OB 1.4 50mm Backup Head Fan mount
This part is a backup fan for Hot end Heat sink.
It happens that each fan has a lifespan of a few 10000's hours.
After a while due to lifespan or dust or what ever else, your fans will start making some noise.
This is an indicator that your fan might get stuck in a near future and that you might need to oil the bearing or change your fan.
With full or half metal hot ends, the fan for the heat sink is MANDATORY. If it fail before printing, it's not a big deal you can change it before starting you hot end.
But if it fails during a print you will break something on your printer as your hot end will get jammed by your expended filament.
This backup fan is made to be a secondary fan in case the first one stopped or get stuck somehow. It should allow at least to end you print without breaking anything and keep your heat sink "cold".
To be used on top of this part:
http://www.thingiverse.com/thing:211597