2.2dt aux belt tensioner 1998

[benali]

I'm having some problems and I'm looking for any advice:
3 mths ago the belt tensioner was replaced by a local mechanic (old tensioner was worn out causing belt to run unevenly). 1 month later a bolt sheared on the mounting plate part of this assembly causing the fixed pulley and belt to fall off, engine had to be dropped to have remaing bolt removed a new bolt fitted and the whole thing re-assembled (existing belt undamaged and re-used).  Everything has been fine untill last night when aux belt dropped off again (shreaded this time). I removed the bottom cover to discover that this time the bolt holding the belt tensioner has sheared off. I'm not a mechanic and would appreciate any opinions on what is going on here before I waste more money on repair bills. Many thanks, Andrew.

[benali]

My mechanic tells me the tensioner is faulty but its just not making sense to me. I've attached a picture of hole the bolt fell out off, you can see that the 1st few threads are damaged presumably where the stud was removed 1st time it happened, I've also attached a pic of the tensioner and broken bolt. Now when the bolt broke this time I noticed a large thrust washer in between the two but this doesn't make sense to me because with the centre bolt tight the whole thing would become locked in position. Does anyone else remember a thrust washer on their tensioner?
Come on you knowledgable people somebody give me some help.
Andrew.