1. Car / Cab and Its Construction:
We know that the carriage in which a passenger travels up and down is known as a Car or Cab.
The car enclosure is usually made of sheet metal members and bolted together to form a cage. In general the car is made either in square or rectangular shape. Very often the residential buildings decide to have a rectangular shaped deep car in order to house a stretcher which may be required to carry sick persons, at times. There are round, Hexagonal, Pentagonal etc. shaped cars also which are custom designed especially for use in shopping malls or public places to improve the aesthetics of the building. These panoramic cars generally are designed with glass enclosures. The Bureau of Indian standards provides recommended dimensions of the car inside, the Lift well and the door opening sizes.
2. What is a car frame >>
A car - frame is a rigid rectangular frame consisting of a cross head, uprights, and plank see the figure below to get a basic understanding of the car frame assembly. The car frame must be constructed to withstand the weight of the car, passengers, platform, safety devices, doors and door operator and all other loads pertaining to the cab system. It should also be designed to withstand safety operation under maximum load conditions described above.
The car frame generally comprises a safety plank, two uprights and a crosshead.
The Cross head, which forms the top of the car frame, consists of structural members generally channel shaped.
Uprights, are the vertical structural members at the sides of the car frame.
Plank is a structural member similar to the cross head forming the bottom of the car frame.
The roller guides or guide shoes are located at the four corners of the car frame. The guides help to guide through the rails. As per BIS, where the platform is directly supported by the plank or sound isolation frame, the vertical center distance between the top and bottom guide shoes should be more than the distance between the guide rail brackets.
2.1 What are the important considerations in a car frame >>
The forces acting on the car frame members are quite complex. If the center of gravity of the rated load in the car coincide with the center of load on the hoist ropes, then there would be tension in the uprights and bending on the cross head and plank. But this condition will not occur as the passengers are not static. This causes bending in the upright and twisting in the cross head and plank.
Since the exact position of the load at a given time is unknown we need to make few assumptions when the elevator is being loaded and during running. Any load that is entering the car n not be more eccentric from the center of the car to the maximum opening width of the car. Similarly, the car once loaded can not be more eccentric from the center of the car to the physical size the cab wall will permit.
The two worst case conditions which give us the stating point for making the calculations. In both the above cases, the car frame is subjected to an overturning moment, the magnitude of which is equal to the load times the eccentricity. The guide shoes resist this overturning effect and impart pressure on the guide rails. This causes the upright to deflect and cause the platform to sag on one side. Generally the cross head and the plank do not pose major design issues but the car frame uprights pose a few design issues. The uprights are designed to occupy less space in order to maximize the r area. In doing so, one has to sacrifice the strength in the uprights. When the rated loads are high causing large car frame bending moments and the available hoist way is small, double uprights are used.
The point of application of the loads with reference to the entrances both while loading and unloading as well as the position of the load in the car while the elevator is running greatly influence the size of the car frame members. In the absence of loading conditions extreme conditions must be assumed for the calculations resulting in much heavier construction than actually required.
In addition, during safety application the car frame must withstand the forces impressed when the safeties grip the rails. Owing to the tolerance in dimensions of the two rails and the difference in lubrication of the two rails, the retarding force on each of the rails will not be the same. The difference in forces cause over turning moment and the over turning moment causes bending of the uprights.
The other extreme condition is at the time of buffer engagement. The buffer is a device located in the pit at the bottom of the hoist-way. The plank channels are subjected to severe bending load. The uprights undergo compression and bending depending upon the position of the load in the car at that moment.
3. What is a Platform?
Elevator car platform is defined as that structure which forms the floor of the car and directly supports the load. The modern elevators utilize an all steel platform construction which is lighter and efficient than its wooden predecessors. In the all steel version, the design might consist of several sections welded together wherein the stringers and floor plates are the modules. The platform or platform frame is attached to the upper side of the safety plank and uprights of the car frame. The platform provides the mounting surface for the cab enclosure. In most cases, the car is isolated from the platform using rubber materials. This rubber isolation provides noise and sound isolation to the car. The compression of the rubber is also used to detect the load in the car by fixing micro switches beneath the rubber isolation. Due to pressure of the rubber materials, the Micro switch which is kept below gets operated and this is used to sense the load in the car. This rubber isolation of the car from the car frame also prevents vibration passed on to the car.
