Today the requirements regarding safety are increasing and passengers are demanding for reliable and comfortable trains and metros. Especially on highly frequented routes as metros or urban railways, the complete traffic can stop in case of an accident or technical problem. It can end in a disaster. The media only will benefit with a headline.
In order to avoid such incidents, more maintenance and preventive care of rolling stock and locos is required. But there is a conflict. Trains are only making money when transporting people and freight, not when remaining in the workshop. Therefore new methods which guarantee more safety and reliability at lower maintenance costs are required.
Safety against derailing depends on two important factors, the wheel load and the lateral force. If the lateral force is rising and the wheel load decreasing then the danger of derailing is increasing. This situation exists frequently at the end of curves or on uneven track. At such moments leading wheels are with reduced wheel loads, or so called wheel unloading.
The existence of track twist on railway lines is fundamental. They are a result of transition layout between levelled track and canted track as well as cross level deviations.
Nencki Wheel weighing facility with twisting track can simulate the wheel unloading
At InnoTrans 2010 Nencki from Switzerland introduced the Wheel Weighing Facility NWW with twisting track. The latest technology has been put into operation in spring 2011 at Beijing Metro Line No. 4. Two similar machines which have been developed by Prose AG, are at MTR and ex KCRC in Hongkong for many years in daily operation. The NWW facility can be installed in the workshop into the service track. Within the area of the front and the rear bogie the track twisting units are installed.
The twisting of the track is carried out by computer controlled lifting and dropping units.
Test method based on EN 14363 standard
The test is according to EN 14363 standard which is normally used for the homologation of new developed rolling stock. The height of the twisting depends on the following criteria:
1. The maximum track twist
According to EN 14363 (18.104.22.168.2) the maximum cross level deterioration before maintenance “in bad conditions” (see ERRI B55 and C 138) is in European countries:
G lim = min (7.0; ( 20/pivot distance) +3 ))
This means that the maximum track deterioration is 7 ‰ for coaches with pivot distance less than 20 m. In case of longer ones it will be calculated according to the above formula.
Based on those conditons the following calculation for the twisting height of vehicle and bogie are made. In case of Non-European countries other methods can be applied.
2. The vehicle and bogie twist
According to EN 14363 (22.214.171.124.3) the twisting of the bogie itself is calculated as follows:
g lim = 7 ‰ in case of distances ≤ 4m
g lim = (20/2a) + 2.0 ‰ in case of distance > 4 m and ≤ 20 m
g lim = 3 ‰ in case of distances > 20 m and ≤ 30 m
g lim = 90/2a in case of distance > 30 m
In case of a pivot distance of 15.6 m the body must be twisted 3.3 ‰ = 52 mm. If the bogie has an axle distance of 2500 mm then the bogie twisting is 17.5 mm. In order to consider the tensioned reaction of dampers and other elastic elements on the bogie, EN 14363 is recommending to add an additional 20% twisting height and show the result in a hysteresis loop. The total bogie twisting height is then 21 mm.
The total combined twisting height is therefore 63 mm for the above described vehicle.
The twisting method is defined in A. 9.3 in EN 14363.
Initially all eight wheels of the vehicle shall be on a horizontal plane. First the vehicle body twist is applied by simultaneous lifting of wheels 11 and 21 by 26 mm (vehicle twist of 52 mm/2/ and simultaneous lowering of wheels 12 and 22 by the other 26 mm. Then a bogie twist is applied by simultaneously lifting wheels 11 and 22 by 10.5 mm and lowering wheels 12 and 21 by the other 10.5 mm. After that the same procedure is done with lowering wheel 11 and 21. The same procedure is done afterwards with wheels 31/31 and 41/42.
On the Nencki NWW the relevant height positions are controlled and determined for the report by linear transducers.
With load cells under each wheel online the wheel load Q is determined. Nencki uses special wheel load cells which are not influenced by lateral force during wheel load measuring.
The test shall be executed with empty air suspension on emergency springs. This is the most critical case regarding safety against derailing.
The critical wheel load = the unloading
In case of the twisting situation shown on the left side, the wheel 12 will have the lowest wheel load Q. This is the most critical point regarding safety against derailing which occurs at the end of curves or in case of uneven track.
Unloading is normal. If the above described twisting is done with each single wheel, the unloading should be similar on each wheel. The difference (∆) between the 4 wheel loads (Q) would be small. In case of a faulty spring, the wheel load on the relevant wheel would be smaller, the ∆ would be bigger.
Calculation of the limits According to the standard (126.96.36.199.7) the limits for Europe are calculated as follows:
ΔQ / Q0 ≤ 0.6
Q0 = (Q11 + Q12)/2 The average wheel load within the same wheel set
ΔQ = |Qmin – Q0| The smallest measured wheel load within the bogie.
Evaluation of the test result
A negative result from ΔQ / Q shows that a wheel load under “unloading conditions” is too small. Possibly reasons could be :
-worn out wheel
-Faulty primary suspension or shimming
-Twisted vehicle body after a “crash”
-Twisted bogie frame
-Faulty elastic elements
Integration of additional test systems possible
While the vehicle is rolled onto the Nencki Wheel Weighing facility NWW, the wheel profile or wheel diameter can be automatically checked and the result integrated into the safety against derailing quality protocol. Not yet realized but from the technical point of view absolutely possible is the test with a complete train composition. Testing of articulated vehicles is also described under EN 14363 A5. With the Nencki NWW concept it is also possible to carry out “load equalization of passenger railroad rolling stock” according to American APTA SS-M-014-06 standards, chapter 5.
Application for conditioned based maintenance
Traditionally a vehicle undergoes the heavy maintenance after a certain number of km or after a time period is reached. The complete vehicle or at least the bogie will stay for several days in the workshop. Expensive maintenance and exchanging of components is done whether it is necessary or not. On the other hand it can happen that a vehicle has for example a faulty spring or elastic elements, defects which can not be evaluated with traditional intermediate control instruments. Such a vehicle is possibly not safe against derailing anymore. But because the period for the scheduled heavy maintenance has not yet been reached, lifes of passengers will be at a risk. Further as a conclusion of this defect the unnecessary wearing of other elements such as expensive wheels and rails can occur.
The above described twisting test can be carried out on the fully automatic computer controlled Nencki NWW within less than half an hour. This can be done during a periodical primary maintenance cycle. If the vehicle is still safe against derailment then it can continue the daily operation without unnecessary and costly maintenance. In case of a negative test result the necessary actions can be initiated.
The challenge for the operators
Such a concept for conditioned base maintenance would mean to part with traditional maintenance methods. Operators may think about new maintenance concepts.
A proverb from Helmar Nahr is saying: The nature of time is the change of things.[ by Walter Kellenberger, Sales Director Railway Technology, Nencki Ltd. Switzerland ]