Three-stage programme puts Combino trams back on track
When higher than expected torsion forces caused cracking in some of the bolted aluminium bodyshells of the Combino fleet in 2002-03, Siemens decided to modify more than 450 trams. Harry Hondius MSc reports on progress with a three-stage programme to deal with the defects
MUCH EFFORT has gone into plans to rectify the structural problems that have affected the Combino tram fleet in recent years.
Developed by Siemens Transportation Systems in Germany, the Combino was commercially successful, with 481 cars ordered by 15 cities. During 2002-03 unexpected problems emerged with the torsion resistance of the bolted aluminium bodyshells, which Siemens formally acknowledged in December 2003, when it warned that there were likely to be serious financial consequences in future years (MR04 p60).
What in 2002 seemed to be a harmless case of some broken Alugrip fastenings ultimately turned out to be a major technical deficiency. Essentially, it appeared that the torsion forces generated in S-curves were much higher than anticipated, causing cracks in the square profiles around the articulations between the car modules. After many studies, tests and consultation with its customers, Siemens decided to deal with the issue in three stages.
How many cars were affected?
Since the launch of the Combino in July 1996 when a demonstrator was unveiled in Germany, 114 Combino Basic cars have been delivered and 310 Advanced versions ordered. In addition to this total of 424, the order for 24 cars from Almeida in Portugal (RG 9.05 p523) was changed to a GT8N design using stainless-steel bodies. Contracts for 22 cars for Verona and 10 cars for Alacant were cancelled, while a number of cars yet to be delivered are being modified from the outset. These include three vehicles with three modules for Erfurt, while 12 five-module cars for the same city have yet to be built.
Another five seven-module cars for Freiburg are on order but not yet built, while a fleet of 40 Type NF12 cars for Budapest will be built to the GT12N design in stainless steel.
That leaves 403 Combinos to be modified. Add to that 36 similar NF10 and 15 NF8 for Rheinbahn and the total reaches 454 cars or 2418 modules.
Once the extent of the problem had been ascertained, Siemens undertook a series of studies and calculations to determine what action should be taken. This work included instrumenting and taking measurements on an Erfurt car and on Car 2091 for Amsterdam, which was a newly-completed vehicle. Component tests were carried out by outside agencies, and discussions were held with clients and their consultants. All this led to a proposal for three stages of work:
- Phase I: relieve the car modules from torsion forces in the upper part of the articulations;
- Phase II: relieve the car modules from forces resulting from the truck/car module connections;
- Phase III: reinforce the modules to withstand the remaining forces in such a way that a 30-year lifespan can be attained without further fissures developing.
During the studies and investigations it became very clear that at no time while the Combino cars were in service were passengers subjected to any additional safety risk from the problem. Detailed examination revealed that several factors influenced the development of the cracks - these included the different types of network, their operational characteristics and the length of the cars.
Different combinations made the car bodies more or less susceptible to fissuring, depending on the number of kilometres the cars had covered. Worst was the bi-directional seven-module car, and best was the NF type with its small-wheeled end bogies, which reduce the forces exerted on the car module bodies.
Siemens became convinced that none of the unmodified cars using Alugrip fastenings would run for 30 years without suffering damage, even when operated under the most favourable conditions. Action was therefore essential.
After making regular inspections of all the cars, Siemens quickly implemented the Phase I measures. Individual modules were reinforced locally where necessary and fissure propagation stopped. This meant that customers could continue to run the cars in service while preparations were made to carry out phases II and III.
Phase I required the upper articulation bearings to be changed. All articulation linkages that allow movements in the vertical and transverse directions have been equipped with an hydraulic damper (Fig 1).
Straight-on bearings pivoting only in the vertical (Y) axis will remain where the car consist of only two modules. As shown in Figs 2, 3 and 4, one other module will be equipped with a pivoting/rolling bearing, allowing rolling movements of the two modules about the longitudinal (X) axis. The 'galloping' bearing will be fitted with a damper. These measures should reduce the torsion loads considerably.
The Phase I changes could be carried out at customers' premises, and all have now been completed. Interestingly, rival tram suppliers took note of the changes and in a number of cases certain multi-section cars were modified in a similar way.
For Phase II the European cars will have to be moved to the Siemens plant in Uerdingen, Germany. The 12 cars supplied to Hiroshima will be dealt with locally, and 59 built for Melbourne will be moved to Melbourne's Preston tram workshops as part of a 50-day operation on each car.
The trucks of the European cars will be separated from the modules and sent to TVT, Maribor, where they will be modified as follows. All trucks will receive four hydraulic dampers on the corners between the truck frame and the car body with bump stops provided (Fig 5). These will come into play when the turning movement of the car body relative to the truck reaches 1·5°. The two parallel linkages, which connect the truck frame with the car body, are reinforced (Fig 6) and will be allowed to turn by 1·5° in each direction.
Specifically for the Amsterdam cars, the floor of the driven modules will also be reinforced. As soon as the car body starts to turn, the dampers will act and dampen the movement, thus reducing the traction forces by as much as a factor of five.
