When the clamp length is relatively short, ratio Lk/d < 3. The short elongation of the screw can't compensate for settings that occur inside the goods. The consequence is that the clamp load will decrease or fully disappear in which case the screw-joint might fail.
Settings in the surface of joined materials
A major proportion of the clamp load can be lost due to settings in threads and joined material surfaces. The amount of settings is related to surface finish, material hardness and number of "joined surfaces". If the risk occurs of loosing clamp load due to large settings for example by dynamic loads or vibrations, a locking-nut should be used.
Example: Galvasised material has a soft coating
Locking nuts should be used when the screw joint consists of one or more soft materials. It is difficult to achieve and to maintain a high level of clamp load because the soft material deforms due to high surface pressure. This plastic deformation continues after the torque value has been applied to the screw. This again is a type of setting and might eventually lead to the complete loss of clamp load. Wood, plastics and aluminum are typical examples of soft materials.
Example: Other solution = apply a washer Material starts to creep: Alu starts from 160°C// Steel starts from 240°C
If the clamp load is to low, dynamic loads normally lead to the loosening of the screw joint in ordinary applications. The use of a locking nut will in most cases prevent this process
Uncontrolled torque value/ method
It's impossible to create a specified clamp load without using proper "torque methods". The scatter will vary significant depending on used tools and the amount of operators involved over a period of time. The conclusion is that some of the screw joints will be "under torqued" and as a result might loosen. In this case locking nuts will reduce the risk of failure for screw joints with low initial clamp load.
80% of all bolts that brake are due to a too low clamp force.
Example: difficult reached places or bad tools -> locking torque will never be the same
Bad connection can mean: a safety problem // hold of production
Dynamic forces make bolts easily brake via fatigue
Consequence is an unsatisfied customer of the end product