A usable disc coupling size chart covers six to fourteen catalog sizes per family, nominal torque from about 0.1 Nm at the smallest servo bore up to 12,000 Nm on multi-MW pump and turbomachinery drives, with bore range scaling from roughly 3 to 19 mm on miniature hubs to 75 to 165 mm on industrial pump hubs [S1][S2][S3].
Process engineers reading the chart should anchor on three columns first: nominal torque, maximum bore, and permissible speed in rpm; misalignment capacity, temperature, and disc count are secondary and product-specific [S4][S5]. For pumps specifically, the SKF coupling catalog demonstrates a 4 kW design power point using a 1.0 service factor and a 18 mm reference bore, illustrating how size and shaft diameter cross-reference in the published selection flow [S8].
Defining the size-chart axes: torque, bore, speed, misalignment
Disc coupling size charts are not simple outer-diameter ladders; they plot nominal torque (Nm), maximum bore per hub (mm), maximum speed (rpm), and angular / parallel / axial misalignment capability (deg and mm) as orthogonal design axes [S4][S6]. A miniature servo disc unit may rate 0.1 to 1 Nm at a 3 to 8 mm bore, while a 12,000 Nm LP3/LP4 double-cardanic pump unit from R+W still fits a comparable chart cell when sized up to 165 mm bores [S2]. Torque density, not the envelope, decides the cell: 304 stainless disc packs stacked 4 to 12 deep at 0.4 to 1.0 mm per disc allow the same chart column to serve both CNC spindles and steam-turbine-driven pumps [S3][S5].
Where disc couplings fit, and where they do not
Disc couplings belong on high-speed, zero-backlash, torsionally stiff drives: servomotor-to-ball-screw CNC spindles, centrifugal pumps, test benches, agitators, and API 671 turbomachinery trains where Rexnord-Thomas-style membrane flexing replaces sliding elastomers [S3][S4]. They are not a fit for shock-loaded mine mills, hammer mills, or any application where reversing torque peaks exceed 3x nominal; those demand a coupling clutch with calibrated slip or an elastomeric element. For ultra-low torque instrument shafts below roughly 0.1 Nm, beam or bellows couplings win on cost and inertia, even though disc variants exist down to that torque class [S3][S4].
Selection criteria compared across the size spectrum
Four criteria separate a servo-rated disc coupling from a pump-rated disc coupling at the same family name: torque per kg, bore-to-OD ratio, speed rating, and misalignment budget per disc pack [S1][S2][S5]. The Miki Pulley SFC / SFF / SFM / SFH families list nominal torques from sub-Nm to a few hundred Nm with integral clamp hubs sized for typical servo shaft diameters, intended for servo motor feeds where low inertia and zero backlash dominate [S1]. The R+W LP1 / LP2 / LP3 / LP4 series start at 350 to 500 Nm and run to 12,000 Nm with conical clamping hubs and optional ATEX variants, sized for process pump and agitator duties where high power density and frictional-locked disc packs are mandatory [S2].
Zero-Max ServoClass splits the middle: 14 sizes in single and double disc models from 0.5 to 250 Nm at -30 to +100 C, with 304 stainless disc members and aluminum hubs, plus a 5-size floating-shaft variant up to 250 Nm for X-Y gantry and screw-jack bridging [S5]. The Ruland and Ondrives US lines reinforce the same pattern: customizable inch or metric bores, lengths and tolerances for high-gain servo control, and explicit warnings that tolerance-stack misalignment beyond the disc pack's flex budget is the dominant field-failure cause [S6][S7].
Typical use cases across the size ladder
At the small end, a 0.5 to 5 Nm disc coupling connects a 100 to 750 W servo motor to a ball screw or planetary gearbox in a semiconductor handler or medical pump, where the 4 to 12-disc pack keeps hysteresis below the resolution of the position loop [S3][S4][S5]. The same disc family in a 50 to 250 Nm size bridges a 5 to 15 kW spindle drive to a precision gearbox; Zero-Max's 14-size ServoClass is representative, with 304 stainless discs and aluminum hubs holding bore-to-OD ratios typical of the servo class [S5].
Above roughly 1,000 Nm, the chart transitions to LP2 floating-shaft pump couplings, where the intermediate tube is built to length and the double-cardanic disc pack absorbs the lateral offset between motor and pump shafts without disturbing alignment of either machine [S2][S9]. At 5,000 to 12,000 Nm, conical-clamp LP3 / LP4 units handle reversing pump drives and high-speed compressor couplings, with optional ATEX certification for explosive-atmosphere chemical and refinery service and a published -30 to +280 C operating envelope [S2]. The same chart row on much larger API 671 units from the historical Thomas-Rexnord line carries several MW on steam-turbine-driven compressor trains, the original application Thomas Flexible Coupling commercialised and that still anchors the high end of the disc-coupling catalog [S3].
Limitations, failure modes, and sourcing notes
The most common disc-coupling field failure is not disc fatigue but bolt loosening or disc-packet overload from a miscalculated misalignment budget, including thermal growth between a hot pump housing and a cold motor that exceeds the 0.4 deg / pack class [S3][S4]. Single disc packs accept angular and axial only; lateral offset demands a double-cardanic LP2 / LP3 / floating-shaft arrangement, and substituting a single-pack unit to save envelope cost is a frequent spec error [S2][S9]. Material limits also bound the chart: 304 stainless discs cap sustained service near 280 C in all-steel LP-series units, while aluminum-hub servo units from Zero-Max stop at 100 C and need a different line above that threshold [S2][S5].
Standards to anchor a specification, where relevant, are API 671 for turbomachinery trains, ATEX 2014/34/EU for explosive-atmosphere disc-pump couplings (an R+W LP-series option), and the basic ISO-style shaft-fit / balance grades carried in the SKF couplings catalog used for 4 kW-class pump sizing demonstrations at 18 mm bore [S2][S3][S8]. For comparing pump-side coupling architectures more broadly, the submersible centrifugal pump impeller design reference is the natural downstream read.
Sizing a real disc coupling from the chart
Step one is the service-factored design torque: design power times service factor from the catalog, then a torque per rpm cross-check against the chart's nominal torque column [S8]. Step two is bore selection: motor shaft diameter and pump shaft diameter set the lower bound, and the chart's max-bore column must clear both with keyway or clamp-hub margin; the SKF example ties an 18 mm pump shaft to a coupling size and a 1.0 service factor in a 4 kW class selection [S8]. Step three is speed: catalog max-rpm must exceed the drive's continuous top speed with a 25 to 30 percent margin for balance-grade residual imbalance [S4][S6]. Step four is misalignment budget: 0.4 deg per disc pack is a typical published angular limit, parallel offset is read from the double-cardanic row only, and the verified misalignment plus thermal growth must stay inside that budget across the full operating map [S3][S4][S9].
Track the next coupling spec against the published 14-size ServoClass ladder, the 12-size R+W LP1/LP2 ladder up to 12,000 Nm, and the 6-size LPA pump family at 500 to 12,000 Nm; a 2026-09 watch item is whether servo-pump duty continues to migrate from elastomeric jaw couplings to low-inertia double-disc units as servo drive bandwidth keeps rising [S2][S5].