4. Roping Methods:
For 1:1 roping, the crosshead members provide mounting for the hitch plate which is positioned under the crosshead.
For 2:1 roping, the crosshead members provide mounting for the hoisting rope sheave, positioned above, within or below the crosshead.
For under slug car frames the 2:1 sheaves are provided under the safety plank.
4.1. What is 1:1 Roped System ?
Shown below is the basic arrangement of elevator car and counterweight, where the car and counter weight hang on either side of the driving sheave of the machine.
Tension in ropes/ belts on car & counterweight side can be calculated by using the following formulas:
T1 = (Mcar + P) * g
Mcwt = Mcar + P/2
T2 = (Mcwt) * g
where 'g' is gravity constant
P = Passenger load
SSL ( sheave shaft load) = T1 + T2
Rope masses are not included in tension calculation in this example.
Roped System 1:1
Ratings of Machine and Motor:
Let us assume a duty load of 10 Passengers with a car weight of 1000Kg
T1 = (1000 + 680) Kg as each Passenger weight is assumed as 68Kg
T2 = (1000 + 340)
The maximum unbalanced load = T1 - T2 = 340Kg
Please note that the load in the cwt is fixed whereas the load in the car is variable. Assuming no passenger in the car.
T1 = 1000Kg ;
T2 = 1340 and T1-T2 = -340Kg.
This load "F" has to be moved UP or DOWN.
When the car is empty, UP movement of car does not require any power as CWT is heavy. Similarly when there is full load in the car, DOWN movement of the car does not require any power as the car is heavy.
Assuming a sheave diameter of 610mm, the radius of the sheave is 305mm.
The Load torque required is F*R
F = 30Kg; R = 0.305m;
Hence Load Torque = 103.7KgMtr
Motor Torque = Load Torque / Gear Ratio * Efficiency
Assuming Gear Ratio = 46; Efficiency = 50%,
Efficiency here is a combination of motor, gear and frictional losses in the rails.
Motor Torque = 103.7/ 46*0.5 = 4.5KgMtr
le 45NMtr (10NMtr = 1KgMtr)
Considering the voltage and frequency fluctuations, overloading etc. one rating higher than the calculated rating is taken.
Motor Power = (Load in Kg * Speed) / 75 * Efficiency
= (340 * 1) / (75*0.5) = 9HP
( The factor 75 is a constant calculated from fundamentals).
Final selection of motor rating is determined based on torque. To be on safer side, higher HP and higher torque over the calculated value is chosen.
4.2. What is Roped System- 2:1 ?
The effect of 2:1 roping is to essentially double the lifting force related to the rope tensions, and another effect is that elevator car speed is a half of machine sheave speed 2:1 roping is widely used in elevator systems.
Flexible cables are used to bring electrical connections from the machine room to the car. Since the wires travel with the car, they are called travelling cables.
Some ropes are used to compensate for the weight of the hoist ropes in order to equalize the loads on the car and counterweight sides at different positions of the car in the hoist way. These are called compensation cables or compensation ropes.
5 What are Deflectors?
In actual case it would not be possible to hang the car on either side of the drive sheave due to various sizes of car and main sheave dia. In such cases deflector sheaves are used to obtain rope drops necessary for CWT suspension, and also to support the car as close as possible to the car's center of gravity. Arrangement with deflector sheave has two main effects:
Sheave shaft load is reduced when compared to 180 degrees wrap.
Wrap angle is reduced, which reduces available traction.
5.1 Single Wrap Traction:
Single wrap traction is a relatively simple arrangement that provides a rope drop and sufficient wrap angle to deliver a required traction.
Arrangement shown below is a 1:1 roping arrangement with deflector sheave.
In the example shown below, the diameter of main sheave and the deflector sheave are the same.
Sheave Shaft Load (SSL) is determined by the vector sum of the 2 tensions acting on machine sheave.
Wrap Angle = 180 - Φ
Vertical force = T1 + T2 * cosΦ
Horizontal force = T2 * cosΦ
SSL = Sqrt[(T1+T2*cosΦ)2 + (T2*sinΦ)2]
Rope masses are not included in tension calculation in this example
Single Wrap Traction
CWT = Car Weight + 45 to 50% of Passenger Weight.