Phase III will be carried out in parallel with Phase II. This will require the interiors to be dismantled with seats, doors and other components systematically stored. The heavily-loaded Alugrip fastenings will be replaced by more solid bolted connections, and all other weak points will be strengthened. These measures are shown in Fig 7.
The sidewalls are reinforced to increase the resistance against longitudinal forces (Fig 8). Note that:
- the new vertical window profiles now penetrate deep into the roof girder and on to the bottom plate to distribute the forces more effectively (Fig 9);
- the door frames are strengthened and connected to the floor by stronger corners (Fig 10);
- the articulation carriers of the wheel-less intermediate modules are reinforced, as are the module ends (Fig 11);
- with the Basic Combino models, the connection with the upper articulations must also be reinforced.
To check the effectiveness of all these measures, Siemens carried out an intensive programme of trials with a completely revamped Amsterdam car 2091 (p615). Sensors positioned at 400 locations were connected to instruments that recorded the data and allowed a comparison between the actual measurements and the calculations made earlier.
A test drive over the Amsterdam network confirmed that the ride quality in curves was much improved, with more of the feel of a conventional bogie tram. The car modules are able to move more freely towards each other and the 'knacking' noise has been eliminated.
Once all this work is complete, the car is reassembled and all components are refitted. Repainting is carried out if necessary and the modules remounted on their trucks.
Siemens calculates that the rebuilt cars will be good for at least 30 years without the risk of body fissures developing. The cost of the whole operation is put at €400m, a sum which very few, if any, industries would have the financial means to sustain.
Weight and noise implications
It is clear that the addition of structural reinforcements means that the cars are heavier. Around 600 kg is added to a basic three-module car and 450 kg for the advanced version. The weight of the five-module cars will rise by 1000 kg or 800 kg, and the seven-module vehicles by 1450 kg or 1100 kg respectively.
With a 3·5% increase in weight, power consumption increases by the same proportion if all other factors remain equal. If the weight increase is critical for the infrastructure, Siemens envisages a package of weight-saving measures for the interior fittings.
The Combino is unfortunately not the quietest among the all-low-floor tram designs, with Cityrunner and Citadis being less noisy. The Combino modification programme presents an ideal opportunity to try and make the cars quieter, perhaps by making changes to the unpowered trucks. The resulting weight increase would be well invested.
Was there an alternative?
The 454 cars involved were built over a period of six years. Assuming that construction of 120 cars a year is the best that could be achieved, it would take four years to build new or replacement cars, provided that ways could be found to stay within the same dimensions and weights.
The present programme of modifications will take two years to complete, so in practice there is no alternative. Of course, Siemens, its independent consultants, safety authorities and customers must be thoroughly convinced that no further fissures will occur during the remaining life of the trams, and Siemens will have to guarantee this.
Perhaps one lesson to be drawn is that when capital equipment with a long lifetime is purchased, the financial strength of the supplier needs to be checked carefully before a commitment is made.
- CAPTION: A newly-built Amsterdam car incorporating all the modifications was used to verify the effectiveness of the structural changes. The car was instrumented with over 400 sensors for testing (inset)
- Fig 1. The upper articulation bearing with an hydraulic damper allows movements in the vertical and transverse directions
- Fig 2. Arrangement of upper articulation bearings on a five-module car. From left to right: pivoting/rolling bearing, pivoting bearing, and a 'galloping' bearing allowing movement about the vertical and transverse axes, as needed to pass over the canal bridges in Amsterdam, for example Source: Siemens
- Fig 3. Simplified model showing how the bearings fitted between modules allow pivoting and rolling movements. a shows the neutral position, b shows the rolling or sway movement and c shows the pivoting movement or rotation about a vertical axis:
1, 2, 3 Ball-and-socket joints
4 Articulation linkage
5 Spring, damper and stop
- Fig 4. Final design of the pivoting/rolling bearing
- Fig 5. Driven truck modification with dampers inserted between the axle bearings and the truck frame. The shock absorber is to the right and the rubber bump stop is also clearly visible
- Fig 6. Reinforced parallel linkage between truck and car body
- Fig 7. Overall view of bodyshell modifications. Roof reinforcement of end modules on the Basic model (top left); double corner connection showing the window support penetrating the roof girder (top right); reinforcement of floor plate on the driven module of an Amsterdam car (bottom right); reinforcement of sidewall in wheel-less modules (bottom centre), reinforcement of articulation end carrier on a wheel-less module (bottom left), reinforcement of end portal frames (left)
- Fig 8. Strengthening of side panels by tensioning. Note the new bolted aluminium corners and end portal frame reinforcement
- Fig 9. New vertical window supports penetrate deeply into the roof girder; The connection with the bottom plate is similar
- Fig 10. New connection bracket between the door frame and the bottom plate
- Fig 11. Reinforcement of the end portal frame and of the horizontal articulation carrier of a wheel-less module (foreground)
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