The Causes of nut-loosening in the Rail sector
Nut loosening in rail and rolling stock is caused by both vibration and settlement as huge stresses and strains are placed on bolted assemblies such as insulated joints, temporary clamps, turnouts and retarder systems.
Vibration affects any rail application that is assembled with nuts and bolts. The effect that vibration has will depend on a combination of factors:
- Age and condition of the fastener
As the bolts become damaged or corroded the torque energy used to tighten them will not efficiently transfer to clamp force. It is clamp force that keeps the joint tight. Even with brand new nuts and bolts which are
correctly torqued, only a small percentage of that torque energy will develop clamp force, with the rest overcoming frictional esistance in the threads.
- The tensile potential of the bolts
In many rail applications the correct tensile load potential of the bolt cannot be achieved. For example, if the bolts in a fishplate were tightened to their full potential, there would be consequential damage to the rail. Because of this the fishplate bolts are unable to resist vibration loosening.
- Application of torque
In many rail applications torque is applied using a non-calibrated wrench. Under these circumstances, incorrect torque is applied and it is impossible to know whether the true potential of the bolt is being realised. Any combination of the above can give rise to that particular joint failing to perform the task it is intended for. The consequences of this can range through unnecessary rail damage to derailments and worse. Vibration nut loosening is graphically illustrated under test conditions on the Tracksure website via the youtube button..
Settlement is where mating surfaces, which appear to be tightly clamped together, become loose with the effects of lateral or vertical movement, impacts, changes in temperature or other causes.Under a microscope it is possible to see two mating surfaces (surface A and B) clamped together as illustrated below. The two surfaces are in fact made up of many peaks and troughs with only certain areas in direct contact. When settlement occurs these peaks are no longer in contact and the joint is no longer held fast.
It is also a fact that when there are two components to be bolted together there are multiple mating surfaces (between components, between bolt and components, between nut and components and between thread and nuts and bolts) and settlement can occur in each of these.
- Human error
Maintenance resource is always scarce and therefore under pressure. The problems of vibration nut loosening and settlement, either singly or together, make the maintenance of bolted assemblies very onerous.The lack of fail-safe systems leaves infrastructure managers open to the plain fact that one mistake can have devastating consequences.