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How Does a One-Way Clutch Work in Overrunning Applications?

A one-way sprag clutch is a critical component in many different mechanical applications and machines. In this article, we’ll discuss how a sprag clutch works in three different overrunning applications.

Check out the article “How a Sprag Clutch Works” if you want to dive into the technical details of a one-way sprag clutch.

Sprag Clutch Refresher

Diagram indicating clamping direction of a GMN sprag clutch. There are arrows pointing clockwise on the outer part of the clutch, and arrows pointing counter-clockwise on the inside of the clutch

A one-way sprag clutch prevents rotation from moving freely in both rotational directions. The three main ways an application uses a sprag clutch to prevent this rotation are:

  • Backstopping
  • Indexing
  • Overrunning

Today we are going to talk about overrunning. But to get an overview of the three types of stopping actions read our article Sprag Clutch: Applications for Use.

What Does an Overrunning Clutch Do?

Diagram indicating clamping direction of a GMN sprag clutch. There are arrows pointing counter-clockwise on the outer part of the clutch, and arrows pointing clockwise on the inside of the clutch

An overrunning clutch allows the rotating component to spin faster than the actual driving component that is producing the driving force.

Typically, an overrunning clutch engages and disengages when the driven RPM exceeds the driving component’s RPM.

Below we detail a few common applications designed for overrunning using a sprag clutch.

Overrunning Clutch Application One: Automatic Transmissions

Automobile transmissions need a clutch to engage and disengage gears. Using a sprag clutch is ideal because it allows the gears to smoothly transition under medium to heavy loads.

An automatic transmission

The automatic transmission requires the shift to have a synchronized engagement of one clutch, while disengaging the other. The automation of this coordinated engagement event is usually made possible by an overrunning sprag clutch. An overrunning sprag clutch “releases” once the reaction force isn’t needed any longer.

Overrunning Clutch Application Two: Torque Converter

A torque converter efficiently transfers an engine’s rotation to the transmission via the transmission fluid (also known as a fluid flywheel or fluid coupling).

Torque converter

The design of a torque converter is made by placing a reactor between a pump and turbine. Inside the reactor is a one-way sprag clutch connected to the centerline shaft.

An overrunning sprag clutch is very important in a torque converter because motion from the engine pump rotates faster than the turbine (which produces lower speeds), the fluid leaving the turbine travels axially towards the reactor and tries to turn it opposite of the power driving motion (the engine pump). A one-way sprag clutch prevents this motion and instead diverts the fluid in the same direction as the engine pump’s rotation.

Overrunning Clutch Application Three: Helicopter Rotors

Helicopter rotors for overrunning clutch application

The rotor on a helicopter is one of its most important parts because it allows the helicopter to take off and move in different directions via airfoils. If an engine failure occurs, it is critical that these airfoils are able to rotate. A freewheeling unit makes this possible.

A freewheeling unit disengages the engine from the main rotor when the engine’s RPM is less than the main rotor’s RPM. This freewheeling unit lets the main rotor continue turning at normal flight speeds.

The Freewheeling Unit

A freewheeling unit commonly consists of a one-way sprag clutch, located between the engine and main rotor transmission. Usually you find this sprag clutch either in the upper pulley (piston helicopter) or mounted on the engine gearbox (turbine helicopter).

When the engine is driving the rotor, it forces the sprag clutch against an outer drum. This prevents the engine from exceeding the transmission RPM. If the engine fails, the sprag clutch will let the outer drum exceed the speed of the inner, which means the transmission can then exceed the speed of the engine.

In this condition, engine speed is less than that of the drive system, and the helicopter is in an autorotative state meaning the blades turn by the action of air moving up through the rotor. This is a very important function which helps land a helicopter if an engine failure occurs.

A Critical Component

An overrunning sprag clutch works in many mechanical applications, here we highlighted some common ones. As you can see an overrunning sprag clutch can be a very critical safety component.

If you need a sprag clutch we know quality matters.

GMN has been manufacturing precision components, like one-way sprag clutches since 1908. We offer the highest quality of manufactured precision components and our sprag clutches have three different product lines full of variety to meet your needs. Let us quote your next project.

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