EMPLOYEE WAS HELPING INSTALL A RUBBER BOLT ON A VIBRATING SCREEN. WHILE THE MECHANIC WAS PLACING THE BOOT EMPLOYEE WAS PRYING ON THE SCREEN BODYWITH A 5' BAR. HE HAD THE END OF THE BAR ON HIS RIGHT SHOULDER AND BOTH HANDS AROUND THE BAR. AS HE LIFTED THE BAR. HE FELT A POP IN HIS SHOULDER.