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Fig. 3 Brake disks on trailer bogie

Finally, a backup mode where the brake plant is con- trolled as a standard pneumatic brake ensures interopera- bility with vehicles equipped with a standard UIC brake. Each axle is equipped with three brake disks for trailing axles (as in Fig. 3), and two for the motorized ones, where electric braking is available, too. In this configuration, the magnetic track brake should be available, since a pressure switch commanded using the brake pipe controls the track lowering (threshold at 3 bar absolute).

The corresponding configuration of the pneumatic brake plant and the inertia values used for calculations are described in Tables 1 and 2.

The pressure applied to brake cylinders and consequently the clamping and braking forces are regulated as a function of train mass (load sensing) and speed (double pressure stage). Load sensing allows optimizing braking perfor- mance with respect to vehicle inertia and weight. Double pressure stage allows protecting friction components against excessive thermal loads (double pressure stage). Both the systems allow preventing over-bral‹ing: according to the regulations [1] and [10], braking forces applied to wheels have to be limited, in order to prevent over-braking, defined as “brake application exceeding the available wheel/rail adhesion”.

In particular, the braking forces are usually regulated,

e.g. on freight trains, using a load-sensing pressure relay, simplified scheme of which is represented in Fig. 4. A sensing device mounted on the primary suspension stage produces a pressure load signal that is approximately pro- portional to the axle load. The reference pilot pressure command, produced by the brake distributor, is amplified by the relay in order to feed brake cylinders, using the leverage schematically represented in Fig. 4. The systems work as a servo pneumatic amplifier with a pneuoio- mechanic closed-loop regulation, aiming to adapt the pneumatic impedance of the distributor output to the flow requirements of the controlled plant. The gain is adjustable since the pivot of the leverage, and consequently, the amplification ratio is regulated by the pressure load signal.

Table 1 Main parameters of the braking plant [5, 6]

Coach   Bogie  Wheel Wheel Brake Number Dynamic Brake acmator Spring counter  Caliper Ratio of

caliper

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250 L. Pugi et al.

Table 2 Vehicle loading conditions and inertia values for braking plant calculation {5, 6)

Coach  Bogie   VOM load (Tare) (t)  TSI load (t)  CN load (normal) (t)  CE load (exceptional) (t) Bogie mass (t) Rotating massfaxle (t)

M1 1 15.9 16.7 17 17.6 9.93 1.5

2 13.9 15 15.4 16.3 9.8 I 1.5

T2 3 13.9 l5