Single-disc (single flex) couplings handle angular and axial misalignment only, with one flex plane rated at roughly 1/2° to 1-1/2° angular offset per disc pack on Lovejoy designs (4, 6, or 8-bolt unitized stainless AISI-301 packs) [S1]. Double-disc (double flex) units add a spacer between two disc packs and a second flex plane, which is what unlocks parallel offset in addition to angular and axial [S1][S5].
SKF's disc-coupling catalogue caps the family at 178 kNm nominal torque with shaft bores up to 289 mm, ambient temperatures generally up to 250 °C, and continuous service above 50 m/s when dynamically balanced, so the single vs double choice is a geometry question, not a torque-budget question [S5]. The trade is simple: single is more compact and lighter per flex plane; double is longer, heavier, but is the only configuration that absorbs parallel shaft offset without forcing one bearing to carry the radial reaction [S3][S4].
What Each Disc Pack Geometry Actually Does
Lovejoy's SU type is a single flex plane with two hubs and one disc pack, listed for angular and axial misalignment only, and is commonly paired back-to-back with a shaft to make a floating shaft coupling when long shaft separations are needed [S1]. Lovejoy's SX type, by contrast, uses 2 hubs, 1 stock spacer, and 2 unitized disc packs, with the two flex planes explicitly stated to accommodate parallel, angular, and axial misalignment at bore sizes up to 13 in (330 mm) [S1].
Regal Rexnord's fundamentals piece classifies disc couplings as the most common flexible-element coupling because of their high misalignment capacity, compact envelope, and zero-lubrication design, and explicitly attributes the parallel-offset capability to the double-flex (two disc packs) layout [S2]. SKF's catalogue extract makes the same statement in two lines: single disc units accommodate angular (a) offset only, while double disc pack units with a spacer allow angular, parallel (d), or combined offset plus some axial (d) movement [S5]. The physics is the same story: one flex plane cannot translate parallel shaft offset into elastic deformation without forcing a reaction load back into the bearings.
Angular Capacity Per Pack: 4-Bolt vs 6-Bolt vs 8-Bolt
Within a single supplier's line, disc-pack bolt count sets the angular-misalignment ceiling. Lovejoy's published disc-pack profile accepts 1/2° to 1-1/2° of angular misalignment, and the same catalogue warns that while the 8-bolt design transmits more torque than the 6 or 4-bolt, the higher-bolt-count pack is not able to accommodate as much angular misalignment [S1].
The same rule carries over to double-disc stacks: total angular capacity is the per-pack figure multiplied by the number of packs, while the parallel-offset capacity is set by the spacer length between flex planes and the bending stiffness of the spacer. R+W America's blog (2020-06) frames the limit directly: a single disc pack is generally rigid in shear, so it cannot compensate for misalignment between two independently bearing-supported shafts [S8]. That shear rigidity is exactly why a second flex plane is the engineering fix for parallel offset, not a stiffer pack.
Comparison Matrix: Single vs Double Disc on Decision Criteria
Across the catalogue sources, four criteria separate the two configurations cleanly. (1) Misalignment types handled: single disc pack covers angular + axial only; double disc pack covers angular + axial + parallel offset [S1][S3][S5]. (2) Compactness and mass: single is more compact in size than the double-flex variety, per Motion Control Tips' technical summary, and that envelope advantage scales with shaft separation [S4]. (3) Reaction loads on bearings: single flex will have lower misalignment capacity and higher reaction loads, because one flex plane cannot absorb parallel offset elastically [S4]. (4) Serviceability: disc pack or spacer may be removed and reinstalled radially on both configurations, so the prime mover and driven machine need not be moved during a pack swap [S5].
Ondrives lists torque up to roughly 530 in·lb in its single-disc and double-disc disc-coupling range as a representative small-frame figure, and the catalogue language confirms both formats are low-maintenance, precisely position the shafts, and reduce vibration while transmitting that torque class [S7]. Lovejoy's SU and SX lines are both ATEX certified, with the SU additionally suited to floating-shaft builds when paired with a hollow spacer shaft [S1]. Where the application is API-610 (e.g. process pumps), Lovejoy supports that standard up to 3,800 RPM on its disc couplings, and the unitized disc-pack design is what carries the balance and piloting requirements called out by API-610 [S1].
Selection Rules: When Single, When Double, When Floating Shaft
Use single disc (single flex) when the dominant misalignment is angular from thermal growth or initial alignment error, the shaft gap is short, and a compact, lightweight coupling is wanted. Lovejoy's SU type is the canonical example: one flex plane, two hubs, one disc pack, rated for angular and axial only, and intended to be doubled up with a hollow shaft to build a floating-shaft coupling when shaft separation grows [S1].
Use double disc (double flex) when shaft separation is meaningful, when parallel offset from base-plate distortion or pipe strain is part of the misalignment budget, or when the application demands two independent flex planes for redundancy of the elastic element. Lovejoy's SX, DI, SXC, and SXCS families are the practical expressions, with the DI "drop-in" spacer format piloted to meet API-610 balance and anti-flail requirements [S1]. Ameridrives' technical note adds a subtlety worth quoting for the diaphragm vs disc comparison: when angular misalignment is imposed on an entire pack, a diaphragm on the centerline of the pack reacts differently than one off the centerline, which is why disc-stack geometry, not just bolt count, governs real-world misalignment tolerance [S6]. Firgelli's 2026-04 mechanism guide frames the same point operationally: disc-pack geometry, material, torque, speed, and operating conditions set the permitted misalignment window, and the entered range is a comparison baseline rather than an absolute guarantee [S9].
Limits, Failure Modes, and What the Standards Do Not Cover
Two failure modes show up repeatedly. First, overloading a single disc pack with parallel offset shears the bolt group and forces the offset into the bearings; that is the failure mode R+W's blog warns against when only one flex plane is present between independently supported shafts [S8]. Second, exceeding the per-pack angular rating, especially on 8-bolt high-torque packs, drives fatigue cracking in the disc-pack laminations; Lovejoy's bolt-count trade-off makes the point that more bolts give torque at the cost of angular headroom [S1].
Third, alignment decay in service. Disc couplings are explicitly inspectable with a strobe light while running (with guards), and the disc pack condition is the visual health check, which is why IBT's 2026-01 reliability piece frames the technology as low-maintenance rather than no-maintenance: bolt preload, spacer concentricity, and balance grade still need to be held [S3]. ATEX certification on Lovejoy SU, SX, DI, SXC, SXCS, SXCST, DIR, DILR, DIRA, and DIRLA lines covers explosive atmospheres but does not change the misalignment budget; the same per-pack angular and total parallel-offset numbers still apply [S1]. Background on the broader coupling family, including how disc couplings differ from gear and jaw types, is in the disc coupling reference page, while related torque-transmission elements are catalogued under gear coupling and jaw coupling.
For applications that mix long DBSE (distance between shaft ends) with parallel offset, the canonical answer is not "more disc packs" but a floating-shaft build: two SU couplings plus a hollow spacer shaft, which restores independent flex-plane behavior at each end of the span [S1]. For comparison of these misalignment-coupling options against elastomeric and gear types, see the single girder crane reference for a typical end-truck drive arrangement, the fluid coupling entry for soft-start applications, and the coupling clutch page for combined torque-transmission and engagement duties. Trackable signal: monitor IBT, SKF, and Lovejoy technical bulletins through 2026 Q4 for revised per-pack angular ratings on 8-bolt unitized packs, and recheck the API-610 balance and piloting language if your train runs above 3,800 RPM.
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