Some washers have a special design that attempts to hel […]
Some washers have a special design that attempts to help keep the nut and/or bolt from coming loose. Known as a lock washer these may be a split ring type, star type, wavy or any one of a number of other designs. A lock washer will be used with the part of the assembly that most likely could turn such as the nut. It could be used under the bolt head in instances where the bolt screws into threads in one part of the assembly.
The self loosening of fasteners is just one aspect of bolted joint design the Designer must consider during the design process. As can be seen in the photo at the side, even if threads are completely locked together by adhesive, problems cannot be prevented if the bolt preload is insufficient to prevent joint movement. The photo shows a M12 bolt that has been partially worn away by movement.
A significant advantage of a bolted joint over other joint types, such as welded and riveted joints, is that they are capable of being dismantled. This feature however, can cause problems if it unintentionally occurs as a result of operational conditions. Such unintentional loosening, frequently called vibrational loosening in much of the published literature, is an important phenomenon and is widely mis-understood by Engineers. It is important for the Designer to be aware of the bolt loosening mechanisms which can operate in order to design reliable joints.
Pre-loaded bolts (or nuts) rotate loose, as soon as relative motion between the male and female threads takes place. This motion cancels the friction grip and originates an off torque which is proportional to the thread pitch and to the preload. The off torque rotates the screw loose, if the friction under the nut or bolt head bearing surface is overcome, by this torque.
Many times washers will play a role that is a combination of the above usages. One common example is to see a lock washer used along with a larger diameter flat washer.