Insufficiently secured bolt connections, which turn loose during usage or even during transportation, can be a cause of minor and major incidents – something that needs to be eliminated at source. On the basis of lightweight design and material-optimised construction, this effect of self-loosening becomes more important than ever. Whether railway, agriculture, automotive, general and heavy engineering, renewable energies or the oil and gas industry, a reliable and safe bolted connection is essential.
The potential outcomes are the annoyance and inconvenience such incidents can cause, as well as high maintenance cost. But the primary concern is the possible danger to life that might arise as a result of insufficient secured bolts.
No yielding
So, what is the answer? First and foremost, threaded connections should be designed in such a fashion that there is no way in which the maximum possible combined loads could lead to the yield point of the mated components being exceeded. The tightening torque must be selected, so that the preload force applied results in a purely frictional connection between the components.
What, then, are the guideline values for safer bolt connections? In construction, the most important parameters are the clamping force, the clamping length and the contact surfaces. Preload force should be as high as possible, at least 75% of the yield strength of the threaded fastener. The clamping length ratio should be around Lk/DNom > 5. Care should be taken to ensure a low number of contact surfaces and sufficient preload force.
Nevertheless, it is not uncommon for threaded connections to fail due to self-loosening when subjected to dynamic loads, particularly where the forces act perpendicularly to the axis of the connection. This can result in defects and damage caused by the partial or complete loss of preload force, in the form of fatigue cracks or unscrewing of connections.
Connections unscrew due to of the internal unscrewing torque within the connection, which occurs when forces act to overcome the frictional connection between the head of the fastener and the part and/or between the fastener and the internal thread. Connections formed using a clearance through-hole (nut and bolt) are particularly at risk of this type of failure, as there is more potential for the fastener to work loose.
Twin-Lock wedge lock washers prevent bolted joints from self-loosening, even under strong vibrations and dynamic loads. Unlike conventional methods, the Twin‑Lock screw locking system utilises the preload force of the joint. For that reason, it is said to be superior to conventional screw locking systems.
Lockdown
With its expertise in screw fastening and secure joinings, the Böllhoff Group offers a wide and varied range of mechanical and chemical fasteners to provide the best support and advice regarding this increasingly complex topic. To this end, it has added Twin-Lock wedge-locking washers to its portfolio of screw-locking elements.
How does it work? Twin‑Lock consists of two identical wedge‑locking washers, which feature radial teeth on one side and splined teeth on the other side. “When the nut/screw is tightened, the radial external teeth of the Twin‑Lock lock washer press positively into the contact surface,” states Böllhoff. “The tilt angle α of the splined teeth is greater than the pitch angle β of the screw thread. Therefore, when the threaded connection is loosened, the preload force initially increases.
“In addition, the coefficient of friction of the radial teeth on the outer surfaces of the washers is much higher than the coefficient of friction on the inside with the splined teeth. This leads to an increase in the preload force when the joint is released, efficiently securing the threaded connection even under strong dynamic loads and vibrations.”
In practice, the combination of the washer pairs is claimed to result in reliable anti‑rotation locking, even in adverse conditions. “The use of Twin‑Lock washer pairs is recommended wherever there is a risk of threaded connections working loose. Twin‑Lock provides effective protection, even under strong vibrations and dynamic loads.”
