Hydraulic power is used very largely in the operation of cranes. As the plunger or ram has a very limited range of motion some means of multiplying distance of travel is required. One simple scheme is to use a set of pulley wheels or sheaves attached to the cylinder and another set to the ram and pass the hoisting chain around them. If we refer back to Figure 16, on page 34, we shall see that seven feet of rope must be pulled in at E in order to raise the lower pulley block one foot. It is very evident that the process can be reversed. Power might be applied to spread the two-pulley blocks apart when a movement of one foot would produce a travel of seven feet at E. That is what is done on the hydraulic crane. The ram is lifted, spreading the pulley blocks apart and thus multiplying the motion of the lifting cable to any extent, depending upon the number of sheaves, so that a travel of but a few feet will result in lifting a load forty or fifty feet.
The same principle is used in hydraulic elevators. The hydraulic cylinder lies horizontally on the basement floor and by means of pulley gearing a short motion of the plunger is sufficient to raise the car several stories.
In a more modern type of elevator the plunger acts directly on the car. The plunger is long enough to reach to the top story of the building which means that it must sink far into the ground in order to let the car down to the first story or basement. A steel pipe is sunk into the ground and serves as the cylinder and in this the long plunger operates. In one of New York’s tall buildings the cars are lifted to a height of 282 feet. The plungers are 6½ inches in diameter and they travel at a speed of over 400 feet per minute. The car which, when loaded, weighs 1,617 pounds, is supported on the top of the plunger. However, counterweights are provided which balance the weight of the car, so that practically the only weight lifted by the plunger is that of the passengers. The advantage of this type of elevator is that it reduces the danger of accident. The motion of the car is steady and easily controlled. The car cannot move down above a given speed and the only possible danger is that the plunger might break. However, a shaft of steel 6½ inches in diameter, even if hollow, is hardly likely to give way even under the most extraordinary loads to which it might be subjected in elevator service.