The bottom tier (24) of a conveyor belt (22) arranged in a spiral stack (26) is supported by and driven by inner and outer drive chains (28) and (30). The inner and outer drive chains are supported on and ride along support rails (32) and (34). The inner and outer drive chains are driven by a drive system (35) composed of inner and outer drive systems (36) and (38), each including a drive motor with a drive gear engaged with the inner and outer drive chains. Each of the motors is controlled by a separate variable frequency drive unit (114) and (116) which in turn are controlled by a control system (44). The control system (44) also controls the operation of a lubrication system (40) that supplies lubricant to the drive chains.