Which device is NOT considered a coupling device?

The slack adjuster is not considered a coupling device. Coupling devices are mechanisms used to connect two components, typically in a way that allows for towing or articulation between them. The fifth wheel, king pin, and pintle hook are all examples of coupling devices.

The fifth wheel is a plate mounted on the frame of a truck, allowing for secure connection to a trailer through a king pin. The king pin is a cylindrical, metal component that is inserted into the fifth wheel, facilitating the connection between the truck and the trailer. The pintle hook is another type of coupling device used primarily in heavier trailers, providing a robust connection point.

In contrast, a slack adjuster is a part of the braking system, specifically designed to automatically maintain the correct clearance between the brake shoes and the brake drum. It does not serve a purpose in coupling two vehicles together; therefore, it falls outside the definition of a coupling device. Understanding the roles of these components is vital for anyone involved in the inspection and maintenance of commercial vehicles, as it helps ensure safe and compliant operation.