There are three major types of elevators: gearless traction, geared traction, and hydraulic. The closed passenger car of a modern elevator rests inside a strong steel frame. The car and frame of an electric traction elevator are lifted and lowered within the elevator shaft by a large electric motor. Guide shoes or rollers on the car frame keep the car in place on the guide rails. Steel cables, or hoist ropes, are attached at one end to the top of the elevator car. They pass over a grooved drive sheave (pulley), which is connected to the electric motor.
The hoist ropes are attached at the other end to a heavy weight called a counterweight. The counterweight slides up and down the elevator shaft on its own guide rails in the opposite direction of the elevator car. Because of this counterweight, which balances the full weight of the car and about half of its passenger load, the electric motor does not have to lift the full load of the car.
Gearless and geared traction elevators differ in the speed at which they travel and the amount of weight they can carry. Gearless traction elevators travel at speeds of 400 to 2,000 feet (120 to 610 meters) per minute and are used in buildings more than ten stories high. Geared traction elevators are slower than gearless traction elevators, usually traveling at speeds of 25 to 450 feet (7.5 to 105 meters) per minute. However, geared traction elevators can carry heavier loads -30,000 pounds (13,500 kilograms) or more.
Hydraulic elevators are used extensively in buildings of five stories or fewer. With speeds rarely exceeding 150 feet (45 meters) per minute, the advantage of the hydraulic elevator is that it does not need any overhead hoisting machinery. The elevator is mounted on a piston that is inside a cylinder extending into the ground to a depth equal to the height the elevator will rise. An electric pump forces oil into the cylinder, causing the elevator to rise. Valves release the oil when the elevator is to descend. Another form of hydraulic elevator is the "holeless" model. A plunger slides up and down on the side of the elevator, and no hole is required beneath the standard shaftway space.
The hoist ropes are attached at the other end to a heavy weight called a counterweight. The counterweight slides up and down the elevator shaft on its own guide rails in the opposite direction of the elevator car. Because of this counterweight, which balances the full weight of the car and about half of its passenger load, the electric motor does not have to lift the full load of the car.
Gearless and geared traction elevators differ in the speed at which they travel and the amount of weight they can carry. Gearless traction elevators travel at speeds of 400 to 2,000 feet (120 to 610 meters) per minute and are used in buildings more than ten stories high. Geared traction elevators are slower than gearless traction elevators, usually traveling at speeds of 25 to 450 feet (7.5 to 105 meters) per minute. However, geared traction elevators can carry heavier loads -30,000 pounds (13,500 kilograms) or more.
Hydraulic elevators are used extensively in buildings of five stories or fewer. With speeds rarely exceeding 150 feet (45 meters) per minute, the advantage of the hydraulic elevator is that it does not need any overhead hoisting machinery. The elevator is mounted on a piston that is inside a cylinder extending into the ground to a depth equal to the height the elevator will rise. An electric pump forces oil into the cylinder, causing the elevator to rise. Valves release the oil when the elevator is to descend. Another form of hydraulic elevator is the "holeless" model. A plunger slides up and down on the side of the elevator, and no hole is required beneath the standard shaftway space.
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