belt is not running true

Hello,

in the last couple of weeks, I've spent quite some time assembling my RepRapPro Mendel, which appears to be going fine apart from one issue: I cannot get the X belt to run true. It just keeps moving towards the machine front side of the idler (moving the carriage by hand in either direction). I have tried playing with the belt tension and the position of the belt in the clamps, but to no avail. Since I cannot find any forum topics on this issue, it seems that nobody else is having this problem, so I'm guessing its solution is probably trivial. I would really appreciate some advise. Which factors are most important for making the belt run true?

Have you got the bracket that the idler is mounted on square to the X axis? You can alter it using the screws that bear on the ends of the x axis smooth rods.

When I was putting mine together I thought that keeping the belts on the idlers would be a problem but once it was lined up properly it's been fine.

I was a bit hesitant to touch the setting screws for the X bars after their careful positioning in an earlier phase of the assembly process. Following your suggestion, I tried it anyway and it does make a big difference.

Thanks a lot!