Most hydraulic machines use some sort of incompressible fluid, a fluid that is at its maximum density. Oil is the most commonly used incompressible fluid for hydraulic machines, including hydraulic cranes. In a simple hydraulic system, when a piston pushes down on the oil, the oil transmits all of the original force to another piston, which is driven up.
In a simple hydraulic system, when one piston is pushed down, another piston is pushed up. Click on the arrow for a demo.
A hydraulic pump creates the pressure that moves the pistons. Pressure in a hydraulic system is created by one of two types of hydraulic pumps:
Variable-displacement pump – Click here to learn more about variable-displacement pumps.
Most hydraulic truck cranes use two-gear pumps that have a pair of inter-meshing gears to pressurize the hydraulic oil. When pressure needs to increase, the operator pushes the foot throttle to run the pump faster. In a gear pump, the only way to get high pressure is to run the engine at full power.
A 70-ton hydraulic truck crane uses a 12.7-L diesel engine that generates up to 365 horsepower. The engine is connected to three two-gear pumps, including:
Main pump – This pump operates the piston rod that raises and lowers the boom, as well as the hydraulic telescoping sections that extend the boom. The main pump is able to generate 3,500 pounds per square inch (psi) of pressure. It generates more pressure than the other two pumps because it is responsible for moving much more weight.
Pilot pressure counterweight pump – A hydraulic truck crane uses counterweights on the back of the cab to keep it from tipping over. These are added and removed by a hydraulic lift that has its own pump. The counterweight gear pump can generate 1,400 psi.
Steering/outrigger pump – One pump controls the steering and the outriggers. The outriggers are used to stabilize the truck during lifting operations. Because steering and outrigger operation are not performed simultaneously, they run off of the same pump. This pump generates 1,600 psi.
Read more: How Hydraulic Cranes Work