NCCER Millwright Practice Exam

Boundary lubrication usually occurs when a machine is which of the following?

1. Operating under high speed

2. Using excessive oil

3. Running at a low speed and load

4. Overheated during operation

The correct answer is: Running at a low speed and load

Boundary lubrication typically occurs when a machine is running at low speeds and low loads. In this state, the lubricant may not form a full hydrodynamic film between the moving surfaces. Instead, the lubrication is provided by a very thin layer, where only a few molecules of the lubricant are present, and it tends to rely heavily on additives or surface coatings to protect the contact surfaces. At low speeds and loads, the relative motion of the surfaces is not sufficient to generate the high pressure or shear necessary to completely separate the surfaces with a thicker film of oil. This condition can lead to direct contact between the solid surfaces, making the quality of lubrication critical to minimizing wear. Therefore, boundary lubrication helps in reducing friction and preventing damage in such operating conditions. In contrast, high-speed operations usually require hydrodynamic lubrication, where a full lubricant film is established. Excessive oil can create other problems, such as aeration or foaming, and overheating typically indicates that the lubrication condition may be failing rather than supporting boundary lubrication.