Off-the-shelf jaw couplings now cover a 48,000:1 torque spread, from a 0.4 Nm (3.5 in-lbs) miniature hub up to a 19,209 Nm (170,004 in-lbs) heavy industrial unit, with bore capacity running 0.125 inch (4.45 mm) to 7 inch (178 mm) in the same product family [S5][S2].
The implication for specifiers is that a single coupling geometry, two metal hubs and an elastomer spider, can be used on an instrument encoder or a 1,500 kW pump driver; the decision is the size code, the hub material, and the spider compound, not a different coupling type [S3][S7].
What the Size Range Looks Like in Real Catalogs
Lovejoy's Jaw Type line, the de facto industry reference set, is offered in 24 stock sizes covering 0.4 Nm to 19,209 Nm and bores 4.45 mm to 178 mm, with more than 850,000 stock bore/keyway combinations across AGMA, SAE, and DIN standards [S5]. The L Type (standard) tops out at 12,500 in-lbs (1,412 Nm) and 2.875 inch (73 mm) bore, while the larger C Type pushes torque to 37,800 in-lbs (4,271 Nm) and bore to 4 inch (102 mm) within the same jaw geometry, illustrating the linear torque-to-bore scaling that drives the size chart [S5].
On the small end, McMaster-Carr stocks clamp-on hubs down to the GS 7 trade size (9/16 inch OD) accepting shafts of 1/8 inch, 3/16 inch, and 1/4 inch, and L035-size set-screw hubs from 4 mm to 8 mm in metric, sized for stepper motors, small encoders, and instrument drives [S4]. The miniature end of jaw coupling sizes typically falls under 1 Nm and is consumed by encoder, small-pump, and instrument-drive applications; the heavy end, around 35,000 Nm in curved-jaw variants, is reserved for heavy pump and compressor trains [S7].
How the Size Code Maps to Torque and Bore
Size designations follow a consistent scaling pattern: smaller codes (L035, L050, GS 7, GS 9) correspond to compact, low-torque couplings, and larger codes (L099, L100, C Type) indicate heavy-duty, high-torque units [S3]. Each code locks three numbers together: nominal torque rating, maximum bore, and outer diameter, so the chart is the single source of truth for selection [S3][S4].
For example, within Lovejoy's L line, bore capacity climbs from 0.375 inch at L035 to 2.875 inch at L100, and the corresponding torque rating climbs roughly in proportion; the AL aluminum line runs 0.125 inch to 1.875 inch bore with a 2,268 in-lbs (256 Nm) ceiling because aluminum hubs cannot transmit the same torque per unit bore as steel [S5][S4]. The SS stainless line and RRS/RRSC spacer line share the 1.875 inch (48 mm) bore / 3,708 in-lbs (419 Nm) torque envelope because both use stainless hubs, with the RRS adding a drop-out spacer for shaft-separation applications [S5].
Selection Criteria Beyond Shaft Diameter
The right jaw coupling size is determined by calculating design torque (nominal torque multiplied by the service factor) and then choosing a coupling whose torque rating and bore capacity meet or exceed the requirement, not by matching shaft diameter to a catalog photo [S1]. The two governing formulas are: Torque (Nm) = (Power (kW) x 9550) / Speed (RPM), and Torque (in-lbs) = (Power (HP) x 63025) / Speed (RPM) [S1].
Service factors (typically 1.0 to 3.0+) account for shock loads, peak torque, starting duty, and the driven-machine category; electric motors on uniform loads sit at 1.0, while reciprocating compressors and heavy shock drives can exceed 2.0 [S1]. Bore tolerance, key size, set-screw versus clamp style, and hub length all influence the final fit, and the spec must be cross-checked against the manufacturer's size chart before release [S6].
Spider Material as a Hidden Size Lever
Spider compound (the elastomer insert) sets the practical torque ceiling, the misalignment capacity, and the temperature window of any given size code, so the same L090 hub can carry very different loads depending on whether NBR SOX rubber, Urethane, Hytrel, or Bronze is specified [S5]. NBR SOX rubber is the general-purpose default; urethane raises torque capacity and abrasion resistance at the cost of higher stiffness; Hytrel gives higher temperature and chemical resistance; bronze (used in high-temperature or spark-free zones) is the most rigid and shifts the coupling toward a near-rigid behavior [S5][S1].
