you can’t keep turning round in one place like a horse grinding sugar cane.
This simile takes up the theme suggested by the previous figurative device; here, though, the horse serves as the power source and walks in a tight radius around a central grinding apparatus in which raw cane is pushed in from the top lengthwise and the pressed out juice is collected in a tub. Likening Janie now to a beast of burden accentuates the suggestion that she has been taken advantage of ("worked") by Tea Cake.