physics

Tractive effort

Tractive effort is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Tractive effort rather than just read about it. In short: In railway engineering, the term tractive effort describes the pulling or pushing capability of a locomotive. The published tractive force value for any vehicle may be theoretical—that is, calculated from known or implied mechanical properties—or obtained via testing under controlled conditions.

Key takeaways

Tractive effort belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Tractive effort to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Tractive effort from memory before moving on to harder problems.

Reference excerpt

In railway engineering, the term tractive effort describes the pulling or pushing capability of a locomotive. The published tractive force value for any vehicle may be theoretical—that is, calculated from known or implied mechanical properties—or obtained via testing under controlled conditions. The discussion herein covers the term's usage in mechanical applications in which the final stage of the power transmission system is one or more wheels in frictional contact with a railroad track.

Defining tractive effort The term tractive effort is often qualified as starting tractive effort, continuous tractive effort and maximum tractive effort. These terms apply to different operating conditions, but are related by common mechanical factors: input torque to the driving wheels, the wheel diameter, coefficient of friction (μ) between the driving wheels and supporting surface, and the weight applied to the driving wheels (mg). The product of μ and mg is the factor of adhesion, which determines the maximum torque that can be applied before the onset of wheelspin or wheelslip.

Starting tractive effort Starting tractive effort is the tractive force that can be generated at a standstill. This figure is important on railways because it determines the maximum train weight that a locomotive can set into motion. Maximum tractive effort Maximum tractive effort is defined as the highest tractive force that can be generated under any condition that is not injurious to the vehicle or machine. In most cases, maximum tractive effort is developed at low speed and may be the same as the starting tractive effort. Continuous tractive effort Continuous tractive effort is the tractive force that can be maintained indefinitely, as distinct from the higher tractive effort that can be maintained for a limited period of time before the power transmission system overheats. Due to the relationship between power (P), velocity (v) and force (F), described as:

P = v F {\displaystyle P=vF} or P v = F . {\displaystyle {\frac {P}{v}}=F.}

Tractive effort inversely varies with speed at any given level of available power. Continuous tractive effort is often shown in graph form at a range of speeds as part of a tractive effort curve. Vehicles having a hydrodynamic coupling, hydrodynamic torque multiplier or electric motor as part of the power transmission system may also have a maximum continuous tractive effort rating, which is the highest tractive force that can be produced for a short period of time without causing component harm. The period of time for which the maximum continuous tractive effort may be safely generated is usually limited by thermal considerations. such as temperature rise in a traction motor.

Tractive effort curves Specifications of locomotives often include tractive effort curves, showing the relationship between tractive effort and velocity.

The shape of the graph is shown at right. The line AB shows operation at the maximum tractive effort, the line BC shows continuous tractive effort that is inversely proportional to speed (constant power). Tractive effort curves often have graphs of rolling resistance superimposed on them—the intersection of the rolling resistance graph and tractive effort graph gives the maximum velocity at zero grade (when net tractive effort is zero).

Rail vehicles In order to start a train and accelerate it to a given speed, the locomotive(s) must develop sufficient tractive force to overcome the train's resistance, which is a combination of axle bearing friction, the friction of the wheels on the rails (which is substantially greater on curved track than on tangent track), and the force of gravity if on a grade. Once in motion, the train will develop additional drag as it accelerates due to aerodynamic forces, which increase with the square of the speed. Drag may also be produced at speed due to truck (bogie) hunting, which will increase the rolling friction between wheels and rails. If acceleration continues, the train will eventually attain a speed at which the available tractive force of the locomotive(s) will exactly offset the total drag, causing acceleration to cease. This top speed will be increased on a downgrade due to gravity assisting the motive power, and will be decreased on an upgrade due to gravity opposing the motive power. Tractive effort can be theoretically calculated from a locomotive's mechanical characteristics (e.g., steam pressure, weight, etc.), or by actual testing with strain sensors on the drawbar and a dynamometer car. Power at rail is a railway term for the available power for traction, that is, the power that is available to propel the train.

Steam locomotives An estimate for the tractive effort of a single cylinder steam locomotive can be obtained from the cylinder pressure, cylinder bore, stroke of the piston and the diameter of the wheel. The torque developed by the linear motion of the piston depends on the angle that the driving rod makes with the tangent of the radius on the driving wheel. For a more useful value an average value over the rotation of the wheel is used. The driving force is the torque divided by the wheel radius. As an approximation, the following formula can be used (for a two-cylinder locomotive):

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Tractive effort

Start with the simplest possible case. Write down what Tractive effort claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Tractive effort before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Tractive effort ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Tractive effort

In research: Tractive effort appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Tractive effort in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Tractive effort is common in secondary-school and first-year university syllabi. It links to neighbouring topics Force, Rolling stock, Vehicles, so understanding it makes those chapters shorter.
In everyday life: Look for Tractive effort outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Tractive effort in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Tractive effort means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Tractive effort out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Tractive effort in simple terms?

Why does Tractive effort matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Tractive effort?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Tractive effort.