x (bodyweight or leg used) + 9x other hand pull with rig as shown. /forum/images/graemlins/crazy.gif