case with a car suspended from cables. By properly designing the outlet orifice the maximum speed of descent can be determined for a given load, and except for the remote possibility of the piston or plunger failing, this speed of descent cannot be exceeded. The danger of the car being drawn into the overhead structure in case of overtravel at the top of the shaft is eliminated by the simple expedient of providing a bypass in the base of the plunger. This is generally arranged so that when the car has travelled about a foot above. the upper terminal landing this bypass in the plunger has travelled to a point above the stuffing box and the water in the casing can escape into the pit thus lowering the pressure in the casing to a point where it becomes impossible to raise the car higher. The objections to this type of elevator are, in addition to the rather low mechanical efficiency, that it is almost impossible to drill the hole for the casing so that it is absolutely vertical. Any inclination of the casing causes cramping or excessive wear on the piston. Another objection is the fact that on long rises the plunger itself has considerable side-play, particularly when the car is stopped suddenly. Aside from their use on sidewalk lifts, where they are still employed to a limited extent, generally being worked directly with water from the city main, this type is not being installed at the present time except in very rare instances.

Hydraulic elevators are also frequently classified as high and low pressure. A low pressure elevator is one in which the water pressure is not over 250 lbs. per square inch and a high pressure elevator one in which the water pressure is greater than 250 lbs. per square inch. Many high pressure elevators use water pressures of 600 and 700 lbs. per square inch.

Electric Elevators. The electric elevator was commercially introduced about forty years ago but owing to the lack of suitable control apparatus was not particularly successful in competition with the hydraulic machine. Improvements in the control, however, eliminated the earlier objections and its use developed rapidly.

The drum type of electric elevator (see Fig. 4) which was first developed and is still extensively used may be described as follows: The motor is connected to a suitable drum provided with spiral grooves by means of a worm shaft engaging with one or two worm wheels; the latter being connected to the winding drum.

FIG. 4. Basement Back Drum Counterweight Type. A. Main hoisting rope. B. Drum counterweight rope. c. Car counterweight rope

spring or springs, thus holding the gearing and cable drum immovable. A solenoid is provided to compress the spring and relieve the tension on the brake shoes. When the controller admits current to the armature of the motor, current is also supplied to the solenoid, which is mounted either above or alongside of the brake, releasing the brake band so that the motor is free to drive the worm gear. On shutting off the current from the motor, the solenoid becomes deenergized and allows the spring to apply the brake.

The traction machine differs from the drum machine, which it has largely replaced, in that the cable in place of being wound on a drum runs over a sheave

wheel and is attached to the car and counterweight respectively. The elevator drive is effected by the traction between the cable and the driving sheave. This type of drive is inherently much safer than the drum type because in installations of all but the highest rises the instant that the car or counterweight reaches the bottom of the shaft much of the driving traction is lost and the cable tends to slide in the grooves of the driving sheave. In most cases it is impossible to pull the car or counterweight into the overhead structure after the other member has reached the lower limit of its travel. For slow or moderate speed installations the traction sheave is driven through a worm and gear wheel similar to that used for driving the winding drum on the older type of machine. For high speed installation this gearing has now been eliminated and the driving mechanism consists of a large, slow-speed motor

FIG. 5. 1 to 1 Double-Wrap UGroove Traction Elevator

with the brake and driving sheave mounted on the same shaft with the motor armature. This type of installation offers the highest possible operating efficiency and eliminates a number of wearing surfaces.

The traction drive is made either full-wrap or half-wrap. These two methods of driving elevator cars by traction sheaves are also known as U-groove and V-groove. In the full-wrap type

of traction machine the traction sheave has twice as many grooves as there are cables. The cables from the car run over the sheaves in one set of grooves, then to the idler sheave placed immediately below or to one side, and then return to the main sheave, running over it in the second set of grooves. From the driving sheave the cables lead to the counterweight. In this method practically 360° of driving contact are secured on the driving sheave for each rope; hence, the name (full-wrap). The grooves in this case are made with a semi-circular bottom slightly larger than the rope diameter, and sides that are practically perpendicular above the groove. A cross-section of one of the grooves is approximately that of the letter 'U'; hence, the designation, U-groove. (See Fig. 5.) The use of sheaves placed on car and counterweight (see Fig. 6) give double the lifting effort at half the speed. This is known as 2-to-1 roping.

FIG. 6. 2 to 1 Double-Wrap UGroove Traction Elevator

In the half-wrap traction, the ropes run from the car over the driving sheave to the counterweight without running over a traction idler sheave. If the sheave spans the distance from the centre of the car guides to the centre of the counterweight guide rails approximately 180° driving traction is obtained. However, where one-half the car width is greater than the sheave diameter, a deflecting sheave is provided so that the counterweight section of the cable is parallel to the counterweight guide rails. In such a case, the arc of contact on the driving sheave is something less

than 180°. However, this angle of contact seldom is less than 135° and even with such angle of contact satisfactory running condition may not be obtained. (See Fig. 7.)

