Sales: +44 (0)1525 840557 - Fax: +44 (0)1525 403918 - E-mail: sales@wheelsure.co.uk
Vibration nut loosening in the rail industry
Vibration nut loosening is a significant challenge in both rail infrastructure and rolling stock. Bolted joints, in numerous applications, at best rely on constant maintenance to function, and at worst fail. The consequences of failure can of course be fatal in the extreme; at the other end of the spectrum the operational costs of excessive maintenance are a huge burden within the industry.
The problem is tragically illustrated by two fatal derailments in the UK; Potters Bar in 2002 and Grayrigg in 2007. Loose nuts contributed to both these fatal crashes. However, the failure of components was, in both cases, exacerbated by reported shortcomings in the maintenance regimes. The coincidence of both failures resulted in loss of life and the less important, but nonetheless very significant, costs to the industry.
“Four main factors contributed to the failure of points 2182A: components were in poor condition; nuts to secure the right-hand end of the rear stretcher bar and the left-hand end of the front stretcher bar were absent from the ends of the bars; and the lock stretcher bar failed by fatigue, broke and became disengaged from the bracket fixing it to the right-hand switch rail.”
Extract from Health and Safety Executive progress report into the Potters Bar accident
Full details: www.hse.gov.uk/railways/pottersbar/htm
“Investigation of the lock and stretcher bars in the facing points at Lambrigg crossover showed that one of three stretcher bars was missing, and bolts that secured the lock bar and another stretcher bar were not in place some of these bolts and the associated nuts and washers were found in the ballast, but others were not”.
Extract from the Rail Accident Investigation Branch interim report into the Grayrigg accident Full Details: www.raib.gov.uk/cms_resources/070226/I012007_grayrigg.pdf
A fatal train crash in Italy in 2002 also saw loose bolts listed as a prime cause demonstrating that the problem is more widespread than the UK.
In the instances described above, nuts were found loose at the side of the track. In many more cases bolt loosening is invisible and leads to bolt hole cracks and potentially serious rail fatigue. In these circumstances derailments occur with, in at least one case in Canada in 2000, serious cost and environmental implications.
According to the Federal Railroad Administration there were just under 1700 derailments in the USA in 2007, approximately 66% of all train accidents. A small but significant number of these count loose bolts or failed joints and bolt hole cracking as the principal cause.
The causes of nut loosening
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.
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. (www.tracksure.co.uk/wheelsure-wheelloss)
Settlement
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.
