Hydraulics is the science related to the behavior of fluids, whether static or dynamic, and the practical uses derived there from. A surprising amount of our current technology is based on hydraulic principles, especially that used in our larger machinery, from automobiles on up. One underlying principle of hydraulics is that many fluids, including water, are incompressible; that is, once any trapped air is removed, a given volume of the fluid can't be physically squeezed into a smaller space. The molecules of the fluid have already aligned themselves in such a way as to take up the smallest volume they can in that physical state.
Hydraulic machines take advantage of one consequence of this principal: the fact that any force applied to an incompressible liquid will propagate through the liquid, without significant loss of strength, to effect whatever's on the other side. To put it simply, the force is transmitted through the fluid from one point to another. This makes for a particularly clever way of doing work at a distance. You press on a column of incompressible liquid here, and it makes something move over there. "Over there" might be your car's braking system, the arm of a backhoe, or the boom of a crane capable of lifting a railroad engine. The purpose of any hydraulic system is to acquire mechanical advantage, and careful juggling of the mechanisms involved in manipulating the hydraulic fluid can increase the initial force significantly. This is known as force multiplication, a common characteristic of machines, right down to the simplest: levers, gears, and the like.
Most hydraulic machines make use of non-corrosive oils as hydraulic fluids. In order for a hydraulic machine to operate at maximum efficiency, it's important that no air bubbles are trapped in the fluid; otherwise some of the input force is wasted squeezing air, which is very compressible. Thus, it's sometimes necessary to bleed the air out of the fluid reservoir. This is often done with car brake systems.
Hydraulic cranes make use of the incompressibility affect to move enormous amounts of weight, up to hundreds of tons. Also called hydraulic truck cranes, because of the truck chassis the crane is mounted on, these monster machines consist of a number of important parts: the boom, the jib, the rotex gear, the outriggers, the counterweights, reinforced-steel cables, and the hook. The boom is the vertical arm of the crane, which allows it to raise its loads; these usually have several telescoping sections to allow them to extend to a greater height. Some cranes also have a jib, a latticework structure at the end of the boom that extends horizontally. The rotex gear is a large gear connecting the cab and boom of the crane to its deck; it turns on a protected turntable bearing so the boom can move from side to side. Outriggers are legs with wide disc-shaped metal pads, or "feet," that extend out to the sides of the crane and brace the crane against the ground, securing it in place. Counterweights are large weights that can be placed on the back of the crane, under the cab, in order to redistribute the load stress and keep the machine from tipping over; the number of counterweights needed depends on the weight of the load. Finally, there's the hook, the huge metal ball-and-hook combo that actually attaches to the load, and the massive reinforced-steel cable system that supports the load between hook and boom. The cable system attaches to a winch just behind the operator's cab. Each cable can support up to 14,000 pounds (7 tons), so if the crane needs to handle larger weights than that, it needs to use multiple cables. A 140-ton crane, for example, would need 20 cables.
Hydraulic truck cranes may use two types of hydraulic pumps to leverage the incompressibility effect: gear pumps, and variable-displacement pumps. Both types are driven by powerful diesel engines. A gear pump pressurizes the hydraulic oil using meshed gears; most hydraulic cranes use two-gear pumps for various purposes, including raising and lowering the boom, to move on the rotex gear, to operate the machine that adds and removes counterweights, and to manipulate the outriggers. Variable-displacement pumps use a much more sophisticated method involving a ring of pistons inside a barrel; due to the design of the system, the hydraulic fluid can be pumped with great force. The power of this type of pump depends on how fast the engine is running; the faster, the more powerful it is. Most backhoes use this type of hydraulic pump.
Surprisingly, the cab of a hydraulic crane is relatively uncluttered. When the operator sits down inside, he controls the crane with two jumbo-sized joysticks, several foot pedals, and other small controls (such as the winch controller). The joysticks control the movement of the boom; the foot pedals control hydraulic pump pressure, and let him control the telescoping sections of the boom. Both types of controls are connected to sets of hydraulic hoses that connect hydraulic rams (large pistons) to mechanisms called spool valves. Another hose connects the valve separately to the hydraulic pump. The joystick or pedal controls which hose is feeding hydraulic fluid to the ram, making the ram slide in or out. Another important instrument found in the cab is the Load Moment Indicator, a string of warning lights that let the operator know when he is lifting the load too high. The operator himself is responsible for determining the angle of lift and the radius of the boom, and inputting this data into the crane's computer system so that it knows when to set off the Load Moment Indicator.
The operator isn't alone in this operation; he depends on the oiler, who makes sure that the crane's parts are all in place and properly secured, and the signalman, who uses hand signals to help the operator keep the load steady and going the right way. The oiler also acts as a spotter, keeping an eye on the lift. Together, these three individuals and the machine they control provide brute strength from some of the heaviest lifting jobs on the planet. The Greek philosopher Archimedes once said, "Give me a lever long enough and a place to stand and I can move the world." With a hydraulic crane, we can do him one better: given a large enough crane and enough counterweights, we can lift it.
