Glossary
39 terms, defined once and used consistently across every article here. Where a term is defined by a standard, the standard is named. Where it is trade usage rather than a definition — "full", "half", "spacer" — the entry says so, because that distinction is how purchase orders end up ambiguous.
A
- AGMAalso American Gear Manufacturers Association
- The American standards body whose documents govern most catalogue coupling practice outside Europe and India. Its 9000-series covers flexible couplings specifically: 9000 for unbalance classification, 9001 for lubrication, 9002 for bores and keyways, 9003 for keyless fits, 9008 for gear-type flange dimensions. AGMA vs DIN vs IS →
- Angular misalignment
- The angle between two shaft centrelines that meet at a point. On a gear coupling it is accommodated by the hub tilting within the sleeve, and it is rated per mesh rather than per coupling — a double-engagement coupling therefore has twice the angular capacity of a single-engagement one of the same size. Misalignment capacity →
- Axial travelalso Axial float, End float
- Movement of the shafts along their common axis, accommodated by the hubs sliding within the sleeve. It exists so a machine train can grow and contract with temperature without loading its thrust bearings — but the sliding happens against tooth friction while transmitting torque, so it generates an axial force of its own. Misalignment capacity →
B
- Backlash
- The free rotational play between hub and sleeve teeth. A new coupling has a designed amount; the useful measurement is not the value but the change since the last inspection, because growth in backlash is the most direct evidence of flank material lost to wear. Tooth wear →
- Balance class
- The potential-unbalance classification of a coupling, determined from its mass, speed and the sensitivity of the system it runs in. Above a threshold that varies by size and manufacturer, couplings require dynamic balancing; below it, as-manufactured balance is accepted. Classified in ANSI/AGMA 9000. How to size a gear coupling →
- Bore
- The machined hole through the hub that fits the shaft. Each coupling size has a maximum bore, set by the material remaining between the bore surface and the root of the hub teeth. Because that limit does not scale with torque capacity, bore is frequently what governs a selection rather than torque. AGMA 9002: bores and keyways →
- BSEalso Between shaft ends, Shaft separation
- The distance between the two shaft ends a coupling must span. It is the dimension that determines whether a compact or spacer coupling is required, and it is specified on the order because the coupling cannot be selected without it.
C
- CG-1, CG-2, CG-3
- The coupling grease classes defined in the AGMA 9001 lubrication standard, distinguished by base oil viscosity, dropping point and NLGI consistency, and matched to coupling type, size and speed. A CG designation is what makes a grease a coupling grease; an NLGI grade on its own does not. Lubrication guide →
- Cold offsetalso Cold alignment target, Thermal offset
- A deliberate misalignment set at ambient temperature, in the direction opposite to expected thermal growth, so that the train comes into alignment once it is hot. A machine aligned to zero cold is a machine misaligned hot, which is the condition it spends nearly all its life in. Installation and alignment →
- Crowned toothalso Barrelled tooth
- A hub tooth whose flank is curved along its length rather than straight, so that contact stays near the centre of the flank as the coupling articulates. Without crowning, contact would move to the tooth edge the moment the coupling misaligned. The crown radius sets the practical articulation limit. Full gear coupling →
D
- DINalso Deutsches Institut für Normung
- The German standards body. On coupling drawings DIN documents are usually component standards rather than whole-coupling standards — DIN 3761 for radial shaft seals, DIN 5480 for involute splined connections — which means a DIN number cited on a coupling drawing frequently governs the seal or the spline and not the coupling. DIN 3761 and DIN 5480 →
- Double engagementalso Full gear coupling
- A coupling with a toothed hub at each end, giving two meshes. Trade usage rather than a standardised term. The second mesh is what makes parallel offset possible at all, so this is the default configuration in general industrial service. Full gear coupling →
- Dropping point
- The temperature at which a grease becomes fluid enough to drip under test conditions. In a coupling it functions as a ceiling: the running temperature of the sleeve has to sit well below it, or grease softens and is expelled past the seals. Lubrication guide →
F
- Flange joint
- The bolted connection between the two halves of a split sleeve, or between a sleeve and a rigid hub. It carries the full transmitted torque, which makes its bolts a rated part of the coupling rather than fasteners to be substituted at site. Full gear coupling →
- Floating shaftalso Jack shaft, Intermediate shaft
- A long intermediate shaft with a coupling at each end, used to span between machines that cannot be placed close together. A pair of single-engagement couplings on a floating shaft provides the two flexing points offset requires, separated by the whole shaft length — which is why half couplings are correct in this arrangement rather than a compromise. Half gear coupling →
- Fretting corrosion
- Wear at a nominally fixed fit — typically the bore or keyway — caused by micro-movement under cyclic load. It produces a characteristic rust-coloured fine powder and matters because it can propagate into a crack. Debris at the fit rather than at the teeth points here, not to tooth wear.
H
- Hub
- The part fitted to the shaft, carrying external teeth on its outer surface in a toothed configuration or a plain flange in a rigid one. Bored and keyed to the shaft it serves, and on interference-fit designs heated to install and effectively permanent once cooled in position.