As the cables in this case can have not more than 180° or 'halfwrap' the conventional U-groove with the semi-circular bottom such as is used on the full-wrap traction sheave will not give sufficient adhesion between the cables and sheave to drive the elevator except when the load is reasonably close to the balance load i.e. when the weight of the car and load is equivalent to

FIG. 7. Overhead V-Groove Traction Elevator

is

the weight of the counterweight. In order to secure sufficient driving traction with the half-wrap type of sheave a V-groove, or some modification of it, adopted. In this case the sides of the groove make a comparatively steep angle, frequently in the neighborhood of 70°. The bottom of the groove is narrower than the diameter of the rope so that there is considerable 'pinching' effect. (See Fig. 7.) The greater the load the more solidly the cable is pulled down into this V-groove. Various manufacturers use modifications of this groove for commercial practice.

Most traction sheaves are made with removable or renewable outer surfaces in which the grooves are actually cut. These are bolted, pressed, or shrunk on the hub so that if the wear be

comes excessive no great amount of time may be lost in replacing the actual driving surface of the sheave. The U-groove construction is favored by some manufacturers and probably will give somewhat longer life without renewing the sheave surfaces. It has the advantage that the traction relation remains practically constant irrespective of wear. It is claimed, however, that because of the use of the idler the overall efficiency of the machine is slightly less than that of the half-wrap machine as the re-action on the machine bearings is double because of the second wrapping of the cables. Some manufacturers make both types and use the U- or V-groove depending upon speed, load, and various other factors.

The gearless machine is practically standard for speeds in excess of 500 or 550 feet per minute although at least one manufacturer builds geared machines up to 650 feet per minute.

Practically all electric passenger and most freight machines are equipped with a solenoid release brake which is spring applied. However, this friction brake Iwould not be sufficient to stop the elevator from high speeds if it alone were to be depended upon. The controller of practically all D.C. high speed machines is so arranged that when the car switch is centred or when the floor selector mechanism cuts off the driving current, the motor itself is used as a dynamic brake, the generated current flowing back into the line. The dynamic braking effect varies as the square of the speed so that at high speed a very powerful braking effect is secured. The friction brake having an approximately straightline speed braking effect becomes a predominant factor when the speed has dropped to a certain percentage of full running speed and at the very lowest speed is practically the sole braking agency. The friction brake also serves to hold the car and load when the car is standing at a landing.

Alternating Current Machines. In the early days of electric elevators practically all such equipment was installed in the congested portions of the larger cities and the current available was in almost every case direct current. The tendency in power generation in more recent years has been to supply the outlying sections of metropolitan areas with alternating current because of the ease with which it may be stepped up to high voltage for efficient transmission over long distances and the ease with which it may be stepped

down to any desired voltage for use. An ever growing demand has arisen for alternating current machines for buildings located in the districts supplied with alternating current. Owing to the difficulty of securing high starting torque and good speed control regulation, the alternating current machines first developed were not particularly successful. However, the growth of the demand for equipment of this kind has been the cause of extensive development of alternating current equipment so that today it compares very favorably with direct current in performance where a high grade of hoisting and control equipment is employed. The alternating current machine now used for moderately high speed work is, as a rule, a double wound induction motor. Generally, high speed windings have either 3 or 4 the number of poles of the slow speed winding. By the use of the two-speed winding a considerable amount of regenerated braking may be secured, the motor acting as the induction generator. With alternating current chines of the single speed type, it is necessary to provide much larger and more powerful brakes than would be the case with direct current machines of the same speed and capacity because on a motor of this type regenerative braking cannot be secured. Many of the early alternating current brakes were objectionable from the standpoint of the noise of operation. Elevator builders have, however, succeeded in overcoming this and the alternating equipment compares favorably in quietness as well as efficiency with direct

current.

ma

With alternating current machines it is necessary to provide phase reversal protection. This generally takes the form of reverse phase relays. The need for this protection is due to the fact that with a three-phase supply, which is becoming almost universal, the reversal of any two leads will reverse the direction of the motor connected in the circuit with it. Such a reversal might be made by building electricians working on the incoming main or by public utilities employees making repairs or line changes at any point from the powerhouse to the entrance of the building in which the elevator is located. One manufacturer has solved this problem in a very ingenious manner. The controls are operated by one or more small 3-phase motors which are permanently tied in to the busses on the elevator control board. These small motors operate the direction

switches through a series of cams turning clockwise for one direction of travel and counterclockwise for the other. If there has been phase reversal, the throwing of the car switch in the up direction will cause the control motor to revolve in the direction opposite to that in which it formerly operated. In other words, it will throw in the down direction switch. However, as the phase reversal has reversed the direction of rotation of the hoisting engine motor, the elevator will actually ascend. In other words, the car will travel in the direction for which the operator has thrown his switch.

