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Essential Components of Lifts, Elevators, type of machines are used in lifts, Brakes, hoisting ropes, Sheave, Counter weights, Door and door operator, Hoisting Motor

Er. Parbhakar Dwivedi
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Following are the essential components of lift utilization.

1. Machines.

2. Brakes.

3. Traction ropes.

4. Sheaves.

5. Divertor pulley.

6. Counter weights.

7. Governor.

8. Guides.

9. Buffers.

10. Door and door operator.

11. Selector.

12. Travelling cable.

13. Hoisting motor.

14. Controller.

15. Car.

16. Safety features.

Machines

Two type of machines are used in lifts. One is geared and the second is gear-less machine.

Geared machine consists of a worm gear reduction unit. Shaft of the hoisting motor is coupled with the shaft of the reduction gear with the help of pulleys. The pulley arrangement is used for braking. The system permits the use of a small but high speed motor. Geared machines are rarely used for speed beyond 0.5 m/s because of excessive problems with noise, vibration and difficulties with wear of the gear. For high rise buildings to reduce the trip time (for reducing waiting interval) one has to go for gear-less machine.

In gear less machine, a grooved driving sheave and a brake pulley are directly mounted on the shaft of driving motor.

Brakes

Magnet operated brakes are generally used. The brakes are generally operated with DC supply. The brake shoes are provided with brake lining, which is of copper woven type as used in automobiles.

In the case of gear less machine, the function of the brake is also to hold and not only to slow down the lift.

Some manufacturers use small three phase motor for the opening and closing of brake.

Ropes

Lift car is raised and lowered by hoisting ropes which pass over the machine driving sheave. Necessary traction is obtained by way of friction between the ropes and the grooved surface of the sheave. Traction types of lifts have inherent safety feature that, when either the car or the counter weight bottoms, the tension in the hoisting ropes is relieved and the driving sheave may rotate without moving the elevator (lift) owing to loss in traction.

At least three ropes are used in parallel for traction purpose. The dia ranges from 1/2 to 1". The ropes are of stranded type, each strand consists of number of steel wires. The core of rope consists of jute, hamp or manilla, impregnated with suitable lubricant. This acts as lubricating medium for reducing the wear of strands due to friction between them.

Apart from traction purpose, ropes are used for speed governor, selector, terminal limit switch, etc. The size of these ropes varies from 1/4 to 3/8".

Factor of safety for the ropes as per IS: 4666 is given below.

Rope speed in m/s Factor of safety
Up to 2.0 10
3.0 11
7.0 12

Sheave

Sheave is a pulley over which wire ropes pass for traction purpose. In geared machine, it is fitted to the output shaft of the gear. In case of gear less machine it is fitted on the shaft of the hoisting motor. The sheave is of V grooved shape. The ratio of sheave diameter to the rope diameter should not be less than that shown below.

Class of rope Dia of sheave/pulley
6 x 19 (12/6/1)
6 x 19 (12/6/1) D (2.95 S+37) with a minimum of 40 D
Plus 6 filler wires
8 x 19 (12/6/1)
Plus 6 filler wires
8 x 19 (9/9/1) Scale

Where, D = Dia. of rope in cm

S = S -Rope speed in m/s.

For service lift, the ratio of sheave diameter to rope diameter should not be less than 30.

Divertor Pulley

It is an idler pulley to change direction of ropes or we can precisely say divert the ropes.

Counterweights

Counterweights are necessary to provide traction and balance the weight of the car plus predetermined additional load, usually 40 to 50% of the maximum car load so as to reduce the size of the motor Counterweights are hanged with the help of wire ropes passing over the driving sheave. On the other side is car. The weights are in the form of cast iron slabs, which are fixed in the frame. The frame is provided with four guide shoes so that the counterweights move vertically within the guide rails.

Governor

The device is provided in the lift machine room to stop the lift when the speed increases beyond the predetermined value. It works on the principles of centrifugal force. It is driven by rope known as governor rope. This rope is attached to the safety gear provided below the car frame. An electrical switch is also provided with the governor. When the speed of the lift car increases beyond the rated speed, this switch gets actuated with the help of a lever fixed to the governor Actuation of the switch causes the stoppage of electric supply to the controller. These speed governors are set to cause the application of safety gear at a speed not less than 115 percent of the rated/contract speed.

Guide

In lift system four guides are provided. Two are for car and other two for the counterweights. Earlier round guide rails were also used. But these days only T guide rails are universally used.

Buffers

Electric lifts are provided with buffers in the pit under the car and counterweight Spring buffers are used for slow speed up to 1.5 m/s and oil buffers for high speed lifts. These buffers are symmetrically located with reference to the vertical center line of the car frame with tolerance of 50 mm. Lift pit should not be punctured for providing buffers so as to avoid seepage of water. These are provided on the channel which is fixed at the bottom of the guide rails.

The use of buffers are to stop the car in case lift over travels beyond terminal limits. To restrict the car from over travelling, terminal limit switch and final terminal limit switch are provided.

Even so the lift car sometimes over travels. These buffers therefore act as final emergency device.

Door and Door Operators

Passenger lifts have horizontal sliding doors. They are either single sliding or center opening types, Central opening types are preferable as they reduce the time for operation of doors. This then reduces the round trip time and subsequently decreases waiting internal. The doors have rollers at the top which ride on a steel track for support and guidance. The doors are guided at the bottom by shoes sliding in a machined self-cleaning slotted metal channel. The door operator is mounted on the lift car and is driven by electric motor and coupled to the car door by belts, chain or levers. The hoist way doors are automatically coupled to the car doors when the lift is at the landing and operate in synchronism with them.

