A lifting pulley is a classic simple machine that operates through the synergy of a circular wheel and a rope. Its core functions include changing the direction of applied force, transmitting power, and enabling labor-saving lifting of heavy objects through mechanical advantage. Based on structural differences, it is mainly classified into three types: fixed pulleys, movable pulleys, and pulley blocks, each playing a unique role in engineering practice.

 

1. Fixed Pulley: The Basic Unit for Direction Change  

The central axis of a fixed pulley is fixed to a support. During operation, it only changes the direction of the force (e.g., converting an upward pulling force to a downward one) without reducing the magnitude of the required force. It is commonly found in the top of tower cranes or wellhead water-lifting devices to optimize operating posture.

 

2. Movable Pulley: The Core of Labor-Saving Mechanism  

The central axis of a movable pulley moves with the load. Its labor-saving principle lies in the fact that the load weight is borne by two segments of the rope, so the applied force only needs to be half of the load weight. For example, the movable pulley that moves with the hook cannot change the direction but can significantly reduce operational intensity.

 

3. Pulley Block: An Integrated Solution for Efficient Labor Saving  

A pulley block is composed of a combination of fixed and movable pulleys. It amplifies the labor-saving effect by increasing the number of rope segments that bear the heavy object. Its mechanical relationship can be simplified to the formula:  

Required Force = Weight of the Object / Number of Rope Segments Bearing the Object  

For instance, when a 100-newton weight is borne by 3 rope segments, only approximately 33.3 newtons of force are needed to lift it. In practical applications, cranes often achieve hundreds-of-tons lifting capacity through倍率 design (e.g., a倍率 of 6 requires 6 movable pulleys and 6 fixed pulleys).

 

4. Impact of Materials and Craftsmanship on Performance  

Pulley materials directly affect durability and efficiency:  

- Cast Iron Pulleys: Low cost, minimal wear on steel ropes, but low strength, suitable for light-load scenarios;  

- Cast Steel Pulleys: High strength, impact resistance, suitable for heavy-duty cranes;  

- Welded/Rolled Pulleys: Lightweight design (40% lighter than cast steel), high precision, and wear-resistant rope grooves, widely used in modern cranes;  

- Non-Metallic Pulleys: Such as nylon or aluminum alloy materials, featuring anti-electromagnetic interference and corrosion resistance, suitable for special environments like ships and metallurgy.

 

5. Key Considerations in Practical Applications  

Although increasing the number of pulleys can enhance the labor-saving effect, it is necessary to balance friction loss (approximately 2%-5% efficiency loss per pulley). In addition, the pulley must match the steel rope diameter (usually 20 times the rope diameter) and use rolling bearings to reduce resistance. Modern cranes tend to adopt lightweight and modular designs, reducing the number of pulleys through high-precision pulley blocks without sacrificing performance.

 

6. Historical and Modern Application Scenarios  

Pulley technology first appeared in Assyrian reliefs in the 8th century BC and was theoretically perfected by Archimedes in ancient Greece. Today, its applications cover:  

- Construction Field: Tower cranes, elevators;  

- Industrial Equipment: Bridge cranes, winches;  

- Daily Life: Flagpole lifting systems, curtain devices, etc.