Why does my belt tensioner break a lot?

I got a 94 ford f150 with a 351, it keeps snapping off belt tensioners, what's happening that it keeps snapping them?

Either you have the wrong belt. The engine might be different from the engine that came with the truck originally. Or the belt is routed incorrectly.

Dan is correct, but the tensioner in this ford is designed to put the proper amount of tension on the belt at all times. Perhaps you are putting the wrong belt on, like it might be a few cm. Short of the correct one, check for that. Also, are you taking the load off of the tensioner when you put the belt on? Make sure you have the belt routed correctly, check the diagram on the fan shroud. If it still keeps snapping, I would say that the tensioners are defective, they did have a huge problem with these things back when they first came out.

Your belt is way too tight. There's more than one adjustment. I can't specifically tell you where since I never did it on a ford. But look it up, your belt is Way too tight.