fluid-coupling

name	fluid-coupling
description	Fluid coupling and torque converter design — slip characteristics, stall torque, efficiency, Föttinger principle, torque multiplication (torque converter), fill level control, power loss, heat dissipation, VOITH/Rexnord selection.
metadata	{"priority":7,"promptSignals":{"phrases":["fluid coupling","torque converter","hydrodynamic coupling","Föttinger coupling","slip coupling","fluid drive"],"minScore":3}}

Fluid Coupling and Torque Converter Design — Complete Skill

Föttinger Principle

Fluid coupling: two-element device (pump impeller + turbine runner); transfers torque through kinetic energy of fluid (oil); output torque = input torque (no multiplication) Torque converter: three-element device (pump + turbine + stator/reactor); torque multiplication possible at low output speed (stall)

Fundamental energy transfer: Circulation: pump accelerates oil centrifugally → oil strikes turbine blades → turbine accelerates → oil returns to pump through stator (if present) At coupling point (no slip): oil circulation = 0; mechanical coupling

Fluid Coupling Characteristics

Slip and Efficiency

Slip: s = (n_pump - n_turbine) / n_pump = 1 - n_T/n_P [dimensionless; s = 0: synchronous; s = 1: stall]

Efficiency: η = P_output / P_input = (T_T × ω_T) / (T_P × ω_P) For fluid coupling (T_T = T_P = T): η = n_T/n_P = 1 - s At design slip (s ≈ 2–4%): η ≈ 0.96–0.98

Speed-torque law: T = λ × ρ × n_P² × D⁵ [N·m; λ = torque coefficient (function of s); ρ = fluid density; D = torus diameter] At constant slip: T ∝ n_P² → quadratic relationship (soft starting characteristic)

Stall torque (n_T = 0, s = 1): T_stall = λ_stall × ρ × n_P² × D⁵ [λ_stall = maximum torque coefficient; typically 5–8× design condition value] Use for motor starting design: stall torque must not exceed motor torque rating × service factor

Power Loss and Heat

Power loss: P_loss = T × (ω_P - ω_T) = T × s × ω_P [W; all goes to heat in oil]

Oil temperature rise during start-up: ΔT = ∫₀^t P_loss dt / (m_oil × c_p) [°C; m_oil = oil mass in coupling; c_p ≈ 1800 J/(kg·K) for mineral oil] Continuous operation: heat rejected to cooling via housing convection or external cooler

Maximum oil temperature: 80–90°C continuous; 120°C transient maximum for mineral oil If exceeded: use forced cooling or reduce start frequency

Fill Level Control

Variable fill coupling: adjust oil fill level → change effective torus volume → control torque and slip

• 100% fill: maximum torque capacity (fixed coupling behavior)
• Partial fill: reduced torque; controlled slip for soft starting or speed control
• Oil reservoir and scoop tube mechanism (Voith): centrifugal scoop controls fill level continuously

Variable speed fluid coupling (VSFC): fill level controlled by scoop position → output speed control Speed range: typically 0.25–1.0 × input speed Efficiency at intermediate speed: η ≈ n_T/n_P = speed ratio (higher than throttle valve for pump control)

Torque Converter Characteristics

Stall Torque Ratio and Coupling Point

Stall torque ratio (K-factor): K = T_turbine / T_pump at n_T = 0 (stall) Automotive: K ≈ 1.8–2.5; industrial: K ≈ 1.3–2.0

Coupling point: speed ratio e_c where T_turbine = T_pump (stator unlocked); η peaks here e_c ≈ 0.8–0.9 of synchronous speed for most converters

Efficiency at coupling point: η_max ≈ e_c × K_eff [K_eff ≈ 1 at coupling point; η_max ≈ e_c ≈ 0.85–0.90]

Stator Function

Stator/reactor: redirects fluid returning from turbine to assist pump input → reaction torque At low speed ratio: stator fixed (one-way clutch locked) → adds reaction torque → K > 1 At high speed ratio (near coupling point): stator freewheels → no reaction → behaves as fluid coupling

Stator blade angle: designed for specific speed ratio; optimal at peak torque multiplication speed

Cavitation in Torque Converters

Cavitation risk: at high speed ratio + high power → vapor pockets form at turbine inlet Prevention: maintain minimum charge pressure p_charge = 3–6 bar (from charge pump) Symptoms: noise, vibration, sudden drop in torque capacity, erosion on blades

Fluid Coupling Selection

Key Parameters

Power rating: P_rated = T_rated × 2π × n_rated / 60 [W; at design slip] Select coupling size: P_rated ≥ P_driven_machine × SF [SF = service factor 1.25–2.0]

Starting torque requirement: Loaded starting: T_load_at_zero_speed (from pump/conveyor inertia and static friction) Coupling must provide T_start ≥ T_load while limiting motor starting current (T_start ≤ ~2.5 × T_rated)

K-factor for sizing: K_factor = n_motor / √(T_motor / ρ) [dimensional; used in manufacturer selection charts]

Service Factors for Common Applications

Application	Service Factor
Centrifugal pump/fan	1.25
Conveyor (light)	1.5
Conveyor (heavy, belt)	2.0
Ball mill / crusher	2.5–3.0
Mine hoist	2.5
Ship propulsion	1.5

Speed Control with Fluid Coupling

Energy savings vs. throttle valve (pump control): Throttle: wastes pressure head as heat; efficiency = (P_useful/P_input) = (Q_actual × ΔP_useful) / (Q_actual × ΔP_total) VSFC: η_VSFC ≈ (n_T/n_P)³ × pump_efficiency at that flow → large energy savings at part flow

Affinity law comparison: At 80% speed: flow = 80%, head = 64%, power = 51.2% of full-speed Throttle at 80% flow: power ≈ 70% (must overcome pressure loss at valve) VSFC saves ~20% power at this operating point

Payback: VSFC typically pays back in 1–3 years on large pump/fan drives vs. throttle control

Maintenance and Monitoring

Oil condition: Change oil every 4,000–8,000 hours; mineral oil ISO VG 32 or 46 for most couplings Acid number: replace when AN > 2 mg KOH/g Viscosity change: replace if ±25% from new

Coupling temperature monitoring: Thermocouple in oil sump or housing; alarm at 90°C; shutdown at 120°C For high-start frequency: thermal model to predict temperature (integrating power loss vs. cooling)

Vibration limits: Residual imbalance: per ISO 1940 G6.3 or G2.5 Bearing vibration: ≤ 2.8 mm/s RMS (ISO 10816 Class II machinery)

Standards

Standard	Scope
ISO 1940-1	Balance quality requirements
ISO 10816-3	Vibration evaluation of fluid couplings
AGMA 9005	Industrial bevel gear drives (fluid coupling service)
VOITH Technical Bulletin	VOITH Turbo coupling selection data
API 671	Special purpose couplings (includes fluid couplings in API service)

Output

Provide: coupling type (fluid coupling/torque converter/VSFC), input speed [RPM] and power [kW], design slip [%] and efficiency [%], stall torque [N·m] vs. motor starting torque, torque ratio K (if torque converter), oil fill type and operating temperature [°C], power loss at design point [kW], cooling method (natural convection/forced oil cooler), size selection (torus diameter D [mm], coupling model), service factor applied, energy savings vs. throttle [%] (if VSFC), and applicable standard (ISO 1940, API 671).