The power operated doors are provided with safety shoes which when comes in contact with a person reverses the door operation. The operating mechanism shall operate with a force not exceeding 123 N. The leading edges of doors are provided with soft and fire resistance material.

Selector

The function of the selector is to give car position information to the controller and operating system so that automatic stops may be made at the selected landing. Selectors are coupled to the car with the help of wire rope or perforated steel tape

For slow speed lifts, switches in the hoist way handle the whole positioning of the car.

The selector consists of many switches which are operated with the help of cams. It has slowing switches stopping switches, car direction switches, car position switches, levelling switches, etc.

Travelling Cables

All electrical connections to the car are made by means of multi core hanging flexible cables, one end of which are connected to a terminal box fitted under the car floor or above the car top, the others to a terminal box fitted in the well at approximately in the mid position. The cable of 10 cores to 22 cores construction are generally used for higher speeds as the heavier cables give a better running performance than lighter cables.

Hoisting Motor

Different types of motors are used for lifts and the selection depend upon.

a. Supply characteristic.

b. Car speed.

c. Quality of service to be provided.

Usually a speed of 600-900 rpm is preferred, whilst at 1000 rpm, there is difficulty in obtaining the necessary degree of silence, even if special precautions are taken in the design of motor room and the mechanical equipment. Further higher the speed, greater the kinetic energy and more powerful braking effort required, but on the other hand, the price increases as the speed increases.

The main requirement of a lift motor are a starting torque equal to at least twice the full load torque, quietness and low kinetic energy. The last feature is necessary to obtain rapid acceleration and deceleration, together with a minimum amount of brake lining wear.

The theoretical power of the motor to drive any lift is calculated as follows.

HP = Out of balance load x speed in m/s/75

Assume that maximum car load is 10 persons (680 kg), the maximum speed of 1 m/sec.

Since counter weight is 50% of car plus contract load, out of balance load will be 1/2 of the contract load.

HP = (680/2x1)/75 =340/75 =4.53

In practice actual HP is considerably more than the theoretical HP. It is essential to ensure that the motor is capable of providing necessary torque to accelerate the total moving mass of the lift system at the desired rate.

Following two type of motors are used in lifts.

a. Alternating current (AC) motors.

b. Direct current (DC) motors.

a. AC Motors

AC motors are almost exclusively squirrel cage induction motors, either single speed or two speeds Squirrel cage motors generally have high resistance rotors to provide high starting torque and limited starting current, because a large proportion of the duty cycle consists of starting from rest. Usually the full load slip of these motors is about 20%. Thus, a motor having a synchronous speed of 900 rpm may run at 720 rpm when the lit is carrying full load in the up direction.

Double squirrel cage rotors are coming into use as well. They enable maintaining high starting torque characteristic, but the full load full speed slip can be reduced to approximately 9%, giving better control and efficiency.

Better speed regulation, lower starting current and smooth acceleration are obtained by using a wound rotor motor which is accelerated by cutting out the rotor resistance in one or more steps. The following classification according to speed requirement can be made.

(a₁) Motor for the single speed (0.5 0.75 m/s) squirrel cage induction motor, resistance start.

(b₂) Motor for the speed (0.75-1m/s).

i. Squirrel cage induction motor. Two speeds are obtained in the ratio of 2:1 by changing the number of poles. Another method of obtaining two windings wound in the same slots. Ratios up to 6:1 are possible with this double wound motor. The high and low speed windings being cut in and out of circuit by means of contactor. These motors are also resistance start.

ii. Slip ring induction motors. Speed changes similar to squirrel cage induction motor, and involves use of two separate rotor windings. The use of two rotor windings may be avoided by carrying the rotor current through internal short circuit path during one of the speeds.

iii. Tandem motors

(a₃) Motor for car speed above 1 m/s

AC servo drive.

AC variable frequency geared machine.

AC variable frequency variable voltage.

b. DC Motors

(b₁) Motor for speed of 0.5- 0.75 m/s. Single speed shunt or compound motor is used.

(b₂) Motor for car speed of 0.75 to 1 m/sec. Two speeds motor having speed ratios of 2:1, 3:1 or 4:1 are used. The motor is shunt regulated.

(b₃) Motor for car speed 1-1.75 m/s. Compound motors employing variable voltage or Wardleonard method of speed control is used with geared machines.

(b₄) Motor for car speed 2.5 m/sec and above. Shunt gear less motors are used for speed control.

Gearless motors. The shaft of this type of motor is directly coupled to the driving sheave without the use of reduction gear. Therefore, the speed of the motor is low. The motor, brake and sheave are mounted on common bed plate to form a single unit. These days variable voltage or Wardleonard principle is used for speed control. The major part of speed variation is accomplished by varying the voltage to the motor, the remaining small change being effected by field control.

Controller

This is located in the machine room. Its exact position is decided to ensure sufficient clearance between the controller, walls and other equipments so as avoid contact of the maintenance persons with the moving parts of the lifts.

The older design of controller consisting of relays, contactors, etc. have given way to microprocessor controller wherein the complete circuitry is on various printed circuit boards. The program logic is stored in the microprocessor which senses various parameters like speed of the lift, over-travel, position of limit switches, group control logic and gives required commands to control the operation of the lifts.

The controller in the machine room basically performs following three functions.

a. Motion control.

b. Operation control.

c. Door control.

Car

The car is the load carrying unit, including its platform, enclosure. Car frame and car door. The car frame is the porting structural frame to which the hoisting ropes or hoisting rope sheaves, car guides, car safety, platform and generally the door operating mechanism are attached. An average passenger requires about 2 sq ft. of floor area to feel comfortable. On its basis, car dimensions are worked out. The standard dimension for various types of lifts are given in IS 3534 and have already been depicted.

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