Electro is on the money Aug 2013.
I had belt failure after 5 yrs replaced with replica type belt through in 10 mins.
Stripped and refitted belt again in situ without slackening motor plate threw again after 15 mins.
Tried again after a few weeks and saw a bow in motor plate due to over tensioned belt.
Slackened to minimal tension lubed all pulleys contact points... seems to have worked so far.
Richy Rich, May 2020
Check it's tention.
Check the alignment of the pulleys. They must be coplanar and the axis parallel.