Belt Conveyor Coupling Defect

belt conveyor
power 22 KW
speed 1500 Rpm (25 Hz)
it used for convying materials.
while vibration data collect we noted increase vibration level to 6 mm/s with small sound and impact ,
in spectrum we noted some harmonic with 3 x dominate
after few days the vibration reach 14 mm/s .
in spectrum we noted harmonic with 6 x dominate .
In TWF we noted high 6 impacte each revolution with kind of misalignment and lossness ,
most commonly issue like this is coupling defects according to rubber element or lose or sever misalignment or cracks ,
after machine stop the cracks apear on top of key .