Assuming T1 = T2 = 100 and Φ = 30; Cos 30 = 0.86 and Sin 30 = 0.5
SSL = √((100+86)2 + (50)2 = 192.6
The above equation shows that the SSL reduces with deflector.
But for the purposes of calculations we always consider SSL = T1 + T2. This is a safer calculation.
6. Sub - Assemblies mounted on car:
6.1 Door Operator:
The Door system is also a part of the total car assembly. As per Indian Standard, the movement of car is allowed only with closed door. (Except in special conditions such as advanced door opening or re-leveling operation) The door system consists of a car door which is opened or closed either manually or automatic. The early designs of manual doors consisted of Collapsible gates. Considering passenger safety, only imperforated doors are currently used for manual door system. On closure of car door, a car gate switch indicates to the controller logic that the car door is closed and the lift can run.
6.2 Are there doors in each landing?
Each landing is provided with a landing door. These doors remain mechanically latched so that when the lift is not in that floor, no one can inadvertently open the landing door and inadvertently fall into the hoist-way. The car gate gets mechanically engaged with the landing gate only while at the corresponding floor level. Due to this mechanical engagement between the car door and the landing door, the car gate moves the landing gate along with it. When the car gate is fully closed, the landing gate also gets fully closed and both of them through their own electrical contacts provide independent signals to the elevator controller that both the car door and landing doors are closed. When the car leaves the floor, a lock gets mechanically engaged with that corresponding landing door and hence the landing door remains closed & latched until the car once again reaches that floor to stop and gets mechanically engaged. All the landing doors remain mechanically locked so that no landing door can be opened when the car is not in that location. When the car reaches the landing, the car door gets mechanically engaged with the corresponding landing door and hence the landing door can be either opened or closed by the motion of the car door.
6.3 Safety conditions to be ensured for the car to run:
Following safety requirement is ensured before an elevator car moves out of a landing:
Both the car and all the landing doors are fully closed. This condition is ensured by sensing electrical signal through a car gate switch to confirm that the car gate is fully closed. All the landing gate switches are electrically wired serially so that inadvertent opening of any one of the landing door will immediately stop the lift and will not allow it to run. It shall not be possible to start the car in motion unless the car door and all the landing doors are fully closed.
All the landing entrances to be protected by doors which will remain locked and cannot be opened from landings except in case of emergency and by authorized persons only. It must be possible to open the landing gate only when the car is present and stopped at that landing. In case of manually operated doors, this safety requirement is achieved by energizing an electro magnet known as "Retiring Cam" which is mounted on the car top. The retiring cam essentially consists of an electro magnet and a cam which actuates an electro mechanical lock in the landing and mechanically unlocks the landing door. When the roller of the landing door electro mechanical lock is pressed by the retiring cam plates, the landing door will be unlocked.
6.4 Power Operated Door:
A door which is opened or closed by motive power other than hand is called as Automatic door. An electric motor is used to close or open the car door either using mechanical levers or by using tapes and pulleys. The motor used is either DC, induction or Permanent magnet type. Present day designs have PM motor whose speed is precisely controlled by Variable voltage variable frequency (VVVF) drive. Both the door drive and the door motor are mounted on the car top. This drive gets commands from elevator controller (either directly or through car top printed circuit board for door opening, Closing, Door Zone etc.
The automatic doors are designed such that the opening of the door can occur only at the landing within a specific zone called the Leveling Zone while the car is at rest. This is accomplished by sending a door zone signal to the elevator controller and the VF drive for the door motor which enables opening of the door.
As mentioned earlier, the car door and Landing door are mechanically coupled so that they open and close together. The coupling is done by cams fitted on the car and Rollers on the Landing door. The car can move from a floor only when the car and landing door electrical contacts are closed and the landing door is mechanically locked. This mechanical locking of landing door is essential to prevent possibility of opening a landing door when a car is not in that landing.
Movement of elevator without closing the doors can result in fatal accidents. Such a condition can arise due to failure of any of the components such as breakage of ropes or pulleys. A copy of the Failure mode effect analysis conducted by the manufacturing company can be obtained to ensure that the elevator controller will not get a wrong door closed signal under any failure condition.
6.4.1 Safety measures while using an auto door:
The power operated doors are designed to exert limited force so that they do not hit and injure a person.
This is accomplished in o ways:
The force inserted by a closing door is designed to be less than 150N. Some of the door systems of present designs re-open the door on sensing the pressure.
