Rated torque on a flexible disc coupling is set by the disc pack (number of bolts, disc diameter, material) and the coupling size, not by the bore diameter. Lovejoy documents the rule directly: when application torque climbs to 50 percent of coupling capacity, the unit's ability to absorb misalignment falls off sharply, so the rating is a disc-pack limit, not a shaft limit [S1].
Bore size controls the side of the problem that engineers get wrong. A given coupling size will list a "rated torque" that holds across the full bore range, plus a separate max-bore value and a torque restriction note where the bushing or key is the weak link. TB Woods documents that "peak overload torque rating is 1.5 times rated torque" and that all published dimensions and weights assume standard hubs at maximum bore [S2][S9]. The chart you read on a datasheet is therefore a two-axis table: coupling size on one axis, rated torque on the other, with bore appearing as a separate column that can either equal the rating or pull it down.
Rated torque is a disc-pack property, not a bore property
Cross-series evidence is consistent on this. Rexnord Thomas 71-8 in sizes 225 through 750 transmits torque through flexible disc elements, with eight bolts per disc pack, and the published torque is the same for every hub configuration that fits within the size's bore window [S4]. The SKF couplings catalogue lists W4 (4-bolt), W6 (6-bolt) and W8 (8-bolt) double-flex disc couplings as separate product tables, with torque and bore set independently inside each table [S5].
When sizing, treat rated torque as a function of three numbers only: disc diameter, number of discs, and bolt circle. The bore column on the chart is the dimension that selects the hub; the torque column is the dimension that selects the coupling size. Conflating the two leads to over-spec on small shafts and under-spec on large ones, both of which fail in service. Lovejoy SX size 185-6, for example, holds 29,205 in-lb nominal and 58,410 in-lb peak (2:1 peak margin) across the entire standard bore range for that size [S7]. The peak-to-nominal 1.5x margin is the same value TB Woods uses across its flexible disc line, so it is a de facto industry convention rather than a vendor quirk [S2].
Bore columns: standard, maximum, and the "torque-restricted" footnote
Three bore values appear on most disc-coupling datasheets, and each one means something different. The "standard bore" is the catalogued pilot or rebore shipped from stock and the one used for weight, WR² and torsional stiffness tables [S9]. The "max bore" is the largest shaft the hub can accept before the keyseat or hub wall becomes the structural limit. The "torque-restricted" bore is the threshold above which the clamping element (taper-lock bushing, QD bushing, or shrunk-fit hub) becomes the weak link and pulls the published torque down.
Crossflex makes this explicit: the datasheet for its ACE81-x26 element states "torque restricted by Clamping Bush capacity, check torque in table below" right next to the bore range, forcing the engineer to read two tables instead of one [S8]. Rexnord's torque-density comparison table reinforces the same point by listing Std Max Bore and Spec Max Bore side by side with torque/weight, so the bore axis and the torque axis are visibly independent and the engineer has to intersect them by hand [S6].
A useful sanity check: if a bore size on the chart is larger than the next size down's standard bore but the rated torque number does not change, the bore is being limited by the keyseat or the bushing, not by the disc pack. That is the signal to either rekey, rebore to a larger coupling size, or fit a taper-lock bushing rated for the application torque.
How AGMA service factors cut the published torque
Catalog torque is a steady, smooth-duty number. Real applications are not. Rexnord Thomas and Lovejoy both apply an application service factor (ASF) before the chart value becomes a selection, and the factor is read off a separate table that depends on the driven machine, the driver, and hours per day [S3][S4]. Common service factor ranges in disc-coupling catalogues run from 1.0 (uniform, electric-motor driven) to 2.0+ (heavy shock, reciprocating driven), and the selected coupling torque must equal or exceed application torque multiplied by the service factor.
For ATEX-classified installations, Rexnord requires the supplement form 0005-08-49-01 to be followed exactly, otherwise the coupling is non-conforming to ATEX on the spot, and the guard must keep a 12.7 mm (1/2 in) radial clearance to the coupling outside diameter [S4]. That guard-clearance rule does not change torque, but it does change which physical size you can install, which in turn can push you to a larger bore column on the chart.
Comparing the main disc-coupling families on the same axes
The chart values only mean something once you fix a family. Four lines dominate the industrial market, and they trade off torque density, misalignment capacity, and bore range differently. The comparison below uses ranges grounded in the catalogues in the research pack; for any given size, use the vendor's own table, not the range. [S1]
Single-flex vs double-flex: a single-flex disc coupling (one disc pack per hub) handles angular and axial misalignment only; a double-flex (two disc packs, with a spacer between them) adds parallel misalignment capacity at the cost of torsional stiffness and usually a slightly lower torque rating for the same disc pack [S5]. Rexnord Thomas 71-8 is the spacer example in the catalogue, with eight bolts per disc and sizes 225 through 750 [S4].
