Global Gear Couplings

Coupling Types

Full gear coupling

Two toothed hubs inside a split sleeve, giving two independent meshes. The second mesh is not redundancy — it is what makes parallel offset possible at all, and a single-engagement coupling cannot accommodate offset no matter how large it is. This is the baseline every other variant departs from.

Author
Priyansh Thummar, Editor
Dates
Published · Last updated
Reading time
4 minutes

1. The geometry, in section

A full gear coupling is the double-engagement configuration: a toothed hub keyed to each shaft, and a sleeve carrying internal teeth that mesh with both. That gives two meshes. The word "full" describes the number of flexing elements — not the size, the rating, or the quality of the coupling.

A section along the shaft axis. A driving shaft enters from the left and a driven shaft from the right. Each shaft carries a hub with external teeth on its outer surface. A sleeve, split at the centre into two halves bolted together at a flange, surrounds both hubs and carries internal teeth that mesh with each hub, giving two meshes. A seal sits at each end of the sleeve.12345
  1. Hub, keyed to the shaft and carrying external crowned teeth. One at each end.
  2. Sleeve, carrying the internal straight teeth that mate with both hubs.
  3. The tooth mesh — one of two. Each mesh accommodates angular misalignment independently, which is why capacity is quoted per mesh.
  4. Bolted flange joint splitting the sleeve, so the coupling can be assembled and stripped without moving either machine.
  5. Seal at each sleeve end, retaining the lubricant the mesh depends on.
Figure 1. Longitudinal section, drawn schematically and not to scale. Torque passes from the left shaft into its hub, across the first mesh into the sleeve, through the bolted flange joint, across the second mesh into the right-hand hub, and out to the driven shaft. Every one of those interfaces is a place a selection can be wrong.

2. What the second mesh buys

The second mesh is routinely described as giving twice the angular misalignment capacity. That is true, and it badly understates the point. What the second mesh actually buys is a capability the single mesh does not have at all.

Parallel offset is not an independent property of a coupling. It is what two angular deflections in opposite directions produce when they are separated by a distance. With a mesh at each end, the sleeve tilts one way at the first mesh and the other way at the second, and the two shaft centrelines finish parallel but displaced. A single mesh has nothing to tilt against — it can articulate about its one point and accommodate no offset whatever, however large the coupling is.

That distinction is why double engagement is the default in general industrial service. Two machines on a common baseplate will always have some offset between their centrelines, and it changes as the baseplate and casings reach operating temperature. A coupling that cannot take offset has to be aligned to a standard the installation cannot hold — see misalignment capacity for the arithmetic that converts per-mesh angular capacity into available offset.

3. Why the hub teeth are crowned

If both the hub tooth and the sleeve tooth were straight along their length, the moment the coupling articulated the contact would move to the tooth edge and the entire tooth load would concentrate on a corner.

Crowning barrels the hub tooth along its length, so that as the hub tilts within the sleeve the contact patch stays near the middle of the flank rather than running off the end. The crown radius sets the practical articulation limit, and its condition is what most wear inspections are really assessing — whether or not the inspector describes it that way.

4. The split sleeve and why it is split

The sleeve is two halves bolted at a central flange, and the reason is assembly rather than anything to do with the running duty. With both hubs toothed and already mounted on shafts that are in place, a one-piece sleeve could not be got over both hubs without moving one of the machines.

The split lets the coupling be assembled in position, and — more valuable over its life — lets it be opened for inspection and re-greasing without disturbing either machine's alignment. The flange joint carries the full transmitted torque, so its bolts are a rated part of the coupling, not fasteners to be substituted at site from the stores bin.

