Another advantage of this type of safety lies in the fact that it is practically not affected by slight changes in the thickness of the rail. This is, of course, due to the very flexible arrangement of the jaws. A slight increase in the thickness of the rail will merely result in a somewhat higher spring compression without much increase in the retarding force of the clamps. The operation of a safety by means of a speed governor always has the disad vantage that no action occurs, except at a certain overspeed. If, for example, the hoist ropes break while the car is at rest, it is rather contrary to common sense to permit the car to fall and gather speed before the safety is applied. For this reason, the flexible guide damp safety is arranged to be actuated in three different manners, viz., first, in the ordinary way by means of a speed governor; secondly, immediately upon the breaking of the ropes and thirdly, at the will of the operator.
Air An air cushion is the en closure of the bottom part of the shaft to a height of from one-sixth to one-third of the car travel with just sufficient clearance for the normal operation of the elevator. At high speed, therefore, the action is that of a piston within a cylinder. It is intended as an addi tional safeguard in case all other safety de vices fail. The speed of a falling car within the air cushion is controlled in various manners, all tending to decrease the area through which air is permitted to escape, as the car nears the bottom of the shaft.
One of the advantages claimed for the air cushion is that it has no moving parts, so that no disarrangement can occur to prevent its operation. This claim, however, is not justified since, of course, there must be a number of doors within the air cushion zone for the in gress and egress of passengers. One of these left open or blown open by the air pressure may be sufficient to put the entire air cushion out of commission.
The main defect, however, is that a car at the top of the shaft is permitted to fall free for a distance of from five-sixths to two-thirds • of the height of the shaft before entering the cushion. As a consequence, the speed of the car at the entrance of the cushion is enormous and large retardations — dangerous to life and limb — have to be allowed to bring the car to rest within a comparatively small distance. By
actual tests, velocities at the entrance of the air cushion as high as 10,000 feet per minute have been measured. Retardations observed have been as much as 10 times gravity, sub jecting a person standing in the car to 11 times his weight. Air pressures have been measured as high as 16 pounds per square inch from which it will be evident that a heavy construc tion of the cushion and doors is required.
Oil With particular reference to high speed electric elevators, it is highly im probable, but yet conceivable, that all of the switches, which constitute the automatic stop ping device at the terminal landings, fail. Un der such conditions, the car will proceed at normal speed and since — in the absence of overspeed—the safety remains inactive, there is no agent to prevent the cage from striking the limits of its travel. Accidents of this kind are made impossible by the installation of oil buffers. One or more of them are placed in the pit and are struck by the cage as it over runs the bottom landing. Another buffer is usually attached to the counterweight and comes into operation when the cage overruns the top landing. The construction of an oil i buffer is shown in Fig. 16. It consists of an outside casing with a cylinder within and piston operating in the cylinder. Casing and cylinder are filled with oil. When the cat strikes the buffer and the piston descends, oil is forced out of the cylinder through holes, so arranged in number and position that the resist ance offered by the fluid is just sufficient to bring the car to an easy stop within the stroke of the buffer. The piston is returned to the upper position by a spring. Consult Baxter. William, 'Hydraulic Elevators' ; Hymans, 'Elevators in the Oliver Building' (in Electric Journal 1911) ; Bethman, H., 'Der Aufzugbau'; Ernst, Adolf, 'Die Lindquist, D., 'Modern Electric Elevators and Elevator Prob lems' (in Transactions of the American Society of Mechanical Engineers, Vol. XXXVII) ; id, The Micro-drive Machine' (1917).