WHY DOES A SCREW NEED TO BE FASTENED?
Slippage under the head or in the thread leads to the screw coming loose.
WHY DO SCREW CONNECTIONS FAIL?
In many cases, screw connections fail because of a loss of pretension. Self-unscrewing and pretension loss is often due to dynamic and static stresses. This can be prevented by using suitable screw fasteners and making structural changes (e.g. the calculation and design of the connection).
DEFINITION OF SCREW FASTENERS
An adequately dimensioned and structurally sound screw connection does not generally require any additional fasteners if it is assembled correctly. However, screw connections can fail, in particular in the event of dynamic stresses. This can be caused by slackening and/or self-unscrewing of the screws.
The self-unscrewing of a screw connection can be attributed to complete or partial pretension loss, which is caused by stress actions (loosening) or relative movements in the joint (unscrewing). Loosening is mostly, but not always, the first stage of a failure. Depending on the direction and total duration of the exertion of forces, a fatigue break may occur in the screw or the connection may loosen independently.