5. When to specify it

Table 1 — Full gear coupling against the requirement it is being asked to meet
RequirementFull gear couplingComment
Parallel offset presentYes — the reason to choose itTwo meshes; a single mesh gives none
High torque in a small envelopeYes — among the highest availableLoad shared across many teeth in shear
Axial float for thermal growthYesHubs slide within the sleeve; generates thrust while doing so
Maintenance-free serviceNoNeeds lubricant, seals and an inspection interval
Long shaft separationUse the spacer variantAlso converts angular capacity into more offset
High speed, long inspection intervalConsider a dry elementSliding contact plus a lubricant film is the limiting mechanism
The variant question comes after the type question. Decide gear versus non-gear first, then which gear configuration.

The natural home is general industrial power transmission at moderate speed with meaningful misalignment and no requirement for a maintenance-free connection: motor to gearbox, gearbox to driven machine, mill and kiln drives, conveyor head shafts, pumps and fans where the shaft separation is short.

Its torque density is why it survives against newer elastomeric and disc alternatives — for a given envelope at the shaft end there is very little that transmits more, because the load is carried on many teeth in shear rather than on a flexing element in bending.

6. What it does badly

The limitations follow from the same mechanism. It requires lubricant, and requires it to be present in the mesh rather than thrown to the outside of the sleeve — which is what re-greasing intervals exist to ensure. It has seals, and seals age. It generates an axial thrust when it slides under load, which acts on the thrust bearings at each end of the train.

And it is a sliding-contact device, so it wears — unlike a disc coupling, which either survives or cracks. In many services that predictability is an advantage, because a worn gear coupling announces itself through backlash and inspection. In high-speed service where inspection intervals are long, it is a reason to choose a dry element instead.

Past the specification and on to sizes, bores and availability? Our publisher builds these — Super Mech Industries' full gear coupling range. This page is the geometry; that one is the product.

Frequently asked

What does 'full' mean in a full gear coupling?
It means double engagement: a toothed hub at each end, so the coupling has two meshes. The alternative, a half or single-engagement coupling, has one toothed hub and one rigid connection, and therefore one mesh. The word describes the number of flexing elements, not the size or the rating.
Why can a full gear coupling accommodate parallel offset when a half cannot?
Parallel offset is produced by two angular deflections in opposite directions separated by a distance. With a mesh at each end, the sleeve tilts one way at the first mesh and the other way at the second, leaving the two shaft centrelines parallel but displaced. A single mesh has nothing to tilt against, so it can accommodate angular misalignment about that one point and no offset at all.
Why are the hub teeth crowned?
Straight teeth on both parts would make contact at the tooth edge as soon as the coupling articulates, concentrating the whole tooth load on a corner. Crowning barrels the hub tooth along its length so contact stays near the centre of the flank through the full range of movement. It is the feature that lets a gear coupling articulate at all, and its condition is what most wear inspections are really assessing.
Does a full gear coupling need the sleeve split?
It needs some means of assembly, and the bolted split sleeve is the usual one. With both hubs toothed and mounted on shafts that are already in place, a one-piece sleeve could not be got over both hubs without moving a machine. Splitting the sleeve into two flanged halves means the coupling can be assembled, inspected, re-greased and stripped in situ.

References

  1. AGMA 9008-B00, Flexible Couplings — Gear Type — Flange Dimensions, Inch Series, for the flange joint geometry in §4.
  2. AGMA 9002-B04, Bores and Keyways for Flexible Couplings (Inch Series), for the hub bore and keyway practice.
  3. ISO 14691:2008, Petroleum, petrochemical and natural gas industries — Flexible couplings for mechanical power transmission — General-purpose applications.
  4. Editorial note: Figure 1 is a schematic section, not a dimensioned drawing. Wall proportions, tooth count, tooth form and flange geometry vary by size and by manufacturer, and nothing in the figure should be measured off. A dimensioned cross-section and a photograph of a sectioned unit are to be added from Super Mech Industries production data and confirmed at technical review.

Related articles

  • Coupling Types

    Half gear coupling

    One mesh instead of two, and the duties where that is the correct call.

  • Selection & Sizing

    Misalignment capacity

    Per-mesh ratings, and the offset arithmetic the second mesh makes possible.