Idler-wheel

Key takeaways

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

Reference excerpt

An idler-wheel is a wheel which serves only to transmit rotation from one shaft to another, in applications where it is undesirable to connect them directly. For example, connecting a motor to the platter of a phonograph, or the crankshaft-to-camshaft gear train of an automobile. Because it does no work itself, it is called an "idler".

Friction drive An idler-wheel may be used as part of a friction drive mechanism. For example, to connect a metal motor shaft to a metal platter without gear noise, early phonographs used a rubber idler wheel. Likewise, the pinch roller in a magnetic tape transport is a type of idler wheel, which presses against the driven capstan to increase friction.

Idler pulley In a belt drive system, idlers are often used to alter the path of the belt, where a direct path would be impractical. Idler pulleys are also often used to press against the back of a pulley in order to increase the wrap angle (and thus contact area) of a belt against the working pulleys, increasing the force-transfer capacity. Belt drive systems commonly incorporate one movable pulley which is spring- or gravity-loaded to act as a belt tensioner, to accommodate stretching of the belt due to temperature or wear. An idler wheel is usually used for this purpose, in order to avoid having to move the power-transfer shafts.

Idler gear

An idler gear is a gear inserted between two or more gears (which include at a minimum a drive gear and a driven gear) to either change the direction of rotation of the output shaft, reduce the size of either or both gears while maintaining the spacing of the shafts, or both.

Gear ratio An idler gear does not affect the gear ratio between the input and output shafts. Note that in a sequence of gears chained together, the ratio depends only on the number of teeth on the first and last gear. The intermediate gears, regardless of their size, do not alter the overall gear ratio of the chain, only change the direction of rotation of the final gear. Thus, each intermediate gear can only change the sign of the gear ratio. The same rule applies to an idler wheel in a non-geared friction drive system. The surface speed of the input shaft is transferred directly to the surface speed of the idler wheel, and then from the idler wheel to the output shaft. A larger or smaller idler wheel maintains the same surface speed (which equals the surface speed of the input shaft); therefore the output shaft is driven at a constant speed regardless of the size of the idler wheel However, there are instances in a friction system where an idler wheel can double as a clutch, or slip if there is a sudden or unusually heavy load on the system, which can cause the ratio of rotations between the wheels to vary.

Applications Reversing An intermediate gear which does not drive a shaft to perform any work is called an idler gear. Sometimes, a single idler gear is used to reverse the direction, in which case it may be referred to as a reverse idler. For instance, the typical automobile manual transmission engages reverse gear by means of inserting a reverse idler between two gears. Since a driven gear (gear "A") rotating clockwise will drive a second gear ("B") counterclockwise, adding a third gear to the string means that gear "C" will be spinning the same direction as "A". A typical transmission is designed with "A" and "B" gears, so when the engine spins, the outputs shaft spins the opposite direction, which drives the vehicle forward. A straight idler gear setup is actually typically an "A" and a "C" gear, which are not in contact with each other until a "B" gear is moved between them. Since the transmission is designed to move the car forwards when the output is spinning in the opposite direction from the input shaft, when added to the "B" idler gear, it forces the "C" gear to spin in the same direction as the "A" gear, and thus the input and output shafts are spinning in the same direction, which drives the car in reverse. Another scenario is a series of rollers, such as used for pressing paper. Each roller has to be powered, but adding a motor to each one is wasteful (and it can be difficult to synchronize rotational speed with independent drive systems). One could simply add a gear onto the end of the shaft of each roller, but that means that each roller would be spinning the opposite direction of the one before (and therefore rubbing against each other as the turn). By simply adding a small idler gear between each larger gear, the result is a series of rollers, all being powered in the same direction.

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Idler-wheel

Start with the simplest possible case. Write down what Idler-wheel 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 Idler-wheel 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 Idler-wheel 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 Idler-wheel

In research: Idler-wheel 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 Idler-wheel 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: Idler-wheel is common in secondary-school and first-year university syllabi. It links to neighbouring topics Mechanical engineering, so understanding it makes those chapters shorter.
In everyday life: Look for Idler-wheel 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.

How to study Idler-wheel in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Idler-wheel 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 Idler-wheel out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Idler-wheel in simple terms?

An idler-wheel is a wheel which serves only to transmit rotation from one shaft to another, in applications where it is undesirable to connect them directly. For example, connecting a motor to the platter of a phonograph, or the crankshaft-to-camshaft gear train of an automobile.

Why does Idler-wheel 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 Idler-wheel?

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 Idler-wheel.