Obstruction to the door closing movement is sensed by operating a momentary pressure operating switch, which provides a door open command to the door drive system.
The obstruction is sensed electronically by infra-red rays mounted vertically along the car door so that the door can open on sensing obstruction even without actually touching the passenger. This arrangement is very commonly used in the elevators manufactured at present.
6.5 Few other assemblies are mounted on the car:
Electronic controls are used to smoothly open and close the doors on receipt of commands from the controller. On reaching the door open limit (Fully Open) or door close limit (Fully closed) the power to the door motor is brought to zero. The present day door operator controllers can learn the door position by counting the pulses of the encoder mounted on the door motor shaft. This helps to achieve smooth opening and closing of the doors.
Car is also fitted with fans, lights and emergency light and false ceiling in most of the cases. The fans are electronically controlled that they go off after a delay when the lift is idle. This helps to save the electrical energy and increase the life of the fan. A car is fitted with emergency light to provide light in the car at times of power outage. Over load announcement and indicators are also common features found in the car to inform overloaded condition to the passengers. The lift does not start under over loaded conditions. Some customers desire voice announcement of floor arrivals in one or multiple language to support the disabled.
The most important assembly is ''car top inspection box assembly''. Often, the elevator mechanic is required to run the elevator from the car top for doing maintenance work. The car top inspection box assembly consists of STOP push button to stop the elevator, a rotary switch to turn to maintenance mode of operation, two push buttons UP and DN to decide the direction of travel and one common push button. Inspection speed should not exceed 0.7s. The service is required to press a common button and a direction button together to move in particular direction. The lift stops once any one of the two switches is not pressed. The inspection operation circuit and fixtures are required to be designed with utmost care to prevent any unintentional movement of the car even in case of failure of any component. It is strongly advised that the elevator manufacturing companies conduct '' Failure Mode & Effect Analysis'' (FMEA) to prevent un intended movement of car even in case a failure of wiring or any one component. There has to be ways to detect a first failure. Failure of any component must be detected, bring the lift to stop (without trapping passengers) and create a break down so that the fault can get rectified before a second different failure occur. The designer should keep in mind to ensure elimination of elevator accidents because lift accidents are normally either fatal or very severe.
Provision is also made on the top of car to load the car with weights. Since many components are mounted on the car, the weights may tilt the balance in static condition. To balance the car under static conditions provision is de to load the car using weights.
Other important assemblies are the guide shoe and roller guides.
Typically Sliding guides are used for speeds up to 2mps and Roller guides are used for speeds above 2.5mps. Roller guide comprise of spring loaded rollers which are in contact with the three sliding faces of the guide rail. The rollers are lined with rubber or polyurethane tires. Noise or vibrations are reduced and the ride quality is significantly improved since the rollers are mounted on ball bearings. For very heavy duty lifts there are designs with six rollers. Roller guides operate on dry rails and hence no oil or any lubricant need be used on rails.
The other major safety component is mechanical safety. These are normally fastened to the underside of the safety plank. Safeties may also be arranged to attach to the elevator structure in the top position. The safety levers are roped to a safety governor which is located in the machine room. When the speed increases beyond the set value the safety levers actuate the safeties which grip firmly with the guide rails and prevent any further mechanical motion. Safety blocks are the ultimate safety for elevators to prevent a free fall of elevator due to ropes cut. Safety in general gets activated only during downward motion. The operation of mechanical safety also sends electrical signal to the controller to cut off the power supply to the motor.
Instantaneous safeties are used for speeds not greater than 1mps. Progressive safeties are used for speeds above 1mps.
6.6 What is a car operating panel (COP):
The car operating panel is an assembly of push buttons, a display and other switches, located inside the car. The push buttons facilitate the passenger to register calls to go to desired floor. The buttons in the car operating panel consists of a contact, which when pressed signals the car top board which in turn communicates serially with the main controller kept in the machine room. The main controller acknowledges the call and communicates to the car top board which in turn lights the LED in the corresponding call button. This is how a light turns ON whenever we press a car call button wires were taken directly to the controller in the machine room. The method of communicating with the master controller either serially or in parallel is based on the cost benefit analysis of running a number of wires to the machine room VS using few serial communication wires. Reducing the number of traveling cable wires also improves the failure rate. All the wires connecting a car to the machine room have to be wired using special traveling cables. This is required due to the number of bending operations the wire undergoes as the wires are connected to the car which moves continually UP and DOWN.
