Coupling Design Torque & Service Factor — AGMA 9002 Explained
AGMA 9002 — Bores and Keyways for Flexible Couplings (Inch Series)
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AGMA 9002 is the American Gear Manufacturers Association standard for bores and keyways of flexible shaft couplings (inch series). It establishes the dimensional tolerances for coupling bores, keyway widths and depths, and shaft fits that ensure interchangeability and proper torque transmission through the hub-to-shaft interface. AGMA 9002-B04 is the North American reference for specifying and verifying flexible coupling bores in industrial drivetrain applications; service-factor selection for coupling sizing is separate catalogue and general engineering practice, not part of this standard.
The MechanixCalc shaft couplings calculator computes the design torque (nominal torque multiplied by a service factor drawn from coupling-catalogue and general engineering practice), the resulting torque safety factor against the catalog coupling capacity, and misalignment checks for radial, angular and axial offset. The tool covers jaw (spider), disc, Oldham, gear and rigid coupling types. Several sub-panels — including misalignment reaction forces, critical speed estimation and the coupling suitability ranking — are engineering estimates with in-app disclaimers; the only bore check is that the shaft diameter falls within the coupling's rated bore range; AGMA 9002 keyway and tolerance conformance is not checked.
Related calculators — AGMA 9002 itself is not implemented
What AGMA 9002 covers
- Bore and keyway dimensional tolerances for flexible coupling hubs — diameter fits, keyway widths and depths to AGMA 9002-B04 (inch series)
- NOT in AGMA 9002: service-factor K_s selection by driven-machine characteristic and shock severity — light uniform (fans, centrifugal pumps) through heavy shock (crushers, reciprocating compressors). Service factors come from coupling manufacturers' catalogue tables and general AGMA coupling practice
- NOT in AGMA 9002: the design-torque calculation T_d = T_nom × K_s and the requirement that catalogue torque capacity exceed T_d
- NOT in AGMA 9002: the torque safety factor SF = T_rated / T_d and the ≥ 1.5 industrial-duty target, which is catalogue practice rather than a standard's requirement
- NOT in AGMA 9002: misalignment limits — radial (parallel offset), angular and axial (end float) — which are set per coupling type by the manufacturer. ⚠️ AGMA 9002 covers BORES AND KEYWAYS ONLY (inch series); of this list only the first bullet is actually within it. The MechanixCalc couplings tool applies the sizing and misalignment checks above as engineering practice, not as an AGMA 9002 conformance calculation
- Bore suitability check: shaft diameter must fall within the coupling's rated bore range, because oversized bores in a fixed coupling envelope reduce hub wall thickness and lower torsional capacity
Governing formulas
T_d = T_nom × K_swhere T_d = design torque (N·m or lbf·in); T_nom = nominal (rated) transmitted torque at the operating speed; K_s = service factor (dimensionless, typically 1.0–3.0 from coupling manufacturers' catalogue tables — accounts for load uniformity, shock level, reversals and starts per hour). The coupling catalog torque capacity must satisfy T_rated ≥ T_d × SF_min.
SF = T_rated / T_dwhere SF = torque safety factor (dimensionless); T_rated = coupling catalog torque capacity (N·m) at the selected bore; T_d = design torque (N·m). Minimum recommended SF = 1.5 for standard industrial duty; higher values apply for safety-critical or shock-loaded applications. A bore larger than nominal reduces T_rated because hub wall thinning lowers torsional stiffness.
Frequently asked questions
What is AGMA 9002 used for?
AGMA 9002-B04 specifies the bore and keyway dimensions — tolerances, fits and keyway proportions — for flexible shaft couplings in the inch series. It ensures that coupling hubs from different manufacturers are dimensionally interchangeable and that the hub-to-shaft interface transmits the design torque without fretting or slipping. Engineers size the coupling for the driven-machine shock category and duty cycle separately, using service factors from coupling-manufacturer catalogues and general engineering practice rather than an AGMA 9002 table.
How is the service factor for a flexible coupling determined?
The service factor K_s is selected from coupling-manufacturer catalogue tables and general engineering practice based on the type of prime mover (electric motor, internal-combustion engine), the driven-machine load category (uniform, moderate shock, heavy shock), and the number of starts per hour; it is not defined by AGMA 9002 itself. Typical values range from 1.0–1.5 for uniform loads (fans, centrifugal pumps) up to 2.0–3.0 for heavy-shock applications (crushers, reciprocating compressors, rolling mills). The design torque T_d = T_nom × K_s must be less than the coupling's catalog capacity divided by the required safety factor.
Does AGMA 9002 cover misalignment limits?
AGMA 9002-B04 itself focuses on bore and keyway tolerances, not on operational misalignment limits. Permissible misalignment (radial offset, angular deviation and axial displacement) is set by the coupling manufacturer, not by AGMA 9002. The MechanixCalc coupling calculator checks all three misalignment types against per-coupling-type catalogue limits held in its own coupling database (these are not ISO 14691 values) and flags any exceedance with a warning.
What is the difference between AGMA 9002 and AGMA 9000?
AGMA 9000 ('Flexible Couplings — Potential Unbalance Classification') covers balance grades for flexible couplings. AGMA 9002 ('Bores and Keyways for Flexible Couplings — Inch Series') covers the dimensional requirements for coupling bores and keyways. The two standards address different aspects of coupling design; service-factor selection for design torque is separate catalogue and general engineering practice, not codified in either standard.
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Related standards
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