To obtain accurate stops level with the land ing it is necessary to slow down the cage to an extremely slow speed. In the microdrive ma chine this is obtained by having a main machine (which may be of any desired type) for ordi nary hoisting purposes and an auxiliary ma chine for the stop. The main and auxiliary machines may be coupled together by a mag netically operated clutch carried on an exten sion of the main motor shaft. At the start, the coupling is released leaving the main ma chine free to hoist the load. Shortly before the stop the current simply to the main motor is interrupted while simultaneously the auxili ary motor is started up and the coupling ap plied. The effect of this operation is that the load is now transferred to the auxiliary ma chine, which drives the drum through its own and the main gear reduction. The cage there fore proceeds to the landing at extremely slow speed controlled by an automatic leveling de vice. The function of this device is not only to stop the cage flush at the landing, but to maintain the alignment between cage and land ing during loading and unloading. If for example a heavy loaded truck is rolled onto the car, the stretch of the hoisting ropes will cause the platform to sink below the landing as soon as the front wheels of the truck rest on the car floor. This will cause the auxiliary motor immediately to restore the alignment be tween car and landing before the rear wheels pass onto the platform. The controlling device most in use with electric elevators consists of a lever in the car operating a number of con tacts. These in turn energize magnets as sembled on a controller panel near the machine. The operator controls only the direction in which the car is to travel, the fast and the slow speed. Operations such as releasing the brake and stepping out the starting resistance occur automatically. In push button controlled ele vators, each landing is provided with a button to call the elevator to that landing. The cage is provided with a series of buttons to dis patch the cage to any desired floor. A passen ger desiring to use the elevator presses the button placed near the elevator shaft, and the car, if not in use, immediately travels to that floor and stops automatically. When the car has come to rest, the door can be opened. The passenger enters, closes the door and presses a button corresponding to the floor to which he wishes to travel and the car at once proceeds thereto. It will be seen that the push button elevator is entirely automatic in its operation, having the advantage of not requiring an at tendant.
Although this type of control was de veloped many years ago, it has in the past found application only to slow speed residence and apartment-house elevators for the lack of a machine capable of making accurate stops level with the landing. With the advent of the microdrive machine, however, this type of control is receiving increased attention and It is safe to predict that it is destined to come into general use.
Safety Appliances.— While the factor of safety in the standard make of elevators is such that accidents rarely occur and practically never where proper attention is paid to the machinery, still all elevators (except plunger elevators) are equipped with safety grips. Of the very large variety of safeties, only those types which have found extensive application will here be described. With wood guides the type of safety generally used consists of an arrangement of planer teeth forced into the guides and ducing resistance by planing or grooving the wood as the car descends. Figs. 9 and 10 show one form of this type of safety. With steel guide rails the types in use are the roll, wedge clamp and the flexible guide clamp safeties. The roll safety—shown on Figs. 11 and 12— employs a corrugated steel roller, adapted to be forced into the apex of an angle, formed by the guide rail and the inclined surface of the safety block. The angle usually is small so as to make the roller self-locking. Th's in turn causes an abrupt stop, so that the safety Can be used only for low car speeds. In the wedge clamp safety, the rails are gripped between the jaws of two clamps. As shown in Fig. 13 the safety is actuated by a drum having a huh pro vided with right and left hand screw threads which engage with two screws. Rotation of the drum in the proper direction moves the screws outwardly, forcing the wedges at the ends of the screws between the rollers of the clamp levers and causing the jaws to grip the rails. The drum is provided with a few wraps of rope, one end of which is fastened to the drum and the other to the governor rope.
Let us now imagine a falling car, equipped with a wedge clamp safety as here described, and analyze what will happen. Let us further
assume that the normal speed of the car is 600 feet per minute. The governor (Fig. 14)— so as not to interfere with moderate speed varia tions from natural causes — will be set to trip at 800 feet per minute; that is to say, as soon as the speed of the car reaches 800 feet per minute, the governor jaws will grip the gov ernor rope, causing the latter to come to rest quickly. As a consequence, the rope on the safety drum, having one end fastened to the governor rope, will unwind while the car keeps on falling. This will cause the safety drum to rotate, actuating the safety mechanism. Before, however, the jaws grip the rail, all of the clear ances must have been taken up. During this celcration of the governor rope. It will there fore he seen that there is, at high speed, a considerable increment in the retarding force exerted by the clamps, resulting in undesirably heavy retardations. Another disadvantage is due to the fact that the safety must be made self-locking so that it will not release its grip on the rails, should the governor rope break. If, therefore, during the slide of the jaws on the rails, variations in the thickness of the rails occur, the jaws can yield only by virtue of their elasticity. That this will cause enormous varia tions in the retarding force in the one or the other clamp is plain and it is therefore no surprise that the platform frequently comes to a stop altogether out of level. These dis advantages have led to the development of the flexible guide clamp safety, first introduced in 1916 and now rapidly superseding the type de scribed above. Each of its clamps has two jaws, one solid and the other provided with a wedge having its face slightly inclined toward the guide rail. Both jaws are pivoted and arc adapted to compress a spring held between the clamp levers. The spring is normally free from compression. A roller is adapted to be brought in contact with the inclined face of the wedge on one side and the guide rail on the other The inclination of the former is such that onct time the car falls another 4 or 5 feet, equiva lent to an increase in speed of approximately 1,000 feet per minute. At the time that the application of the safety begins, the car speed therefore has assumed very considerable over speed, amounting in the present example to 1,800 feet per minute. This is decidedly a dis advantage. The jaws now grip the rail and, as a consequence, the rotation of the safety drum stops. The governor rope, previously at rest, is therefore suddenly accelerated to the car speed of 1,800 feet per minute and begins to slip through the governor jaws. The force then exerted on the periphery of the safety drum, which is a direct measure for the retarding force exerted by the clamp, is the sum of fric tion caused by the grip of the governor on the governor rope and of the force necessary to suddenly accelerate the same to car speed. Now it will be clear, that in the present example the safety should be designed to stop the car at any speed above 800 feet per minute. At a speed of 800 feet per minute, however, the effect of the sudden acceleration of the gov ernor rope is small and most of the work is done by the friction caused by the grip of the governor on the governor rope. If, however, the car actually falls, action of the safety be gins at a speed of 1,800 feet per minute with a very considerable effect of the sudden ac this contact is established, the roller continues to climb upwards until its motion is arrested. In doing so, it first forces the solid jaw to engage with the rail, after which it will cause the wedge—and therewith the jaw containing the wedge — to recede from the rail. The lat ter jaw thereby swings around its pivot and compresses the sprang. The rail, therefore, will be gripped between the solid jaw on one side and the roller on the other side with a force corresponding to the spring compression. Since the travel of the roller is limited, the maximum amount of spring compression is also fixed and, with that, the retarding force which the clamps exercise can be arranged to be just sufficient for a smooth stop from any speed. It will be seen that the operation of the safety begins immediately from the moment that the roller makes contact with the wedge and rail. The time lag between the operation of the actuating mechanism and the gripping of the rails, exist ing with the wedge clamp safety, is here prac tically eliminated; indeed, the flexible guide clamp safety responds immediately to any de mand for its operation.