alternator belt keeps breaking

[brokenvan]

can anyone suggest why my alternator belt keeps breaking?
its on a vauxhall movano (van) i had a new alternator fitted in dec last year at a local garage. afterwards, everytime i started the motor up, there was a massive squealing. consensus at work was that the belt was way too tight. i took the van back to garage and they told me the pulley bearings were shot, these were replaced. about a month afterwards my bellypan was ripped off,no speeding through puddles, just normal driving, again chaps at work suggested garage had not replaced bellypan correctly.
all was fine until march when there was a noise like driving over gravel coming from under the bonnet. the belt was wrapped around the pulleys, all chewed up. at the time i was travelling at about 70mph on a french motorway. i had a genuine renault belt fitted.
last saturday, again doing 65-70 on a french motorway, noise like gravel and the belt has come off again.
now assuming a new belt was fitted when the alternator was replaced, a belt is lasting me about 3 months.
bearing in mind i'm just about able to do an oil change, can anyone suggest what could be the problem?
i'm a bit reluctant to return to the garage as they don't have the best rep around here (some things you find out too late)
very many thanks for any help.

[Andywitz]

Hi, I came across a similar problem on a car i worked on recently that had had an alternator fitted by another garage too.
When i fitted the new belt i noticed that it was running at an angle because the alternator was sat further back than the crank pulley. It turned out that the garage that had fitted the new alternator had left a spacing washer out before they fitted the alternator pulley leaving it too far back causing the alignment problem & high belt wear.

Worth checking out maybe.

Andy