| name | flexible-coupling |
| description | Flexible coupling design — jaw/disc/gear/elastomeric couplings, torsional stiffness, misalignment capacity, service factor, peak torque, AGMA 9002, coupling selection by drive type, balancing, critical speed interaction. |
| metadata | {"priority":7,"promptSignals":{"phrases":["flexible coupling","coupling design","torsional stiffness coupling","misalignment coupling","AGMA 9002","coupling selection"],"minScore":3}} |
Flexible Coupling Design — Complete Skill
Coupling Types and Characteristics
Jaw (Spider/Claw) Couplings
Construction: two metal hubs with interlocking jaws; elastomeric spider (polyurethane or NBR) fills gaps
Torsional stiffness: depends on elastomer hardness; Shore 92A spider ≈ 3× stiffer than Shore 64A
Misalignment capacity: parallel: 0.25–1.0 mm; angular: 1.0–1.5°; axial: 1–4 mm
Damping: high (elastomer); good for shock absorption
Fail-safe: hubs can drive through if spider fails (lobes can contact)
Typical torque range: 5–5,000 N·m (size dependent)
Disc Couplings
Construction: metallic disc packs (stainless or alloy steel) alternately bolted to each hub
Torsional stiffness: very high (near-rigid in torsion); k_t = E × I / L_disc_pack
Misalignment capacity: parallel: 0.25–0.75 mm; angular: 0.5–1.5° per disc pack; axial: ±4 mm
Damping: very low (metal-to-metal); resonant peaks are sharp
Applications: precision machinery, turbomachinery, servo drives; no lubrication required
Torque range: 100–100,000+ N·m
Gear Couplings
Construction: outer sleeve with internal gear teeth meshes with external gear on hub; crowned teeth allow misalignment
Torsional stiffness: very high (near-rigid)
Misalignment capacity: angular: up to 1.5° per gear set (2× if double-engagement); parallel: up to 3 mm
Lubrication required: grease-packed or oil-continuous; critical for long life
Speed range: up to 20,000 RPM with balanced design; per AGMA 9009
Applications: high-torque, high-speed; turbines, compressors, rolling mills
Elastomeric Tyre Couplings
Construction: rubber tyre element grips two flanges; toroidal shape
Misalignment capacity: highest of common types — parallel: 2–3 mm; angular: 4°; axial: ±5 mm
Damping: very high; excellent vibration isolation
Applications: pump drives with high misalignment; generator sets; rough service
Bellows Couplings
Construction: thin-wall metallic bellows (stainless or titanium); welded to hubs
Torsional stiffness: very high; near-zero backlash
Misalignment: angular: 3–5°; parallel: 0.25–0.5 mm; axial: ±0.5 mm per convolution
Applications: encoders, servo drives, precision positioning; high-temperature service
Coupling Selection Methodology
Service Factor (AGMA 9002)
Design torque:
T_design = T_nominal × SF [N·m; SF = service factor from AGMA 9002 table]
Service factor by driver/driven type:
| Driver | Driven (Light Shock) | Driven (Moderate Shock) | Driven (Heavy Shock) |
|---|
| Electric motor (uniform) | 1.0 | 1.5 | 2.0 |
| Electric motor (slight shock) | 1.5 | 2.0 | 2.5 |
| IC engine (4-cyl+) | 2.0 | 2.5 | 3.0 |
| IC engine (1–3 cyl) | 2.5 | 3.0 | 4.0 |
Additional factors:
- Frequent reversals: add 0.5 to SF
- Ambient temperature > 50°C (elastomers): derate by factor from manufacturer
- Load reversals and dynamic shock: check peak torque separately
Torque Rating
Continuous torque:
T_continuous = P / ω [N·m; P in W; ω in rad/s]
Peak torque check:
T_peak ≤ T_coupling_rated_peak (typically 2–3× continuous rated torque for elastomeric; 3–5× for disc)
Vibratory torque (resonant condition):
At resonance: T_vibration = Q × T_excitation [Q = quality factor = 1/(2ζ)]
Must not exceed coupling fatigue rating (typically 50% of continuous rating for number of cycles)
Torsional Analysis
Torsional stiffness (coupling contribution):
k_t_coupling [N·m/rad] from manufacturer; varies with torque level for elastomers
Two-mass torsional system:
ω_n = √(k_t × (J₁ + J₂) / (J₁ × J₂)) [rad/s; J₁, J₂ = polar moments of inertia]
Torsional natural frequency check:
f_n_torsional must be ≠ N × (excitation frequency) for N = 1, 2, 3, 4
Separation margin: f_n ≥ 1.25 × f_excitation (resonance avoidance criterion)
Soft couplings (elastomeric): lower k_t → lower f_n → can shift resonance below operating range
Stiff couplings (disc/gear): high k_t → high f_n → rigid coupling approximation at low frequencies
Misalignment Capacity
Parallel misalignment generates radial forces on bearings:
F_radial = T × sin(α) / r ≈ T × α / r [small angle; α = misalignment angle; r = coupling radius]
For jaw coupling: restoring stiffness of elastomer generates bearing load
Angular misalignment generates alternating torque:
ΔT = T × tan(α) ≈ T × α [alternating torque at 2× rotational frequency for single-flex]
Double-flex couplings (two disc packs): parallel offset with no angular velocity fluctuation
Combined misalignment:
Allowable parallel = √(δ_parallel² + (r × θ_angular)²) ≤ coupling_limit
Most manufacturer curves assume 50% parallel + 50% angular simultaneously
Lateral Critical Speed Interaction
Overhung coupling affects shaft critical speed:
m_coupling × e_unbalance + m_coupling × d_overhang → additional lateral force at first critical
Check: coupling weight × overhang moment < 5% of bearing load (rule of thumb)
API 671 (special-purpose couplings): coupling critical speed ≥ 1.25 × maximum continuous speed
Balance grade: G6.3 per ISO 1940 for general; G2.5 for turbomachinery (API 671)
Keyway vs. Keyless Connection
Keyed hub:
Keyway weakens shaft; stress concentration K_t = 1.5–2.5 at keyway root
Torque capacity: T_key = L × w × τ_allow × d/2 [L = key length; w = key width]
Keyless tapered bushing (Locking Assembly):
Friction coupling: T = μ × F_clamp × r_shaft [μ ≈ 0.12 dry; F_clamp from bolt torque]
More uniform stress distribution; easily removed; preferred for high-cycle fatigue
Shrink fit hub:
T_max = μ × p × π × d² × L / 2 [p = interface pressure from interference calculation]
Standards
| Standard | Scope |
|---|
| AGMA 9002-C16 | Bore and keyway tolerances for flexible couplings |
| AGMA 9009-D02 | Nomenclature for flexible couplings |
| ISO 1940-1 | Balance quality requirements for rotors |
| API 671 | Special-purpose couplings for petroleum |
| API 610 Annex F | Coupling data sheet for pump couplings |
| NEMA MG-1 | Motor shaft/coupling interface requirements |
Output
Provide: coupling type (jaw/disc/gear/bellows), nominal torque T_nominal [N·m], service factor SF (AGMA 9002 driver/driven type), design torque T_design [N·m], peak torque T_peak [N·m] vs. coupling rating, misalignment capacity (parallel [mm], angular [°]) vs. installation misalignment, torsional stiffness k_t [N·m/rad], torsional natural frequency f_n [Hz] vs. excitation frequency (separation margin [%]), coupling weight [kg] and overhang [mm], balance grade (ISO 1940), hub-shaft connection type (keyed/keyless/shrink), and applicable standard (AGMA 9002, API 671, ISO 1940).
