Many passengers are thinking about what exactly ensures a smooth and safe movement of the train underground. The answer lies in the complex engineering system of wheel-rail interaction, which is not much different from land-based rail transport, but has its own unique features.
Unlike cars where the traction is provided by tires, subway They are based on the steel surface of the rails. This creates minimal rolling resistance, allowing heavy compounds to be dispersed with high efficiency. The basis of the movement is wheel-pair - rigid construction of two wheels pressed on the axle.
In this article, we will discuss in detail how the undercarriage of the subway is arranged, what materials are used to make rails and why the wheels are given a special conical shape. Understanding these principles will help to appreciate the scale of engineering thought hidden from the eyes of the average passenger.
Design of the metro wheel pair
The foundation of the rolling stock is wheel-pair. It is not just two wheels on the axle, but a high-precision knot that can withstand enormous loads. Unlike car wheels, subway wheels do not have pneumatic tires and are made of special high-strength steel. The axle and wheels form a single whole, which excludes the slipping of one wheel relative to the other on turns.
The key element here is comb - the inner ledge on the rim of the wheel. It prevents the wagon from leaving the track, keeping the train in the track even when passing steep turns. The distance between the inner faces of the wheel pairs is strictly standardized and corresponds to the width of the track.
- 🚆 Monoblock construction: the wheel and axle are a single unit, which increases reliability.
- 🛑 The crest of security: internal burtices that prevent the wheel from jumping off the rail head.
- ⚙️ Conical surface: the wheel profile has a cone, which allows the center of gravity to shift when entering the curve.
It is important to note that the wheel rolling surface is constantly subjected to wear. Used to extend the service life bandage - technology in which a removable steel ring is pressed onto the wheel rim, which can be replaced when thinning, without changing the entire wheel. It is a cost-effective solution for metros with heavy traffic.
Railroad: the basis of movement
If the wheels are the feet of the train, the rails are the road it walks on. In the subway, it's used. railway standard gauge, which is not much different from the railway, but has a reinforced design. The rails are made of alloy steel, resistant to abrasion and dynamic loads.
The main type of rails used in the metro has a mass of a linear meter of 50 or 60 kilograms. Heavy rails are necessary to withstand frequent passage of heavy trains without deformation. The rails are laid on sleepers (often reinforced concrete or plastic), which in turn are fixed to the ballast or concrete base of the tunnel.
⚠️ Attention: Electric current in the metro is supplied through a contact (third) rail located on the side of the running rails. The voltage there reaches 825 volts, so it is strictly forbidden to be on the tracks not only because of moving trains, but also because of the risk of electric shock.
To ensure smooth running, the joints between the rails are welded into long lashes, which eliminates the characteristic knocking of the wheels. This technology is called linkless. It also reduces the noise level in the tunnel and the vibration transmitted to the building structure.
The principle of interaction of wheel and rail
The movement of the train is possible due to the force of friction of the rolling, which in the case of a steel wheel on a steel rail is extremely small. This allows one locomotive to pull tens of tons of cargo. However, low friction creates problems during braking and acceleration, so engineers use different clutch control systems.
When a train enters a turn, thanks to the conical shape of the skiing surface, the outer wheel travels a longer path than the inner one, although both rotate at the same angular speed. This is due to the displacement of the point of contact of the wheel with the rail to a larger diameter of the cone. This mechanism is called self-centering.
The most important parameter is wheel-piece. If the profile is worn incorrectly, there is a phenomenon of "wagging" or even the risk of derailment. Therefore, profile deformation is carefully monitored during each maintenance.
- 📉 Minimum friction: allows you to save energy when moving.
- 🔄 Self-centeredness: the cone profile stabilizes the trolley in straight sections.
- ⚖️ Load distributionThe weight of the wagon is transferred evenly to the rails through point contact.
When braking is used recoverywhen the motors are operating in generator mode, but for a complete stop, pad or disc brakes are applied, pressed directly against the wheels or a disc on the axle.
Carts: body support
The body of the car does not lie directly on the wheels. It is based on special frames called cart. The trolley is a complex design that includes wheel pairs, spring suspension, traction engines and a braking system. It is the trolley that ensures smoothness of the course and extinguishes vibrations.
In modern subway cars, such as the series Moscow 2020 or NevaCarts with primary and secondary spring suspension are used. Primary (between the axis and frame of the cart) is usually rubber metal or spring, and secondary (between the frame of the cart and the body) is pneumatic. Airbags allow the body to "swim" over irregularities of the way, providing comfort to passengers.
