The TGV SR10 Suspension
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This material is taken from a 1990 issue of the French Revue Generale du Chemin de Fer (RGCF). It is loosely paraphrased and translated. If this text is too technical for you, you can study up on railway suspension systems at the Railway Technical Web Pages by Piers Connor, referenced in the links section under the "General" heading. The SR10 suspension described in this article is now in use on all TGV trainsets in various forms. Even the first-generation TGV Sud-Est trainsets were retrofitted with this design.
Introduction
The articulated trainset architecture presents many advantages, but can at first sight present some challenges in the design of a secondary suspension that do not arise in a conventional architecture. These challenges include:
The lack of overhangs beyond the load bearing points of the secondary suspension leads to a less favorable frequency response than for a conventional railway car with two bogies. The end-supported car body in the articulated design causes a natural excitation of the fundamental vibration mode, with the lowest natural frequency and most noticeable discomfort to vehicle occupants.
The placement of the suspension points at the ends of the car body leads to a higher hunting frequency (about the yaw axis) for a given lateral suspension stiffness. This higher frequency body hunting mode increases the cross-coupling to the bogie's own hunting mode, because of the lessened spacing of the respective natural frequencies of car body and bogie. To avoid this coupling, a stiffer lateral suspension can be used, but this reduces comfort and causes its own problems at high lateral accelerations when running through curves at 150 to 200 km/h on conventional lines. In the TGV architecture, the stiffening of the lateral suspension by progressive elastomeric springs does not solve the problem because it introduces non linear behaviors in the vertical axis which adversely impact ride comfort.
The articulated nature of a TGV trainset where car bodies are linked by a pin joint leads to coupling between car bodies. The chain formed by consecutive cars thus has vibration modes of its own in addition to the modes inherent in each car.
This abundance of vibration modes causes a more complex and varied frequency response for a given dynamic input.
Despite these various handicaps, the performance obtained with the first generation of TGV trainsets (the TGV Sud-Est) is quite acceptable, judging from the response of passengers to the new service. The arrival of the second generation TGV Atlantique and the increase of service speeds to 300 km/h warranted an in-depth optimization study of the articulated suspension by SNCF.
Pneumatic Suspension
Knowing the shortcomings of the articulated architecture, the designer can be tempted to sidestep the problems altogether. One approach is to remove the coupled modes of the car bodies by removing the pin joint between them and instead suspending each separately on the bogie frame, with traction and buffing forces transmitted by a conventional coupling. At constant suspension stiffness, halving the suspended mass leads to a square root of two increase in the natural frequency. Alternately a more flexible suspension can be developed to preserve the modal characteristics, something which is technically difficult in the lateral axis. This latter approach was tested by modifying the TGV 001 prototype but did not prove entirely satisfactory.
Another research avenue is to create an efficient dynamic filter by making the natural frequency of the secondary suspension very low, below 1 Hz. This has the benefit of filtering out parasitic modes such as the vibration of the steel spring used in the original TGV suspension, a large (300 kg each) spring with very little internal damping and a natural frequency of about 20 Hz. The size of the spring is all the larger because each bogie supports a load of approximately 35 metric tons, much more than a conventional design. The large mass leads to a lowered natural frequency more prone to coupling with other dynamic modes.
From this stand point, a pneumatic suspension offers the advantages of very low mass (only a few kilograms for the membrane), good internal damping, and a very high flexibility made possible by an external air reservoir. By controlling air pressure, a pneumatic suspension also affords a load leveling capability which is made necessary by the increased flexibility. However, this type of suspension presents two challenges:
If the pneumatic connection between the membrane and the air reservoir is not carefully designed, the vertical flexibility can be frequency dependent. Above a certain cutoff frequency the flexibility drops off and the reservoir no longer has an effect, leaving the dynamics essentially determined by the membrane's flexibility. This phenomenon is sometimes used to provide additional damping, but in this case it can reduce the quality of the vibration isolation.
The membrane itself exhibits a lateral flexibility that is not constant with displacement, unlike the linear behavior of a steel spring. On a stress-strain diagram, a steel spring describes a straight line through the origin, while a pneumatic membrane describes a hysteresis cycle. Strictly speaking, this makes the simple notion of flexibility meaningless for a pneumatic membrane, unless one defines it over a hysteresis cycle. The membrane has an effective transverse flexibility that is the opposite of the desired behavior, exhibiting low flexibility for small displacements (where high flexibility is desired to decouple bogie hunting from the car body) and high flexibility for large displacements (where more stiffness is preferable to maintain a better balance between lateral displacement and passenger comfort).
The design finally selected for this suspension is composed of a pneumatic membrane connected to a high capacity reservoir. The articulation of a TGV trainset, with its inter-car support frames, affords ample space to accommodate large reservoirs directly above, and in contact with the membrane, something that would not be possible in a conventional architecture.
The high vertical flexibility obtained with this system requires the use of a anti-roll member to prevent excessive leaning in curves while preserving the desired dynamic properties. The natural frequency of the suspension (in a loaded configuration) is on the order of 0.7 Hz, and appropriate design of the membrane to reservoir connection keeps the vertical flexibility nearly constant over the relevant frequency range.
