BLASTING WITH WATER
A novel use of water pressure has been developed in England. In certain mines it is dangerous to use dynamite for blasting purposes owing to the presence of explosive gases, and successful experiments have been made with hydraulic cartridges. This consists of a cylinder of steel fitted with a series of little plungers arranged in a row in the cylindrical wall of the cartridge. As in powder blasting, a series of holes are drilled in the face of the rock and the cartridges with the plungers retracted are fitted into the holes. Then the cartridges are connected to a high-pressure water supply. The water forces the plungers out, exerting enough pressure to burst the rock. Not only is this system perfectly safe, but it is economical, because the gallery does not have to be cleared of workmen before every blast. There is more certainty in the use of water cartridges, and the danger, common where dynamite is used, of having the rock drill or pick strike and explode a stick of dynamite which failed to go off with the rest of the charge in a previous blast is avoided.
Hydraulic pressure is also used in jacks for lifting heavy weights. The principle of the hydraulic jack is the same as that of the hydraulic press.
RAISING A BRIDGE SPAN WITH WATER
An interesting illustration of the use of these liquid levers was afforded in the construction of the Quebec Bridge. This huge bridge, it will be recalled, consists of two cantilevers which stretch out from opposite shores of the St. Lawrence River and support between them a center span 640 feet long and weighing 5,400 tons. The span was built on barges, towed down to position between the cantilever arms and then lifted up 150 feet to the floor level of the bridge. Eight 1,000-ton hydraulic lifting jacks were used, two at each corner of the span. They were ideally suited to this kind of work. The bridge span had to be lifted with utmost care and the motion had to be simultaneous on all four corners. If one corner were raised faster than another the span would be twisted and subjected to serious strains. In the first attempt the fastening gave way at one corner and the span crumpled up and plunged down into the river. But a year later at the second attempt the span was hoisted successfully to position. The jacks had a lift of two feet, and, counting the time required to secure the huge plate chains by which the span was suspended, move the jacks down and give them a fresh hold, it took fourteen minutes to complete each two-foot lift. The work was done only during the daylight hours and on the third day the span was finally brought into position and made fast to the cantilever arms by means of twelve-inch pins driven home at each corner.
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.
HYDRAULIC ELEVATORS
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.