A325 hex bolt

A325HexBolt


DTI Washer

DTI_WASHER

TC BOLT ASSEMBLY

LABELED_TC_BOLT

introduction

Tension Control (TC) bolts are a unique type of bolt assembly that are
    designed for ease of installation and inspection.

Lets first look at the standard Hex Bolt

    (Shown on the right)

A regular hex head bolts that requires a direct tension indicator washer
(DTI) to determine if it has reached the correct tension. A DTI is a washer with bumps on it, as the tension increases these bumps begin to flatten out and to find it they are at the correct tension one must use a feeler gauge to determine how much they have been flattened which can be very time consuming when there are thousands of bolts to check.



Now lets compare this painstaking process to tension control bolts.

    (shown on the right)


A tension control bolt has a section of the bolt called the spline, the spline has a shear plane between it and the rest of the bolt which allows it to snap off when the correct tension is achieved. As long as the bolts are manufactured properly and comply with very specific codes then the spline will snap off at or just above the minimum required tension on all of the bolts. The great part about this is that instead of having to meticulously check each DTI washer one can simply glance at a bolt and visually see from a distance that the spline has been snapped off and therefore it has reached the proper tension. The specifications of these bolts helps regulate proper manufacturing, installation, and inspection; however it’s the physics behind these bolts that is intriguing and allows them to function properly.