The other commonly used buttons in a COP are Alarm, Interphone, FAN switch etc. The COP is also used for giving Door Open, Door Close commands. The COP has hands free intercom which facilitates intercommunication between the passenger inside the car and the elevator machine room or Security. The Alarm button is connected to a hooter which is kept on the car top. The Alarm has to be active even when power fails in order to get attention in case of entrapment inside the car. The interphone and Alarm are provided to help a trapped passenger inside a lift.
Typically the COP will have Floor Call buttons, Door Open Button, Door Close button, Alarm button, Auto/ Attendant switch, Independent control, Interphone, Fan Switch, Interphone switch and position indication display.
CAR TOP BOX: The car top box communicates with the master controller serially. The other electronic assembly which communicates serially with the car top box is the car call panel CCP. The CCP is mounted in the car operating panel COP and all the car call buttons, Door Close, Door open and other buttons are had wired to this CCP. The car top box also controls the door drive unit. The car top box sends ''Door Close'' or ''Door Open'' commands to the door drive.
7. ROPES AND TRACTION:
A wire rope has three main elements
Wire
Strand
Core
The basic component of a wire rope is a wire, which is made up of steel in various sizes. The number of wires in a stand depends o the usage of the wire rope. A defined number of wires are spun helically around a central wire, which is called a strand. A number of such strands are then helically spun around a core to form a wire rope. The way the wires are spun to form the strands and the way these strands are spun around the core determine the overall performance characteristics of the wire rope.
The geometrical arrangement of wires in a stand is called its construction. The most common strand constructions are Seale, Warrington and Filler.
While ordering ropes, it is important to mention the following
Nominal rope diameter (eg 10mm)
Rope Construction ( eg 8 X 19 W meaning 8 stands of 19 wires each Warrington arrangement)
Type of core
Tensile Grade
Surface Coating
Lay- Type and direction.
7.1 Rope Selection:
The number of expected bending cycles a rope has to face is a key information before selection of a rope.
Foresee that a particular installation will face continuous use with high acceleration and deceleration.
The following parameters are used for rope selection:
Code required Factor of safety
Actual Factor of Safety
Number of ropes
Static tension per rope
Traction ratio
Popular rope sizes are 8mm, 10mm, 13mm and 16mm.
Ropes should be selected per the breaking loads provided by their manufacturers, and the Factor of Safety (FOS) required by the local codes. The most important aspect of rope selection is the verification of the factor of safety and must generally comply with a factor of safety = 10 for speeds up to 2mps and greater for higher speeds.
The BIS specifies the factor of safety for suspension ropes In the case of the traction drive, the factor of safety shall be based on static contract load plus the weight of the lift car and accessories.
All the ropes at one installation must be from the same manufacturer and of same material, grade, construction and diameter preferably cut from the same reel.
The factor of safety of the wire rope can be calculated by the following formula:
K = Number of runs of the rope. For 2:1 roping this shall be 2
N = Number of ropes
S = Manufacturer's rated strength of one rope.
W = Maximum static load imposed on all the car ropes at its rated load on any position of car in the hoist-way.
7.2 Example - Calculation of factor of safety
Elevator Specs | Manufacturer's Catalog |
Number of ropes(N) | 3 | Rope unit weight (Kg/m) | 0.35 |
Duty Load ( Kg) DL | 1088 | 10mm Rope braking force (N) | 40,000 |
Rise (m) | 60 | Trav Cable unit weight (Kg/m) tc | 1:1 |
Overhead (m) | 4.5 | Comp Rope unit weight (Kg/m) tcomp | 2.24 |
Pit depth (m) | 1.6 | | |
Dia of Ropes mm | 10 | | |
Roping | 2:1 | | |
CWT Over balance | 45% | | |
Total Car weight (Kg) Carw | 1300 | | |
Calculations:
Traveling cable weight Trw= R*tc*0.5 | 33 | |
Total Counter weight = Carw+0.45*DL+0.5Trw | 1806 | |
Total Comp chain weight Compw = R*tcomp | 134 | |
Total Rope length= 2*Pit depth + 2* Rise + 4* OH | 141.2 | |
Weight of ropes= Rope length * unit weight * No of ropes | 148 | |
Car on top level with 125% load= 1.25*DL+Trw+Compw+Carw | 2827 | |
Car at bottom level with 125% Loaded = Carw+1.25*DL+Rope weight | 2808 | |
Safety Factor= N*(Min braking force / Max weight on car side) Assuming N=2 | 21.3 | |
Acceptable limit as per BIS | 12 | |
Even with N=2, it meets the Safety Factor.
