I have to find the tension in belt, in belt pulley mechanism...
And your question is?
In order to get good feedback, please explain as much as possible about your assembly, what you are trying to achieve, what you have tried so far, and what you have run into.
In the mean time, here is a little bit about belt calculations:
There is no way to do this currently. The belt feature is nearly useless as a result. Please upvote my suggestion to add this capability: Tension Measurement for Belts