 Each end of each caisson is supported on small wheels , which run on rails on the inside face of the eight @-@ metre diameter holes at the ends of the arms . The rotation is controlled by a train of gears : an alternating pattern of three eight @-@ metre diameter ring gears and two smaller idler gears , all with external teeth , as shown in the picture . The large central gear is fitted loosely over the axle at its machine @-@ room end and fixed in place prevent it from rotating . The two smaller gears are fixed to each of the arms of the wheel at its machine @-@ room end . When the motors rotate the central axle , the arms swing and the small gears engage the central gear , which results in the smaller gears rotating at a higher speed than the wheel but in the same direction . The smaller gears engage the large ring gears at the end of the caissons , driving them at the same speed as the wheel but in the opposite direction . This cancels the rotation due to the arms and keeps the caissons stable and perfectly level . 