But less than 3# 10mm ropes are not acceptable as per BIS
Hence use 3#, 10mm Ropes.
For a long rope life, ensure
Lubrication
Proper alignment of sheaves
Ropes are not twisted during installation
Equal adjustment of rope tension at the installation.
7.3 Recent Trends:
One of the leading manufacturers of elevators have introduced Polyurethane Coated Steel Belts in their elevator systems. As per the manufacturer it replaces the heavy woven steel cables that have been the industry standard since 1800. These belts are only 3mm thick and hence make it possible to have smaller sheaves thus making it possible to mount the machine in the hoist way itself thus eliminating the need for machine rooms.
Similarly another manufacturer has introduced ''Suspension Traction Media'' by which the sheave diameter can be reduced up to 85mm. This Ultra Rope has a carbon fiber core surrounded by high friction coating.
8. Driving Machine Sheave:
The machine sheave is just a pulley with grooves around the circumference. The sheave grips the hoist ropes, with car on one side and counterweight on the other side. On account of rope tension, each element of the hoist rope in contact with the driving sheave is pressed against it and becomes a source of friction. When you rotate the sheave, the ropes move along. The friction provided by the groove pressure is known as available traction. It is generally accepted that the maximum available traction is dependent upon three major factors:
Angle of wrap of the ropes around the traction sheave;
Shape of groove profile;
Coefficient of friction between rope and sheave material.
Available Traction (TRav) must be greater than the Required Traction (TRreq) for the sstem for the proper operation as described above.
But if the elevator is stalled due to any reason (Safety operation, Counterweight on buffer etc.), it must be assured that traction of the system can be overcome so as not to drive the non-stalled component to an unsafe condition. Under such condition, TRav must be less than TRstalled.
8.1 Groove Types:
The U groove sheave can be considered as the desired sheave of choice for optimum rope life. The larger D/d ratio makes bending easier, provides a large arc of contact between rope and sheave, reduces operating stresses, and generally optimizes rope life. Unfortunately, in spite of its merits for improving rope life, this type of groove does not provide sufficient traction and hence is used mostly only for high speed applications with double wrap arrangement.
As undercuts become larger, groove pressure increases, traction increases, and unfortunately, rope and sheave wear and rope fatigue accelerates.
Generally the angle of the V groove is between 32 and 40 deg. Traction increases with reduction in V angle. This type of groove places the largest amount of pressure on the ropes and sheave grooves, resulting in the greatest amount of traction, but also the greatest amount of rope abrasion.
Undercut U- and V-grooved sheaves adversely affect rope performance. But these grooves increase traction and therefore do not require a large diameter sheave, hence these groove types are more preferred by elevator system designers.
8.2 Calculation of available traction:
The available traction relation depends on the type of sheave grooving, the arc of contact the rope makes with the driving sheave, and the speed of the elevator
The available Traction TRav = e^f.α
Where,
e is the base of natural logarithm
f is the friction factor of ropes in the groove
α Angle of ropes on the traction sheave in radians
e= 2.718
The friction factor F of the ropes in the grooves is dependent on the groove profile.
8.3 Sample Calculation of Available Traction (Dynamic and Stalled)
Stalled traction is when the counterweight sits on the buffer whereas Dynamic is at normal condition of elevator operation.
From the above table it is clear that
Available Traction increases with increased wrap angle.
Traction increases with increased under cut.
V cut provides bet traction.
Available traction can be increased by increasing the actual coefficient of friction of the material.
Note that all the above parameters are dependent on one another.
Compromising on any of the above factors should not change the final traction value. With this background, elevator systems designers need to be very careful in estimating traction and establishing their designs.
8.3.1 Required Traction:
When the car and the counterweight hang on the main sheave, the weight on both sides will not be the same. The weight difference varies as the car position changes with respect to the counter weight. The weight of car, counter weight, weight of ropes, weight of compensating cable, weight of traveling cable-all these weights influence the difference in weights on both ends of the sheave. The available traction, which was calculated earlier, must be more than the required traction in order not to allow slip of the ropes and allow the ropes to rotate along with the car. In addition to the weights, in a dynamic situation, the acceleration and deceleration of the car also influence the required traction.
