Hi Shan, welcome to Outdoorking.

The Ranger Autodrive's drive mechanism is conceptually the same as the Greenfield system - I think it is found on some other machines too. It relies on a ballrace thrust bearing in between the forward and reverse clutches. Here is part of the Illustrated Parts List for the 01866 Autodrive:
[Linked Image]
Item 8 is the thrust bearing.

Obviously I do not know whether your particular mower has been correctly assembled, but this is the way it is supposed to be.