ELEVATORS or LIFTS
Post 339 ⇒ by Gautam Shah →
Elevator is an intermittent transfer system that moves goods and people in a direction perpendicular to the gravity. This is unlike the inclined stairs and escalators or almost parallel to the gravity systems like corridors, passages, roads, lanes and automated walkways. Elevator systems are modulated transfer systems in contrast to the uninterrupted transfer entities, and so have limited capacity of conveyance.
The efficiency of a transfer system is determined by the fact whether the system is parallel, inclined or perpendicular to the gravity. The additional effort required to work against or towards the gravity, respectively retard or add to the efficiency.
The intensity of transfer depends on whether the system operates continuously or intermittently. Continuous systems such as the escalators, automated walkways, conveyer belts, are governed by the speed of movement, while the intermittent systems such as the elevators, buses, railways are affected by the size capacity of the module, speed and frequency of service. Both systems, however have some traffic capacity limitations.
An elevator system is capable of elevating or raising a load to a height. Lifting devices such as rope and a smooth axle were used to draw water from wells and lift building stones. The smooth axle was replaced by a wheel or pulley. The first documented proof of a lifting device is by Roman architect Vitruvius, who reports Archimedes (Greece 236 BC) building it. Ancient and medieval elevators used drive systems based on hoists or winders. These were inefficient methods. The use of a screw drive and multiple pulley systems, were the most important steps in lifting technology.
Elevators are manual, mechanical or power driven systems. Manual elevators are now used for small weights and low elevations. Manual elevators are free-fall and pulled or assisted-rise, or controlled fall-rise types. Some are partial combinations of both types, i.e. the rise may be free due to the counter weights, but fall may be controlled (such as dumb waitors from top floor kitchen facilities), or the system may work other way round. Fall is controlled by provision of a counter dead weight or parallel module. Mechanical grips that control the accelerations of speed during fall are also used. Fall is required to be controlled to avoid damage to the bucket or car module of the elevator, its resting place, noise due to impact, vibration during uncontrolled descent, spillage of liquid and breakage to fragile items like crockery etc. Fall is also required to be controlled so that stops match the required datum. Control of the rise is not very problematic in manual elevators, because there is no self acceleration during ascent. Rise of a weight (dead or a purposive load) to be elevated, is accurately controlled by the amount of energy expended. However some control mechanism to maintain the rise once gained, is required. Such dumb waitors are used between kitchen and serving areas, godowns and delivery areas, documents and spare parts transfer in commerce and industries.
Bucket-lifts were common for water wheels in India, Persia and many other countries. Bucket-lifts were also used to lift ore and coal from deep mines, and transfer miners. An air lift system works on a suction mechanism to transfer materials.
The classifications here are oversimplified discussions for the non technical people.
Mechanical Elevators use some form of chain pulley system or other devices to reduce the energy required. Such elevators are capable of lifting larger loads than dumb waitors. In some cases the elevators are designed as quasi mechanical systems, where rise is through a mechanical or power driven system, but the fall is natural but controlled. Such systems are used where, loads heavier than the manual capacity are to be lifted, power supply is erratic or not available, lifting requirements are occasional.
Power driven elevators are generally motorized, where an electric motor creates a hydraulic pressure or winds a traction rope to elevate a weight. These elevators work on DC current to regulate the start, speed and stop.
Hydraulic Elevators are not extensively used now, however, are ideal for low rise building where design limitations preclude overhead supports and machine rooms or load-bearing walls around lift well. The machine room can be located nearly anywhere, and let one maintain a flat roof line and save money on construction. Geared traction elevators are ideal for low-to medium-rise buildings, where high speeds are not essential, though speeds up to 400 ft/min. can be planned. Gear-less traction elevators are recommended for high-rise applications requiring the ultimate in service-speeds of 500ft/min and up.
Passenger elevators are wide and shallow in depth, with centre-opening double doors or single side doors. Service-elevators, like for hospitals, are narrow and deep. Combination passenger and service elevators should be almost square. Freight elevators’ size and shape are determined by the dimensions of goods to be carried and by the loading / unloading methods used, but nominally allow carriage of 1200×2400 sheet materials.
While providing elevator system in a building, two basic norms are considered. The frequency and quantity of service required, and the quality of services desired. The quantity of services is determined by the passenger handling capacity during peak periods, while the quality of service is measured in terms of time required (waiting time) to avail a lift. The quality of elevator service is determined by the building planning, intensity of peak hour requirements, extent and duration of traffic, frequency and urgency of service required, type and control mechanisms, type of landing doors, etc.
Operation of lifts: Modern lifts have an electronically intelligent control system that actuates starting, stopping, door opening and closing, direction of motion, acceleration, speed and retardation, emergency smooth fall of the lift car. A variety of motors are used to power the lifts. The simplest is single speed resistant AC motors, the reduction gear is attached to the pulley, braking is semi mechanical, or through an additional motor, generally a DC motor. In a little more complex setup reduction gears are attached to the motor and a secondary winding is provided to stop the motor. More complex and accurate system consists of a DC motor. With DC motor input voltage can vary the output speeds, step-less gear system is often integral part of the motor. Such systems provide very high speeds, accurate stopping, less jerks and smooth operation.
Elevators are prerequisite requirements for buildings taller than 3 floors. Elevators are often a legal requirement in public buildings because of wheelchair access laws.