These are powerful machines capable of excavating into hard rocks, but require large bentonite plants and large cranes to hold the cutter. Steering mechanisms are built into the grab body to provide a high degree of verticality control. Hydraulic cutters are also known as Hydromills
This type of grab has hydraulically operated closing jaws and so is able to exert considerable excavating force to excavate dense sands, stiff clay, or weak rock material. These grab bodies can also include steering mechanisms.
This grab uses only pulley forces to close the grab jaws and so is more suited to softer ground. It is also called Rope grab or Mechanical grab as it does not employ hyadraulic force.
This grab type is rigidly suspended from the carrier machine whereas the three other types are rope suspended. The Kelly grab can provide reasonable excavation force and verticality control but may not be as fast as suspended grab excavation.
