Fluid Coupling, also known as Hydraulic Coupling, is a hydro-kinetic device used to transmit rotating mechanical power between shafts. It works on the principle of acceleration and deceleration of a hydraulic fluid. Unlike rigid mechanical couplings, fluid couplings provide smooth power transmission with shock absorption, making them widely used in heavy industries such as thermal power plants, mining, conveyors, crushers, and automotive systems.
A fluid coupling always has a slight speed difference between the input shaft (driver) and the output shaft (driven). This speed difference is called slip, and it is necessary for torque transmission.
Essential Parts of a Fluid Coupling
1. Impeller (Pump Section)
- Connected to the driving shaft (electric motor or engine).
- Acts as a pump, imparting kinetic energy to the fluid through centrifugal force.
2. Runner or Rotor (Turbine Section)
- Connected to the driven shaft (machine load).
- Acts as a turbine, absorbing energy from the circulating fluid and transmitting torque.
3. Casing (Housing)
- Encloses both impeller and runner.
- Provides an oil-tight reservoir and protects the fluid circuit.
Working Principle of Fluid Coupling
Starting Condition
- When the electric motor starts, the impeller rotates, throwing oil outward due to centrifugal action.
- The runner remains stationary initially.
Acceleration Phase
- The impeller pumps oil towards the runner, and circulation begins.
- A conical baffle diffuses the fluid flow, causing a gradual torque build-up.
- This allows the motor to accelerate quickly to rated speed without overloading.
Torque Transmission
- Oil circulates continuously between impeller and runner in a closed loop.
- This circulation forms a helical spring-like flow path.
- Once transmitted torque equals resisting torque, the runner starts rotating and accelerates the driven machine.
Slip
- Even in steady-state operation, the runner speed is slightly lower than the impeller speed.
- This slip (2–5%) is essential for fluid flow and torque transfer.
Types Of Fluid coupling
1. Constant fill / fixed speed Type
- Oil volume inside the casing remains constant.
- Provides smooth start and overload protection.
- Commonly used in conveyors, crushers, and fans.
2. Variable speed / Scoop Type
- Oil quantity is varied using an external scoop tube.
- Provides speed control and soft start capability.
- Used in applications requiring variable load handling (e.g., pumps, compressors, large conveyors).
Performance Characteristics
Slip (s)
s=((Ni−No)/Ni)×100
where = Input speed (rpm), = Output speed (rpm
Typical slip: 2–5% at rated load.Efficiency (η)
η≈(1−s)×100
Higher slip → lower efficiency.
Torque Transmission
Torque depends on fluid density (ρ), impeller speed (N), and oil fill volume.Power Loss
Due to slip, part of energy is lost as heat, requiring oil cooling in large units.
Advantage of Fluid Coupling
- The power transmission is free from vibration and noises.
- Power transmission is smooth even in extreme condition.
- Motor or engine starts unloaded.
- Overload protection.
- Controlled start up speed without shock loading of power transmission system.
- The maximum torque can be adjusted by varying the amount of oil filled in the casing.
- It can be used in both vertical and horizontal application.
Disadvantage of Fluid Coupling
- There is always slip. There is always slight difference in speed of impeller and runner.
- It cannot develop torque when the driving shaft and driven shaft are rotating in same angular velocity.
- Under stalling condition, the coupling dissipates energy as heat it may lead to damage.
Applications of Fluid Coupling
- Thermal Power Plants – conveyors, crushers, ID fans, FD fans, and BFP drives.
- Automobiles – automatic transmissions, buses, locomotives.
- Mining & Material Handling – belt conveyors, bucket wheel excavators, stacker reclaimers.
- Cement & Steel Industries – rotary kilns, crushers, mixers.
- Marine & Offshore – propulsion systems, pumps, compressors.
Problems, Causes and Corrective Actions in Fluid Coupling
Problem 1: Excessive Slip
- Cause: Low oil level, wrong viscosity oil, or overload on the driven machine.
- Corrective Action: Refill with correct quantity of recommended oil (ISO VG 32/46), use only specified grade oil, and reduce or inspect the load on the driven equipment.
Problem 2: Overheating of Fluid Coupling
- Cause: Continuous overload, inadequate oil circulation or cooling, wrong oil viscosity, or frequent start/stop cycles.
- Corrective Action: Remove overload, inspect cooling system, use proper oil grade, and reduce frequent starts to allow cooling.
Problem 3: Oil Leakage
- Cause: Damaged oil seals, worn-out gaskets, loose casing bolts, or cracks in the housing due to vibration.
- Corrective Action: Replace faulty seals or gaskets, tighten bolts, and repair or replace casing if cracked.
Problem 4: No Power Transmission (runner not rotating)
- Cause: Coupling empty of oil, fusible plug melted due to overheating, or wrong installation.
- Corrective Action: Fill coupling with specified oil, replace fusible plug after identifying overheating cause, and check installation alignment.
Problem 5: Excessive Vibration or Noise
- Cause: Misalignment between motor and driven shaft, worn bearings, unbalanced impeller/runner, or poor foundation.
- Corrective Action: Realign shafts, replace faulty bearings, balance rotating parts, and improve foundation grouting.
Problem 6: Frequent Thermal Plug Failure
- Cause: Prolonged overload, insufficient cooling, or incorrect oil level.
- Corrective Action: Monitor and limit overload, enhance cooling system, and maintain recommended oil filling level.
Problem 7: Slow Acceleration of Driven Machine
- Cause: High inertia load, low oil level, or damaged impeller vanes.
- Corrective Action: Start load gradually, fill to correct oil level, and inspect or repair impeller.
Problem 8: Abnormal Wear of Internal Parts
- Cause: Contaminated oil (dust, water, particles) or poor maintenance practices.
- Corrective Action: Drain and refill with clean oil, install filters if needed, and follow preventive maintenance schedule.
Frequently Ask Questions (FAQs) on Fluid Coupling & Hydraulic Coupling
To enable the fluid to flow from impeller to rotor it is essential that there is a difference in ‘Head’ between the two and thus it is essential that there is a difference in speed known as slip, between the two. Slip is an important and inherent characteristic of a fluid coupling resulting in several advantages. As the slip increases, more and more fluid can be transferred from the impeller to the rotor and more torque is transmitted.
Characteristics of operating fluid affect the transmission behavior of a coupling. The higher the density of the operating fluid, the better the transmission capacity. The higher the viscosity of the operating fluid, the more unfavorable the transmission behavior. The viscosity Index and flash points of operating fluid are also important. It must be ensured that the operating fluid is compatible with coupling components and their materials and operating conditions.
Thermal protection on this type of coupling provides safety to the fluid coupling and directly to the driver and driven machine. Fusible plug is the most common thermal protection. If the temperature of fluid in the coupling increases for any reason (which includes overloads) then the fusible metal in plug melts & all the fluid in the coupling drains out thus stopping power transmission and over loading the prime mover.
Normally, mineral oil of viscosity class ISO VG 32 is used for constant fill type coupling and ISO VG 46 for variable speed coupling.
Slip usually ranges between 2% – 5% under normal operating conditions.
Oil viscosity and density., Amount of oil filled., Operating temperature., Load inertia and resisting torque.
Usually between motor and gearbox to ensure soft start, reduced shock loading, and overload protection.
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