What characteristic is common to planetary gears?

Planetary gears are indeed commonly used in high-density load applications because of their unique design and functionality. They consist of a sun gear at the center, with one or more planet gears that revolve around it, and a ring gear that encircles the planet gears. This configuration allows for the distribution of load across multiple gears, resulting in increased torque output and higher efficiency.

The way they are arranged enables them to handle heavier loads compared to other gear types. This is particularly beneficial in machinery and vehicles where compact space and high load capacity are required, such as in automatic transmissions, heavy machinery, and robotic systems. The ability to manage significant loads while maintaining relatively small dimensions is a key reason why planetary gears are favored in heavy-duty applications.

In contrast, the other characteristics mentioned do not accurately capture the primary advantages of planetary gears: they are not typically designed for speed increase alone (rather, they can also be used for speed reduction), and while they do have efficient designs, they can be relatively complex compared to simpler gear systems. Thus, their suitability for high-density load applications is a defining feature.