Loosening and releasing screw connections
Loosening or releasing of a screw connection can be initiated by setting processes (permanent change in length). In this process, the surfaces in the threads, since these surfaces are never 100% flat, conform to each other and thus deform plastically. This leads to a reduction in the elastic deformations in the screw connection and as a result also to a reduction in the pretensioning force introduced during assembly. For this reason, care must also be taken to ensure that the strength of all components involved in the connection is approximately the same.
This ultimately leads to a reduction in the residual clamping force under load and in the worst scenario to loosening of the screw connection.
The reduction of the preload force thus results in play in the connection. Even loosening of the connection can lead to an increased risk of fatigue failure. Due to the resulting space for movement, the screw is deformed and possibly bent with every load. The constant deformations lead to material fatigue or overload, and a fatigue fracture occurs.
In the case of dynamically transversely loaded bolted joints, there is the possibility of completely automatic loosening if the clamping force can no longer maintain the frictional connection in the joint. The transverse displacements that occur lead to a pendulum movement of the bolt and thus to a relative movement in the nut thread. If the amplitude is sufficiently large (maximum deflection from the rest position), sliding occurs under the head and nut contact surface. The nut or bolt is loosened.
Normally, the frictional resistance due to the clamping forces between the nut and bolt and the braced parts is sufficient to adequately secure the connection when properly installed. In the case of oscillating or impacting loads, additional securing, e.g. by locking teeth or bonding, may be necessary. If the pretensioning force or clamping force is lost due to excessive loading, the connection still remains secured.
Another important issue is the temperature at which the material is used. If the wrong material is used permanently at an elevated temperature, this can lead to significant stress relaxation. This significantly reduces the pretensioning or clamping force and the screw connection can loosen.
There are three groups for securing bolted joints:
- Securing against unauthorized loosening of the connection
- Securing against independent unscrewing
- Securing against independent loosening
VDI 2230 Part 1 - Systematic calculation of highly stressed bolted joints - Cylindrical threaded connections
Wiegand, H. / Kloos, K.-H. / Thomala, W. (2007) Bolted joints - Fundamentals, calculation, properties, handling; 5th edition, Springer Verlag; Heidelberg Berlin
"Settling" refers to the loss of preload force in a bolted connection caused by plastic deformation of the bolt (lengthening) or the clamped components (shortening). These processes reduce the clamping force and the connection loosens.
Accordingly, the settling safety device must ensure that the settling behavior of the connection is compensated for by a sufficiently large elastic spring deflection of the safety element. In any case, the expected settlement amount must be taken into account. In addition, the stiffness of the set retainer used must be greater than or equal to the stiffness of the bolt material, otherwise the retainer will not engage until a large part of the pretensioning force has already been lost (e.g. DIN 127 spring lock washer).
Based on current experience, the following settling protection methods can be classified as effective:
- Bolts with pressed-on, resilient head washer (e.g. flange bolts, spring head bolts)
- Nuts with pressed-on, concave support washer
- Clamping washers and disc springs according to DIN 6796, DIN 6908 or DIN 2093
The locking elements listed under the heading loosening protection usually maintain the pretensioning force of the bolted joint throughout the entire life cycle of the bolt.
This group includes:
- Schrauben und Muttern mit Verriegelungszähnen (z.B. VERBUS TENSILOCK®)
- Scheibenpaar mit beidseitig aufgewalzten Rippen (z.B. NORD-LOCK®)
- Flüssige oder microverkapselte Klebstoffe auf dem Gewinde (z.B. Loctite®)
Loss protectiondevices refer to the locking elements that cannot prevent partial loosening. However, they do prevent a bolted connection from completely coming apart. This type of securing is only useful for connections with transverse loads.
This group of locking elements includes:
- Nuts with clamping part (e.g. DIN 980, DIN 982, DIN 985, DIN 986, DIN 6924, DIN 6925)
- Screws with plastic insert or coating
- Screws with deliberately introduced diameter or thread defects
- Castle nuts (e.g. DIN 935, DIN 937, DIN 979, DIN 94) up to strength class 8.8 !!!
- Wire securing
The category of manipulation protection is better known under the keyword safety screws and basically has the purpose of preventing or making it more difficult for unauthorized persons to loosen or open a screw connection.
Most fuse approaches here focus on alternative drive solutions. In addition to various "standard drives", individual solutions can of course also be implemented.