Elevators

views updated

ELEVATORS

ELEVATORS. Primitive elevators were first used in ancient Rome. These grew out of the rope and pulley and used a platform instead of a hook or net to raise cargo. The development of the modern elevator began with the industrial revolution and the use of the steam engine to power the mills. Steam-powered machines—used for spinning, weaving, or metalworking—were not individually powered. They were driven by a central steam engine through line shafting consisting of long lines of rotating shafts located overhead. At each machine location, a belt and pulley transmitted power to the machine. Most mills at the time were multistoried, requiring a hoisting machine to move material between floors. The central steam engine powered this machine, as it did all other machines in the mill. The operator pulled a rope that controlled the rotation of the hoisting machine, giving the operator control over starting and stopping. Later, when elevators were installed in locations other than mills, it became apparent that a separate steam engine would have to be installed for the elevator.

Almost all hoists or elevators functioned by winding a rope on a drum to raise the platform. Because of the ever-present danger of rope breakage, however, they were not used to carry passengers. Elisha Otis (1811–1861), a mechanic in a mattress factory in Yonkers, New York, pioneered

the technology of elevator safety and paved the way for the modern passenger elevator. During the second year of the New York Crystal Palace exhibition in 1854, he gave dramatic demonstrations of a safety device that would grip the elevator guide rails if the hoist ropes parted. Showman as well as inventor, Otis would have himself raised on the elevator and then direct an assistant to cut the hoist rope with an ax. The safety always worked. In 1857 Otis built the first elevator exclusively for passengers and installed it in a china and glass store in New York City.

Use of the drum machine for elevators was restricted to relatively low-rise buildings because the length and weight of the drum imposed a severe restriction on the building structure and on the height that the elevator could travel. By 1870 a rope-geared hydraulic system was developed whereby a piston acting through a system of sheaves (complex pulleys) raised and lowered the elevator. One foot of motion of the piston could cause the elevator to move two, three, or even ten feet, depending on the alignment of the sheave and rope system. This was a major design improvement and eliminated the winding drum with its inherent limitations.

By 1880, both these systems were in general use. In some locations, they could use city water pressure, obviating the need for a steam engine to drive the necessary water pump. At about this time, engineers also developed a plunger hydraulic elevator, which was practical except in tall buildings, where it suffered from the same drawbacks as the drum machine. This elevator required that a hole be drilled in the ground to the same depth as the height of the building in order to accommodate the hydraulic cylinder. At first, the development of the electric motor had only a minor effect on the operation of elevators when it was used to replace the steam engine that powered the pumps used on hydraulic systems. Otis then designed a drum machine that used an electric motor to drive the drum, and the first such electric-powered elevator was installed (1889) in the Demarest Building in New York City.

The major breakthrough in elevator design and the beginning of the modern high-speed elevator occurred about 1900 with the development of the traction machine. The traction machine depends on friction between the driving sheave or pulley and the hoist ropes (or, more frequently as technology improved, metal cables). With the traction machine, elevators can serve buildings of any height and are limited in speed only by the height of the building. Prior to 1900, all elevator machines, whether steam or electric powered, transmitted the driving power through a train of gears to the driving sheave or drum. This method is suitable for moderate elevator speeds under 500 feet per minute. At higher speeds, gear wear and gear noise make the application impractical. In 1904 the Otis Company installed the first gearless traction machine in Chicago, which was to make high elevator speeds practical. In this machine, the drive sheave is mounted directly on the motor shaft and there are no gears. The result is a quiet and smooth-running elevator, even at speeds of 2,000 feet per minute.

After the gearless traction machine, the major developments in elevator engineering were control refinements in order to provide a smooth ride at high speeds. Further control developments were made during the 1920s and 1930s by Otis, Westing house, and other companies to minimize the skills needed by the elevator operator. These systems provided automatic landing of the elevators at floor level and automatic floor selection by pressing buttons for the desired stop. Completely automatic elevators that did not require the presence of an operator were developed in the 1920s, but did not come into general use until 1950. By 1960 the installation of any elevator requiring an operator was a rarity.