A novel, handheld vibration monitoring system has been developed, which enables maintenance engineers at Siemens Mobility to check train traction motors for early signs of damage to the bearings.
The two handheld FAG Detector III vibration monitoring systems supplied by Schaeffler UK have already paid for themselves within just two months, by minimising disruption costs to rail operators and by significantly reducing the number of traction motor repairs due to bearing deterioration.
“If we detect early signs of wear to traction motor bearings, we can repair or replace that motor at our own convenience before more significant damage occurs,” says Peter Ridgway, Production Support Engineer at Siemens Mobility, Rolling Stock. “The cost of repairing a damaged traction motor would be many times the cost of a standard overhaul.”
Prior to investing in the FAG Detector III handheld vibration monitors, Siemens had been experiencing bearing deterioration problems on some traction motors, which are fitted to the Desiro electrical multiple units. The bogies on these trains are all driven by the same traction motor. What Siemens needed was some method of monitoring the condition of the traction motor bearings in order to predict their remaining operational life and to take the appropriate corrective action before damage is caused to the motor.
“In theory, these traction motors have a service life approaching one million miles before they need overhauling, but we were experiencing some bearing issues earlier than this. In March last year, I therefore started to search for a suitable solution and came across Schaeffler’s UK website, which was very helpful,” said Ridgway.
After talking to Schaeffler UK’s Engineering Manager Dr Steve Lacey, Ridgway decided to work with Schaeffler on developing a suitable condition monitoring solution. Between March and October 2010, Schaeffler carried out a comprehensive program of measuring vibration from around 100 traction motors. This involved measuring vibration on the traction motor housing and assessing the condition of the bearings and other associated mechanical components.
The vibration measurements were carried out whilst the trains were positioned over an Underfloor Wheel Lathe at two of Siemens’ rail maintenance workshops in Southampton and Ilford. These wheel lathes are used to perform routine wheelset skimming in order to maintain the correct tread profiles.
As Dr Steve Lacey states: “At the onset of the investigation, I wasn’t very hopeful that simple vibration-based parameters could be used to give a reliable indication of motor condition. However, after carrying out detailed vibration measurements on the traction motor housing, it became apparent that simple characteristic vibration parameters related to the raw signal and signal envelope could be used to give a very reliable indication of the condition of the motor, even in the presence of high background vibration from other sources within the transmission system and the Wheel Lathe such as the gearbox and axle box, as well as the belts, conveyors and bearings within the lathe itself. We believe this novel solution for assessing the condition of rail traction motors is an industry first and we’re very excited about the potential benefits that our FAG Detector III can bring to rail operators.”
Following this in-depth analysis and reporting, Schaeffler developed special software for its FAG Detector III. The aim was to provide Siemens with a device that would be easy to use and that didn’t require a vibration monitoring expert to interpret the measurement results.
The modified Detector III now enables maintenance engineers at Siemens to implement vibration measurements on a routine basis. The device has a new ‘Rail-Measurement’ menu option with pre-set limits for the Desiro traction motors. The train normally comprises four cars (coaches), each with two bogies, two axles per bogie. Each train has eight driven axles each with their own asynchronous traction motor.
Vibration measurements on the Class 444 and 450 trains (power collection DC third rail) were undertaken on the Southampton Wheel Lathe at a surface speed of 80m/min. For the Class 360 train (power collection AC overhead), measurements were obtained on the Wheel Lathe at Ilford, at a surface speed of 60m/min, which was the maximum speed available. The Desiro motor bogie has two axles, each fitted with a 250kW traction motor driving the wheelset through a 4.15:1 ratio gearbox.
“Initially, we took measurements horizontally on the traction motor housing at both the drive end and non-drive end positions by attaching an accelerometer with a magnetic base. Prior to attaching the accelerometer, the surface area had to be cleaned with a solvent to remove as much grease and dirt as possible,” recalls Dr Lacey.
“We then collected vibration data using our FAG Bearing Analyser over three frequency bands: 0-200Hz, 0-1kHz and 0-5kHz and using 8,096 data points. The results demonstrated that, even amongst the very high background noise and vibration levels, it is possible to detect vibration frequencies that are related to the drive end bearing,” adds Dr Lacey.
It was proved that simple based parameters such as the RMS (root mean square) of the signal envelope (RMS Env) and the carpet level of the envelope spectrum (CL Env) gave very reliable indications of motor condition. In most cases it was the drive radial ball bearing that seemed to be experiencing most of the problems, including some deterioration of the raceways and rolling elements. The cause of the damage is still being investigated but is thought to be due to a number of possible reasons, including degradation of the lubricant and possible electrical erosion.
Peter Ridgway concludes: “Siemens is committed to undertaking maintenance to the highest standards of integrity. We have been extremely impressed with Schaeffler’s commitment to this project and cannot fault the company’s drive and dedication to finding a reliable and practical solution for us. The cost of purchasing the two FAG Detector III units is modest compared to the cost savings we will make due to fewer motor repairs, eliminating the possibility of in-service traction motor failures and the disruption that would cause to our rail operators and networks.”
“In addition, we can now properly survey the fleet of trains to assess the condition of their traction motors and to make more informed decisions as to when to overhaul a particular motor.”