Schedule of Fits and Clearances

Wear occurs at any time when there is motion between two parts. This motion can be intentional, such as when a shaft rotates in a plain bearing or when a roller moves back and forth over a track. Wear can also be accidental, where two parts that should be immovable chafe together.

If the parts are intended to move together, then the maintenance documentation will have a Schedule of Fits and Clearances, based on the limit system, issued for each mechanism used on the aircraft.

If the parts are not intended to move together, it will depend upon inspection procedures to discover the problem and repair schemes will be initiated in an attempt to prevent recurrence.

The Schedule of Fits and Clearances contains tables, which specify the limits on wear and other characteristics such as:

• Ovality (of a hole or shaft)
• Bow of a shaft
• Twist of a shaft.