Simple pulley

Simple pulley
Type	Power amplifier / rope tensioner
application	Lifting loads, tensioning ropes
Ashley No.	3225
English	Double whip
List of nodes

The simple pulley is a technique to improvise the principle of the pulley with existing means when climbing or in mountain rescue .

application

Three different applications
A = simple pulley block, B = self-winding , C = rescue hoist

The simple pulley system with climbing or static ropes is used in youth work and in outdoor training to tension ropes, for example the support rope, the handrail ropes and the safety rope on a rope bridge or on a cable car .

It is also used for lifting or pulling loads, for example in mountain rescue , height rescue and in rope-assisted tree climbing technology ( tree care ). As a self- winding mechanism, the Münchhausentechnik offers an alternative to climbing with Prusik slings .

Manufacturing

A pulley serves as a deflection point, for example fixed to a tree with a belt loop . The sling should form a triangle of forces, the angle of which on the roller is less than 60 degrees. The pull rope is passed through the pulley. A second pulley is attached to the load rope, for example with a Prusik knot , a cross clamp knot , or a karabiner clamp knot . The cross clamp knot and the carabiner clamp knot also work with a sling. Alternatively, a can breast or ascender or a Tibloc be used.

Backstop

If the block and tackle is used to pull or lift loads, a backstop must be built into the rope. For example, a Garda loop with two identical carbines, a Kara-eight loop with a carbine and a figure of eight , a breast or ascender or a Tibloc . The backstop then blocks the load rope after each lift and the second pulley can be pushed further away for the next lift.

• Backstop
• 

  Backstop blocked

• 

  Backstop free

• 

  with pulley and rope clamp

• 

  with Garda loop

Alternatively, a Garda backstop can be used instead of the first pulley . Or a pulley with a built-in backstop. Or cheaper, but a little more complex, as it has to be pushed in by hand with each stroke, a Prusik loop .

See also : "Backstop" in the article pulley block

Translation and friction loss

• When pulling against the load :

The theoretical transmission ratio is 1: 2. So only half the force (50%) has to be applied on the pull rope. However, this ideal is not achieved through friction losses:

Redirection	force to be expended	translation
theoretically	(50%)	(1: 2)
Bolt-mounted roller	58%	3: 5
Snap hook	66%	2: 3

The difference is converted into heat and abrasion. Regardless of this, the distance on the pull rope is twice as long as on the load rope.

• Construction as a rope tensioner
• 

  first pulley
  (in the loop on the tree = fixed point on the left)
  changes the direction of pull

• 

  second pulley on Prusik sling
  (load on the right,
  pull rope to the bottom left)
  tensile force 1: 3

• 

  Securing with
  half blow on slip

• 

  Demo of the forces
  Left (yellow) = fixed point in the
  middle = "hand pull"
  Right = rope tension

Forces of the rope tensioner on a tree,
with different structures.
Tension ratio on the tension spring (A) = 1: 2 and (B) = 1: 3

• When pulling in the direction of the load (e.g. the rope tensioner mentioned above):

Here the transmission ratio is 1: 3. Only a third of the “hand pulling force” is required on the pulling rope to tension the rope with three times the force over two pulleys.

Alternatives

• A pulley block with a snap hook as a deflection is easier (but has more friction than with rollers).
• The carter's knot does not use any metal deflections, but a rope loop and is therefore only suitable for low loads (see dangers ).
• Rollers instead of reversing carabiners reduce friction losses and thus increase efficiency.

hazards

• If rope were to run over rope or through a loop of tape, the pressure and movement would create too much frictional heat, which would burn the material.
• The backstop fails with a larger load, for example a rope clamp at approx. 400 daN.
• The Gardaschling squeezes the rope and reduces the breaking load.

Modifications

Loppers with 1: 3 cable pull

Other forms are described in the theoretical article Pulley .

• Three or five times the translation can be found in loppers for pruning fruit trees .
• The double pulley has four times the translation .
• The Swiss pulley system has five times the translation .

See also : List of nodes

Web links

• various pulleys in “Mountaineering and climbing - what does physics say about it?” PDF, 486 kB
• Simple pulley for cave rescue

Individual evidence

1. ↑ Frictional losses on the reversing carabiner. Retrieved August 25, 2015 .