Traction elevators are the most common type of elevators. Elevator cars are pulled up by means of rolling steel ropes over a deeply grooved pulley, commonly called a sheave in the industry. The weight of the car is balanced by a counterweight since 1900. Sometimes two elevators are built so that their cars always move synchronously in opposite directions, and are each other's counterweight.
Nowadays, some traction elevators are using flat steel belts instead of conventional steel ropes. Flat steel belts are extremely light due to its carbon fiber core and a high-friction coating, and does not require any oil or lubricant. Because of these qualities, elevator energy consumption in high-rise buildings can be cut significantly. Some elevator manufacturers have use this technology such as Otis (Gen2's polyurethane belts), Schindler (Suspension Traction Media for both 3300 and 5500 series), and Kone (UltraRope).
Geared traction elevators
Geared traction machines are driven by AC or DC electric motors. As the name implies, the electric motor in this design drives a worm-and-gear-type reduction unit, which turns the hoisting sheave. While the lift rates are slower than in a typical gearless elevator, the gear reduction offers the advantage of requiring a less powerful motor to turn the sheave. These elevators typically operate at speeds from 38 to 152 meters (125-500 ft) per minute and carry loads of up to 13,600 kilograms (30,000 lb). An electrically controlled brake between the motor and the reduction unit stops the elevator, holding the car at the desired floor level.
Contemporary cheaper installations, such as those in residential buildings and low-traffic commercial applications generally used a single or two speed AC hoist machine (abbreviation is AC/(speed number)). The widespread availability of lower-cost solid state AC drives has allowed infinitely variable speed AC motors to be used universally (for ACVV/AC - VVVF), bringing with it the advantages of the older motor-generator based systems, without the penalties in terms of efficiency and complexity. The older MG-based installations are gradually being replaced in older buildings due to their poor energy efficiency.
Winding drum elevators
- Main article: Winding drum elevators
Gearless traction elevators
Gearless traction electric elevator first invented by Otis in 1913, first installed in Woolworth Building in New York City. This type of drive system could be employed in buildings of any height and operated at much higher speeds than steam-powered elevators. This design has proven so durable that even now, when a building is modernized—while the elevator control system is replaced with the most up-to-date electronics—it is rarely necessary to replace a well-maintained gearless machine. These elevators typically operate at speeds greater than 500 feet per minute (or 2.5m/s).
In a gearless traction machine, five to eight lengths of wire cable, known as hoisting ropes (or wire ropes), are attached to the top of the elevator and wrapped around the drive sheave in special grooves. The other ends of the cables are attached to a counterweight that moves up and down in the hoistway on its own guiderails. The combined weight of the elevator car and the counterweight presses the cables into the grooves on the drive sheave, providing the necessary traction as the sheave turns.
To reduce the load on the motor, the counterweight is calculated to match the weight of the car and a half-load of passengers. As the car rises, the counterweight descends, balancing the load. This reduces energy consumption because the motor is required to lift no more than the weight of half a car load at any time. The grooved sheave in this traditional gearless system is quite large, from 0.6 to 1.2 meters (2–4 ft) in diameter. The electric motor that runs it must be powerful enough to turn this large drive sheave at 50–200 revolutions per minute in order to move the elevator at the proper rate.
Safety is provided by a governing device that engages the car’s brakes, should the elevator begin to fall. A powerful clamp clutches the steel governor cable, which activates two safety clamps located beneath the car. Moveable steel jaws wedge themselves against the guiderails until sufficient force is exerted to bring the car to a smooth stop.
Elevators with more than 100 ft (30 m) or the speed is 2.5m/s or above of travel have a system called compensation. This is a separate set of cables or a chain attached to the bottom of the counterweight and the bottom of the elevator cab. This makes it easier to control the elevator, as it compensates for the differing weight of cable between the hoist and the cab. If the elevator cab is at the top of the hoist-way, there is a short length of hoist cable above the car and a long length of compensating cable below the car and vice versa for the counterweight. If the compensation system uses cables, there will be an additional sheave in the pit below the elevator, to guide the cables. If the compensation system uses chains, the chain is guided by a bar mounted between the counterweight railway lines.
Machine room less traction elevators
- Main article: Machine room less elevator
The traction version of the machine room less (MRL) elevators are usually implemented with the smaller gearless traction motor that can place the inside the shaft to make the spaces not reduced by the large size motor.
Types of suspension ropes
Suspension ropes used on traction type elevators usually attached to the crosshead of the part of car sling and extending up into the machine room looping over the sheave on the motor and then down to the counter weights. Hoisting cable are generally 3 to 7 in number. These ropes are usually 1/2”or 5/8” in diameter.
The term roping system can be defined as the arrangement of cables supporting the elevator and which has many types or arrangements as follows.
Wraping for traction sheave
Rope passes over sheave once and connected to counterweight. It is used on mid and low-speed elevators with geared traction motors.
Rope wound over sheave twice in high speed elevators for the high rpm gearless traction motor to make an additional friction. It is used on high speed elevators.
When rope connected to counterweight where cable travels as far as car in opposite direction. It is used on geared traction or high speed elevators.
There are some variants of 2:1 roping, for higher capacity:
- 4:1 roping, rope moves four times as far as cab compared with the 2:1 roping, for freight elevators.
Notes and references
- Toshiba Elevator and Building Systems Corporation - Elevator Basics: Rope elevator (with machine room)
- Toshiba Elevator and Building Systems Corporation - Elevator Basics: Machine-Room-less Elevators
|Elevator drive systems|
|Traction • M.R.L. • Winding Drum • Hydraulic (Oildraulic)|
|Reference: hkelev - Elevator drive systems
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