A teaching aid bogie
The present project has been conducted by three engineer students at the Louvain School of Engineering whithin the frame of their master thesis (Ecole Polytechnique de Louvain, Louvain-la-Neuve, Belgium, june 2009). The main objective was essentially educational. It results in an experimental bench which aims at highlighting the wheel/rail guidance phenomenum of railway vehicles that is used for several decades. Furthermore, a slideshow located next to the device explains the various dynamic behaviour revealed by the bench: vehicle centering, limit cycles, ... and even derailment!
This demonstrator is not only intented to engineer students following the vehicle dynamics course but also to any body interested in visualizing and understanding the wheel/rail guidance via various experimental scenarios established for this scaled bogie (around 1/5 scale). And immeditatly, the first demonstration have demonstrated the real educational interest of this device, for both specialist and non-initiated visitors.
What is a bogie?
On a common train, metro or tram, the bogie ensures the link between the carbody and the track. Its main function is to ensure the vehicle guidance in safe conditions. It is commonly composed of a frame, two rigid wheelsets and two suspension stages: the primary suspension' acting between the frame and the wheelset and the secondary suspension which essentially ensure the passenger comfort.
The test rig is illustrated in the opposite figure. A scaled bogie (around 1/5 scale), composed of an aluminium rigid frame and two wheelsets whith conical running thread (rolling surface) laying on two roller pairs which replace the real track. The rollers are actuated by an electrical actuator that allows to impose different rotational speed to the bogie wheels.
The bench is able to illustrate, for a constant speed:
the bogie centering submitted to a lateral inital displacement;
the lateral stiffness of the bogie submitted to a constant lateral load;
the lateral dynamic behaviour of bogie:
stable when running under a critical speed
unstable when running above a limit speed
possibly entering limit cycle between the two
Lateral displacement and yaw angle are measured by sensors and displayed in real time on a screen next to the bench.
These experiments mainly demonstrate the « hunting motion » which combines lateral displacement and yaw angle (rotation along the vertical axis) of the bogie : this motion, stable for low speed, enters limit cycles when the speed is increased. This phenomenon originate from the forces occuring at the wheel/rail interface? These forces are quite complexe and induces, from a mathematical point of view, high non-linearities in the equation of motion. For the highest speeds, the resulting instabilies engender large vibrations and harsh impact on the structure which focus viewer attention, better that any simulation, on the potential dangerousness of the phenomenon.
All these operation modes have aslo been simulated on a computer using a dynamic model implemeted with dedicated softawre (multibody programs like ROBOTRAN, developed at the CEREM, and SIMPACK).
To strengthen the educational aspect of the bench, two devices were bult.
The first one aims at accounting for the "lateral stiffness" the a bogie induces when rolling, thanks to the thread conicity: the visitor can pull itself the bogie in the lateral direction thanks to a handle and appreciate the large return force resulting from the wheel/rail contact.
The second one illustrate the behaviour in a track with cant: an electrical cylinder placed at the ground level tilt the whole structure with a 10° angle to "simulate" this situation. It can be observed that the bogie remains centered between the two rollers.
General view The following video illustrate the general functioning of the bench, with a view on the control screen on which the motion monitoring is displayed in real time. A mirror orientated at 45° is placed on the top of the view to vizualize more easily the yaw motion. The bogie is placed at the eyes height so as to comfortably observ the wheel/rail contacts. The mechanisms is protected by a plexiglas in order to avoid accident.
Bogie centering following a lateral disturbance
When the bogie is running at a speed lower than the so-called "critical speed", it will automatically damped any lateral disturbance by doing its hunting motion. The two following videos demonstrate this phenomenon when the bogie is submitted to an initial lateral displacement. The typical screeching of the wheel on the rail clearly sounds ans remind the real bogie entering railway stations!
Limit cycle and derailment
Above a given velocity, the bogie loose is natural stability and the hunting motion is not damped anymore: it oscillates from left to right causing harmful, indeed dangerous, for the passengers. This phenomenon is illustrated on the left video.
For the highest speed, the oscillation amplitude increases endlessly and the wheels hit the rail with their flange. This is (will be!) illustrated on the right.
Conclusions and perspectives
A test rig has been built in order to highlight and characterize the kinematic and dynamic behaviour of a railway bogie. It must be recalled that it has a strictly educational vocation and aims at demistifying the wheel/rail guidance phenomenon which relies on the small wheel conicity (1/20 or 1/40) and not on the flange contact as it can be guest from play activities (toys, miniature train, etc.)
This educational bogie is used as well as for mechanical course at the Louvain School of Engineering as for plublic demonstration, for instance in collaboration with the railway industry actors.
The students ...
Quentin Bailleux (Mechanical engineer)
Vincent Dolne-Pholien (Mechanical engineer)
Sébastien Piret (Electromechanical engineer)
The promoters ...
Paul Fisette (iMMC, CEREM)
Nicolas Docquier (iMMC, CEREM)
Bruno Dehez (iMMC, CEREM)
... and other invaluable helping people !!!
Jovino Cambon (UCL manufacturing laboratory, LAFAB, iMMC)
Axel Jottard (UCL manufacturing laboratory, LAFAB, iMMC)
Alban Maton (UCL manufacturing laboratory, LAFAB, iMMC)
Jean de Frutos (UCL manufacturing laboratory, LAFAB, iMMC)
Thierry Daras (UCL smart sensors and actuators laboratory, LACTION, iMMC)
Alex Bertholet (UCL mechanical testing, structure and civil engineering laboratory, LEMSC, iMMC)
Antoine Bietlot (UCL mechanical testing, structure and civil engineering laboratory, LEMSC, iMMC)