4-bolt vs 6-bolt vs 8-bolt: bolt count sets torque capacity in steps. SKF publishes W4, W6 and W8 as separate tables because the same coupling size can carry materially different torque depending on disc stack, and the W8 line is where the highest torque-per-bore values appear [S5]. Lovejoy's SU-6, SX-6 and SX-8 lines follow the same logic: 6-bolt and 8-bolt packs, with SX-8 sized for the heavier end of the catalog [S1].
Close-coupled vs spacer: a close-coupled Thomas 54RDG or DBZ-A/B sits between two shaft ends with a short overall length, so its bore range is the limiting dimension more often than on a spacer design. A spacer coupling (DBZ-C, Series 52, Series 71) inserts distance between the hubs to allow removal of a pump impeller or a bearing without disturbing the driver, and the spacer is what the bore must clear, not the disc pack [S3].
Clamping-bush vs integral hub: Crossflex and most European disc couplings use a separate clamping bush (Taper Lock, QD, or equivalent) that mounts the hub to the shaft, and the bushing's rated torque caps the assembly. The chart torque is the disc-pack torque; the orderable torque is the lesser of the two. Rexnord Thomas and Lovejoy SX/DI lines are predominantly integral-hub designs, where bore and keyseat are the limit instead of a bushing [S8][S3].
Failure modes when bore and torque are misread
Three failure patterns show up repeatedly in disc-coupling service. First, keyseat shear: the bore is within the catalog max, the key is standard, and the application torque exceeds the key's capacity. The disc pack survives; the key folds. Second, hub-bore elongation: a repeated shock load drives a clearance fit into a fretting fit, the bore wall cracks at the keyseat, and the disc pack never sees its rated torque. Third, bolt fatigue on a torque-restricted bore: the clamping bush is undersized for the actual load, the hub slips on the shaft, and the disc-pack bolts absorb the impact. [S5]
Each of these traces back to a single error, reading the bore column as a torque qualifier instead of as a fit qualifier. The chart's rated-torque number applies to the disc pack at any bore within the size; the keyseat, the hub length, and the bushing each carry their own torque ceiling, and the lowest of the three wins. Lovejoy's 50 percent-of-capacity rule is the conservative operating ceiling for sustained misalignment tolerance, not for torque, and the two limits are additive in the sense that both must be respected independently [S1][S7].
Sourcing and standards that govern the chart values
Catalog torque ratings are vendor-tested under AGMA and ISO coupling-test conventions, but the values themselves are not third-party certified line items. What is independently auditable is the safety envelope around them: ATEX conformity for explosive atmospheres (per the Rexnord 71-8 supplement 0005-08-49-01 and the 12.7 mm guard-clearance rule) [S4], OSHA and ANSI guarding for general industrial service [S4], and the EU Machine Safety Directive for CE-marked drives. Bore tolerances themselves typically follow AGMA 9002 or ISO 286-1 fit recommendations, which a vendor will call out on the drawing rather than on the catalog page.
Forces, moments and balance grades for the coupled machinery are usually pulled from API 610 (process pumps) or API 541 (form-wound motors) on the process side, with the coupling selected to absorb the residual misalignment and torque ripple after the equipment has been built to those specs. When the chart is being read alongside a torque sensor calibration or a torque wrench tester verification, the bolt preload on the disc pack (called out in the Thomas locknut-tightening table) is the cross-check number, not the catalog torque [S3].
Where to start on a new selection
Pull three numbers from the application: required torque (with service factor applied), shaft diameter on each end, and the misalignment budget (angular, parallel, axial). Read the chart at the coupling size that meets torque at the catalog rating, then verify that both shaft diameters fall inside that size's bore range with the keyseat or bushing rated for the application torque. If they do not, step up one coupling size and re-check; the disc pack is cheap insurance relative to an unscheduled shutdown. For drives where a coupling clutch or a fluid coupling is being evaluated against the disc pack, treat the chart as the disc-coupling side of that comparison and size the alternatives on the same three numbers. A peer reference for spec-driven component selection, including the conveyor drive train that often hosts a disc coupling on the head or tail pulley, is covered in this conveyor bulk-density and lift spec map. [S3]