Why Locomotives Don't Have Tires

Practical Engineering
6 Feb 202416:17
EducationalLearning
32 Likes 10 Comments

TLDRThe video script delves into the fascinating world of locomotive engineering, revealing the intricate mechanics behind the traction and adhesion that allow trains to overcome immense forces. It explores how the tiny contact patches between steel wheels and rails generate enough friction to propel massive freight trains. Through engaging demonstrations and insightful explanations, the script unravels the complexities of wheel-rail interactions, creep phenomena, and advanced traction control systems employed by modern locomotives. By combining theoretical concepts with practical examples, the script offers an engaging narrative that captivates viewers with the ingenious engineering behind train locomotion.

Takeaways
  • 🏎️ Formula 1 cars experience extreme lateral accelerations of up to 5 g-forces around corners, putting immense strain on both the driver and the car.
  • 🛤️ Traction, or the friction between the tires and the road surface, is crucial for cornering and braking in F1 racing, and Pirelli invests heavily in tire compound development.
  • 🚂 Unlike F1 cars, locomotives can generate over 50 tons of tractive effort through the tiny contact patches between steel wheels and steel rails.
  • ⚖️ Friction is the 'last frontier' of vehicle/track interaction and is essential for acceleration and braking, but also contributes to rolling resistance.
  • 🔢 Friction boils down to two factors: the normal force (weight) and the coefficient of friction between the surfaces.
  • 📐 Locomotive wheels experience a phenomenon called 'creep,' where the wheel rim spins faster than the locomotive's travel speed due to elastic deformation.
  • 🎚️ Modern locomotives use sophisticated creep control systems to vary tractive force and maintain peak friction, improving efficiency.
  • 🔄 While higher friction coefficients aid traction, there's a trade-off with durability, leading to the use of steel wheels on steel rails.
  • ⚖️ Locomotive weight is critical for developing sufficient friction to pull trains, but must be balanced against track and infrastructure limitations.
  • 🎥 The video promotes the creator's Nebula platform, highlighting its focus on ad-free, independent content tailored for niche audiences.
Q & A

  • What is the main topic of the video script?

    -The script discusses how locomotives are able to generate enough traction force to pull heavy freight trains despite the relatively small contact area between the steel wheels and rails.

  • How does the weight of a locomotive affect its traction?

    -The weight of the locomotive (or normal force between the wheels and rails) is one of the two main factors that determine the traction force. Increasing the weight of the locomotive increases the traction force, which allows it to pull heavier loads.

  • What is the role of the coefficient of friction in locomotive traction?

    -The coefficient of friction between the wheels and rails is the second main factor that determines traction force. A higher coefficient of friction results in greater traction force for a given weight.

  • What is the phenomenon of 'creep' in locomotive wheels?

    -Creep refers to the fact that locomotive wheels spin slightly faster than the speed of the locomotive due to the elastic deformation of the wheels and rails in the contact area. Part of the wheel rim sticks to the rail while the rest slips, causing the wheel to spin faster than the locomotive's translational speed.

  • How do modern locomotives maximize traction?

    -Modern locomotives use sophisticated creep control systems that monitor each wheel individually and adjust the tractive force to stay near the peak of the traction-creep curve. This allows them to extract the maximum possible traction from the available weight and friction coefficient.

  • Why don't locomotives use rubber tires for increased traction?

    -While rubber tires offer higher friction coefficients, they wear out quickly and require frequent replacement, which is impractical and expensive for freight locomotives that travel hundreds of thousands of miles. Steel wheels on steel rails offer a balance of durability and adequate traction with proper weight distribution and creep control.

  • What techniques are used to improve wheel-rail friction?

    -Railways use various techniques to improve wheel-rail friction, such as dropping sand on the tracks, using air or water jets, applying chemical mixtures, and even lasers to clean and increase the friction between the wheels and rails.

  • What is the 'stick-slip' phenomenon mentioned in the script?

    -Stick-slip refers to the oscillation between sticking and sliding that can occur when the static friction coefficient is higher than the dynamic friction coefficient. This can lead to issues like rail corrugation and unwanted noise in locomotives.

