white knight wk767 tumble dryer grinding noise?

hi

im having problems with my dryer its making grinding noise,i opened the top and back panel to see if the belt has snapped but no but instead in the back bottom side a round thing that connects with the drum that came loose bolt has come out so i decided to put it back on again after i fix it it runs ok for under 5 mins then the bolt becomes loose again can someone tell me what do u call this thing i have no clue