Counterweights
Power requirements of the driving machine for moving the car are reduced by hanging a counterweight on the hoisting ropes. Use of a counterweight also is advantageous for maintaining traction between the hoisting ropes and the driving sheave. The weight of the counterweight usually is made equal to the weight of the unloaded car and the ropes plus about 40% of the rated load capacity of the car (Fig. 16.14).
A counterweight usually is made up of cut steel plates set in a steel frame.
Moving up as the car moves down and down when the car moves up, the counterweight is kept in a fixed vertical path by upper and lower guide rollers that are attached to its frame and roll along a pair of guide rails.
Roping for Elevators
The American National Standard Safety Code for Elevators, Dumbwaiters, Escalators and Moving Walks, ANSI A17.1, requires that a car be suspended from at least three hoisting ropes for traction-type machines and two ropes for windingdrum machines. At least two ropes are needed for a counterweight. All these ropes should be at least 1â„2 in in diameter.
A wire rope for an elevator installation comprises a group of steel strands laid helically around a hemp core. Each strand, in turn, consists of steel wires placed helically around a central wire and has a symmetrical cross section.
For a given weight of a car and load, the method of roping an elevator has a considerable effect on car speed and loading on the hoisting ropes machine bearings, and building structural members. The simple arrangement of hoisting ropes, cars, and counterweight shown in Fig. 16.14a, for example, is called 1:1 roping, because car speed equals rope speed. The ropes are attached to the top of the car frame, wind around the driving sheave, bend around a deflector sheave, and then extend downward to the top of the counterweight. This rope arrangement is also known as single-wrap roping because the ropes pass over the driving sheave only once between the car and the counterweight. The 1:1 single-wrap roping often is used for high-speed passenger elevators.
For single-wrap roping, the rim of the driving sheave is given wedge-shaped or undercut grooves, to obtain sufficient traction. The sheave grips the ropes because of a wedging action between the sides of the grooves and the ropes. The pinching, however, tends to shorten rope life.
For good traction with less rope wear, double-wrap roping (Fig. 16.14b), is frequently used for high-speed passenger elevators instead of single-wrap. For double-wrap roping, the ropes are attached to the top of the car frame, wind twice around the driving sheave and secondary sheave, and are then deflected down to the counterweight by the secondary sheave. Because of the double wrap, less grip is needed at the driving sheave. As a result, its rim may be given U-shaped or round-seat grooves, which cause less rope wear. The 1:1 double-wrap roping shown in Fig. 16.14b, however, applies twice the load to the driving sheave for the same weight of car and counterweight as does the single-wrap roping in Fig. 16.14a and requires a heavier design for affected components.
For the double-wrap roping shown in Fig. 16.14c, rope speed is twice the car speed. The arrangement, called 2:1 roping, is suitable for heavily loaded, slow freight elevators. For this arrangement, a higher-speed, less costly motor can be used for a given car speed than with 1:1 roping. The ropes in this case are not attached to the car and counterweight as for 1:1 roping. Instead, the ropes wind around idler sheaves on car and counterweight, and the ends of the ropes are anchored at the top of the hoistway at beams. As a result, the load on the driving and idler sheaves is only about one-half that for 1:1 roping.
In most buildings, driving machines are located in a penthouse. When a machine must be placed in a basement (Fig. 16.14d), the load on the overhead supports is increased, rope length is tripled and additional sheaves are needed, adding to the cost. Other disadvantages include higher friction losses and a larger number of rope bends, requiring greater traction between ropes and driving sheave for the same elevator loads and speeds; modestly higher power consumption; and potentially greater rope wear.
Figure 16.14e shows a type of 2:1 roping suitable for slow, low-rise elevators.
In contrast to the roping in Fig. 16.14c, only one end of the hoisting ropes is deadended at the top of the hoistway. The other end is attached to the counterweight.
Also, the ropes pass around idler sheaves at the car that are placed on the underside of the car frame.
Elevator Operating Systems
The method of actuating elevator control is called elevator operation. Many types of operation are available and some are complex and sophisticated. These may cost more than the simpler systems for installation and operation, but the sophisticated systems accomplish more automatically and handle traffic more efficiently. Following are descriptions of several types of operation:
Car-Switch Operation. With a manually operated car switch or continuouspressure buttons in the car, an operator controls movement and direction of travel of the car. To ensure that the operator controls car movement, the handles of levertype operating devices should return to the stop position and latch there automatically when the operators hand is removed. In automatic car-switch, floor-stop operation, the operator releases the lever or button to stop the car at a landing. Slowing and stopping are then accomplished automatically.
