With reference to fluid couplings for the main propulsion shaft:
(a) explain the principle of operation;(5)
(b) state how the transmitted torque may be varied;(2)
(c) state THREE advantages.(3)
Fluid Coupling in Marine Propulsion
A fluid coupling is a hydrodynamic device used to transmit rotational power from one shaft to another without a mechanical connection. In marine applications, it’s often employed between the main engine and the propeller shaft.
2. Hydrodynamic couplings for ships – Voith
How it works:
- The input shaft drives an impeller, which circulates a fluid (usually oil) within the coupling. 1. Fluid coupling – KSB www.ksb.com
- The kinetic energy of the fluid is transferred to a turbine connected to the output shaft. 1. Fluid Coupling – Dhole Patil College Of Engineering Pune dpcoepune.edu.in
- As the speed of the input shaft increases, so does the fluid velocity, resulting in increased torque transmission.
Advantages of Fluid Couplings:
- Smooth power transmission: Eliminates shock loads and vibrations. 1. Fluid coupling – Wikipedia en.wikipedia.org
- Overload protection: Acts as a clutch, preventing engine damage during overload conditions.
- Reduced wear: Protects the drivetrain components from wear and tear. 1. Hydrodynamic couplings for ships – Voith voith.com
- Load sharing: Facilitates load sharing between multiple engines. 1. Hydrodynamic couplings for ships – Voith voith.com
Disadvantages of Fluid Couplings
Lower efficiency: Compared to direct mechanical couplings, fluid couplings are less efficient.
Power loss: Some power is lost due to fluid friction.
Types of Fluid Couplings in Marine Propulsion
There are primarily two main types of fluid couplings used in marine propulsion:
1. Constant Fill Fluid Couplings
- Fixed oil volume: The coupling contains a fixed amount of oil.
- Torque transmission: Torque is transmitted through the oil as it circulates between the impeller and turbine.
- Load-free start-up: Allows for smooth engine start-up without load on the propeller.
- Dampening: Effectively reduces vibrations and shock loads.
- Common applications: Used in various marine vessels, from tugboats to larger commercial ships.
2. Fill-Controlled Fluid Couplings
- Variable oil volume: The amount of oil in the coupling can be adjusted.
- Torque control: By varying the oil level, the coupling’s torque transmission characteristics can be changed.
- Clutching function: When the oil level is reduced, the coupling can be decoupled, acting as a clutch.
- Applications: Often used in vessels requiring precise speed control or load sharing between multiple engines.
Both types of fluid couplings offer advantages in marine propulsion, and the choice depends on the specific requirements of the vessel.
Torque Transmission and Variation in Fluid Couplings
Torque Transmission
In a fluid coupling, torque is transmitted through the hydrodynamic action of the fluid.
- Impeller: The input shaft drives an impeller, which imparts kinetic energy to the fluid. 1. Fluid Coupling – Dhole Patil College Of Engineering Pune dpcoepune.edu.in
- Fluid circulation: The fluid circulates within the coupling, creating a vortex. 1. FLUID COUPLING, THE PRINCIPLES OF OPERATION – YouTube www.youtube.com
- Turbine: The kinetic energy of the fluid is transferred to the turbine, which is connected to the output shaft. 1. Fluid Coupling – Dhole Patil College Of Engineering Pune dpcoepune.edu.in
As the speed of the input shaft increases, so does the fluid velocity and the resulting torque transmitted to the output shaft.
Torque Variation
Torque variation in a constant fill fluid coupling is primarily achieved through:
- Engine speed: Increasing the engine speed increases the input torque and, consequently, the output torque.
- Fluid viscosity: Changing the fluid viscosity can affect torque transmission, but this is generally not practical in most applications.
In fill-controlled fluid couplings, torque variation is more direct:
- Oil level adjustment: By changing the amount of fluid in the coupling, the torque transmission characteristics can be altered. 1. FAQ On Fluid Couplings | Fluidomat Australia www.fluidcoupling.com.au
- Hydraulic control: Some couplings use hydraulic systems to control the oil level, allowing for precise torque adjustment.
It’s important to note that while torque can be varied in fill-controlled couplings, the efficiency of the coupling might be compromised at lower oil levels.
Advantages of Fluid Couplings in Marine Propulsion
Smooth power transmission: Eliminates shock loads and vibrations.
Overload protection: Acts as a clutch, preventing engine damage during overload conditions.
Reduced wear: Protects the drivetrain components from wear and tear.
Load sharing: Facilitates load sharing between multiple engines.
Engine protection: Provides a cushion against torsional vibrations.
Reduced shock loading: Protects the propeller and shafting from sudden load changes.
Disadvantages of Fluid Couplings in Marine Propulsion
- Power loss: Some power is lost due to fluid friction, resulting in lower efficiency compared to direct mechanical couplings.
- Speed slip: There is always a slight speed difference between the input and output shafts.
- Increased complexity: Fluid couplings add complexity to the propulsion system compared to direct couplings.
- Maintenance requirements: The fluid coupling itself and its associated components require regular maintenance.
While fluid couplings offer several advantages, it’s essential to weigh these against the potential drawbacks for a specific marine application.