Because many teeth share the load and steel teeth in shear are extremely strong.

Elastomeric couplings transmit torque through a rubber element, which is compliant and therefore limited in the stress it can carry. A gear coupling transmits through steel teeth in direct contact, with a large number of teeth engaged around the full circumference at once.

That gives a very high torque density: for a given outside diameter, a gear coupling handles substantially more torque than most alternatives, which matters when the coupling has to fit between two machines whose shafts are already positioned.

It also makes them torsionally stiff - they transmit torque almost without wind-up, which is desirable where precise angular relationship matters and undesirable where the drive train needs compliance to absorb shock.

Where shock absorption or vibration damping is needed, an elastomeric or grid coupling is the better choice despite the lower torque density.

Through crowned teeth that can rock inside the sleeve teeth, with the number of meshes determining what kinds of misalignment are accepted.

The hub teeth are crowned - barrel-shaped along their length - rather than straight. That crown lets each tooth pivot within its mating tooth space, so the hub can sit at a small angle to the sleeve while all the teeth remain properly engaged.

A single-engagement coupling has one mesh and accommodates angular misalignment only. A double-engagement coupling has a mesh at each end with a sleeve between them, and the combination of two angular allowances lets the two shafts also be laterally offset - the sleeve simply sits at an angle to both.

Axial movement is handled by the teeth sliding relative to each other, within a limit set by the tooth length.

The permitted misalignment per mesh is modest in angular terms, so shafts still need proper alignment - the coupling absorbs residual error and thermal growth, not installation error.

Because the flexibility comes from steel sliding on steel, and unlubricated steel teeth wear extremely quickly.

Every time the coupling rotates through one turn with any misalignment present, each tooth slides back and forth in its mating space. That is continuous relative motion under high contact load, and it happens at shaft speed for the whole life of the machine.

With adequate lubricant a film separates the surfaces and wear is negligible. Without it, the teeth wear rapidly, backlash grows, and the coupling eventually fails - sometimes suddenly and destructively at speed.

That makes lubrication failure, rather than overload, the usual cause of gear coupling failure. Leaking seals, missed regreasing intervals and the wrong grease are the common routes.

It is also why non-lubricated flexible couplings - elastomeric, disc, grid - are frequently preferred where the maintenance regime cannot be relied on, even at the cost of torque density.

A coupling-specific grease, because centrifugal force separates ordinary greases at coupling speeds.

A grease is oil held in a thickener matrix. In a rotating coupling, centrifugal force acts on that mixture continuously, and with an ordinary grease the denser thickener migrates outward while the oil is left behind. The teeth - which are at the outside - end up packed with thickener that has little lubricating value, while the oil that should be lubricating them sits elsewhere.

Coupling greases are formulated to resist that separation, with the oil and thickener having similar densities and a consistency chosen for the application. They are also formulated for the high contact pressures involved.

Using a general bearing grease is a common and expensive shortcut: the coupling appears to have been greased correctly, and the teeth wear anyway.

Some large couplings are continuously oil-lubricated from the machine's own system instead, which avoids the problem entirely.

At the manufacturer's interval, based on speed, misalignment and operating temperature - typically annually or more often on hard duty.

The interval depends on how quickly the grease degrades, which depends on how much sliding occurs per revolution (misalignment), how fast it turns, and how hot it runs. A coupling running with more misalignment works its lubricant harder.

Regreasing is not simply topping up. The correct procedure generally involves removing the old grease, because degraded grease and wear debris left in place contaminate the new charge, and because a coupling packed too full can burst its seals.

Inspect the seals at the same time. An O-ring or gasket that has hardened is the reason grease escapes and dirt enters, and replacing it is far cheaper than replacing the coupling.

Record the interval and the grease used. A coupling whose grease history is unknown is best regreased and put on a schedule rather than assumed to be adequate.

Higher torque density and axial capability, at the cost of needing lubrication.

A disc coupling flexes metal discs to accommodate misalignment. It needs no lubrication at all, is torsionally stiff, and has no backlash - which suits servo and precision drives - but it accommodates less misalignment and generates axial restoring forces as it flexes.

A grid coupling transmits through a serpentine steel grid seated in slots, which gives some torsional flexibility and shock absorption. It needs lubrication like a gear coupling but damps torsional shock, which a gear coupling does not.

A gear coupling gives the highest torque for its size and handles substantial axial movement freely, which matters on large machines with thermal growth.

The practical decision often comes down to maintenance: where regreasing will genuinely happen, gear couplings are excellent; where it will not, a disc coupling avoids the failure mode altogether.

Grease leakage and discoloration first, then backlash, noise and vibration.

Look for grease thrown around the coupling guard - that means a seal has failed, and the coupling is losing lubricant and taking in dirt. Grease that comes out black, gritty or separated into oil and paste indicates it has been working hard or has been in too long.

As teeth wear, backlash increases. On a stopped machine it can be felt by rocking the shafts against each other, and it grows over successive inspections.

Running symptoms are increased vibration, particularly at twice running speed, and a change in noise. Heat at the coupling is another indicator - a coupling should not run appreciably warmer than its surroundings.

Alignment is worth re-checking whenever these appear. A coupling wearing faster than expected is often reporting misalignment that has developed since installation, and replacing the coupling without correcting that simply repeats the failure.