1. RAILS :
Both the car and the counterweight must be guided by at least two rigid steel guide rails. The functions of guide rails are as follows :
- to guide the car and the counterweight in their vertical travel and to minimize their horizontal movement,
- to prevent tilting of the car due to eccentric load, and
- to stop and hold the car on the application of the safety gear.
The guide rail is considered a continuous beam with variable supports.
As per Indian standard, the Guide Rails shall be of “T” section. The strength of the guides, their attachments and joints shall be sufficient to withstand the forces imposed due to the operation of the safety gear and deflections imposed due to uneven loading of the car.
In the calculation of the guide rails, two operating conditions should be taken into consideration (1) safety gear operation, (2) running conditions with the load unevenly distributed on the car floor. The buckling stress has to be calculated as per guidelines by Indian Standard.
Guide rails are typically, 5 meter (16-feet) long “T” – sections manufactured in different sizes for different duty loads, speeds and classes. For rail end-to-end connections, plates are used to hold and these are called fishplates. The Rails and are attached to the building with rail brackets. These brackets known as rail brackets keep the plumb, square and maintain the correct distance between the guide rails(DBG) so that the car maintains proper ride quality.
Guide rails are a significant factor in achieving ride quality. The characteristics of rail which impact the ride quality includes blade straightness, rail height, tongue and groove width, blade thickness, surface finish, tongue and groove positioning etc.
Given proper machining and accessories, the installation of the rail system is the critical and final stage of the process of making a good set of rails.
The better rail systems are those requiring the least amount of time and effort during field installation. The quality of an installation is dependent on the alignment of the rails, both horizontally and vertically.
Typical Guide Rail with markings of dimensional details
Rails are lubricated with oil to reduce the frictional resistance and enhance the riding smoothness. The present day shoes have built in lubricator tanks with feeding wicks through which oil trickles to the sliding surface of the guides. Oil or grease is likely to absorb dust and also changes its properties due to temperature and other environmental condition.
2. Safety Gear & Governor:
The Governor monitors the movement of the cab and safety stops the elevator if the elevator car over speeds. The over speeding could speeding could be due to malfunction of speed control of the elevator motor or due to free fall of the cab due to breakage of elevator ropes. The Governor activates an over-speed switch if the cab moves greater than the rated speed. The switches are used to stop the elevator and apply the brake.
The worst case of over speeding of the cab is when the traction ropes breaks and when the cab falls down due to gravity. Under such worst case conditions, the governor, which is located in the machine room or Overhead depending on the elevator design activates the safety block mounted on the car frame and the safety block stops the elevator movement by locking on to the elevator rails.
The governor rope runs over the governor sheave and down to the elevator car and is attached to the safety trip mechanism. The governor rope continues to the pit and runs over a pit mounted sheave and turns back to the governor sheave located in the machine room or overhead. This governor rope arrangement forms a continuous loop while the elevator moves up an down the hoist-way.
If an elevator over speeds either because of a major malfunction or the suspension ropes are severed then the governor sheave accelerates as the car accelerates until a preset speed limit is attained and a speed sensing device in the governor is tripped. At that point the governor’s rope griping jaw is activated to seize the governor rope. As the car continues to descend, the stopped governor rope moves the fixed car safety operating lever to engage the safety jaws installed at the ends of the safety plank. Friction between the safety jaws and the guide rails slow the car to a halt.
The safety jaws are of two types, instantaneous and progressive. Instantaneous Safety Gear is a device in which gripping action on the guide is almost immediate and the cab almost stops instantly. Instantaneous Safety is normally used below a speed of 1mps.
A progressive gear is a safety in which deceleration is effected by a braking action on the guides and progressively brings the car to a stop within the limits prescribed by the standards. Progressive safety is normally used for higher speed lifts as a sudden stop may cause severe jerk and may injure the passengers inside the cab.
The governor for few high sped designs, are encoder capable. Mounted on the governor, the encoder tracks the precise location of the elevator and relays the information back to the controller.
This arrangement is normally used in high speed lifts and to ascertain the elevator position after a power failure.
Once the car comes to a stop due to safety operation, the safety gear can be released by moving the car in upward direction. The wedges will slide back to their original position.
3. BUFFERS:
Buffers are located in the pit as a final safety device to bring the descending car or counterweight to rest on it if they move past the normal downward limit of travel. The buffers are designed to absorb the kinetic energy of motion of the car.
Buffer are used both on the car as well as Counterweights to absorb the energy either in UP or DOWN travel.
Buffers are of two types: Spring and Oil buffer. Spring buffers can be used for elevators with lower speeds and Oil buffers for higher speeds.
As a thumb rule the buffer should be adequate to support a static load of 2.5 to 4 times the mass of the car together with the passenger load without compressing the buffer solid.
