identify this particular type of part
A quick learner from Philadelphia, PA says:
August 24, 2016
There is a metal part covered in black plastic with what looks like an open washer on both ends. The weight of the window sits on it and glides across the frame to open the window.
Tom from SWISCO responded:
August 25, 2016
I'm not sure what it is you have, exactly. Can you show me a picture? That would be a huge help. From there, we will try our best to find you a replacement.