The kiln is usually turned by means of a single Girth Gear surrounding a cooler part of the kiln tube, but sometimes it is turned by driven rollers.  The gear is connected through a gear train to a variable-speed electric motor.  This must have high starting torque to start the kiln with a large eccentric load.  A 6 x 60 m kiln requires around 800 kW to turn at 3 rpm.  The speed of material flow through the kiln is proportional to rotation speed; a variable-speed drive is needed to control this.  When driving through rollers, hydraulic drives may be used.  These have the advantage of developing extremely high torque.  In many processes, it is dangerous to allow a hot kiln to stand still if the drive power fails.  Temperature differences between the top and bottom of the kiln may cause the kiln to warp, and refractory is damaged.  Hence, normal practice is to provide an auxiliary drive for use during power cuts.  This may be a small electric motor with an independent power supply, or a diesel engine.  This turns the kiln very slowly, but enough to prevent damage.