What is an Elevator or Lift?
Elevator or Lifts are used for vertical transportation. We can also define elevator as '' A hoisting and lowering mechanism equipped with a car or platform which moves along the guides in a vertical direction and which serves two or more floors of a building''.
Why do we need Elevators?
Elevators Have become a necessity by virtue of the human comfort and convenience, and or by statutory regulations. The increase in the number of high and medium rise buildings in the last five decades has given rise to large number of elevators. During the last decade, large improvements have taken place in the engineering of elevator system with introduction of superior drives, logic control systems and elegant cars with enhanced safety.
Invention of Elevator:
Crude elevators with steam or hydraulics as source of power were available during 1800s. The first hydraulic elevators were designed using water pressure as the source of power. In 1853 Elisha Graves Otis created the first safety hoist. He installed a rope-break safety device called the safety brake(the equivalent of the modern safety gear) into the elevator. With the Otis safety brake, in the case of rope failure, a sprig would force a ratchet to engage saw-tooth iron bars and safety secure the car from falling. In 1854, Otis demonstrated the safety brake by boarding his elevator at the Crystal Palace in New York and cutting the traditional hemp rope himself. The safety brake worked flawlessly, making a dramatic presentation. ''All Safe gentlemen'', Elisha Otis announced as the brakes kicked in. By executing this stunt, Otis had heralded the birth of the elevator industry. Otis established a company for manufacturing elevators and went on to dominate the elevator industry. Today the Otis Elevator Factory is one of the world's largest manufacturers of vertical transport systems.
When was the first electric powered elevator displayed?
At the Mannheim Exposition of 1880, as the industrialized world was adopting electrical power, the German company Siemens exhibited an electric powered elevator. The mid-19th century marked the dawning of the age of electricity, and developments in elevator technology was being driven by the appearance of the first high-rise buildings in the United States, which necessitated the development of elevators in order to make them practical. As such, the United States emerged as the center of elevator technology development for decades.
What is the normal speed of elevators installed in India?
Popular speed of elevators in India are from 0.63 meters per second (2.2 KM/H) to 2.5 meters per second (9 KM/H).
What is the fastest speed of elevator?
According to the my knowledge till now, the fastest speed is 72KM/H (20mps) by Mitsubishi lifts for Shanghai tower, China. Burj Khalifa in Dubai has Otis elevators traveling at 36KM/H (10mps).
How does the elevator function?
To learn about the function of elevators, we shall consider the analogy of fetching water from a well. In Indian villages, many would have observed how the water is fetched from a well. A bucket is tied to a rope and is passed through a pulley. The bucket tied to the ropes is lowered using the pulley and once it reaches the bottom, water gets filled inside the bucket and then the bucket is pulled up manually.
From the above analogy, it is easy to develop the concept of operation of elevators. First replace the bucket with a lift cage, rectangular in shape, which will be used to carry the passengers. This lift cage is called a car, in elevator terminology. If the rectangular lift car has to move up and down the well, the well also has to be rectangular in shape. This rectangular shape of well is known as elevator shaft or Hoistway.
Car & Counterweight:
In our above analogy, we had replaced the bucket with rectangular elevator car. In elevators, one side of the rope is tied to the car and the other side of the rope is tied to a counter weight. The counter weight consists of a counter weight frame filled with cast iron weights. Nowadays concrete filler weights are also used primarily for reducing the cost.
A counterweight is an equivalent counterbalancing weight that balances the car and its passenger load. Typically, the counterweight is the weight of the car plus 40-50% of the car's rated capacity.
Counter weight is rectangular in shape and moves up and down the same hoistway. Since the car and counter weights are tried to the opposite ends of the rope, the counter weight moves down when the car moves up and vice versa.
Why the car and counterweight tied by a rope on either side of the pulley do not fall?
One end of the rope is attached to the elevator car, looped around a sheave(pulley) and the other end tied to the counterweight frame. A sheave is just a pulley with groove around the circumference. The sheave grips the hoist ropes, so when you rotate the sheave, the ropes move too. The friction between the ropes and the sheave generates the traction which gives this type of elevator its name. The deep groove in the sheave provides the necessary traction and helps to hold the car and counterweight in their positions without freewheeling. This traction is the very basis of elevator technology.
What is the material used in a rope?
