science

Sprag clutch

Sprag clutch is a science 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 Sprag clutch rather than just read about it. In short: A sprag clutch is a one-way freewheel clutch. It resembles a roller bearing but, instead of cylindrical rollers, non-revolving asymmetric figure-eight shaped sprags, or other elements allowing single direction rotation, are used.

Key takeaways

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

Reference excerpt

A sprag clutch is a one-way freewheel clutch. It resembles a roller bearing but, instead of cylindrical rollers, non-revolving asymmetric figure-eight shaped sprags, or other elements allowing single direction rotation, are used. When the unit rotates in one direction the rollers slip or free-wheel, but when a torque is applied in the opposite direction, the sprags tilt slightly, producing a wedging action and binding because of friction.

Applications

Automatic transmissions A sprag clutch is used in some automatic transmissions as a method of allowing the transmission to change gears smoothly under load. Various models of General Motors' Turbo-Hydramatic transmission have used this system as well as many transmissions from Ford, such as the Ford C6 transmission. A sprag clutch is used in most older automatic transmissions and some newer ones for shifts which require the synchronized engagement of one clutch with the disengagement of another. Using an overrunning clutch instead of a hydraulically actuated one makes this synchronization automatic. The sprag clutch simply "lets go" once the reaction force it provides is no longer needed. In a Simpson planetary gearset, common to many automatic transmissions, this happens on the 1-2 shift, and reverses on the 2-1 downshift. The 2-3 shift is done by engaging a single clutch, and nothing is released. Some newer electronically controlled transmissions make "clutch to clutch" shifts, without any sprags. This can also be used exclusively in first gear (transmission shift lever in D, but auto valve body or management selecting 1st) on some autos. That way it will automatically provide forward drive, but will not provide any engine braking. This is done not so much to avoid engine braking per se, but rather to allow a low throttle opening 2-1 downshift as a car decelerates, to avoid a loud (and potentially abrupt) and unnerving jolt as a result of the downshift. On transmissions so equipped, manual selection of 1st gear typically engages an additional band that grips the same section as the one way clutch would engage, and thus allows for engine braking.

Turbine Starter A sprag clutch may be used in the starting turbine of a turbojet engine, so that the running main engine does not keep the starter engaged at high speeds.

Helicopters A sprag clutch is used in many helicopter designs to transfer power from the engine to the main rotor. In the event of an engine failure, the sprag clutch allows the main rotor to continue rotating faster than the engine so that the helicopter can enter autorotation.

Motorcycle applications

Starters A sprag clutch is used in the electric starter motors of modern motorcycle engines to replace the Bendix gear common to automobile starters.

Primary drive

Many modern sport and racing motorcycles use sprag clutches in the primary drive between the engine and transmission. This prevents the rear wheel from losing traction during rapid downshifts when the engine speed is not matched to road speed. If a sprag clutch is not present, much greater care is required when downshifting, because losing rear wheel traction can cause dangerous highside accidents.

Bicycle hubs A few brands of bicycle components have started making bicycle hub that use a sprag clutch instead of the classic multi-pawl or Ratchet design.

Conveyors On conveyor drives, a sprag clutch is used for indexing and anti-runback.

Overrunning Overrunning occurs when two or more motors can be used to drive the same machine, including through the same shaft. A typical setup is as follows. The service drive, or prime mover, provides a combination of high speed and high power for normal operation. The secondary drive, or pony drive, can be run at low speed and low power, usually to position the conveyor for maintenance. The overrunning clutch allows the secondary drive to remain stationary during normal high speed operation of the conveyor drive.

Indexing An indexing motion provides for accuracy in the drive system. A link is installed between the prime mover and one race of the sprag clutch. The reciprocating motion of the prime mover is converted to an indexing movement on the other race of the sprag clutch.

Backstopping Backstopping prevents a loaded inclined conveyor from running back when the drive system is stopped. The outer race of the sprag clutch is fixed to the machine frame, usually via a torque arm. The inner race rotates freely in the forward direction of the conveyor drive. When the machine tries to run backward, the sprag clutch prevents this motion. In this application the sprag clutch is often called a "backstop" or "holdback".

Hoist load brake A hoist load brake is used as a secondary brake to prevent industrial hoists from catastrophically dropping their loads in the event of mechanical failure.

Pumping systems A sprag clutch is used in some pumping systems where a standby prime mover is required in case of a failure by the main drive. An example is a pump with a single rotating shaft through the unit connected at one end by an electric motor and the other by a standby diesel engine isolated by a sprag clutch.

Ratchets and Wrenches Newer designed ratchets and wrenches using this design to work in the tightest spaces with no teeth and pawl required like traditional ratchets and wrenches.

Lubrication Sprag clutches may be oil or grease lubricated. Most sprag clutch manufacturers don't allow the use of lubricants containing extreme pressure additives.

See also Ratchet (device) Slipper clutch

References

Illustrations

Sprag clutch: One-way bearing combining sprags and bearing rollers in one race — One-way bearing combining sprags and bearing rollers in one race

Worked examples

Example 1 — a first encounter with Sprag clutch

Start with the simplest possible case. Write down what Sprag clutch claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, 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 Sprag clutch 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 Sprag clutch 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 Sprag clutch

In research: Sprag clutch appears in science 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 Sprag clutch 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: Sprag clutch is common in secondary-school and first-year university syllabi. It links to neighbouring topics Clutches, Motorcycle transmissions, so understanding it makes those chapters shorter.
In everyday life: Look for Sprag clutch 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 Sprag clutch in 20 minutes

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

Frequently asked questions

What is Sprag clutch in simple terms?

Why does Sprag clutch matter?

Because it connects several science 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 Sprag clutch?

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 Sprag clutch.