I
- Interference fitalso Shrink fit, Press fit
- A hub bore machined slightly smaller than the shaft, so the joint is held by elastic pressure rather than by the key alone. Installed by heating the hub to expand it and driving it home in one movement; it cannot be adjusted afterwards without heating it again. Installation and alignment →
- IS 3238
- The Indian Standard specification for gear-type flexible couplings, published by the Bureau of Indian Standards. It is the document a domestic Indian purchase order is most likely to name, and it uses metric fit conventions rather than the inch-series practice of the AGMA documents. IS 3238 →
- ISO 14691
- The ISO standard for flexible couplings in general-purpose petroleum, petrochemical and natural gas service. Its special-purpose counterpart is ISO 10441; the boundary between them is a question of service criticality rather than of coupling type. ISO 14691 →
K
- Keyway
- The axial slot in shaft and bore that carries the key transmitting torque. It removes material from the same wall that limits maximum bore, so a deeper-than-standard keyway can push a bore past a size's capability even when the nominal diameter is within it. AGMA 9002: bores and keyways →
M
- Mesh
- One engagement of hub teeth with sleeve teeth. It is the unit misalignment capacity is quoted in, and the distinction that matters most in specification: a per-mesh rating read as a whole-coupling rating produces a selection that is out by a factor of two, always in the unsafe direction. Misalignment capacity →
N
- NLGI grade
- A grease consistency classification from the National Lubricating Grease Institute, running from very soft to very hard. It describes stiffness only. Two greases of the same NLGI grade can behave entirely differently inside a rotating coupling sleeve, which is why the CG classes exist alongside it. Lubrication guide →
- Nominal torque
- Torque at the coupling derived directly from the driver's rated power and speed, before any service factor is applied. For power in kilowatts and speed in revolutions per minute it is 9550 × P ÷ n, in newton metres. Torque calculation →
O
- Overhung moment
- The bending moment a coupling's mass applies to the nearest bearing, as a product of that mass and its distance from the bearing. It is the reason an oversized coupling is not a free choice, and the reason spacer length is not extended without checking what the bearing is being asked to carry.
P
- Parallel offsetalso Radial offset, Parallel misalignment
- The perpendicular distance between two shaft centrelines that remain parallel. It is not an independent capability of a coupling: it is what two opposed angular deflections produce when the meshes are separated by a distance, which is why it requires two flexing points and disappears entirely on a single-engagement coupling. Misalignment capacity →
- Peak torquealso Momentary torque
- The transient torque produced by starting, a short circuit, surge or a jam, checked against a coupling rating that is separate from the continuous one. The service factor does not apply to it, and applying it there is a common double-counting error. Torque calculation →
- Pipe strain
- Load applied to a machine casing by pipework that had to be pulled into position to meet its nozzle. It moves the shaft centreline and keeps moving it as the line heats and cools, so it is an alignment problem that no amount of shimming at the feet can resolve. Installation and alignment →
R
- Reverse dial
- An alignment method that reads the rim of each hub from a bracket mounted on the other, deriving both offset and angularity from two radial readings. Preferred over rim-and-face on gear couplings because it is insensitive to axial float — which a gear coupling permits by design. Installation and alignment →
- Rim-and-face
- An alignment method reading radial offset from the coupling rim and angularity from a machined face. Quick and adequate on short, rigid connections; its face reading is corrupted by axial float, which limits it on gear couplings. Installation and alignment →
S
- Selection torque
- Nominal torque multiplied by the service factor. This is the figure compared against a coupling's published continuous rating — never against its peak rating. How to size a gear coupling →
- Service factor
- A multiplier classifying the character of the load — torsional shock, reversal and duty cycle — on both the driven and driving side. It is not a safety factor and not an allowance for uncertainty: it describes loading that is known about, not loading that is unknown. Service factor guide →
- Single engagementalso Half gear coupling
- A coupling with one toothed hub and one rigid connection, giving one mesh. Accommodates angular misalignment about that single point and no parallel offset at any size. Trade usage rather than a standardised term. Half gear coupling →
- Sleevealso Cover, Housing
- The outer member carrying the internal teeth that mesh with the hubs. Usually split into two halves bolted at a central flange, so the coupling can be assembled, inspected and re-greased without moving either machine. Full gear coupling →
- Sliding velocity
- The rate at which a hub tooth slides along its mating sleeve tooth, set by misalignment and speed rather than by torque. It is the quantity that actually governs whether the lubricant film survives, and therefore the reason published misalignment ratings fall as speed rises. Misalignment capacity →
- Soft foot
- A machine foot that does not sit flat on its support, so tightening its bolt distorts the casing. Any alignment measured on a machine with soft foot is a measurement of a distorted machine, and it will not repeat. Installation and alignment →
- Spacer coupling
- A coupling with an extended centre section separating the two meshes. The extra separation converts the same per-mesh angular capacity into proportionally more parallel offset, and allows a pump to be withdrawn without moving its driver. It costs overhung moment and axial space, not torque capacity.
T
- Torsional natural frequency
- The frequency at which a machine train oscillates in torsion. Coupling mass and torsional stiffness are inputs to it, which is why substituting a much larger coupling than the calculation requires is a change to the machine rather than a harmless margin.
W
- Worm tracking
- A fine wavy or serpentine line traced along a tooth flank, left by the contact point migrating as the coupling articulates through misalignment beyond what the crown accommodates. Frequently mistaken for a machining mark. Where it appears, the cause is misalignment. Tooth wear →
A note on vocabulary
A surprising amount of disagreement about couplings turns out to be disagreement about words. "Full" and "half" are not defined in any standard — they are trade shorthand for double and single engagement, and they are understood differently in different markets. A drawing that cites a DIN number may be specifying a seal rather than a coupling. A misalignment figure quoted without saying whether it is per mesh or per coupling is out by a factor of two.
Where an entry above says a term is trade usage, name the underlying property on the purchase order instead: number of meshes, angular capacity per mesh at the operating speed, maximum bore, and the standard the bore and keyway are to be machined to.