4. BRAKE:
In lift application the brake is normally electromagnetic type. The major versions in use are Shoe type and Disc type. The brake is applied by spring force in normal condition. It is released by energizing the coil of the electromagnet which in turn will release the drum or disc allowing it to rotate. The electromagnetic coil is designed to overcome the spring force. In the event of power failure the brake automatically applies due to spring force.
As per standard the brake should have sufficient torque to safety stop a car with 125% overloaded also hold the load of the system at stop. Static torque to absorb kinetic energy of all moving parts.
The design of the magnet coil is normally done with DC voltage. In AC coils the operation will be abrupt while closing or opening which produces heavy jerk. DC coil operation is much smoother and softer. Normally brake coils are provided with levers for manual release in case of emergency.
Lift are equipped with various controls to slow down and practically stop the lift dynamically without the mechanical brake coming into operation. Function of the brake is to hold the car in its position after it comes to level and stop. In case of single speed lifts with no speed control, the lift is stopped purely by mechanical control resulting in poor leveling accuracy and frequent wear and tear of brake liners.
5. LIMIT SWITCHES:
These are limit switches mounted in the shaft and operated by the movement of the car. Generally minimum of 3 limit switches are mounted at the top and bottom floor regions. The first switch encountered near the topmost or bottom most floor is the terminal slow down switch. During normal conditions, the floor positions are counted by the controller with the help of the photo electric switch mounted on the car top and initiate slow down at the respective landings. In the event of failure of this counting, the terminal slow down switches initiate slow down of elevator to prevent over travel. The second set of limit switches apply the brakes in the event of failure of counting of pulses by the processor and also the failure to slow down with help of terminal slow down switch. If the car approach the terminal landing at normal speed without slowing down the second set of limit switches operate when the car over travels by about 50 to 60mm beyond the terminal floor. These are known as final limit switches. These switchers disconnect the power to the controller and drive and the lift can be restarted only if the car is physically moved to disengage the final limit switches. All these switches ensure that the car does not over travel But in the event the car over travels the car or counter weight, depending on the direction of travel, sit on the buffer and the kinetic energy is absorbed by the buffer. The buffer also has a switch which once again trips the elevator motion.
The other safety switches are:
- Stop button inside the car: This button was available in the car operating panel and the car can be brought to an abrupt strop by pressing this button. This button causes more inconvenience such as stopping with heavy jerk, stopping out of floor level and passenger entrapment. In the recent designs, this switch has been removed from the car operating panel except in certain states like west Bengal where it is a code requirement.
- Over speed governor switch: This switch is located in the governor and trips and brings the car to a halt when the car speed exceeds about 1.15 times the rated speed. This type of over speed may be due to malfunction of controller or the drive. The Governor trips the safety block if the speed exceeds1.4 times the speed. This kind of excessive speed may be due to rope breakage and free fall of elevator.
- Safety Gear Switch: Whenever the safety gear is tripped due to governor over speed detection, the safety gear, in addition to operating the mechanical safety, also switches an electrical safety to cut off the lift movement by the controller.
- Stop switch on the car top: This switch is normally used by the service mechanic when he travels on the car top.
- Stop switches in the Pit: Normally two switches are provide, one inside the hoist-way very close to the sill and accessible from the landing. The other switch is located on the walls of the pit. The service person, while entering the pit puts off the switch near the sill and after entering the pit switches off the other switch mounted on the wall of the pit also. While coming out of the pit, he resets the switch inside the pit first and then the one near the landing sill after coming out of the hoist-way.
All these safety switches are connected in series and will be connected suitably to stop the motion of the lift.
Yet another emergency device is the door pen key. As we know, all doors in closed condition and mechanically locked is the basic safety requirement. In order to rescue the trapped passenger, it may be required to open the respective landing door near which the car has stopped. For this purpose, special keys are provided by the manufacturer and kept in a secured place. This an emergency safety device and must be used only by authorized personnel and well trained in rescue operation. This is a very difficult operation as many accidents occur while doing this kind of emergency operation.
6. Emergency devices:
In elevators there is a risk of passenger getting trapped inside a car either due to power failure or due to failure of the lift control system. A battery operated alarm button is provided in the car operating panel to sound an alarm in order to get external help to rescue the stranded passenger. Hands free interphones are also provided to talk to the security or any other assigned person in the building.
7. Requirements for fireman’s lifts:
Fireman operation helps the Fireman to use the lift and conduct rescue operation when the building is on fire.
- For buildings having height of 15m or more, at least one lift will meet the requirements of fireman lift.
- The lift assigned for fireman operation must have minimum of 1.44 sqm of floor area.
- The capacity of the lift has to be at least 8P (544 Kg).
- Doors shall have automatic operation ( both car and landing) with a minimum of 800mm opening.
- Landing doors shall have minimum of 1 hour fire resistance.
- The speed of the elevator should be such that the lift travels from bottom to top floor within 1 minute.
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