FAA Aviation Mechanic General (AMG)Materials, Hardware and ProcessesHard
A technician torques a bolt to a table value that assumes dry, unlubricated cadmium-plated threads. If the technician instead applies engine oil to the threads before torquing to that same table value, what is the most likely effect on the bolt's actual clamping force (preload)?
- APreload decreases below the required clamping value
- BThe torque wrench reading becomes inaccurate but preload is unaffected
- CPreload increases above the intended value, risking overstress
- DPreload remains unchanged since torque value stays the same
Show answer & explanationAnswer & explanation
Correct answer: C. Preload increases above the intended value, risking overstress
Torque values in standard tables assume a specific friction condition (typically dry threads). Lubricating the threads reduces friction, so more of the applied torque converts into clamping force (preload) rather than being consumed by friction, causing the bolt to be stressed beyond its intended preload and risking failure or thread damage.
Why the other options are wrong
- A. Lubrication reduces friction, which increases preload rather than decreasing it.
- B. The torque wrench reading itself remains accurate; it is the resulting preload that becomes incorrect.
- D. Preload is directly affected by the change in thread friction, so it is not unchanged.
Torque-Friction-Preload Relationship
Standard torque tables assume a specific thread friction condition; changing lubrication (wet vs. dry) alters the relationship between applied torque and actual bolt preload.
- Torque tables typically assume dry, unlubricated threads
- Lubrication reduces friction, increasing preload for a given torque
- Using the wrong friction assumption can overstress or understress a fastener
Memory trick: Oil the threads, and the bolt squeezes tighter than the chart intended