A belt is a loop of flexible material used to link two or more rotating shafts mechanically, most often parallel. Belts may be used as a source of motion, to transmit power efficiently or to track relative movement. Belts are looped over pulleys and may have a twist between the pulleys, and the shafts need not be parallel. In a two pulley system, the belt can either drive the pulleys normally in one direction (the same if on parallel shafts), or the belt may be crossed, so that the direction of the driven shaft is reversed (the opposite direction to the driver if on parallel shafts). The belt drive can also be used to change the speed of rotation, either up or down, by using different sized pulleys. As a source of motion, a conveyor belt is one application where the belt is adapted to carry a load continuously between two points. History The mechanical belt drive, using a pulley machine, was first mentioned in the text the Dictionary of Local Expressions by the Han Dynasty philosopher, poet, and politician Yang Xiong (53–18 BC) in 15 BC, used for a quilling machine that wound silk fibers on to bobbins for weavers' shuttles. The belt drive is an essential component of the invention of the spinning wheel. The belt drive was not only used in textile technologies, it was also applied to hydraulic-powered bellows dated from the 1st century AD. Power transmission Belts are the cheapest utility for power transmission between shafts that may not be axially aligned. Power transmission is achieved by purposely designed belts and pulleys. The variety of power transmission needs that can be met by a belt-drive transmission system are numerous, and this has led to many variations on the theme. Belt drives run smoothly and with little noise, and provide shock absorption for motors, loads, and bearings when the force and power needed changes. A drawback to belt drives is that they transmit less power than gears or chain drives. However, improvements in belt engineering allow use of belts in systems that formerly only allowed chain drives or gears. Power transmitted between a belt and a pulley is expressed as the product of difference of tension and belt velocity: where, T1 and T2 are tensions in the tight side and slack side of the belt respectively. They are related as where, μ is the coefficient of