A mechanical device such as a pully or wheel that does not transmit power, but supports a moving belt etc.http: //www.merriam-webster.com/dictionary/idler%20wheelhttp: //www.merriam-webster.com/dictionary/idler+pulleyOED 2nd edition 1989
disapproval If you describe someone as an idler, you are criticizing them because you think they are lazy and should be working. The Duke resents being seen as a moneyed idler. = loafer. someone who is lazy and does not work
An idler is a pulley or roller that does not produce any mechanical advantage, nor transmit power to a shaft Idlers are used to lead a chain around a bend (as in short wheel base recumbents), or to take up slack in a drive chain (as in a tandem chain tensioner, or the pulleys in a derailer