was not required for the hoist, which would hold its position when the driving belt was disengaged. This arrangement meant less chance of a car and occupants falling to the bottom of the elevator shaft due to mechanical failure in the drive system.
Safety was further enhanced by other innovations by Fox and Co. One was the replacement in 1852 of traditional hemp rope by stronger and more wear-resistant steel wire rope. The other innovation was a safety brake, which could stop the car from free falling in the event of rope failure. This brake, however, was not automatic and depended upon quick action by an alert operator.
Falling cars were still a severe hazard, but after 1850 new developments in elevator technology greatly reduced the number of occurrences.
In the mid-nineteenth century, William Adams and Co. manufactured freight elevators in Boston. In 1859, one of their freight platforms in a group installation dropped to the bottom of its shaft. An engineer for the firm, inspecting the damage, found that it was not as severe as might be anticipated. He concluded that the hoistway, as built, happened to be relatively airtight, and as a result, the air as it was compressed below the falling platform acted as a cushion and slowed its fall. This suggested a way to mitigate these disasters, and in fact the idea was patented and hardware developed and marketed.
Elisha Graves Otis Invents Safeties
Another very active key figure in the evolving elevator industry in mid-nineteenth century America was Elisha Graves Otis (1811–1861). His Improved Elevator of 1854 incorporated an automatic safety mechanism, which in the event of rope failure as shown in Figure 1-3, would activate automatically.
All elevators, of course, had guide rails, which were necessary to prevent the suspended car from swinging from side to side, striking the hoistway walls. The Otis Improved Elevator was a variation on existing rack-and-pinion drives, in which the rack was attached to the guide rails. In the new design, the teeth curved upward rather than extending perpendicular from the rack. The brake, relocated below the cross beam at the top of the platform or car, consisted of safety dogs connected to a spring and the hoisting rope. Because the rope, as long as it remained intact, supported the freight platform or passenger car, the spring remained compressed and held the safety dogs away from the rack and the elevator functioned as expected. In the event of a break in the rope or if for any reason it lost tension, the safety dogs would engage the upward angled rack teeth, preventing the car or platform from falling.
Otis was an accomplished mechanic and very inventive builder of elevators, always sensitive to safety issues. However, on the financial side his business failed to prosper despite the success of his Improved Elevator with its advanced safety mechanism. Beginning around 1860, nearly all traction elevators incorporated his braking system in one form or another.
FIGURE 1-3 Elisha Graves Otis cuts the hoisting rope, demonstrating his new safeties, still used, that prevent an elevator car from free falling. (Wikipedia)
Just three months after receiving his patent, Elisha Otis died of natural causes. His business flourished under the ownership of his sons, who reconstituted the firm as N.P. Otis and Brother. The company prospered under the inventive and financial skills of Norton and Charles Otis. They quickly adapted to the new post-Civil War environment, in which the focus now included passenger elevators built for the new generation of higher-rise hotels, shops, and office buildings.
At about the same time that these developments in traction elevator safety and reliability were occurring, in England and continental Europe as well as in the U.S., hydraulic elevators were emerging in low-rise applications. Here we are talking about water pistons, as opposed to the hydraulic oil machines of today. Typically, the water supply was from a high-capacity pump system or reservoir. The water pressure would cause the car to rise to the top floor or as high as required. Then, a discharge valve permitted the car to descend at a measured pace due to its own weight.
Hydraulic elevators had some intrinsic advantages in low-rise applications. Those running off a natural or impounded reservoir had no further fuel costs, and unlike steam power, there were not the tasks of moving in coal and disposing of ashes. They were simple and quiet. In the event of piston failure, the car or platform would not free fall, its speed of descent regulated by the size of the rupture.
Bedrock or a high water table could make for a difficult installation. Builders of hydraulic elevators could then, however, resort to hybrid designs, standing the cylinders vertically above grade outside the buildings or laying them down horizontally. These installations required additional wire rope and pulley mechanisms, compromising the advantages of simplicity and safety.
Just as the nineteenth century was a time in which elevators evolved from primitive lifts to becoming a defining fact in the great cities of America and Europe, so in the ninth decade of that century did the electric motor assume new forms, enabling it to replace coal-burning steam power.
Throughout the 1870s, hydraulic (water) elevators were installed in great numbers. Drive configurations and structural variations proliferated as did the number of manufacturers building them. Additionally, there were many exclusively wire-rope machines being built and installed, with great innovations that made them safer and more efficient. Still, steam power, which was noisy, hot, and required frequent human intervention, powered most elevator installations.
Then, beginning around 1880, the DC electric motor changed everything.
The first electrical distribution system was Thomas Edison’s 110-volt DC utility in lower Manhattan, intended for indoor residential and commercial use. It was energized in 1882, followed four years later when George Westinghouse began building an AC system, enabling the use of transformers to increase the voltage for efficient transmission and lower it for users. AC eventually eclipsed DC, but meanwhile Edison commenced large-scale DC motor production and for many decades these motors remained in use in many applications for which they were better suited than AC motors, notably in elevators.
DC motors could be run off an AC power supply by means of a simple motor-generator set, often in a single enclosure with no exterior shafts, and later by tube-type and inexpensive solid-state diode rectification. The reason a DC motor was at the time preferable to an AC motor was that, although both could be reversed, the speed of an AC motor could not be easily varied, as required to operate an elevator. In contrast, DC motor speed is varied simply by adjusting the voltage applied to the armature or current applied to the field circuit.
Nikola Tesla, working with George Westinghouse, developed three-phase AC power distribution and he invented the highly efficient and maintenance free three-phase induction motor, shown in Figure 1-4, which quickly permeated industrial facilities worldwide. But since it was essentially a single speed device, it was not suitable for elevator power until the 1960s, when the variable frequency drive (VFD) was introduced. This consisted of electronic circuitry that permitted users to run AC induction motors at lower (or even higher) than rated speed by means of pulse-width modulation (PWM), which we will discuss in detail in Chapter 3.
FIGURE 1-4 Tesla’s AC induction motor was not suitable for elevator use until the 1960s, when the variable frequency drive made speed control possible. (Wikipedia)
When electric motors were first suggested for elevator power, the public was skeptical. There had been a number of power line fatalities as new distribution systems were being constructed,