What Is a Flexible Coupling and When Do You Need One?

What is a flexible coupling?

A flexible coupling is a mechanical part that joins two rotating shafts, such as a motor shaft and the shaft of the machine it drives, so that power passes from one to the other. Unlike a solid, rigid joint, a flexible coupling allows a small amount of misalignment between the two shafts and can absorb shock and vibration as the machine runs.

Two shafts are almost never in perfect line, and they can also be different sizes. A flexible coupling connects them and takes up that difference, so the drive keeps running.

What a flexible coupling does

A good flexible coupling handles several jobs at once:

• Transmits torque from the driving shaft to the driven shaft.
• Tolerates misalignment, whether the shafts are at a slight angle, offset side to side, or spaced slightly apart along their length.
• Damps shock and vibration, smoothing out sudden loads so they are not passed straight through the drive.

How a flexible coupling flexes

The flexing comes from a rubber or polyurethane element held between the two halves of the coupling. The element takes up the small mismatch between the shafts and absorbs shock as the drive runs. A highly elastic coupling uses a soft, deep element, so it takes up more movement and damps more vibration than a stiff joint can.

Because the element is a separate part, it is also the part that wears. On larger couplings it is designed to be replaced on its own, without stripping the drive.

When to use a flexible coupling

Reach for a flexible coupling when:

• Two separate shafts have to be connected, for example a motor to a pump, blower or gearbox.
• Perfect alignment cannot be guaranteed or held over time.
• The drive experiences shock loads or vibration that you do not want reaching the motor or the driven machine.
• You want a serviceable joint where a worn flexible element can be replaced without stripping the whole drive. This applies to the larger sizes.

A rigid coupling is the better choice only when the shafts are held in very precise alignment and no shock damping is needed. This is uncommon in general machinery.

What a supplier needs to know

To match a coupling to a drive, a supplier will ask for:

• The torque the drive has to carry.
• The shaft sizes on both sides.
• The type and amount of misalignment expected.
• The operating speed.
• The environment. Heat, oil and chemicals all affect rubber elements.

Matching the coupling to these conditions keeps the drive running smoothly and protects the bearings on both shafts.

Signs a flexible coupling needs attention

The flexible element is a wear part. Catching wear early avoids damage to the shafts and bearings on either side. Watch for:

• More noise or vibration from the drive, which can mean the element is worn or the shafts have drifted out of line.
• Cracking or crumbling of the rubber or polyurethane element.
• Play or backlash when the drive changes direction, a sign the element is no longer taking up the load cleanly.

What you replace depends on the size of the coupling. On a larger coupling the flexible element is changed on its own. A smaller coupling is usually replaced as a whole unit.

To see the flexible couplings AVM supplies, visit the flexible coupling category.

Frequently asked questions

What is a flexible coupling used for?

It connects two rotating shafts to pass power between them while allowing slight misalignment and absorbing shock and vibration.

What is the difference between a rigid and a flexible coupling?

A rigid coupling locks the shafts solidly together and needs precise alignment. A flexible coupling allows small misalignment and damps shock, protecting the bearings and shafts.

When should I use a flexible coupling instead of a rigid one?

Use a flexible coupling whenever alignment cannot be held perfectly or the drive sees shock and vibration, which covers most general machinery.

Do flexible couplings wear out?

The rubber or polyurethane element wears over time. On a larger coupling it is replaced on its own. A smaller coupling is usually replaced as a whole unit.

How do I know if a flexible coupling is worn?

Look for extra noise or vibration, cracking of the rubber element, or play when the drive reverses.

What is a highly elastic coupling?

A flexible coupling with a soft, deep rubber element. It takes up more shaft movement and damps more vibration than a stiffer coupling, which suits engine and heavy machinery drives.