The driving performance test involves driving a self-tapping locking screw specimen into a test plate until one full thread has completely passed through the plate, without the screw breaking or sustaining damage.
The torsional strength test involves clamping the shank of a self-tapping locking screw specimen within a split-die fixture-or a similar device-that is threaded to match the screw's threads. A calibrated torque-measuring device is then used to apply torque to the screw until it fractures; this fracture must not occur within the clamped, threaded section.
A tensile test is performed on the screw specimen to verify its minimum ultimate tensile load. Fracture should occur within the shank or the unengaged threaded length of the screw, rather than at the junction between the head and the shank. Prior to fracture, the specimen must be capable of sustaining the minimum tensile load specified for its corresponding performance class.
Hydrogen embrittlement is a critical issue that requires strict attention during the surface treatment processes for self-tapping locking screws. During the acid pickling stage-where screws are agitated in dilute hydrochloric acid-the amount of hydrogen absorbed by the steel increases linearly with the square root of time until it reaches a saturation point. Furthermore, during electroplating and phosphating processes (including electrolytic degreasing), if the cathodic current efficiency falls below 100%, a significant quantity of hydrogen atoms is generated; these atoms adsorb onto the screw's surface, leading to hydrogen permeation and rendering the steel brittle due to hydrogen absorption.
The recommended hydrogen embrittlement relief (baking) time for self-tapping locking screws is 6 to 8 hours, with temperature ranges typically set at 160–200°C for phosphated parts and 200–240°C for electroplated parts. However, in actual production, the specific baking duration must be determined based on a multitude of manufacturing variables, including core hardness, surface roughness, plating duration, coating thickness, pickling time, and acid concentration. Ideally, this hydrogen relief treatment should be performed immediately after electroplating and prior to any subsequent passivation treatment.