A new form of alternating current control consists of an induction regulator which is in effect a current transformer in which the secondary may be revolved through 180 electrical degrees. This is so connected to the motor that the induced current may be made to buck or boost the line voltage. As the secondary voltage on a 220-volt line is made 110 this gives a starting voltage of 110 volts and the full speed running voltage of 330. This, of course, requires a special motor. The revolving of the secondary of the induction regulator is accomplished automatically and produces an exceedingly smooth and rapid acceleration. At the present time, however, it is rather expensive to build and install.

Controllers.-Fundamentally,

modern elevator controllers consist of a board provided with a series of magnetically operated switches for starting and stopping the car in either direction of travel, for automatically accelerating the load, and for providing the necessary circuit to bring the car to rest gradually. Usually, the controller is provided with a potential switch which is normally held engaged so long as the switch on the incoming circuit is closed. This potential switch is opened only in case of some abnormal condition arising through the operation of the elevator. The magnet which holds this switch in place is supplied with current through a circuit which ordinarily runs through the upper and lower and final limit switches, the safety switch on the car, and a switch on the governor or safety. In addition to this, other auxiliary safety switches may be placed in series in the same circuit so that the opening of any one of them will shut off current from the entire controller. This potential switch also acts as novoltage switch and may be equipped to drop out on overvoltage or over-load. This potential switch usually opens both

sides of the incoming line. After passing through the potential switch the current feeds in parallel to two direction switches which are mechanically interlocked by various ingenious mechanisms so that it is impossible for both the up and down direction switch to be in operation at the same time.

In the usual type of resistance control, which until a few years ago was almost the universal type and is still very largely used, the current fed to the armature is limited by a series of resistances, generally cast iron grids, mounted on the rear of the control board. Various sections of these resistances are cut out of circuit by a series of automatic switches known as acceleration switches. A number of different systems may be used in securing this automatic acceleration. A modern arrangement consists of a series of separate switches which may be arranged either to give constant acceleration regardless of load in the car and known as the time-limit system or a system in which the speed of acceleration is dependent upon the load in the car known as the currentlimit type. The latter is frequently employed where there is a penalty on high current demands by the public utility company. A set of switches somewhat similar to the accelerating switches is provided to regulate the dynamic brake of the motor during a stop.

In order to secure the high starting torque necessary to produce rapid acceleration many controllers are arranged so that a heavy series field is provided during the starting and accelerating period. This series field is usually short circuited when the last accelerating switch contact goes in. In order to obtain a reasonably constant speed up and down with any load that may be placed in the car, field regulation is frequently resorted to. A common arrangement is to provide a contact on the speed governor which will operate at a slight increase over the normal speed. The closing of this contact gives a heavier field on the motor and decreases the speed.

The chief objection to resistance control is the fact that a considerable amount of power is wasted as heat in the starting resistance. An effort to produce higher operating efficiency led to the introduction of the multivoltage machine. A motor generator set is provided which supplies current to all the elevator machines in the group, there being available several voltage steps, as for example 60, 120, 180, and 240. By applying these various voltages successively to accelerate

variable voltage, or unit multivoltage. In this system each elevator motor is provided with a separate motor-generator set. The output to the generator is fed to the elevator motor. The entire control of the elevator is secured by increasing, decreasing, and reversing the field of the generator. The inherent

or automatic control. A series of pushbuttons in the hallways and a set of similar buttons, one for each floor, on the car are used to control its motion. The operation of a hall button calls the car to that landing. After the passenger enters the car, closes the door and pushes a button for the floor to which he wishes

a system known as collective pushbutton control in which the hall buttons are arranged to stop a car going in the direction in which the person pressing the button wishes to travel. When the car arrives at the landing the passenger may enter and the closing of the car gate will automatically start the car again to

wards its original destination. This form of control has multiplied the service which may be given by pushbutton elevators. It is particularly valuable for use in apartment houses, hospitals, and similar institutions.

With high car speeds much greater skill is required of operators to bring the car to rest reasonably close to a landing than is the case with cars of slower speeds. Even with skilled operators the percentage of landings at which it was necessary to 'inch' the car was high. As this 'inching' process is expensive both in time and current consumption. a system for making the landing stop automatically was developed.

