A chain hoist is operated by hand. An operator will pull down on one of the chain loops on one side of the chain. This will turn a pulley mechanism inside the chain hoist housing. When this pulley turns, it will lift up the end of the other chain which usually has a hook on the end. By pulling down on one chain, the manual hoist is actually able to increase the mechanical work that is being done. This is caused by the gear ratio inside the manual chain hoist.
Inside the chain hoist housing are two gears. One is smaller than the other. Most chain hoists use a 20cm and 25cm gear. The two gears are attached, so when one moves, the other moves. The chain is looped over the smaller gear, and then hangs down in a loop (the loop you pull on). The chain then continues on over the larger gear and down to a point or another loop, depending on the type of hoist you are using. The operator pulls on the section of chain which is looped over the smaller gear.
When someone pulls down the chain as explained in Section 1, the smaller gear will turn. However, since it is smaller in size, it turns faster than the larger gear. It makes more rotations than the larger gear in a cycle. Since the larger gear is turning slower, it creates more force, in effect transforming the "pull" on the chain into a larger force. This is a mechanical advantage. An operator can put less force on the smaller gear, but still lift large objects. That is because the larger gear transforms that force into a much larger one.