Signal Operation. The car can be started only by an operator pushing a start button in the car. The operator can register stops in advance by pressing and releasing a push button in the car corresponding to the predetermined floor number.
Persons calling for the elevator can similarly register a stop by pushing an up or down hall button. The car automatically stops at landings for which signals were registered, regardless of the direction of car travel or of the sequence in which buttons at various floors were pressed. When a landing is served by two or more elevators, the first available car approaching the floor in the specified direction makes the stop automatically.
Automatic Operation. An operator is not needed for automatic operation. Starts and stops are signaled by passengers in the car or by hall buttons or by an automatic operating mechanism. The car starts either in response to this mechanism or when a passenger presses a car or hall button. Responding to signals from car or hall buttons, the car travels to and stops at the signaled landings, and car and hoistway doors open automatically. Following, in order of increasing sophistication, are descriptions of several types of automatic operation.
Single Automatic Operation. The car starts when a passenger presses and releases a car button corresponding to a landing. The car then travels to that floor and stops. The car also starts when a hall button is pressed, travels to that landing, and stops. After any button has been pressed, depression of any other button has no effect on car movement until the stop signaled by the first button has been made.
Selective Collective Automatic Operation. When a car button corresponding to a landing is pressed, the car travels to that floor but also, on the way, makes other stops signaled. Hall calls are answered in the order in which landings are reached in each direction of travel, regardless of the sequence in which signals were received.
Up calls are served when the car is traveling upward, and down calls are answered when the car is traveling downward.
Group Automatic Operation. This is an extension of selective collective operation to a group of cars serving the same landings. A supervisory control system automatically dispatches cars to answer calls and coordinates the operation of the group. A call is answered by the first car to approach a landing in the proper direction. In response to a timer, the car leaves a terminal at predetermined intervals, which can be varied in accordance with traffic requirements. Group operation with automatic dispatching increases the number of passengers the elevators can carry in a given time.
Elevator Group Supervision
The supervisory control system for group automatic operation of elevators should be capable of making adjustments for varying traffic conditions. It should control car motion so that cars in the best location for answering calls do so. In the past, group control systems employed an elaborate relay network to program elevator motion. The advent of microprocessors, however, has dramatically changed elevator control systems.
The state-of-the-art group control system employs limited artificial intelligence features, which control the dispatching of high-speed elevator groups more efficiently.
One such supervisory control system contains a database based on practical knowledge, traffic data, and experience of elevator group-control experts. The system is able to maximize every elevator operation by application of the database and through knowledge it has obtained from ongoing traffic monitoring functions, such as call quantity and car loading. Decisions also involve use of fuzzy logic, which allows the elevator group control to make decisions based on fragmentary and fuzzy intelligence concepts. For example, use of the incorporated intelligence and common sense in the decision-making process maximizes the effectiveness of the system and assists in determining whether or not potential car assignments will result in shorter waiting times or cause congestion in elevator lobbies.
In another approach, the system is designed to alleviate lobby congestion during heavy up-peak traffic periods. Elevators make fewer stops per round trip, with the result that cars return to the lobby faster. The floors about the lobby served by the group are divided into sectors of contiguous floors. The number of sectors is normally one less than the number of elevators in the group. As an elevator returns to the lobby during an up-peak period, it is assigned to service one of the sectors.
Passengers can easily determine which floors each car is serving by checking the Information Display System screens located next to or above each elevator entrance.
The same car will not necessarily serve the same sector on successive trips. Care is taken to ensure that each sector will receive equal service by the assignment of sectors in a round-robin manner.
In still another approach, unlike any other control system presently in use, the system is arranged to know the passenger quantity and destination before the call is answered by the elevator. This is achieved through the use of a keypad call-entry system installed at each landing. The prospective passenger enters the destination into the keypad, and the elevator control system immediately assigns an elevator to that call. The passenger is notified graphically which elevator will respond to the call. Since the efficiency of elevator movement is greatly improved, this system offers the opportunity to reduce the quantity of elevators used to meet specific traffic conditions.
Additional Elevator Safety Devices
Automatic elevators require several safety devices in addition to those usually installed in elevators with operators. These devices include:
An automatic load weigher to prevent doors from closing and the car from starting when it is overloaded Car and hall buttons that passengers can push to stop the doors from closing and to hold them open Means for preventing doors from closing when the entrance is obstructed Emergency power system that is activated as soon as the primary system fails Lights to indicate landings for which calls have been registered Two-way communication with a supervisor outside the hoistway