For having proper traction of the system, the following equation must be fulfilled.
The Worst Case required traction TRreq(dyn) < TRav
The worst case TREQ(DYN) occurs when
Empty Car is decelerating to stop on top most floor.
125% loaded car decelerating to stop at bottom floor.
Hence we need to calculate the traction under the above two conditions to determine the system traction.
For calculation of the traction, we need the following parameters
Car weight = empty car weight + Duty Load
Counterweight = Car weight + OB x Duty + TC unit weight x Rise / 4
ROPE weight = Rope unit weight x # ropes x (Rise + Overhead) x 2
TC weight = TC unit weight x Rise / 2
Comp weight = Comp unit weight x Rise
Accel = 0.5m/s;
OB ( Over balance) = 45%
From the Example 1, we already have the following elevator specifications:
Elevator Specs
Number of ropes | 3 |
Duty Load (Kg) DL | 1088 |
Rise (m) | 60 |
Speed (m/s) | 1,5 |
Gravity (g) (m/s^2) | 9.81 |
Acceleration (m/s^2) | 1.0 |
Dia of Ropes mm | 10 |
Overhead (m) | 4.5 |
Pit Depth (m) | 1.6 |
CWT Over balance | 45% |
Roping | 2:1 |
Total Car weight (Kg) Carw | 1300 |
From Manufacturer's catalog
Rope unit weight (Kg/m) | 0.35 |
Trav Cable unit weight (Kg/m) tc | 1.1 |
Comp Rope unit weight (Kg/m) tcomp | 2.24 |
Min braking force of 10mm rope (N) | 40000 |
Calculations:
(Car & CWT sheaves not considered in this calculation)
Traveling cable weight Trw = R*tc*0.5 | 33 |
Total Counter weight = Carw + 0.45 * DL + 0.5Trw | 1806 |
Total Comp chain weight Compw = R * tcomp | 134 |
Total Rope length = 2* Pit depth + 2* Rise + 4* OH | 141.2 |
Weight of ropes = Rope length* unit weight * No of ropes | 148 |
Condition 1
Empty Car going up stopping at top landing
T1 = (Total mass of Empty Car + Traveling cable + Comp Chain) x (Gravity - Acceleration)
= (1300+33+134) x (9.81-1) = 12924
T2 = Total mass of CWT x (Gravity + Acceleration) + Total mass of Hoist Ropes 8 Gravity + Total mass of CWT * Roping^2* Acceleration = 1806*(9.81+1) + 148*9.81+148*(2)2*1=21566
T2/T1 = 21566 / 12924 = 1.67
Condition 2
125% loaded car going down stopping at bottom landing
T1 = ( Total mass of Empty car + 1.25 * Duty Load) * ( gravity + Acceleration) + ( Total mass of hoist ropes * Roping^2 * Acceleration)
= (1300 + 1.25 * 1088) * (9.81 + 1) + ( 148 * 9.81) + ( 148 * 2^2 * 1) = 28755 + 1452 + 592 = 30798
T2 = ( Total mass of CWT + Weight of Comp Chain) * ( Gravity - Acceleration)
( 1806 + 134) * (9.81 - 1) = 17091
T1 / T2 = 30798 / 17091 = 1.8
Greater of the above two ratios is 1.8 Available Traction with U undercut 105degree and wrap angle of 180degree is 2.
Hence TRav > TRreq,dyn and meets the requirement |
Traction under Stalled condition:
It is also important to ensure that when the Counterweight is sitting on the buffer, the available traction must be less than the required traction so that the traction should not be large enough to lift the car.
TRreq,stall > TRav.stall
T1 Stall = Total mass of (Empty car + Travelling Cable + Comp Chain) * Gravity
= (1300 + 33 + 134) * 9.81 = 14391
T2 Stall = Total Mass of rope * Gravity
= 148 * 9.81 = 1452
TRreq.stall = T1stall / T2 stall = 4391 / 1452 = 9.9
Tav.stall for U sheave with 105degree undercut and 180degree wrap angle is 4.66 Hence TRreq.stall > TRav.stall |
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