The Force that Holds Us Back

Over five hundred years ago, Leonardo da Vinci was the first person to study friction systematically. Da Vinci’s main result is still used today by many engineers:

  • Friction is proportional to the normal force.
  • When two objects are pressed together twice as hard, the friction also doubles.

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The force of friction is directly proportional to the applied load (Amontons’ 1st law). The force of friction is independent of the apparent area of contact (Amontons’ 2nd law), and kinetic friction is independent of the sliding velocity (Coulomb’s law).

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Friction in Pulleys: Why It Matters in Rigging and Lifting

Pulleys are essential tools in the world of rigging, construction, and industrial lifting. They make it easier to lift heavy loads by redistributing force, improving mechanical advantages, and minimizing strain on the worker. However, friction—that invisible force—plays a significant role in how pulleys operate and must be understood to ensure safe and effective lifting practices.

What Is Friction in Pulleys?

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Friction in a pulley system occurs at two key contact points:

  1. Between the sheave (the rotating wheel) and the axle (bearing or bushing)
  2. Between the rope (or wire rope) and the sheave

While pulleys are designed to reduce friction, it’s impossible to eliminate them. The amount of friction present affects the efficiency of the pulley system and the amount of force required to move a load.

 

Types of Friction in Pulley Systems

  1. Bearing Friction

This is the resistance that occurs where the sheave rotates on its bearing or axle. High-quality ball bearings can greatly reduce this friction, while worn or improperly lubricated bearings increase it.

 

  1. Rope-on-Sheave Friction

This friction is generated as the rope bends and moves along the sheave. Even smooth-running sheaves cause some resistance. The tighter the bend (the smaller the sheave diameter), the more resistance the rope encounters.

  1. System Friction (Total Load Resistance)

When multiple pulleys are used in a compound system (mechanical advantage), friction accumulates. Each additional sheave adds a small percentage of force lost to friction; this adds up quickly and must be accounted for in load calculations.

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Real-World Effects of Friction

  • Reduced Mechanical Advantage: A pulley system that appears to offer a 4:1 mechanical advantage may, in reality, deliver only 3.2:1 due to friction.
  • Increased Effort: The person or device lifting the load must exert more force than expected.
  • Heat and Wear: Excessive friction generates heat and accelerates wear on sheaves and ropes.
  • Safety Concerns: Misjudging the effects of friction can lead to equipment overload, dropped loads, or even failure of the pulley system.

 

 

How to Minimize Friction in Pulley Systems

  • Use High-Quality Pulleys: Bearings and materials make a big difference. Ball-bearing pulleys are ideal for high-efficiency systems.
  • Inspect and Maintain Equipment: Dirt, corrosion, or worn parts increase friction and reduce reliability.
  • Choose the Right Sheave Size: Larger sheaves reduce rope bending and wear, improving efficiency.
  • Proper Rope Selection: A rope with a smooth sheath or low stretch can reduce friction and improve control.
  • Lubricate Bearings: Regular maintenance ensures smooth rotation and reduces resistance.

Measuring Friction Loss

In rescue, industrial, or tower environments, teams often estimate friction loss as 5% to 10% per pulley. That means a system with three pulleys could lose 15% to 30% of its theoretical mechanical advantage, significantly affecting rigging decisions.

Friction is a silent but powerful factor in pulley systems. While pulleys are designed to make lifting easier, failing to account for friction can compromise performance and safety. Understanding how and where friction occurs—then taking steps to reduce it—ensures a safer, more efficient work environment.

Always inspect, calculate, and plan with friction in mind—because in rigging, every ounce of force and every percentage point of efficiency counts.

 

Stay Trained and Rig On My Friends,

Anjali and Kathy

By on July 17, 2025   /   Tower Safety