idle wheel

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A gear, wheel, or roller interposed between two similar parts to convey motion from one to the other without change in speed or direction of motion.

from The Century Dictionary.

• noun A wheel (C, fig. 1) placed between two others (A and B) for the purpose of transferring the motion from one axis to the other without change of direction; a carrier-wheel.
• noun A wheel that performs a duty other than the transmission of power, as the preservation of a strain on a belt, etc.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a pulley on a shaft that presses against a guide belt to guide or tighten it