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When a little screw won't work....Use a bigger screw!
Actually, that does not look like good advice in this case. The photo clearly shows that torque applied to the handle applies a bending load to the screw.
Even worse, the bending load is applied to the threaded portion of the screw. Failure is inevitable. A bigger screw would last longer before breaking.
Better would be to redesign that part so that the torque on the handle is applied to an arm that is clamped on, or maybe keyed, or maybe even a roll pin. Has anyone broken the roll pin that holds the handle onto that same shaft? Another option would be a shoulder bolt with the shoulder submerged a few diameters into the shaft. Then the threads would not see the bending load.