• Standardisation of bores for uniform shafts of electric motors
• During start-up of the engine a low inertia is effective so that less power is needed
• Slow start-up with use of delay chamber because the oil is sucked into the workspace
• No axial extension of the coupling when using a brake disk or drum  
• Protection of the flexible coupling

The fluid coupling consists of a limited number of main components. Located on the drive side, a hollow-shaft is connected to an inside paddle wheel. On the driven side, the output is achieved via a two-part aluminium outer shell with an integrated paddle wheel. One radial and one needle bearing make ensure the bearing assembly while shaft seals ensure against external influences. A standard fluid coupling contains a fusible safety plug (145°C – red) as well as an oil filler plug.

The operating principle of the fluid coupling is based on the Föttinger principle.
Regarding the inner wheel drive, the hollow shaft and connected inside paddle wheel act as a pump while the outer shell acts as a turbine. The power provided by the engine generates a rotational flow of fluid via the hollow shaft and the inside paddle wheel. This flow energy, in turn, gradually causes the outer shell to turn, thus transferring the rotational energy to the driven side, possibly through the connection of a flexible coupling or pulley. Between input and output there occurs a slippage of 2-6%.

The fluid coupling type K can be extended by a small (SCF) or a large (DCF) delay chamber. The start-up torque is reduced and the start-up time is extended, so that a smooth start-up is guaranteed. The following start-up factors can be implemented with the different delay chambers:
• ALFA K (without delay chamber): start factor 1.8-2.
• ALFA K-SCF (with a small delay chamber): start factor 1.5-1.8
• ALFA K-DCF (with a large delay chamber): start factor 1.2-2.5 

To monitor temperature and speed several accessories are provided to complete the fusible safety plug.

Fusible safety plug (FP)
In case of overheating, the fusible link of the fusible safety plug melts allowing the oil to discharge and the torque transmission disabled.
The standard fluid coupling contains a fusible safety plug with a melting point of 145°C (red). Fusible safety plugs with the following temperatures are available: 96°C (blue); 120°C (white); 145°C (red); 180°C (green)

Fusible safety plug with pin (FTP)
To avoid oil leakage the use of a second fusible safety plug with pin is recommended, in addition to the standard fusible safety plug. This plug needs to be one temperature class lower than the standard screw. When reaching the trigger temperature of the fusible link of the pin, the plug is pushed out by a spring to actuate a mechanical switch which turns off the motor or triggers a signal. Fusible safety plugs with included pins are available with the following temperatures: 96°C (blue); 120°C (white); 145°C (red).

Contactless pulse generator (PM)
To avoid oil leakage, the use of a second temperature sensor is recommended, in addition to the standard fusible safety plug. The pulse generator needs to be one temperature class lower than the standard screw. When reaching the trigger temperature of the pulse generator it transmits the signal to the opposite sensor without any contact. This opposite sensor switches the motor off or triggers a signal. Pulse generators with the following temperatures are available: 100°C (blue); 120°C (white); 140°C (red); 160°C (yellow).

Mechanical temperature control (ET+FTP)
If the pin triggers the FTP because the temperature is exceeded, this actuates the mechanical switch installed. The motor is switched off or a signal is triggered.

Electronic speed control (SCD)
The electronic speed control can be installed as an additional coupling protection (for inner wheel drives). It compares the actual speed with the target speed. If the proportion is too high the motor switches off or a signal is triggered.

Contactless temperature and speed control (T09+PM)
If the temperature of the PM is achieved, it transmits a signal to the sensor, the motor switches off or a signal is triggered. In addition to the temperature control, a speed control is possible as well (only for inner wheel drives). The actual speed and the target speed are compared. If the difference is too big the motor switches off or a signal is triggered.

REF: https://www.ktr.com/en/products/power-transmission-technology/couplings/fluid-couplings/inner-wheel-drive/