I agree with historyljc. If the bolts loosen up, they will have more tendency to break. Use a little Loc-Tite on the nut if this is the problem.
Also, if the holes are enlarged and there is too much slop, the bolts will shear more easily. On our main slide, the motor drives shafts made up of square metal tubing. I had one bolt that had loosened a while back, and the hole in the square tubing had become enlarged. I fought with that bolt breaking repeatedly for almost 2 years. I finally took the piece of tubing with the enlarged hole, exchanged it end for end, and drilled new, clean holes in the new end. No problems since.
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