Loos Cableware

The industry leader in aircraft cable accessories, pulleys, swaging tools, and cable cutters

How Do I Get Long Service Life from Pulleys?

Even though the working success of an entire aircraft cable system is much more complex than choosing the correct pulley, understanding aircraft cable performance and longevity in relation to pulley size is very important. Pulley size is especially important in critical applications, such as aircraft flight controls, lifting & hoisting systems, and exercise equipment, where unnecessary fatigue and premature failure could result in damage to property or users.

The D:d ratio

When designing systems that will use aircraft cables over pulleys, you lessen fatigue and extend service life by choosing the highest possible D:d ratio for your application.

What is the D:d ratio?

The D:d ratio is a guideline established to assist in maximizing cable life & prevent accelerated failures. The upper case D stands for the diameter of the pulley being used, while the lower case d stands for the diameter of the cable (overall diameter on jacketed aircraft cable). The larger the pulley that you design for use with a given cable size, the higher the D:d ratio.

What does the D:d ratio mean?

In most scenarios the larger the D:d ratio, the longer the wire rope service life. It’s a method of understanding the relative service life of a cable on a particular diameter pulley when compared with other available diameters. This means that the resulting design guidelines do not provide absolute service life figures, but a gauge of service life relative to other design choices. For example, if a designer chooses to select different operating parameters, such as a smaller pulley, relative service life reductions are expected. There are many other variables that factor into the success of an aircraft cable and pulley system, especially in relation to fatigue and failure. Therefore, testing of your design is key in determining how all the forces affect fatigue and cable longevity.

We recommend minimum D/d ratios (see the general design guidelines below) to assist in your design.  The reason we provide the minimum ratios is that we understand in a lot of applications, you will not have the room to have large pulleys and sheaves. Be aware that if your system is designed with a lower D:d ratio, you can expect fatigue and failure sooner than if you used a larger pulley or smaller aircraft cable. We would also recommend more frequent inspections to check for signs of fatigue.


If you have questions about aircraft cable system designs involving pulleys, or any other technical aspects of cable systems, please contact us by email at sales@esheaves.com  or contact our sales team for more information. We can work to find the solution that best suits your application needs. Below is a table that highlights our recommended cable diameter related to pulley or sheave diameter.