Anchor Loading - Mechanical Advantage and Friction

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Ever wonder why your anchor feels the load differently depending on how you pull? Understanding mechanical advantage and friction isn't just theory—it's the difference between safe rigging and anchor failure.

Summary:

This video breaks down how mechanical advantage systems and friction affect the actual load on your anchors, with real-world testing that reveals the gap between theory and practice.

Theoretical Mechanical Advantage (The Math): Use a simple fraction formula where the mechanical advantage is the denominator:

Lifting up: Load on anchor = (MA - 1) / MA
- Example: 5:1 system = 4/5 of load (80%)
Resting (with PCD): Load on anchor = MA / MA (100% of load)
Pulling down (with redirect): Load on anchor = (MA + 1) / MA
- Example: 4:1 system = 5/4 of load (125%)

Real-World Test Results (250 lb load):

5:1 lifting up: Theory says 200 lbs on anchor; actual was 190 lbs (friction helped)
4:1 pulling down: Theory says 312.5 lbs on anchor; actual was 350 lbs (friction hurt)

Friction's Impact (10 lb test weight):

Rock Exotica pulley (92% efficient): 24 lbs on anchor when pulling down
Carabiner (inefficient pulley): 34 lbs on anchor—42% more load due to friction
Munter hitch (maximum friction): 140 lbs on anchor when hauling; only 13 lbs when lowering (friction becomes your friend in descent)

Key Takeaway: When you redirect to pull down instead of up, you're no longer helping the anchor hold the load—you're adding your pulling force to it. Combined with friction losses through inefficient pulleys or carabiners, your anchor can see significantly more load than theoretical calculations predict. Always account for both mechanical advantage AND friction when assessing anchor strength requirements.