The progression from an elevator machine powered by the line shafting of a
mill to one in which the power source was independent would appear a
simple and direct one. Nevertheless, it was about 40 years after the
introduction of the powered elevator before it became common to couple
elevator machines directly to separate engines. The multiple belt and
pulley transmission system was at first retained, but it soon became
evident that a more satisfactory service resulted from stopping and
reversing the engine itself, using a single fixed belt to connect the
engine and winding mechanism. Interestingly, the same pattern was followed
40 years later when the first attempts were made to apply the electric
motor to elevator drive.
[Illustration: Figure 8.--In the typical steam elevator machine two
vertical cylinders were situated either above or below the crankshaft, and
a small pulley was keyed to the crankshaft. In a light-duty machine, the
power was transmitted by flatbelt from the small pulley to a larger one
mounted directly on the drum. In heavy-duty machines, spur gearing was
interposed between the large secondary pulley and the winding drum. (Photo
courtesy of Otis Elevator Company.)]
[Illustration: Figure 9.--Several manufacturers built steam machines in
which a gear on the drum shaft meshed directly with a worm on the
crankshaft. This arrangement eliminated the belt, and, since the drum
could not drive the engine through the worm gearing, no brake was
necessary for holding the load. (Courtesy of Otis Elevator Company.)]
By 1870 the steam elevator machine had attained its ultimate form, which,
except for a number of minor refinements, was to remain unchanged until
the type became completely obsolete toward the end of the century.
By the last quarter of the century, a continuous series of improvements in
the valving, control systems, and safety features of the steam machine had
made possible an elevator able to compete with the subsequently appearing
hydraulic systems for freight and low-rise passenger service insofar as
smoothness, control, and lifting power were concerned. However, steam
machinery began to fail in this competition as the increasing height of
buildings rapidly extended the demands of speed and length of rise.
The limitation in rise constituted the most serious shortcoming of the
steam elevator (figs. 8-10), an inherent defect that did not exist in the
various hydraulic systems.
[Illustration: Figure 10.--Components of the steam passenger elevator at
the time of its peak development and use (1876). (From _The First One
Hundred Years_, Otis Elevator Company, 1953.)]
Since the only practical way in which the power of a steam engine could be
applied to the haulage of elevator cables was through a rotational system,
the cables invariably were wound on a drum. The travel or rise of the car
was therefore limited by the cable capacity of the winding drum. As
building heights increased, drums became necessarily longer and larger
until they grew so cumbersome as to impose a serious limitation upon
further upward growth. A drum machine rarely could be used for a lift of
more than 150 feet.[5]
Another organic difficulty existing in drum machines was the dangerous
possibility of the car--or the counterweight, whose cables often wound on
the drum--being drawn past the normal top limit and into the upper
supporting works. Only safety stops could prevent such an occurrence if
the operator failed to stop the car at the top or bottom of the shaft, and
even these were not always effective. Hydraulic machines were not
susceptible to this danger, the piston or plunger being arrested by the
ends of the cylinder at the extremes of travel.