In opera

tion this is much like the old pushbutton except that an operator is employed who presses a series of buttons on the car for the floors at which landings are to be made. At a signal from the dispatcher the operator presses a button or throws a lever which closes the hoistway door and car gate, the closing of this gate starting the elevator hoisting engine. When each of the floors for which a button has been pressed is reached the car automatically slows down and stops. The closing of the door and starting of the car are controlled as before by the operator. Passengers on intermediate floors going in the direction in which the car is travelling may stop the car by pressing a hall button. The car may be brought to the exact floor level by the use of a small auxiliary machine known as the micro machine, or by running the main machine at slower speeds for the last few inches of travel.

In order to bring the car exactly level with the floor without the necessity of operating the main machine (known as 'inching') an auxiliary machine has been developed which comes into action as soon as the main machine stops. It is controlled by cams in the hoistway and switches on the top of the car and is so arranged that the car if stopped within the zone of the landing cams will bring the car within 8-inch of the landing level. This is accomplished by providing a miniature elevator machine geared to the main machine by means of the high ratio worm gear. The main machine is built with internal-expanding rather than external-contracting brakes. After the main machine has stopped the micro machine revolves the armature, brake assembly, and driving sheaves of the main machine through the worm and gear until the car is at the landing. By bringing the car level with the landing floor the danger of passengers

tripping while entering or leaving is practically eliminated and it is also of considerable advantage where truck loads of material are run on and off elevators. The same action is now accomplished in certain installations by the main machine through the use of hoistway cams or other suitable mechanism.

A number of modifications of the pushbutton and of signal control have been worked out. One of these is known as call and dispatch system in which the operator located at a small board resembling a telephone switchboard may handle several elevators sending them to certain floors to be loaded and unloaded and in general handling the installation much as a railroad dispatcher handles traffic over the section of the road under his control. An interesting use of this system was an installation made for a large restaurant where the cooking was done on certain upper floors, the food sent to a general distributing floor where the individual orders were prepared and these individual orders sent in small button-controlled dumb waiters to waitresses located on three lower floors. This system was arranged so that an order dispatched to one floor could not be removed or called by waitresses on the other floors. Modifications of such a system have been worked out for large parking garages in cities it being possible for one operator at a switchboard to dispatch an automobile to any empty storage space in the building, the automobile being loaded and unloaded from the elevator car mechanically, no operator being required on the car.

Car Construction.-The majority of all passenger and freight elevators are provided with a rectangular frame of steel known as a sling. The cables are attached to the upper member, known as the crosshead, and on this frame or sling are mounted two pairs of guide shoes, one pair above and one pair below the frame. These run on the guide rails and serve to keep the car in its correct position in the hoistway. A safety device for stopping the car in case of failure of hoisting cables or overspeed is attached to this sling generally on the under side but occasionally fastened to the crosshead. The car platform normally rests across the bottom member of the sling and in most cases is supported by inclined rods or braces attached to the platform and to the crosshead near the top. On certain large elevators two slings are provided running on two sets of guide rails. In this case the safety device is

provided with four sets of jaws, two beneath each sling, all being geared together by suitable shafting and gears.

Occasionally very large platform elevators are not equipped with a sling but are suspended by cables at the four corners and in case of exceptionally large elevators such as freight-car lifts there may be additional supporting cables at various points on the side. There are several serious objections to the platform type of car; first, the failure of the cable or cables on one corner may permit the car to tilt enough to dump the loaded car down the hoistway; second, it is almost impossible to provide adequate safety devices on this type of equipment.

Guide rails for elevators may be either of wood or metal. Wood rails are generally of rectangular sections and the material employed is ordinarily maple or dense yellow pine. Wood rails are all limited to comparatively light loads, low speeds, and short rises. The objections to them are the tendency to warp or twist out of line and the lack of homogeneity. Steel rails are by far the more common. They consist of T-shaped sections and ordinarily are machined on the faces and top of the head, this being the portion used for guiding the car. The rails are either tongued and grooved or dowelled and the joints are secured by fish plates. It is necessary to secure very exact alignment of rails if satisfactory operation is to be secured. Rails are ordinarily made in three sizes which weigh approximately 61⁄2, 15. and 30 pounds.

In order to provide a means of stopping and holding the car should the hoisting cables fail and to provide a means of bringing the car to rest in case of overspeed, elevators are equipped with safeties. The type of safety most generally employed consists of a pair of rail clamps placed under the car. These

clamps are pinned together and are provided with arms projecting in back of the pin. By driving a wedge between these projecting arms the faces of the grips are made to clamp on the guide rails. The mechanism for applying the pressure to these jaws usually consists of a drum upon which a number of turns of cable are wound. This drum is mounted on a shaft provided with left and right hand threads the ends of these shafts being fitted to the wedge blocks or

cams.

When the cable is unwound from this drum the shaft is rotated and the wedge of cams pushed (or in some cases pulled) between the extensions of the