The lateral suspension tuning was more difficult, in light of the membrane's undesired lateral behavior. Through the research program a pneumatic membrane was developed that offered minimal hysteresis and a high enough flexibility that the car body's hunting frequency is on the order of 0.75 Hz (neglecting damping) and thus well below the bogie hunting frequency. This specially designed membrane incorporates a metal skirt with a geometry carefully determined in order to reduce flexibility with increasing lateral displacement. This configuration does not call for additional lateral bumpers, which as mentioned above adversely affect the vertical axis. The lateral flexibility is reduced by a factor of 2 over a displacement from the neutral position to the bumper stops, which themselves are still required to limit the lateral motion at maximum displacement.
Bogie-to-Body and Body-to-Body Connections
Any suspension design is composed of a restoring element (steel spring or as in this case a pneumatic spring) that forces the vehicle back to its equilibrium position, and a damping element (in general a hydraulic damper) which damps out the body's oscillation by proper tuning to the spring's frequency. The dampers are absolutely necessary and are generally mounted in parallel with the corresponding restoring element; however, this placement causes problems. Above the damper's natural frequency, the damper responds as a solid element and transmits parasite vibrations from the bogie to the car body. Theoretical calculations as well as experience have shown that removing the dampers, while unacceptable because car body oscillations become poorly damped, significantly reduces the residual vibrations inside the vehicle.
Bogie yaw dampers are less susceptible to this phenomenon because they are well decoupled from vertical and transverse movements of the car body. Their function, in a design where bogie hunting is already well decoupled from body hunting, is to improve bogie stability. Their influence on vibration levels is almost eliminated if the bogie design further benefits from a natural stability sufficient for the desired speeds, which the long wheelbase design of the TGV bogies already provides.
For the remaining dampers, the articulated architecture turns its liabilities into assets. As any vertical or transverse movement of two car bodies articulated on a common bogie induces a relative rotation between the car bodies, and hence a relative displacement between the car ends, the damping in these axes can be done entirely between car bodies by using longitudinal dampers at each corner of the car end. This makes it possible to remove entirely the vertical and transverse dampers that link the bogie directly to the body in a traditional architecture, thus eliminating the parasitic vibrations transmitted by these dampers.
This arrangement provides excellent damping (all the more important because of the very low natural frequencies obtained with the pneumatic suspension could cause passenger discomfort) and maintains excellent dynamic filtering properties, resulting in excellent ride quality.
Furthermore, the damper characteristics and their configuration affords a significant improvement of the transient response of the suspension (switches/points, track defects on curves taken at high cant deficiency) because the bogie's motion is unconstrained by transverse dampers.
Finally, the excellent decoupling obtained between bogie hunting and car body hunting led to a redefinition of the yaw damper, which remains the only damper linking bogie and body directly, in the sole function of damping bogie hunting. When decoupling is not so great, yaw dampers serve to damp both bogie and body hunting. In this case, they are optimized for maximal effectiveness near the natural bogie hunting frequency, which is about 4 Hz and much above the body hunting frequency.
Results
An exceptional ride stability and level of comfort has been achieved through a successful combination of computer modeling, subsystem testing and full scale line testing on TGV Sud-Est trainset number 10.
Trainset 10 performed test runs in all configurations likely to be encountered in revenue service (empty, loaded, with new wheels, with wheels at the wear limit of 450000 km, on high speed lines, on standard lines, etc.) In addition, several high speed test runs demonstrated that the SR10 suspension offers considerable performance margin above commercial service speeds. The speed record runs of TGV Atlantique trainset 325 amply demonstrated this. Trainset 325 was equipped with a stock SR10 suspension, the only modification consisting of four yaw dampers rather than the usual two per bogie. A speed of 515 km/h was reached with an astonishing level of vertical and transverse comfort. At 400 km/h the dynamic environment inside the vehicle was equivalent to a standard Corail coach at 160 km/h. Trainset 325 ran through curves at over 360 km/h with cant deficiencies above 180 mm, as well as ran over switches/points at 500 km/h without even coming close to the comfort guidelines set out by SNCF. This suspension presents a significant advance:
In the vertical axis, the comfort figure of merit as calculated by SNCF was multiplied by two. The effective vertical acceleration spectrum over a band from 4 Hz to 30 Hz was cut in half compared to the old suspension.
In the transverse axis, the comfort figure of merit was multiplied by 1.5. The effective transverse acceleration spectrum over a band from 4 Hz to 30 Hz was cut by a factor of 2 to 2.5 (depending on the point of measurement)
The transient response to track defects, while difficult to quantify, was qualitatively greatly improved as the suspension swallowed up these irregularities
According to the ISO2631 standard defining effective acceleration measurements, the mean vertical and transverse acceleration at 300 km/h on the LGV Sud-Est was just 0.15 m/s^2.
Conclusion
For SNCF, the development of the SR10 suspension represents both a breakthrough and a vindication.
The vindication, first: the articulated architecture with its many advantages turned out to be an appropriate solution to the problem of passenger comfort at high speeds, and turned its liabilities into assets. Articulation has important advantages, such as a very low natural frequency for the suspension, the accommodation of large air reservoirs between cars, and the reduction of parasitic vibration through the elimination of dampers that mechanically link the bogie to the car body.
Finally, the SR10 suspension is a breakthrough that came at the end of a long research program and allows SNCF to fit its TGV trains with a suspension worthy of the speed, safety and comfort that customers can expect.
End RGCF article
Last Update: April 2000
