The in-house escalator maintenance team at Tube Lines has developed a novel escalator "smart step" to help the company understand wear and tear on escalators.
Tube Lines looks after 226 escalators on the Jubilee, Northern and Piccadilly lines of London Underground and escalators play a vital role in ensuring passenger journeys run smoothly. With 1.3 million passengers each day, high availability is vital. Between refurbishments, Tube Lines said it is able to achieve greater than 99% availability on the escalators it maintains, which are in use 20 hours per day.
Steps are designed to last up to 20 years but Ali Albadri, lead engineer for the smart step project said, "Some steps are lasting only three years. When there is a problem it is very expensive to replace." In 2006 Tube Lines replaced over 700 steps in addition to 2000 that were renewed as part of planned refurbishment work.
The reason a step usually lasts just a few years is due to fatigue cracking caused by the step constantly travelling round the escalator. If unchecked this could result in unplanned escalator repairs or station closures and pose a safety risk. This has meant that the maintenance team has had to run non-destructive testing to ensure safe operation and plan ad hoc step replacement.
A team of engineers at Tube Lines was set up to look at why steps cracked. The team found that little was known about the actual stresses experienced by a working escalator step, eg, those that result from the differential wear through "stand on the right" passenger loading. "We needed a way to see what was going on inside the escalator," said Albadri. The team therefore identified the need for a diagnostic tool to accurately measure the stresses experienced in service - the "smart step" concept was born.
The aim of the project was to build a probe that could measure and record stress on a working escalator, allowing escalator designers and maintainers to take various measurements while an escalator was in operation, to verify component life and optimise component replacement intervals. The team worked with specialists in data capture and stress analysis to develop a measurement tool to collect strain data.
It took around six months for Tube Lines to build a prototype smart step. The step comprises 30 sensors and data logging equipment fitted within a standard escalator step and can be operated safely in normal passenger service. The strain gauges are connected to a high speed data recorder to capture each stress reading 100 times per second, to build up a full picture of stresses experienced on the step. Miniature video cameras are fitted to record the lateral motion of the step within the tracking system. The recorder can store over 20 hours of data, which is then downloaded to a laptop computer after the step has been in use for a full day of operation.
The step has been installed at Colliers Wood station and Tube Lines is assessing how the device could be used to monitor escalators continuously, without the need to download data from the data logger each night. Tube Lines says smart step should result in maintenance cost savings. By pin-pointing defects at an early stage, it will allow escalator performance and availability to be maximised.
Adrian Davey, head of IT at Tube Lines, said, "The challenge now is to turn smart step into a business solution." Among the issues he is looking at is how to get data off a moving escalator and how to use wireless technology underground. Once data is collected it will need to be loaded into an SQL Server database at the Tube Lines.
Tube Lines says the smart step technology could be applied throughout the escalator market and it is in the process of preparing a patent application. The team has also shared the findings of the smart step project with colleagues in train maintenance. This has led to the development of a "smart axle" being designed and implemented on a Piccadilly Line train. The tool will be used to determine track forces in order to offer prospective train builders core data.