lift

A lift hill, or chain hill, is an upward-sloping section of track on a roller coaster on which the roller coaster train is mechanically lifted to an elevated point or peak in the track. Upon reaching the peak, the train is then propelled from the peak by gravity and is usually allowed to coast throughout the rest of the roller coaster ride's circuit on its own momentum, including most or all of the remaining uphill sections. The initial upward-sloping section of a roller coaster track is usually a lift hill, as the train typically begins a ride with little speed, though some coasters have raised stations that permit an initial drop without a lift hill. Although uncommon, some tracks also contain multiple lift hills.
Lift hills usually propel the train to the top of the ride via one of two methods: a chain lift involving a long, continuous chain which trains hook on to and are carried to the top; or a drive tire system in which multiple motorized tires (known as friction wheels) push the train upwards. A typical chain lift consists of a heavy piece of metal called a chain dog, which is mounted onto the underside of one of the cars which make up the train. This is in place to line up with the chain on the lift hill.
The chain travels through a steel trough, and is normally powered by one or more motors which are positioned under the lift hill. Chain dogs underneath each train are engaged by the chain and the train is pulled up the lift. Anti-rollback dogs engage a rack (ratcheted track) alongside the chain to prevent the train from descending the lift hill. At the crest of the lift, the chain wraps around a gear wheel where it begins its return to the bottom of the lift; the train is continually pulled along until gravity takes over and it accelerates downhill. The spring-loaded chain and anti-rollback dogs will disengage themselves as this occurs.

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