Elevators, beginning in the nineteenth century and continuing to the present, have evolved from primitive platforms that lifted freight in factories and mines, to powerful multi-car systems that convey materials and human passengers to offices and residences approaching a mile above street level. There is every reason to believe that they will continue to grow. A space elevator, shown in Figure 1-7, has even been envisioned. It would transport freight and humans from the surface of the earth to geocentric orbit and beyond.
FIGURE 1-7 Idealized diagram of a space elevator, not to scale. (Wikipedia)
A cable or tether would be anchored to the earth and extend into space. Vehicles would climb this cable, lifting passengers and freight to orbiting space stations without benefit of huge shuttles. The cable would extend well beyond the system’s center of mass, to a heavy counterweight, which due to its position well beyond the geostationary orbit, would exert a powerful counter-gravitational force, pulling the cable tight.
A continuous stream of climbers (not human, but mechanical) could ascend the cable, passing others returning in the opposite direction to earth. Escape velocity would not have to be attained. It would be a relatively slow journey, requiring about a week. Konstantin Tsiolkovsky conceived the space elevator in 1895, near the end of the century that defined elevator technology. Rather than a cable under tension because it was hanging from the counterweight, it was a tower, sitting on the ground and subject to great compressive force.
There are some major difficulties in building the space elevator, primarily having to do with fabricating the cable, which has to support its own enormous weight without breaking. As of now, no known material can do that. But carbon nanotubes should work. Development costs are high, but once built, the space elevator will transport passengers and freight quite economically. Space elevators will eventually be constructed on other planets, moons, and asteroids. For these lighter bodies, the challenges are less formidable but the rewards are not as great.
Lighter materials such as Kevlar would be suitable for constructing extraterrestrial cables. In 1975, Jerome Pearson introduced the idea of a tapered cable. Maximum tension on a space elevator cable would be at geosynchronous altitude, so the cable would have to be thickest there and taper carefully as it approaches earth. The concept of a space elevator became more realistic after the development in 1990 of carbon nanotubes. In 2000, Bradley Edwards proposed a flat ribbon rather than the round cable of previous designs, because it would be less vulnerable to damage from meteors and space debris. Moreover, climbers could use rollers to travel upward. Since then, numerous feasibility studies have concluded that the space elevator is a valid concept, and it will profoundly affect human history as we continue on our greatest journey.
1. Who made the first extant reference to an elevator?
A. Archimedes
B. Vitruvius
C. Plato
D. Aristotle
2. Early nineteenth century elevators:
A. were powered by work horses
B. were steam-powered
C. were powered by electric motors
D. rose to a height of 20 stories
3. William Strutt’s elevator:
A. was in a coal mine
B. had no pulleys
C. ran off a flat belt
D. carried passengers and freight
4. Hotels and office buildings used elevators beginning around:
A. 1875
B. 1865
C. 1855
D. 1845
5. Henry Waterman’s elevator in Manhattan:
A. did not require an outside attendant
B. was powered by an electric motor
C. was a hydraulic elevator
D. had a clutch that disengaged when the operator released the handle
6. In George Fox and Co.’s freight elevators:
A. there were frequent mechanical failures
B. meshing spur gears with a worm gear became obsolete
C. a separate brake for the hoist was required
D. wire rope replaced hemp rope
7. Elisha Graves Otis:
A. was enormously successful financially
B. pioneered the use of safeties
C. specialized in hydraulic elevators
D. built traction engines throughout the United States
8. In a hydraulic elevator installation:
A. if bedrock was encountered, a hybrid design was needed
B. cylinders could be installed vertically only
C. noise was a severe problem
D. complex rope and pulley assemblies were required
9. Nineteenth century hydraulic elevators:
A. used no coal
B. used oil for hydraulic fluid
C. rose to unprecedented heights
D. would free fall if the piston failed
10. Electric motors replaced steam in elevators:
A. before the American Civil War
B. in the 1920s
C. after 1900
D. beginning around 1880
For answers, go to Appendix A.
Virtually all modern elevators fall into one of three categories, with some exceptions, variations, and models that combine elements of two or even all three types. These types are traction elevators, hydraulic elevators, and machine room-less elevators.
Traction elevators are the most common type. They are characterized by multi-strand steel “ropes” that in a typical design are attached to a hitch plate at the top of the car. There may be six or more of these cables, each capable of lifting the car and occupants. The cables pass over and are driven by a deeply slotted sheave, two or more feet in diameter, at the top of the shaft, with the other ends attached to a counterweight. (In one design, two cars move synchronously in opposite directions, each functioning as the other’s counterweight.)
Instead of traditional ropes, some manufacturers, notably Otis, Schindler, and Kone, have introduced very light steel belts, with carbon fiber cores and high-grip coating. Lubricant is not necessary, and energy consumption is reduced, especially in high-rise applications.
Traction elevators, shown in Figure