Shaft Couplings

What is a shaft coupling?

A shaft coupling is a device used to connect two rotating shafts so that power and torque can be transmitted from one to the other, such as from an electric motor to a pump, gearbox, fan, or conveyor. As well as joining the shafts, most couplings are designed to accommodate small amounts of misalignment between them, absorb shock loads, reduce vibration, and protect connected equipment from damage.

In real machinery, two shafts are rarely perfectly aligned. Mounting tolerances, thermal expansion, and wear all cause small offsets and angles between shafts. A rigid connection would put heavy loads on bearings and seals, leading to vibration and early failure. Flexible couplings solve this by allowing a controlled amount of movement while still transmitting drive smoothly.

A typical flexible coupling has:

• Two hubs - one fitted to each shaft, usually made from cast iron, steel, or aluminium
• Flexible element - a rubber, polyurethane, or plastic insert between the hubs that transmits torque, absorbs shock, and allows misalignment
• Bore - the hole in each hub that fits the shaft, either pilot bored, finished with a keyway, or tapered to suit a tapered locking bush
• Fixing - set screws, keys, or tapered locking bushes that secure each hub to its shaft

Common coupling types include:

• Jaw couplings - two hubs with interlocking jaws and a flexible spider element between them
• HRC couplings - two cast iron hubs with jaws and a rubber element, often supplied with taper bore or pilot bore hubs
• Tyre couplings - a flexible rubber tyre connecting two flanges, allowing large amounts of misalignment
• Rigid couplings - solid couplings used where shafts are precisely aligned

Key features:

• Transmits torque - connects driving and driven shafts so power is passed between them
• Accommodates misalignment - allows small angular, parallel, and axial misalignment
• Absorbs shock - flexible elements cushion sudden loads and starts
• Reduces vibration - dampens vibration and noise in the drive
• Protects equipment - reduces loads on bearings, seals, and shafts
• Easy maintenance - flexible elements can often be replaced without moving the motor or driven machine
• Range of sizes and bores - to suit small drives through to heavy industrial machinery

In simple terms, a shaft coupling joins two shafts so one can drive the other, while absorbing shock and allowing for small misalignment.

Why are shaft couplings essential?

Shaft couplings are essential because they are the link between a power source and the equipment it drives. A poorly chosen or worn coupling can cause vibration, noise, overheating, bearing and seal failure, and even shaft damage. A well-chosen flexible coupling protects the whole drive train, absorbs shock and misalignment, and helps machinery run smoothly and reliably.

Their main benefits are:

• Reliable power transmission - connects motors and driven equipment securely
• Protects bearings and seals - reduces loads caused by misalignment
• Absorbs shock loads - cushions start-up, stopping, and sudden load changes
• Reduces vibration and noise - flexible elements dampen vibration through the drive
• Easy installation - allows for small installation and alignment errors
• Quick maintenance - worn elements can be replaced quickly in many designs
• Fail-safe options - some designs keep driving even if the flexible element fails
• Flexible bore options - pilot, finished, and taper bores suit a wide range of shafts

Shaft couplings are used across a wide range of industries and applications, including:

• Pumps - connecting electric motors to centrifugal and positive displacement pumps
• Fans and blowers - direct drive fans and air handling equipment
• Gearboxes and drives - connecting motors, gearboxes, and driven machines
• Conveyors - drive units and transfer systems
• Compressors - connecting motors and engines to compressors
• Mixers and agitators - process and food machinery
• Agricultural machinery - pumps, drives, and processing equipment
• Industrial machinery - production lines, machine tools, and general drives

The main types of shaft couplings

Shaft couplings are usually grouped by their design and how flexible they are.

Jaw couplings:

• Jaw couplings - have two hubs with interlocking jaws and a flexible spider element in between. They are compact, simple, and widely used for general drives. They are also fail-safe, as the jaws continue to drive if the spider wears or fails
• Spider elements - the flexible inserts for jaw couplings, available in nitrile rubber, polyurethane, and other materials. Softer spiders absorb more shock and misalignment, while harder spiders transmit more torque
• Jaw coupling sizes - made in a range of sizes, often identified by references such as L035, L050, L070, L075, L090, and L100, with larger sizes transmitting more torque and accepting larger shafts

HRC couplings:

• HRC couplings - have two cast iron hubs with jaws and a rubber element, giving a robust, fail-safe, and easy-to-maintain coupling. Commonly identified by sizes such as HRC 70, 90, 110, 130, 150, and above
• HRC elements - replacement rubber elements, commonly made from natural rubber for good shock absorption and flexibility
• HRC hub types - available with taper bores to suit tapered locking bushes, or pilot bores that can be machined to the required shaft size

Other flexible couplings:

• Tyre couplings - use a flexible rubber tyre between two flanges, accommodating large amounts of misalignment and absorbing heavy shocks
• Curved jaw couplings - jaw couplings with curved jaws that give a more even load on the spider for higher torque and smoother running
• Chain couplings - two sprockets joined by a double roller chain, compact and able to transmit high torque
• Gear couplings - use toothed hubs and sleeves for very high torque in heavy-duty drives
• Disc couplings - use flexible metal discs for high-precision drives with zero backlash
• Bellows and beam couplings - precision couplings for servo motors, encoders, and motion control

Rigid couplings:

• Sleeve couplings - a simple sleeve that clamps or keys two shafts together, used where shafts are accurately aligned
• Clamp couplings - split couplings that clamp both shafts, for precise, rigid connections

By bore type:

• Pilot bore - a small starter bore that is machined out to the exact shaft size and keyway needed
• Finished bore - supplied ready-machined to a standard shaft size, usually with a keyway and set screw
• Taper bore - designed to accept a tapered locking bush, allowing the coupling to suit many shaft sizes by changing the bush

How to choose the right shaft coupling?

