The principal use to lift items with a device like this is known as mechanical advantage. This is a force that allows the device to multiply the weight it lifts through the mechanism. This increases the amount of force through the pulleys, abortion which change direction of the force that is applied to the rope of the chain that is used in conjunction with the pulley in the chain block. The more pulleys there are the higher the mechanical advantage, which in short means the more a particular block can lift. Usually, two or more pulleys are used in a chain block to increase this mechanical advantage and this has been the case for millennia, though the same principal is still used today, albeit with electrical power in a lot of cases.
Tackle and Hoist
The devices really came into their own a couple of hundred years ago on ships and docks. These are still found here today and suit the needs as the environment involves a lot of toing and froing and also is quite limited in space. A chain block tends to be small and take up little room and so is ideal for this reason also.
Of course, nowadays the chain hoist is a lot more complex and has a lot more bells and whistles than the original form of tackle and hoist. Saying that these are often an option and the simplistic form of hoist can still be seen in many businesses.
Chain hoists are an evolution on the tackle hoist and use gearing to make the whole process a lot easier. This mechanism is pulley based and uses a gear ratio inside to make lifting the items a lot easier.
These devices house all the mechanical parts and they are usually quite tidy in size. They also have quite complex gearing ratios and this is a step further onward from the traditional hoist. They usually use a 20cm and a 25cm gear and these are both attached. When one moves the other tends to move. The chain is attached to these and hangs down, when the operator pulls a chain connected to the small gear it begins to hoist via the chain attached to the larger gear.
As the chains are pulled the smaller gear tends to turn faster, due to its size. The larger gear is of course slower and creates more force. This means that the chain is easier pulled and this is mechanical advantage in operation. This is the basis of how a device like this works.