Selector charts are a one-page shortcut to a working coupling size, not a substitute for the catalogue: they plot rated torque against bore, speed, and a small set of misalignment numbers, and they assume the engineer has already done the torque math [S1][S2].
A correct pass through a disc coupling chart requires five inputs in this order: continuous and peak torque, operating RPM, service factor, shaft bore, and the angular/parallel/axial misalignment you can actually hold during installation, plus a check of torsional stiffness if the drive is a servo or CNC axis [S1][S2][S3].
Step 1: Compute design torque before you touch the chart
The chart is sized off design torque, not nameplate torque, and the SI form used across Formura, KTR, and Rexnord literature is Torque (Nm) = HP x 9550 / RPM, then multiplied by the application service factor [S1]. Service factors are duty-class numbers tied to the driven machine: pumps, compressors, conveyors, and mixers each carry their own multiplier that pushes you one or two frame sizes above the bare calculated number [S1][S3]. Peak torque and inertia load during start-up must also fit inside the chart's published peak or shock rating for the chosen size, not just the continuous rating [S2]. If your drive is a servo or positioning axis, the design point is the cyclic peak torque the amplifier delivers, not the RMS torque [S2].
Step 2: Read the torque-versus-RPM grid and the speed ceiling
Most disc coupling charts are organised as a table of rated torque per size with a separate speed column, sometimes published as a maximum balanced RPM that shrinks as bore increases because the rotating mass grows [S2]. The chart cell you select must satisfy both: design torque below rated torque, and operating RPM below the size's published speed limit, which is a function of balance grade, bore, and the disc pack's critical speed [S2][S3]. The Regal Rexnord selector walks the same path: torque, then speed, then bore, then misalignment, then balance [S3]. Rexnord's online coupling selector at the shop portal then turns those four answers into a configured part number plus 2D/3D CAD downloads, but the chart logic it runs is identical to the printed table [S4].
Step 3: Match bore, keyway, and DBSE on the chart
Once a frame size clears torque and speed, the chart's bore column is checked against both shaft diameters; hub options are discrete catalogue values, so a 60 mm shaft and a 75 mm shaft on the same coupling will force either a tapered or stepped-bore family, not a single standard part [S1][S2]. Spacer length, called DBSE (distance between shaft ends), is a defined catalogue dimension on disc couplings and is read off the same chart; for pump and compressor builds a long spacer is needed for seal access, and chart rows for the same size usually list available DBSE values such as 100 mm, 140 mm, and 180 mm [S2]. For an introduction to the disc coupling family itself, the encyclopedia page covers the hub, disc pack, and spacer geometry that the chart references.
Step 4: Verify angular, parallel, and axial limits on the chart
Disc couplings flex through the disc pack, so each chart row lists three numbers: angular misalignment in degrees, parallel offset in millimetres, and axial float in millimetres, and the published limits are size-specific, not extrapolated from outside diameter [S1][S2]. Typical catalog values on small-frame disc couplings sit in the 1.0 to 3.0 degree angular band and 0.2 to 1.5 mm parallel offset, dropping as frame size grows because the disc pack spans a longer moment arm [S2]. If your installation cannot be aligned to those numbers, you step up one size, switch to a jaw coupling or gear coupling for higher misalignment, or use a spacer design to lower the effective misalignment seen at each shaft end [S2][S6].
Step 5: Stiffness, backlash, and balance grade on the same chart
For servo, CNC, and positioning duty the chart has a second layer that many engineers skip: torsional stiffness in Nm/rad and backlash in arc-minutes or arc-seconds, both of which are configuration-specific, so KTR RADEX-NC publishes one stiffness for the EK single-disc layout and a different (usually lower per Nm of torque) stiffness for the DK double-disc layout on the same size [S2]. Miki Servoflex publishes stiffness on its family page, while the Thomas disc family routes engineering enquiry until tables are published, so cross-brand charts cannot be compared on stiffness alone [S2]. Do not infer stiffness from coupling OD; request the catalogue number for the exact configuration and size [S2]. For the broader trade-off between disc, gear, and elastomeric designs, the coupling clutch encyclopedia entry lines them up on torque density, damping, and maintenance.
Selector chart compared with adjacent sizing methods
A printed or PDF selector chart, a vendor's online selector, and a torque-derating worksheet give the same answer when used correctly, and the differences are speed, not physics [S3][S4][S5]. A disc chart assumes an all-metal flex element, zero lubrication, and a defined DBSE, so it undersizes a high-damping general industrial drive where a fluid coupling is the better match [S2][S6]. Lovejoy's plain-English selector asks for bore, speed, service factor, and misalignment before it asks for a part number, which mirrors the same five inputs the disc chart uses and is a useful sanity check before committing to a size [S5]. The trade-off in practice: chart-only sizing is fastest, vendor online selectors add CAD output and price, and a derating worksheet is mandatory when ambient temperature, cyclic peak, or balance grade pushes outside the chart envelope [S3][S4].
Failure modes that send engineers back to the chart
Three failure patterns drive a return to a smaller or larger size: bolt loosening from cyclic peak torque above the chart's published shock rating, disc pack fatigue from running at sustained torque inside the rated band but outside the speed curve, and premature wear on the spacer hardware from axial float being absorbed by a single-disc layout that should have been a double-disc design [S1][S2][S6]. If a selector switch is used in the same drivetrain to reverse the motor, confirm the chart's peak rating covers the reversing duty cycle, not just steady-state rotation, because reversing loads concentrate wear on the disc pack bolt circle [S1][S2]. For a working spec band on torque, misalignment, and lubrication across disc, gear, and fluid families, the disc coupling sizing reference collects the catalog numbers and the failure thresholds that force a re-spec [S6].
Trackable signals to watch over the next sourcing cycle: published peak-torque ratings for double-disc EK/DK layouts, which most catalogues still list as a multiple of continuous torque, and any update to the AGMA service factor table that would shift the disc chart's torque math for pump and compressor duty [S1][S2][S3].
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