Primary and secondary railway suspensions: what are the differences?

Primary and secondary railway suspensions are essential to ensure stability, safety and comfort when a train is running. Although they belong to the same system, they operate in different positions and perform complementary functions in controlling the forces generated by the contact between wheels and tracks.

Understanding the difference between primary and secondary suspension allows us to understand how a train manages to maintain stable behavior even at high speed, when cornering and in the presence of irregularities on the railway line.

How does a train's suspension system work?

During movement, wheels and axles are subjected to continuous stress. Joints, switches, curves, variations in the track and imperfections in the line generate shocks and vibrations which, without an effective suspension system, would be transmitted directly to the trolley, the cash desk and the passengers.

The system must therefore:

• attenuate shocks and vibrations;
• maintain correct contact between wheel and rail;
• control the movements of the trolley;
• support the mass of the vehicle;
• limit the oscillations of the case;
• improve passenger comfort;
• contribute to the stability of the convoy.

To achieve these results, suspension is generally divided into two levels: primary and secondary.

What is primary rail suspension?

The primary suspension is located between the axle bushings and the bogie frame. It represents the first level of isolation with respect to the stresses coming from the track.

When a wheel encounters an irregularity, the primary suspension intervenes to absorb and control part of the impact before it is transmitted to the rest of the vehicle.

Its main functions are:

• control the movements of the axles;
• filter high frequency stresses;
• maintain contact between wheel and rail;
• contribute to the stability of the trolley;
• limit the forces transmitted to the frame;
• promote correct dynamic behavior when cornering.

The primary suspension must find a precise balance: it must be flexible enough to absorb irregularities, but also controlled enough to keep the geometry of the truck stable.

What is secondary rail suspension?

The secondary suspension is located between the bogie and the body of the rail vehicle. Its main task is to isolate the case from vibrations and movements that have already passed through the first level of suspension.

It intervenes in particular on passenger comfort and on the control of cash register movements, such as:

• vertical oscillations;
• lateral movements;
• roll;
• pitch;
• changes in structure;
• stresses generated when cornering.

Secondary suspensions can use pneumatic springs or other elastic elements, supported by shock absorbers developed to control their oscillations.

A properly designed secondary system allows the chassis to maintain a more stable and comfortable behavior without compromising the train's ability to adapt to the track.

What is the main difference?

The difference between primary and secondary suspension mainly concerns the position and type of movement controlled.

The primary suspension operates directly in the axle area and manages the stresses coming from the wheel-rail contact. The secondary suspension instead works between the undercarriage and the body, reducing the transmission of vibrations and controlling the movements perceived by the passengers.

The two suspensions should not be considered independent systems. Their effectiveness depends on how they work together and the correct calibration of each component.

What is the role of railway shock absorbers?

The elastic elements support the vehicle and absorb irregularities, but naturally tend to continue to sway. Rail shock absorbers dissipate this energy and control compression and rebound, preventing excessive or prolonged movement.

Depending on the application, they can intervene on the control:

• vertical;
• side;
• transverse;
• of roll;
• of yaw;
• of movements between trolley and cash register.

Alongside the primary and secondary suspension shock absorbers there may be yaw dampers, designed to limit the twisting movements of the undercarriage and contribute to stability at higher speeds.

Each railway application has different characteristics in terms of mass, speed, bogie geometry, route and operating conditions. There is therefore no universal calibration suitable for all trains.

The design must consider:

• static and dynamic loads;
• oscillation frequencies;
• operating speed;
• track conditions;
• cornering behaviour;
• temperature;
• durability and fatigue resistance;
• safety and comfort requirements.

Way Assauto designs and manufactures shock absorbers for primary, secondary and anti-snaking railway suspensions, combining mechanical experience, research and control processes compliant with industry standards.

To learn more about Way Assauto solutions dedicated to railways or submit a specific application, contact our technical team!