Traction motors can be installed directly on the trolley (axial support suspension) or on the frame of the car (drive suspension). The second option is considered more progressive, as it reduces unsprung masses, which reduces the wear of the track and wheels.
Why do carts creak sometimes?
The screech occurs due to the friction of the wheel ridge against the rail in steep turns or when the wheel pair slips. Often this is a sign of profile wear or the need to lubricate the side of the rail.
Each cart has its own system. depreciationwhich is used to quench fluctuations of different frequencies. Without this system, the train would have shaken so much that it would be impossible to stand on its feet, and the structure of the car would have collapsed quickly from the fatigue of the metal.
Comparison of traffic systems: Metro vs Tram
Although both trams and subway trains use steel gauge, there are significant differences in the chassis design between the two. The Metropolitan requires higher speeds and greater carrying capacity, which dictates its standards.
Below is a table comparing the key parameters of the chassis:
| Parameter | Subway car | Tramway. | Car |
|---|---|---|---|
| Type of support | Steel wheel. | Steel wheel. | Pneumoshin |
| Diameter of the wheel | 840 - 960 mm | 600 - 750 mm | Depends on the model. |
| rut | 1520 mm (CIS standard) | 1524 mm (often) | Different. |
| Suspension | Pneumatic | Spring/Resort | Spring/Pneumo |
As you can see from the table, the metro wheels are much larger than tram wheels. A larger diameter allows for higher speeds and reduces rolling resistance. Besides, subway It is more complex and massive, since it must withstand a large axial load.
When designing new metro lines, the minimum radius of rounding the path is taken into account, since the rigid wheelbase of the trolley cannot pass too steep a turn.
Wear and maintenance of the chassis
Constant friction of the metal against the metal leads to inevitable wear. Wheel profile Over time, it wears out, the comb thins, and flat spots (potholes) may appear on the surface of the ski due to wheel locking during emergency braking. This causes a strong knock and vibration.
To restore the geometry of the wheels undergo the process of turning on special machines. There are depots. wheel-pastwhere the replacement of worn-out pairs takes place. The regularity of the check depends on the run and operating conditions of the line.
- 🔍 Defectoscopy: ultrasonic control of the metal integrity of the axle and rim.
- 📏 Profile measurement: check of conformity of the wheel shape to the standard.
- 🛠️ FloodingRemoval of the top layer of metal to restore smoothness.
⚠️ Attention: Operation of a wagon with a faulty chassis or critical wear of the ridge is prohibited. This can lead to derailment (derailment), which in the conditions of a metro tunnel threatens catastrophic consequences.
Modern diagnostic systems allow you to monitor the condition of wheel pairs in real time. The vibration and temperature sensors transmit data to the dispatcher, allowing the problem to be detected before it becomes critical.
Innovation: from steel to rubber
The classic steel-on-steel scheme dominates the world’s subways, but there are exceptions. For example, in the Paris metro and partly in Montreal and Mexico City, trains are used on the trains. rubber-tyre. They drive on special concrete paths, and steel wheels are only used for insurance and passing switches.
Rubber wheels provide better grip, allowing you to overcome steeper climbs, and create less noise. However, such a system requires more complex and expensive maintenance of the track and the tires themselves. In Russia and CIS countries, traditional traditional is widely used. steel-gauge.
☑️ What do the mechanics check before they leave?
The future is likely to be in hybrid solutions and magnetic levitation (maglev), where there is no contact with the rail at all. But for now, it is the reliable combination of steel wheel and rail that remains the “gold standard” of underground transportation.
The efficiency of the subway is based on minimal friction rolling steel wheels, which allows you to transport huge masses of passengers with minimal energy consumption.
Frequently Asked Questions (FAQ)
Why aren’t subway wheels made of rubber like a bus?
Rubber tires have high rolling resistance, which requires significantly more energy to drive. In addition, the steel wheel lasts for decades, and the rubber wheel wears out in months, which is economically unsuitable for such volumes of transportation.
What happens if the wheel loses its comb?
Without a ridge, the wheel will not be able to be held on the rail in turns and on the arrows. This will lead to derailment, stoppage of traffic on the line and potential damage to infrastructure.
How often do I change the wheels on the subway?
Complete replacement of wheel pairs occurs after the development of a resource that can be hundreds of thousands of kilometers. However, the intermediate flow (profiling) is carried out regularly in the depot to eliminate irregularities.
Why do we sometimes get sparks when we brake?
Sparks can occur when sandboxes are working (sand is supplied under the wheels to improve traction) or when brake pellets are heavily rubbed against the wheels / brake disc during emergency braking.