  • Why is locomotive weight so important for railway engineering?

    -Locomotive weight is crucial because it directly determines the traction force available, which in turn governs the ability of the locomotive to overcome resistance and pull heavy train loads. It is a key factor in railway engineering and track design.

  • What is the purpose of the 'Practical Construction' series mentioned at the end?

    -The 'Practical Construction' series, available on the Nebula streaming platform, documents the process of installing a water line below railroad tracks in a way that can withstand the immense forces from passing trains, without interrupting rail traffic.

Outlines
00:00
⚙️ The Engineering Marvels of Formula 1 and Locomotives

This paragraph introduces the extreme engineering involved in Formula 1 racing and locomotives. It highlights the immense forces and traction required for F1 cars to corner at high speeds, as well as the remarkable tractive effort of locomotives despite their small contact patches with the rails. The paragraph sets the stage for exploring the engineering principles behind how locomotives achieve traction without the need for tires.

05:02
🧪 Exploring the Fundamentals of Friction

Through a hands-on demonstration with a sled and various materials, this paragraph delves into the concept of friction and its governing factors: the normal force and the coefficient of friction. It illustrates how these factors influence the traction force, and how environmental conditions can impact the coefficient of friction. The paragraph emphasizes the importance of weight and friction coefficient in managing tractive effort for locomotives.

10:07
🚂 The Science Behind Locomotive Traction

This paragraph provides a deeper understanding of how locomotive wheels interact with the rails to generate traction. It introduces the concept of creep, where the wheels spin faster than the locomotive's linear motion due to elastic deformation in the contact patch. The paragraph explains how modern locomotives leverage creep control systems to optimize traction by staying at the peak of the traction-versus-creep curve. It also addresses the trade-offs between using different materials for wheels and rails.

15:11
📺 Promoting Nebula: A Platform for Independent Creators

This paragraph shifts gears to promote Nebula, a streaming platform tailored for independent content creators. It highlights the advantages of Nebula over traditional platforms, such as freedom from algorithmic constraints and the ability to produce content without catering to clickbait. The paragraph encourages viewers to support independent creators by subscribing to Nebula, offering a discounted subscription rate and emphasizing the value of ad-free, thoughtful content.

