Idler pulley

I just replaced the idler pulley-arm assembly in my White Westinghouse dryer. You can see the assembly here: http://www.repairclinic.com/0081.asp?RccPartIDx0775
My question is - Is there nothing to hold the white plastic pulley on the post and prevent it from slipping off? Or is it just the tension from the belt that is supposed to hold it on? Because mine seems to operate with the pulley wheel very close to the end of the post.
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