17.05.17
Why are S&C layouts failing?
Source: RTM Apr/May 17
Dr Sin Sin Hsu, programme engineering manager IP Track Development at Network Rail, analyses why switch and crossing (S&C) layouts fail prematurely and how a Good Practice Guide to S&C design helps mitigate the risk.
S&C performance in terms of reliability and asset life has deteriorated across the UK network in recent years. Delay minutes in 2014-15 for S&C failures causing disruption to traffic and customers cost Network Rail over £60m, not including maintenance costs. To some extent this is due to increases in tonnage and speed, and to newer, heavier rolling stock. However, there are more fundamental underlying issues with components and designs which manifest themselves primarily at the wheel-rail interface – where millimetres matter.
Standards provide safe limits for operation, but don’t necessarily give guidance on good design of components or systems to achieve performance requirements and maximise service life.
An increasing number of new layouts are failing within months, or even weeks, of installation. New switches in the Reading remodelling are a case in point. Great West Junction (Basingstoke) was renewed in 2007, but ongoing speed restrictions due to geometry deterioration and component failure have plagued the layout ever since. Such failures are preventable.
What are the causes – and the solutions?
Analysis of defects in switches reveals that by far the most prevalent (over 3,000 per year) is switch blade damage, with an escalating trend over the past four years. Failures are also occurring at common and obtuse crossings, with a growing trend of foot cracking in cast crossings.
These trends need to be arrested and performance and reliability improved. To achieve this goal, it is necessary to identify root causes of failure modes and propose robust solutions.
Thorough site investigations and detailed analysis, using tools such as VAMPIRE and Track-Ex, demonstrated that the majority of failures could have been avoided through better layout and alignment design and component specification.
A series of case studies, all of which looked at critical design features at specific sites and the application of alternative design to mitigate the inherent problems, are documented in the S&C Track Design Good Practice Guide (free to download from the PWI website, or the Network Rail Standards website).
The Guide promotes techniques and evidence-based engineering by using a ‘Red, Amber, Green design checklist’ for designers and specifiers to improve safety and reliability, and therefore more sustainable S&C performance. Examples of proposed solutions are:
- Reducing lateral forces on switch rails by increasing the turnout radius
- Improving vehicle steering though S&C by increasing cant deficiency
- Prolonging component service life for traffic type and speed, e.g. high axle-loads, avoid using sharp-angled crossings in high-speed S&C
- Providing consistent track support under S&C – and adjoining plain line
- Designing out, or simplifying, high-maintenance S&C, e.g. double slips, two-levelling
- Minimising high-impact loading at crossings, by optimising wheel transfer
- Suitable switch profile and horizontal geometry: compatibility with wheel profiles
- Appreciating the importance of correct machining and assembly tolerances
- Appreciating that manufacturing limitations may necessitate a ‘maintenance activity’, e.g. grinding, at or shortly after layout installation
Shalford Junction: a case study
A one in six cast crossing in a new double junction at Shalford was being damaged, within months of installation. Poor wheel transfer resulted in significant impact loading, leading to nose damage, voiding and damage to the nose bearer, and loosening of rail fastenings.
Furthermore, the crossing was a two-levelled design, on a rake of 20mm, with baseplates thickened by 20mm, giving 40mm overall cant. Such designs skew the wheels relative to the rail running plane, with adverse effects on wheel transfer – from wing rail to nose.
The running band (shown by using white paint) showed that each passing wheel imparted two impact loadings at the crossing nose. Firstly, through the downward ‘jump’ from wing rail to nose tip. The wheel briefly ‘takes flight’, then lands with a secondary impact – of lesser magnitude, but over the same bearer. This phenomenon is due to a deficiency in the design of the crossing.
Within six months, the nose had been battered to the shape of the flange root of the wheel. The thickened baseplates rotate the wheelset approximately 1⁰ clockwise relative to the running plane, creating a clash between the wheel flange and crossing nose. With metal-on-metal contact, the impact loading is increased, until one side of the interface becomes conformal – exactly as found at Shalford.
A replacement crossing for Shalford Junction has been manufactured with three design changes: inclined wing rail to better support the wheel at the wheel transfer area; single continuous nose topping (previously the topping consisted of two gradients which caused the secondary impact); and tolerance on the topping machining dimension has been tightened to +0.5/-0mm for this particular crossing.
These measures will give smoother wheel transfer at the nose and reduce, or even eliminate, the secondary impact, thus improving the performance of the crossing.
For More Information
W: www.thepwi.org