The choice of spider therefore effectively re-sizes the coupling: an L050 with a Hytrel spider can carry a higher continuous torque than an L050 with NBR SOX, which is why size charts always list the spider compound alongside the torque rating [S5]. Hub material adds a second lever: sintered metal, aluminum, bronze, steel, stainless steel, and ductile iron hubs are all available in the same size code, with stainless and aluminum adding corrosion resistance at a lower torque ceiling [S5][S4].
Comparing the Common Size Families on Decision Criteria
Within the same physical size, the four common jaw coupling families differ in torque capacity, max bore, RPM limit, and corrosion behaviour, so the choice is criteria-driven, not aesthetics-driven. The table below maps Lovejoy's L, AL, SS, and C families against the four criteria a specifier actually weighs [S5].
L Type (standard steel): Max torque 12,500 in-lbs (1,412 Nm), max bore 2.875 inch (73 mm), general purpose, fail-safe, no published corrosion rating. AL Type (aluminum): Max torque 2,268 in-lbs (256 Nm), max bore 1.875 inch (48 mm), lightweight and corrosion-resistant (passivated bores). SS Type (stainless): Max torque 3,708 in-lbs (419 Nm), max bore 1.875 inch (48 mm), corrosion-resistant for washdown or mild chemical duty. C Type (higher-torque steel): Max torque 37,800 in-lbs (4,271 Nm), max bore 4.000 inch (102 mm), fail-safe with radially removable elastomeric cushions, intended for heavy pump and compressor service [S5]. For miniature or sub-1 Nm service, see the jaw coupling encyclopedia entry for the size-code-to-torque mapping below the L035 threshold [S7].
Failure Modes Tied to Wrong Sizing
An undersized coupling is the single most common cause of premature jaw coupling failure: the spider degrades rapidly, the metal hubs can fracture under load, and the connected motor or gearbox is exposed to shock [S1]. An oversized coupling is not a free upgrade; it adds rotating mass, raises cost, can reduce vibration damping, and may not provide the intended flexibility at the operating point [S1].
Spider failure is the key safety argument for jaw couplings in the first place: because the jaws of the two hubs still engage even with the elastomer destroyed, the coupling is fail-safe, torque continues to transmit, and the operator can detect the failure by noise or vibration before a catastrophic event [S5][S1]. A beating sound or visible vibration during operation is the diagnostic signal to stop and recheck alignment, per Lovejoy's installation guidance [S2]. For the rules that govern minimum hub length and shaft engagement depth, see the hub length and minimum shaft engagement spec rules reference.
Stock Availability and Lead Time as a Spec Driver
More than 850,000 bore combinations and most types available from stock in 24 hours make the jaw coupling one of the few flexible couplings where the fastest path is usually a standard catalog item, not a custom build [S5]. McMaster-Carr lists over 225 jaw-coupling SKUs across L, AL, and GS trade sizes with same and next-day delivery on stocked hubs, which sets the practical floor on lead time for non-OEM service [S4].
Spacer-style RRS, RRSC, and RRC variants add a drop-out center for shaft-separation applications, where the pump or gearbox must be moved axially without disturbing the driver, and these are typically built to order with longer lead times [S5]. The RRC is the larger version of RRS, with a spacer available in glass-reinforced plastic, cast iron, or aluminum depending on the torque and corrosion requirement [S5]. For adjacent mechanical-power-transmission reference data, the ISO 17396 T and AT trapezoidal metric profile pulleys guide covers the matching driven-side spec.
Tracking signal: watch for updated Lovejoy size-chart revisions (the JW-21 catalog was reissued in December 2025) and any new miniature-size spider compounds targeting servo and encoder duty under 1 Nm, where the L035-size end of the range continues to see the most product churn. The next trackable move is a published C Type bore extension above 4 inch, which would push the upper boundary of the same jaw geometry beyond 4,271 Nm in the same catalog family [S5].
For component-level specifications, see high voltage tester, and torque sensor.