What does "her good nature wore out/ like a fan belt" mean in "Barbie Doll"?

The use of "a fan belt" is a direct reference to a machine
that has worn out from too much use.  The image of the woman being conditioned to
possess "good nature" is one that reflects a constant demand to be nothing other than
"good natured."  In this light, the Piercy suggests that the socialization of women is
one that treats them as objects, constantly needing them until they are of no use and
disposed when they are no longer useful or productive.  This connects to her overall
idea that modern women who are constructed as "Barbie Dolls" are products of a
patriarchy that seeks to dehumanize and objectify women, using them for specific ends
and then discarding them when those ends are met:  "In 'Barbie Doll' the girlchild
fulfills the patriarchal prescription for obedience by destroying herself. She
perpetuates patriarchal power in death by being transformed into someone she could not
be in life."  In constructing women's perception in this manner, the use of the "fan
belt" is a reminder that all machines have a life span, and when they reach a certain
point of extended use, all machines die out.