The Pulley Effect

aka Parallelogram of Forces

Parallelogram of forces
When a climber falls, the belayer has to block the rope and counter the pulling force caused by the fall. The last quickdraw must thus withstand two equal forces: one on the climber and one on the belayer side. This doubling of forces is known as Pulley effect, or Parallelogram of forces.

Climbing ropes are dynamic and absorb part of the impact force, transmitting no more than 12kN to the climber. The rope pulling the last quickdraw at the end of a fall will probably exert a force of 12kN on the climber side, and 12kN on the belayer side too.

In reality many factors contribute to lower the force on the last quickdraw. The belayer side of the rope often pulls only with two/thirds of the force, reducing it to 8kN, and the angle of the rope also plays a crucial part. While 0 degrees angles double the force, 90 degrees angles increase the force of 1.5 times only and finally 120 degrees angles do not increase the force at all.

The reason why European Standards require all climbing equipment to withstand a force of about 22kN is due to the combination of the dissipation of the impact force by rope elongation and the doubling effect caused by the parallelogram of forces.

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