What is the importance of the recurrence of the mechanical imagery in chapter twenty

One possible explanation is that Nurse Ratched and the staff she is in charge of wants the men to behave in mechanical, almost robotlike fashion. Also, the hospital uses many mechanical devices to "control" the patients so they are dealt with in this way.

Chapter Twenty, just a paragraph long, marks the change in the ward after McMurphy gives up his struggle against Nurse Ratched. She once again reasserts her control over the rest of the patients, for McMurphy knows that to oppose her is to ensure that he will never leave the ward. The ward is part of the larger machine that swallows these men up but doesn't spit them out.