| 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).
