I have a lot of Bessey F clamps. They are nice clamps but nearly all of them work loose under load and/or vibration. Eventually the sharp edges of the cast sliding piece round over slightly and no longer bite on the serrations in the rail. My solution was to drill 6mm holes at about 35mm centres along the rail. I fit a socket head cap screw into the appropriate hole and secure it with a nut. It's a PITA but without it most of my Besseys would be useless. I can post a pic if needed.