Double Nutting and Jam Nuts

Start with bolt load

The first fix for self-loosening is enough bolt load in the first place (J-8.1). A joint tightened to the right target per App O is much less likely to loosen. Choose the method by the risk of loosening at that joint.

Which nut goes where

• The thin jam nut goes inside, between the flange and the full heavy hex nut. The outer heavy hex nut carries the load (J-8.2.2).
• Do not use a thin jam nut as the outer nut.
• Remove raised markings from the nut bearing faces so the nuts seat flat.
• Double nuts on both ends can unjam together. Use a cap screw or a fixed nut at one end.
• Thread tolerance (2A/2B fit) changes the stress needed to lock the nuts by up to ten times, so follow the procedure rather than feel.

Outline of the J-8.2 procedure

1. Check thread slack (axial backlash) on a sample nut pair.
2. Fit the jam nut and heavy hex nut, with the fixed end on the other side.
3. Tighten in star pattern passes to target, then check passes.
4. Lock the outer nut against the jam nut as the procedure sets out. Add an intermediate pass if the last pass needs more than twice the previous one.
5. The procedure is suitable for spiral wound gaskets on standard piping flanges.

Positive locking (J-8.3)

• Tack weld of 3/8 in. diameter or less, on a part of the stud that carries no load.
• Safety wire or cotter pin.
• Peened threads or tabbed washer plates.
• Proprietary locking products, used to the maker’s instructions.

Ordering

Allow for the jam nut height when ordering studs. The Stud Bolt Length Calculator has a double-nut option. Patriot Bolt supplies heavy hex and heavy hex jam nuts in matching grades.