According to the foregoing there are, there fore, four general classes of elevators — belt, steam, hydraulic and electric.
This class of ele vator, Fig. 1, is usually installed for slow-speed freight service in factories, is reliable and vet low in cost. The speed seldom exceeds 75 feet per minute. The machine is usually bolted to the ceiling of one of the floors. The middle one of the three flat-faced pulleys shown in the illustration is tight on the shaft and is adapted to actuate the hoisting drum through the intermediary of a worm-and-worm gear. The two outer pulleys are loose on the shaft and are belted, one by straight belt and the other by crossed belt to a line shaft pulley. To operate the elevator in the one or other direc tion the straight or crossed belt is shifted onto the tight middle pulley. The machines are pro vided with brake, slack cable device and auto matic stop at terminal landings.
Steam Steam machines for elevator service may be dismissed with the ref erence that they are no longer in use. This has been due to the very large consumption of power with no compensation for the disad vantage in the matter of ease of control.
Hydraulic The hydraulic ele vator installed in large numbers up to about the year 1900 is the •so-called vertical hydraulic type (Fig. 2). In this type a cylinder of a diameter of 8 to 24 inches is placed in a vertical position in the elevator shaft or in any other convenient location. Within this cylinder works a piston. The pull exerted thereon by the water pressure is transmitted through the piston rods to a number of sheaves, which in turn operate on the hoisting ropes. The sheaves introduce a gear ratio varying from 2:1 to 12:1 between the car and piston travel. The ele vator is controlled by a lever placed in the car, which actuates the operating valve. For the ascent, the valve admits water to act on the piston, at the same time discharging the water underneath. The descent of the car occurs by reason of its unbalanced weight, the water above the piston being allowed to flow through a circulating pipe to the space underneath. About the year 1900 the plunger type of hy draulic elevator (Fig. 3), (before that date applied only to low rises) began to be intro duced for high speed passenger service. In this type a cylinder of a length equal to the car travel is set vertically in the ground. In this cylinder works a piston or plunger of the same length, carrying the car on its top. The weight of car and plunger is partially counter balanced by a weight attached to the car f rame. Although this type of elevator had a larger power consumption and was more difficult to control than the vertical geared elevator, it succeeded in driving the latter from the field.
This was due to the popular belief in the greater safety of the plunger elevator, since the load to all outward appearance was sup boned on a steel column and not suspended from ropes as in other types. As a matter of fact the plunger consisted of ordinary com mercial tubing, finished to size and screwed together. That columns of this nature and of lengths up to about 300 feet could carry loads without buckling was due to the counterbal ance, which for high rises exceeds the weight of live load and car. In reality therefore part of the plunger was in tension, and the load, in stead of being supported on a steel column, was again suspended from ropes overhead. As the plunger rose out of tie cylinder, its buoyancy decreased.
To compensate, there fore, the weight of the counterweight ropes per foot is made equal to half of the weight of water dis placed by one foot of plunger. In a high rise plunger elevator the masses to be set in motion and stop ped are quite consid erable. For a rise of 200 feet, for example, the plunger may weigh 4,000 pounds, the car 4,000 pounds, the ropes 2,000 p o.0 n d s, counter weight, 4,000 pounds, live load, 2,500 pounds. For this reason, the control of the elevator at high speeds became diffi cult, notwithstanding the allowance of a liberal amount of un derbalance and sur plus pressure. The control of the elevator is effected by means of a lever actuating an oper ating valve (Fig. 4), which at the will of the operator permits water to flow into the cylinder for the ascent, or out of the cylinder for the descent In addition, two automatic top valves (Fig. 5) are furnished, automatically to stop the elevator at the limits of its travel. Another type of hydraulic elevator machines, which may be dismissed with simple reference, is the horizontal hydraulic machine. It consists of a cylinder set horizontally. As in the vertical geared machine, the piston oper ates a number of multiplying sheaves. This type is distinguished as of the pushing or pulling type depending on whether the piston rod is in compression or tension. The desired water pressure is generally obtained by steam or electncally driven pumps. On low pres sure systems of about 150 pounds per square inch, the pumps deliver the water into a pres sure tank, which absorbs the pump pulsations and serves as a storage of power. In high pressure systems, generally of about 800 pounds per square inch, the pumps deliver the water into a weighted accumulator. The water dis charged by the machines is collected in a dis charge tank from which again the pumps are supplied.