Abstract: A drive mechanism for a barrier or a stop or gate device includes a drive unit having a drive motor (17) to which at least one single-stage step-down gear, worm gear and/or spur gear with an articulation to a pivoting crank (38) is connected, with the crank being mounted on a primary shaft (46) pivoting preferably around a horizontal axis at bearing points on both sides. The shaft is provided with a fixing device (40) for the barrier beam (41) or gate element which can generally pivot at an angle of approximately 90° in addition to abutments for the compensation springs (48) which act upon their spring crank. The drive mechanism further includes a single-part, single-piece drive casing (G) with open, flat securing flange strips (2) on all sides and a receiving flange (14) to secure the drive motor (17) which can be dismantled and removed. The motor shaft (36) of the drive motor is introduced into a gear case (9) which can be closed by a cover (10) and which contains a set-down gear.