No.19 Mechanical Advantage
There are different types of Mechanical Advantage (MA).
Leverage, Purchase, Reeving, Bow Pulls etc etc. Lets look at ways to set rope so that we can increase power.
This is called Reeving and the principle is as old as mankind and can be found in nearly every discipline man undertakes, from shoe-tieing to pyramid building.
It is a HUGE subject and Arborists can use just a few simple ideas to increase safety and productivity.
Hey, does that mean the idea is a Mechanical Advantage!!?!
We must differentiate between MA and Re-direct.
MA is worked out by the number of moving ropes at the moving object.
A Re-direct is attached to a static point and does not move.
1. 2:1 Friction plays a big part in MA, less friction means more pulling power.
More friction means more holding power.
Should we set a pulley when we cut the tree down or just through the rope over a branch?
How much pulling force do we need?
2. 1:1 The rope has been re-directed only, even if the anchor side has a pulley we get zero additional pulling power.
3. 2:1 Now we have MA. You see, 2 parts of the rope are moving at the moving object.
This can tied with knots if no extra equipment is available, a
lightweight system and very effective in many situations.
4. 2:1 Adding a redirect to control the placement of the tree does not add MA.
5. 4:1 The Purple rope has been Piggy-Backed onto the Green 2:1. A Piggy-Back doubles what ever MA it attaches to.
2:1 = 4:1. 4:1 = 8:1 etc etc.
5b. 14:1 We have Piggy-Backed a 7:1 Block and Tackle.
There are many types of Block and Tackle.
All are attached at an anchor and have a floating end that is pulled toward the anchor.
If we turned the 7:1 Block and Tackle around what would the MA be? Correct answer wins a Wooden Hand tee shirt!
6. 2:1 We can rig with rope-on-rope, thimbles or pulleys. Pulleys either have a sheave or a bearing and bearings offer the least friction.
The X-rigging rings offer low friction and are very strong and need no karabiner or shackle attachment.
X rigging ring tied into a midline knot (alpine butterfly) to form a 2:1.
We use this as a side guy-rope when felling Acacia next to the railway line.
7. 1:1 Setting more re-directs does not increase MA.
8. 1:1 There are many ways to control gravity.
MA is one of them but we can gain control when
i) the rope is set well
ii) the branch is cut well in relation to the rope setting
9. 2:1 Setting a 2:1 at the branch balance point or further toward the branch tip gives great control when cutting.
10. 2:1 This is a Double Whip Tackle and is a true 2:1 if the rope runs at 180 degrees parallel to itself.
As the rope opens out into a ‘V’ shape we lose power.
11. Double Whip Tackle Much less power than 2:1 but great control once the branch has been cut.
When the green line is pulled horizontal the anchor points in the tree top see much force.
We have turned the system into a ‘Bow’ system.
As the branch is lowered the angles decrease and so does the force at the anchor.
Remember, a force input must go somewhere and unbeknown to the novice treeworker force can be changed from our favour to against it very quickly.
Take a minute to look at this drawing to imagine the change of forces.
12. Double Whip Tackle with floating Anchor.
The anchor on the left will experience double the weight of the branch when the rope legs run at 180 degrees.
That is similar to MA right?
Re-read point 10. When the rope runs at 180 degrees and there is zero friction we have a pure 2:1 MA.
It is the same with force at the anchor.
Remember, friction at the anchor decreases the force at the anchor.
The anchor on the right is Floating, it has been Piggy-Backed so in theory that anchor will experience up to 4 times the force.
13. Speedline A simple speedline.
Attach a pulley at the balance point of the branch and lift.
We applied the same load (Osaka san) to different MA systems to find the true MA.
System MA KN Actual MA
1. Direct (Osaka san) 1:1 . 68kn 1:1
2. Pinto 2:1 1.32kn 1.94:1
3. Mummut ‘rescyou’ 6:1 2.7kn 3.97:1
4. Rock Exotica ‘aztek’ 4:1 2.9kn 4.2:1
5. 3 Fold 6:1 2.74kn 4:1
6. 3 Fold & Piggy Back 12:1 4.62kn 6.79:1
7. Petzl 4:1 2.7kn 3.97:1
8. SMC 4:1 2.74kn 4:1
9. Rope on Rope 2:1 1kn 1.47:1
2:1 with Pinto Pulley
2:1 Alpine Butterfly
6:1 Mammut Rescyou
4:1 Rock Exotica ‘Aztek’
4:1 Home made with Petzl pulley
6:1 3 Fold Home made with Rigging Thimbles
A. Rock Exotica AZTEK
B. SMC Pulley
C. Petzl Pulley
D. 3 Fold
E. Ropes !
12:1 Piggy-Back on the 3 Fold