In order to assure the unique wedge locking function of the Nord-Lock washers, the hardness of the mating surfaces should not exceed the hardness of the Nord-Lock washers.

* In order to assure the unique mechanical locking function of the Nord-Lock washers, the hardness of the mating surfaces must be lower than the hardness of the Nord-Lock washers.

The table shows the range of minimum and maximum hardness values according to ISO 898-1 for the stated standard bolt grades.