The car is raised and lowered by traction steel ropes. Recently few elevator manufacturers have replaced the ropes with Coated Steel belts. However we shall continue with the concept of roped elevators which are still very popular worldwide.
How the rectangular car moving inside the shaft does not tilt, spin etc.?
On either side of the car, guide shoes are fitted which guide through the guide rails which are fixed on the walls of elevator shaft using brackets. The similar arrangement is made to the counterweight (CWT) frame also for smooth movement of CWT frame up and down.
Guide rails are necessary on either side of the car for following reasons:
To guide the car in vertical travel and to prevent horizontal movement or lurching of the car/ CWT as much as possible, and to prevent tilting of the car due to eccentric load, such as, when a group of passengers stand on one side or rear of the car.
When a car is loaded with passengers, there is an eccentric load on the car; it is prevented from tilting by the guide shoes (or rollers guides) pressing on the rails. The rail now acts as a beam supported by the brackets and it must have sufficient strength to carry these forces and also have sufficient stiffness to keep the front edge of the platform level with the landing as loads enter or leave the car. In the case of passenger elevators, where the eccentric loads are small these properties are not that important as with freight elevators where eccentric loads are usually very large.
Greater the load, larger is the required cross sectional area of the guide rail. In addition to the cross sectional area, it must be supported at certain regular intervals or it tends to buckle, if the points of support are too far apart. The distance between the points of support to prevent buckling depends on the moment of inertia and on the cross sectional area of the guide rail.
The guide rail has also got a very important function to stop and hold the car in the event of free fall of the car. The guide rail acts as a column during safety application.
How the car and Counterweight (CWT) move up and down?
The sheave which holds the car and the counterweight on either side, is connected to the shaft of an electric motor. When the motor turns one way, the sheave raises the elevator car, when the motor turns the other way, the sheave lowers the elevator. In the earlier designs of electric elevators, 3 phase induction motors and gears with or without speed controls were used. But in recent designs Permanent magnet DC motors are used.
What is the normal supply voltage to the motor?
The normal power supply is 3 phase, 400/415 Volts 50 Hz. We shall start our discussions with 3 Phase induction motors and later move on to PM AC motors and their speed controls. In India, 3 phase induction motors typically work on 400/415V, 50 Hz power supply. Elevator motors typically used to have 4 or 6 Poles.
How is the rated speed achieved?
We know that the RPM of the induction motor is determined using the formula 120 f / P, where f stands for frequency of power supply and P stands for the number of poles in the motor. Using the above formula, the RPM of the 4 pole and 6 pole inductions motors work out to 1500rpm and 1000rpm, (25 and 16.66 revolution per second respectively).
The minimum diameter of the ropes allowed as per Indian standards is 8 mm and the Indian standard specifies that the minimum diameter of the driving sheave has to be 40 times that of the rope to get the optimum wrap angle. By this code, the minimum diameter of the sheave works out to 320mm and a circumference of 1006mm, which means for one revolution of the sheave the lift will travel 1.006 meters. From our calculation in earlier Para, he 6 pole motor makes 16.66 revolutions per second, making the elevator to travel at a speed of 16.75mps. But, even one of the fastest elevator in the word at Burj Khalifa runs only at 10mps. Typically, a residential elevator runs with a minimum of 0.63mps and max of 2.5mps. The speed of the motor is reduced using gears.
What is the function of a Gear in elevators?
To achieve this speed, we require a reduction gear coupled between the motor shaft and the driving sheave. The worm gear used in elevators, not only decreases the rotational speed of the traction pulley, but also change the plane of rotation. By decreasing the rotation speed, with the use of a gear reducer, we are also increasing the output torque, therefore, having the ability to lift larger objects for a given pulley diameter. A worm gear is chosen over other types of gearing possibilities because of is compactness and its ability to withstand higher shock loads. It is also easily attached to the motor shaft, sometimes through use of a coupling. The gear reduction ratios typically vary between 22:1 and 69:1.
Summary:
The elevators achieve their vertical motion from an electric motor. Steel cables or hoist ropes run from the top of the elevator car, over the drive sheave to the top of the counter weight. The downward force caused between the gravity acting on the weight of the car and counterweight creates friction between the steel cables and the driving sheave, thus creating traction. As the sheave rotates, the elevator car is raised or lowered. The car and counterweight guide rails guide the movement of the car. Rails also provide safety in case of free fall due to breakage of ropes, by action of safety block mounted on the car and the rail.
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