Choosing the right coupling means matching it to the torque, speed, shafts, and operating conditions. Work through these points in order:

• Torque - work out the torque to be transmitted. As a guide, torque in Nm equals the power in kW multiplied by 9,550, divided by the speed in rpm
• Service factor - multiply the torque by a service factor to allow for shock loads, frequent starts, and the type of machine being driven. Heavier, shock-loaded drives need a higher service factor
• Speed - check the coupling is rated for the maximum operating speed
• Shaft sizes - check the diameter of both shafts, as they may be different, and make sure each is within the coupling’s maximum bore
• Bore type - choose pilot bore if you will machine the bores, finished bore for standard shafts, or taper bore for use with tapered locking bushes
• Misalignment - consider how much angular, parallel, and axial misalignment the coupling needs to accept
• Shock and vibration - choose softer flexible elements or tyre couplings where shock and vibration need to be absorbed
• Fail-safe requirement - choose jaw or HRC couplings where the drive should continue if the element fails
• Space - check the overall length and outside diameter fit the available space between the machines
• Environment - consider temperature, oil, chemicals, and moisture, and choose a suitable element material
• Maintenance - choose a design where elements can be replaced easily if regular maintenance is expected
• Quality and brand - choose couplings from reputable suppliers for accurate machining and reliable performance

How to fit a shaft coupling

Fitting a coupling correctly helps it last longer and protects the connected equipment:

• Isolate the machine - make sure the motor is switched off and isolated before starting work
• Prepare the shafts - clean the shafts and keyways and remove any burrs
• Fit the hubs - fit each hub to its shaft with the correct key, and secure it with set screws or a tapered locking bush
• Set the gap - set the distance between the hubs to the manufacturer’s recommended gap, so the element fits correctly
• Fit the element - insert the spider, rubber element, or tyre between the hubs
• Align the shafts - check alignment with a straight edge, feeler gauges, dial indicator, or laser alignment tool, and adjust the machines until alignment is within the coupling’s limits
• Tighten everything - tighten all screws and bolts to the recommended torque
• Fit the guard - always fit a suitable guard over the rotating coupling before running the machine

FAQ - All you need to know about shaft couplings

What does a shaft coupling do?

A shaft coupling connects two rotating shafts so that torque can be transmitted from one to the other, while absorbing shock and allowing for small misalignment.

What is a jaw coupling?

A jaw coupling has two hubs with interlocking jaws and a flexible spider element between them. It is compact, simple to fit, and fail-safe, as the jaws continue to drive if the spider fails.

What is an HRC coupling?

An HRC coupling is a robust flexible coupling with two cast iron hubs and a rubber element. It is easy to maintain, fail-safe, and available with taper bore or pilot bore hubs.

What is a spider element?

A spider element is the flexible insert that sits between the jaws of a jaw coupling. It transmits torque, absorbs shock, and allows for small misalignment.

What is the difference between nitrile and natural rubber elements?

Natural rubber elements offer excellent flexibility and shock absorption for general use. Nitrile rubber elements offer better resistance to oils and greases.

What does pilot bore mean?

A pilot bore is a small starter hole in a coupling hub, designed to be machined out to the exact shaft size, usually with a keyway added.

What is a taper bore coupling?

A taper bore coupling has hubs with a tapered bore that accept tapered locking bushes. This allows the coupling to fit many shaft sizes simply by choosing a bush with the correct bore.

How much misalignment can a coupling handle?

It depends on the type and size. Flexible couplings can usually accept small amounts of angular, parallel, and axial misalignment, while tyre couplings can accept more. Always check the manufacturer’s limits and align the shafts as accurately as possible.

How do I know what size coupling I need?

Work out the torque from the motor power and speed, apply a suitable service factor, then choose a coupling rated above that torque. Check the coupling’s maximum bore suits both shafts.

How do I know when a coupling element needs replacing?

Signs include cracks, splits, wear, hardening, or swelling of the element, increased noise or vibration, and visible play between the hubs. Replace worn elements promptly to protect the rest of the drive.

Why did my coupling element fail early?

Common causes include misalignment, overloading, incorrect hub gap, the wrong element material, oil contamination, high temperatures, and frequent shock loads. Check alignment and coupling selection when replacing the element.

Do shaft couplings need a guard?

Yes. Rotating couplings are a serious entanglement hazard and must always be fitted with a suitable guard before the machine is run.