What is a Compound Pulley System and Its Mechanical Advantage?

What is a Compound Pulley System

A compound pulley system uses a fixed pulley and a movable pulley so that an operator is able to move a heavy load with less physical effort. The multiple pulley system is able to multiply the strength and pulling power of the winch, lowering the strain on the winch and the object being pulled. Such a system can be found on many heavy lifting cranes and overhead winching machines. The block and tackle is also a type of compound system.

In the most simple terms, the weight reduction of a given load is reduced by the number of pulleys used in a compound pulley system. With this in mind, a system using four pulleys to lift an object will feel like the user is actually lifting one-fourth of the item’s weight. This is why acrane is able to operate with a smaller cable and winch, saving valuable space on the machine as well as allowing the crane to be physically smaller and lighter, while still remaining operational and able to lift greater amounts of weight.

The compound system also affects the ability to pull. When using a winch to recover a vehicle that is stuck in deep mud or has become trapped in a steep ditch, for example, the winch operator will often use a compound pulley system to help ease the vehicle out while exerting minimal strain on the winch and cable. By using this type of system, a much smaller winch can often do the job of a much larger device while remaining cool and drawing much less power.

On battery-powered winch motors, this means extended pulling power from a single battery. It also means less chance of damaging the winch motor due to over-heating of the electrical circuits from straining and stressing the motor. With a hydraulic winch motor, there is much less chance of breaking a fluid line or damaging the pump due to a difficult pull. The pull will require a little more time when using a compound system, although most users agree that the benefits outweigh the problems and repairs. The system can assist with heavy tasks and allow work to be completed with much less strain placed upon any system.

Mechanical Advantage

If you have not read the companion article: Understanding Mechanical Advantage in the Single Sheave Pulley Systems Used in Rescue Operations, it is recommended that you do so before beginning this one. It explains the means by which a pulley can double its input force, examines common examples of both simple and compound rope and pulley MA (mechanical advantage) systems, and explains how to calculate mechanical advantage ratios in those systems.

One of the key points emphasized in that piece is that pulleys are force magnifiers wherever they are attached, be it the rescue load or the anchor system. It is crucial that rescue personnel understand this point. When deciding on how to configure a rescue MA system, it is too easy to allow one’s focus to be directed to the working end of the system without remembering to give due consideration to the stresses that will be applied to the anchor system. In other words, the advantages gained by using a pulley MA system come at a cost, and that cost is highly variable, depending on the MA system selected and how it is configured.

Remember too, that the forces generated by a mechanical advantage system are transferred to the rescue load through every piece of equipment in the system, any of which could fail. In particular, we would do well to worry about the lead, 3-wrap Prusik that serves as the interface between the hauling system and the main haul rope. It is probably the weak-link in the system. This Prusik can also serve as a force governor for the whole system, the canary-in-the-coalmine so to speak. Tests indicate that an 8 mm, 3-wrap Prusik, on a 1/2” rescue rope will begin to slip at between 7 and 9.5 kN (1,575- 2,125 lb.). For this reason, it is wise to task someone with keeping a watchful eye on that Prusik and instruct them to call a halt at the first sign of slippage.

While it is fun to contemplate MA systems that produce outputs such as 5:1, 6:1, 7:1, and even 9:1, in actual practice, the use of such systems should be accompanied by a sense of extreme trepidation. Remember: keep an eye on that lead Prusik!

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