Mindmap
Keywords
💡Traction
Traction refers to the friction or grip between the wheels of a vehicle and the surface it travels on. In the context of the video, traction is described as one of the most important factors in F1 racing and locomotive engineering. For F1 cars, achieving optimal traction through the tires is crucial for cornering and braking at extreme speeds. Similarly, the tractive effort (forward force) of locomotives relies heavily on the traction between the steel wheels and rails.
💡Adhesion
Adhesion refers to the force that holds two surfaces together due to molecular attraction. In the context of railway engineering, adhesion refers to the friction or grip between the locomotive wheels and rails, which allows the wheels to transfer the tractive effort (pulling force) to the rails without slipping. The video highlights that locomotives rely solely on adhesion for traction, as the steel-on-steel contact between wheels and rails is a crucial factor in their operation.
💡Coefficient of friction
The coefficient of friction is a numerical value that quantifies the degree of friction or resistance between two surfaces in contact. In the video, the coefficient of friction is mentioned as a key factor in determining the traction force between locomotive wheels and rails. It is influenced by various factors, such as surface roughness, contaminants, and environmental conditions. The video explains how engineers aim to optimize and control the coefficient of friction to maximize the tractive effort of locomotives.
💡Creep
Creep refers to the phenomenon where the locomotive wheels spin faster than the actual speed of the locomotive along the rails. This occurs because the surface layer of the wheel is stretched forward by the rail, causing the wheel to rotate faster than the linear movement. The video demonstrates this effect using a brush and rug analogy, showing that the bristles in contact with the rug flex, effectively increasing the circumference of the brush and causing it to spin faster for the same linear movement.
💡Contact patch
The contact patch is the small area of contact between a wheel and the rail. The video states that the contact patch for a locomotive wheel is roughly the size of a US dime (about 2-3 square centimeters or half a square inch). Despite this tiny contact area, locomotives can generate immense tractive effort by optimizing factors like weight and coefficient of friction. The video uses the analogy of hanging two cars from a dime glued to a wall to illustrate the significant forces involved in the contact patch.
💡Stick-slip
Stick-slip is a phenomenon where a surface alternates between sticking to and slipping over another surface due to differences in static and dynamic friction coefficients. In the context of locomotives, stick-slip can lead to undesirable effects such as rail corrugation and unwanted noise. The video explains how modern locomotives use sophisticated creep control systems to monitor and adjust the tractive force to avoid excessive slip and stay at the optimal traction point on the traction vs. creep curve.
💡Tractive effort
Tractive effort, also referred to as pulling force, is the forward force that a locomotive can exert to pull a train. The video emphasizes that the tractive effort of a locomotive is limited by the adhesion (friction) between the wheels and rails. It discusses how engineers calculate the required tractive effort based on factors like the steepest track gradient and the available adhesion (friction coefficient) to determine the necessary locomotive weight and number of locomotives needed to move a train.
💡Normal force
The normal force is the force perpendicular to the contact surface between two objects, arising from the weight or external forces pressing the objects together. In the context of the video, the normal force refers to the weight or downward force of the locomotive acting on the wheels, which is a crucial factor in determining the traction force (along with the coefficient of friction). The video explains that increasing the normal force (weight) of the locomotive can increase the available traction force.
💡Asperity
Asperity refers to the microscopic peaks and valleys on the surfaces of materials in contact. The video briefly mentions asperity interlocking as one of the complex factors that contribute to the friction between surfaces. In the context of locomotive wheels and rails, the asperities on the steel surfaces can interlock, affecting the overall friction and traction behavior at the contact patch.
💡Dispatchable adhesion
Dispatchable adhesion is a term used in the video to refer to the reliable or dependable coefficient of friction that can be expected for a specific locomotive and operating conditions. The video states that engineers calculate the required locomotive weight by dividing the necessary tractive effort by the dispatchable adhesion (expected friction coefficient). This accounts for factors like locomotive age, weather conditions, and rail contaminants that can impact the actual adhesion or traction achievable.
Highlights

Formula 1 cars are among the fastest in the world, particularly around the tight corners of the various paved tracks across the globe.

Drivers can experience accelerations of 4 to 5 lateral gs around each lap.

Traction is one of the most important parts of F1 racing and the biggest limitation of how fast the cars can go.

A single modern diesel freight locomotive can deliver upwards of 50 tons of forward force (called tractive effort) into the rails, but it's somehow able to do that through the tiny contact patches between two smooth and rigid surfaces.

The area that's physically touching between a wheel and rail, called the contact patch, is roughly the size of a US dime: maybe 2 to 3 square centimeters or half of a square inch.

Friction really boils down to two numbers, one that's simple (weight, or more generally, the normal force between the two surfaces), and a coefficient that's a little more complicated.

Environmental contaminants like oil, grease, rust, rain, and leaves lower the coefficient of friction, making it harder to keep the wheels stuck to the track.

Even since the days of steam locomotives, sandboxes have been used to drop sand on the tracks to increase the friction between wheels and rails.

Locomotive wheels under traction exist somewhere in between sticking to a rail through friction and sliding on it from not enough friction.

The surface layer of the wheel is stretched forward by the rail, but toward the back of the contact area, there's not enough adhesion, and they separate as the elastic stress is released.

The locomotive wheels actually spin faster than the locomotive is moving along the rails, an effect called creep.

For lots of materials, the "dynamic" friction coefficient when something is sliding is less than the coefficient of friction when there's no relative movement, giving rise to the effect called stick-slip.

Sophisticated creep control systems can monitor each wheel individually and vary the tractive force to try and stay at the peak of the traction versus creep curve, eeking out a few more percentage points on the friction coefficient.

The answer to why not use rubber tires is that everything comes with a tradeoff - following the logic of more durable tires leads to ending up with a steel wheel on a steel rail.

Independent creators like those on Nebula are the future of great video, with a different model that changes the incentives and rewards away from catering to algorithms and towards producing thoughtful, niche content.

Transcripts

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