The section of the Tower presenting the least difficulty to elevator
installation was that above the juncture of the four legs--from the second
platform to the third, or observation, enclosure. There was no question
that French equipment could perform this service. The run being perfectly
straight and vertical, the only unusual demand upon contemporary elevator
technology was the length of rise--525 feet.
The system ultimately selected (fig. 37) appealed to the Commission
largely because of a similar one that had been installed in one tower of
the famous Trocadero[13] and which had been operating successfully for 10
years. It was the direct plunger system of Leon Edoux, and was, for the
time, far more rationally contrived than Backmann's helicoidal system.
Edoux, an old schoolmate of Eiffel's, had built thousands of elevators in
France and was possibly the country's most successful inventor and
manufacturer in the field. It is likely that he did not attempt to obtain
the contract for the elevator equipment in the Tower legs, as his
experience was based almost entirely on plunger systems, a type, as we
have seen, not readily adaptable to that situation. What is puzzling was
the failure of the Commission's members to recognize sooner Edoux's
obvious ability to provide equipment for the upper run. It may have been
due to their inexplicable confidence in Backmann.
[Illustration: Figure 31.--The French Girard pumps that supplied the Otis
and Roux systems. (From _La Nature_, Oct. 5, 1889, vol. 17, p. 292.)]
The direct plunger elevator was the only type in which European practice
was in advance of American practice at this time. Not until the beginning
of the 20th century, when hydraulic systems were forced into competition
with electrical systems, was the direct plunger elevator improved in
America to the extent of being practically capable of high rises and
speeds. Another reason for its early disfavor in the United States was the
necessity for drilling an expensive plunger well equal in length to the
rise.[14]
As mentioned, the most serious problem confronting Edoux was the extremely
high rise of 525 feet. The Trocadero elevator, then the highest plunger
machine in the world, traveled only about 230 feet. A secondary
difficulty was the esthetic undesirability of permitting a plunger
cylinder to project downward a distance equal to such a rise, which would
have carried it directly into the center of the open area beneath the
first platform (fig. 6). Both problems were met by an ingenious
modification of the basic system. The run was divided into two equal
sections, each of 262 feet, and two cars were used. One operated from the
bottom of the run at the second platform level to an intermediate platform
half-way up, while the other operated from this point to the observation
platform near the top of the Tower. The two sections were of course
parallel, but offset. A central guide, on the Tower's center-line, running
the entire 525 feet served both cars, with shorter guides on either
side--one for the upper and one for the lower run. Thus, each car traveled
only half the total distance. The two cars were connected, as in the
Backmann system, by steel cables running over sheaves at the top,
balancing each other and eliminating the need for counterweights. Two
driving rams were used. By being placed beneath the upper car, their
cylinders extended downward only the 262 feet to the second platform and
so did not project beyond the confines of the system itself.[15] In making
the upward or downward trip, the passengers had to change from one car to
the other at the intermediate platform, where the two met and parted (fig.
39). This transfer was the only undesirable feature of what was, on the
whole, a thoroughly efficient and well designed work of elevator
engineering.
[Illustration: Figure 32.--The Otis distributor, with valves shown in
motionless, neutral position. Since the main valve at all times was
subjected to the full operating pressure, it was necessary to drive this
valve with a servo piston. The control cable operated only the servo
piston's valve. (Adapted from Gustave Eiffel, _La Tour de Trois Cents
Metres_, Paris, 1900, p. 130.)]
[Illustration: Figure 33.--General arrangement of the Roux Combaluzier and
Lepape elevator.]
[Illustration: Figure 34.--Roux, Combaluzier and Lepape machinery and
cabin at the Tower's base. (From _La Nature_, Aug. 10, 1889, vol. 17, p.
168.)]
In operation, water was admitted to the two cylinders from a tank on the
third platform. The resultant hydraulic head was sufficient to force out
the rams and raise the upper car. As the rams and car rose, the rising
water level in the cylinders caused a progressive reduction of the
available head. This negative effect was further heightened by the fact
that, as the rams moved upward, less and less of their length was
buoyed by the water within the cylinders, increasing their effective
weight. These two factors were, however, exactly compensated for by the
lengthening of the cables on the other side of the pulleys as the lower
car descended. Perfect balance of the system's dead load for any position
of the cabins was, therefore, a quality inherent in its design. However,
there were two extreme conditions of live loading which required
consideration: the lower car full and the upper empty, or vice versa. To
permit the upper car to descend under the first condition, the plungers
were made sufficiently heavy, by the addition of cast iron at their lower
ends, to overbalance the weight of a capacity load in the lower car. The
second condition demanded simply that the system be powerful enough to
lift the unbalanced weight of the plungers plus the weight of passengers
in the upper car.
As in the other systems, safety was a matter of prime importance. In this
case, the element of risk lay in the possibility of the suspended car
falling. The upper car, resting on the rams, was virtually free of such
danger. Here again the influence of Backmann was felt--a brake of his
design was applied (fig. 38). It was, true to form, a throwback, similar
safety devices having proven unsuccessful much earlier. Attached to the
lower car were two helically threaded vertical rollers, working within
the hollow guides. Corresponding helical ribs in the guides rotated the
rollers as the car moved. If the car speed exceeded a set limit, the
increased resistance offered by the apparatus drove the rollers up into
friction cups, slowing or stopping the car.
[Illustration: Figure 35.--Detail of links in the Roux system. (From
Gustave Eiffel, _La Tour de Trois Cents Metres_, Paris, 1900, p. 156.)]
[Illustration: Figure 36.--Section of guide trunks in the Roux system.
(From Gustave Eiffel, _La Tour de Trois Cents Metres_, Paris, 1900, p.
156.)]
The device was considered ineffectual by Edoux and Eiffel, who were aware
that the ultimate safety of the system resulted from the use of supporting
cables far heavier than necessary. There were four such cables, with a
total sectional area of 15.5 square inches. The total maximum load to
which the cables might be subjected was about 47,000 pounds, producing a
stress of about 3,000 pounds per square inch compared to a breaking stress
of 140,000 pounds per square inch--a safety factor of 46![16]
[Illustration: Figure 37.--Schematic diagram of the Edoux system. (Adapted
from Gustave Eiffel, _La Tour de Trois Cents Metres_, Paris, 1900, p.
175.)]
[Illustration: Figure 38.--Vertical section through lower (suspended)
Edoux car, showing Backmann helicoidal safety brake. (Adapted from Gustave
Eiffel, _La Tour Eiffel en 1900_, Paris, 1902, p. 12.)]
A curiosity in connection with the Edoux system was the use of Worthington
(American) pumps (fig. 40) to carry the water exhausted from the cylinders
back to the supply tanks. No record has been found that might explain why
this particular exception was made to the "foreign materials" stipulation.
This exception is even more strange in view of Otis' futile request for
the same pumps and the fact that any number of native machines must have
been available. It is possible that Edoux's personal influence was
sufficient to overcome the authority of the regulation.
[Illustration: Figure 39.--Passengers changing cars on Edoux elevator at
intermediate platform. (From _La Nature_, May 4, 1889, vol. 17, p. 361.)]
[Illustration: Figure 40.--Worthington tandem compound steam pumps, at
base of the Tower's south pier, supplied water for the Edoux system. The
tank was at 896 feet, but suction was taken from the top of the cylinders
at 643 feet; therefore, the pumps worked against a head of only about 250
feet. (From _La Nature_, Oct. 5, 1889, vol. 17, p. 293.)]
[Illustration: Figure 41.--Recent view of lower car of the Edoux system,
showing slotted cylindrical guides that enclose the cables.]