Zip Line

A type of pulley with a grooved wheel known as a sheave is used in zip-lines, and the pulley turns as it travels along, thus reducing friction and enabling greater speed than would otherwise be possible.

The zip-line trolley is the frame or assembly together with the pulley inside that run along the cable. Zip-lines also have some kind of device to allow the cargo or rider take advantage of the pulley system. This could include a harness, seat, a cabin or often just a handhold in smaller playground applications, that attaches to the pulley by a pivoting link or carabiner which secures the load, allowing the person or cargo to travel down the line.

Zip-line spring braking system
To be propelled by gravity, the cable needs to be on a fairly steep slope. Even then the rider or cargo will often not travel completely to the end (although this will depend on the load), and some means of safely stopping the car at the bottom end is usually needed with the larger zip-lines. Users of zip lines must have means of stopping themselves.[26] Typical mechanisms include:[28]

Friction created between the pulley against the cable.
Thick, purpose-built leather gloves.
A mat or netting at the lower end of the incline.
A passive arrester system composed of springs, pulleys, counterweights, bungee cord, tire or other devices, which slows and then stops the trolley’s motion.
A “capture block” which is a block on the cable tethered to a rope controlled by a person who can manually apply friction on the rope to slow the user down.
Gravity stop, exploiting the sag in the cable, where the belly of the cable is always lower than the termination point. The amount of incline on a zip-line controls the speed at which the user arrives at the termination point.
Hand brake at the end of the zip-line.

There are certain precautions that can be taken. Riders are physically attached to the cable by a harness which attaches to a removable trolley. A helmet is required on almost all courses of any size. All zip-line cables have some degree of sag, so the proper tensioning of a cable is important and allows tuning the ride of a zip-line.