A concave nut is a special locknut that has a recessed section. It is used in combination with a convex nut to make up the HARDLOCK Nut (two nut system). The concave nut has the purpose of wedging the bottom convex nut forcing it perpendicularly into the threads. This removes the gap between the nut and bolt threads increasing friction, preventing the locknut from coming loose. The concave nut is usually installed after the convex nut, allowing the protruded section and recessed section to join together.
When installing the concave nut before the convex nut, precaution must be given. The Concave nut has less threads than a regular nut making it weaker than a bolt of the same material strength. If possible, the concave nut should not be installed straight onto the clamped surface where dynamic loading is at play.
The Concave nut should never be installed to the same torque as a regular nut unless the torque is specified by HARDLOCK.
Please see our recommended torque specifications for the HARDLOCK Nut (RIM TYPE) here: