How do you lift millions of pounds hundreds of feet into air? Very carefully, of course. But moving such massive payloads is no joking matter, where safety must come first. Therefore, you probably won't hear any complaints that the world's largest mobile crane rotates at only 1 /8 rpm.

The huge machine that tackles these difficult jobs is the PTC III, from Mammoet Group, Rotterdam, Netherlands. Like the mammoth (Mammoet is Dutch for mammoth), the cranes that Mammoet develops are known for their huge size, strength, and ability to thrive in hostile conditions. Recently, Mammoet commissioned the largest mobile crane in the world, the PTC III. It stands 200 m tall and can lift 1600 t at speeds to 80 m/min. It was developed to handle heavy conveyor systems used in the oil industry for maintenance work.

But size, weight, and strength may not be the most impressive aspect of this machine. PTC stands for Platform Twin-ring Containerized, where twin-ring refers to the crane's construction, and containerized meaning that it can be disassembled and transported by standard means. The crane is supported on a steel ring, with 54 wheels on four bogies to allow its tower — driven by gears through hydraulic motors — to rotate through 360°. When disassembled, the entire crane fits within 88 standard marine shipment containers that can be transported readily by ship, rail, or truck.

Ideal application for hydraulics
The idea of a hydraulic drive for the rotary drive came up very quickly, explained Jan van Seumeren Jr., technical director at Mammoet. "Naturally, the winches and drive wheels could have been driven electrically. But we would have needed very large motors and generators for on-site supply." Large electric motors could have prevented the PTC III front meeting space and/or weight requirements for shipping. Furthermore, heavy, bulky motors could have made assembly and disassembly more difficult.

The PTC III relies on a 20-ft tall, diesel-driven power supply that drives electric generators and 17 hydraulic pumps. The pumps drive 29 hydraulic motors that each drive a planetary gear drive. The hydraulic motors provide the mechanical power to drive the wheels through the gear drives, and each gear drive multiplies torque from the motor and allows it to rotate at a more efficient speed than if the motor drove the wheel directly.

Collectively, the wheels transmit the torque to rotate the tower up to 1°/sec. Van Seumeren says the gear drives are lubricated with hydraulic fluid, and the the entire hydraulic system holds roughly 8000 l of fluid. Pumps, motors, and gear drives were all provided by Bosch Rexroth BV, Boxtel, Netherlands.

Bosch Rexroth's involvement was not only as a supplier, but as a development partner that became involved in the project at an early stage, says van Seumeren. "We selected as many components as possible from the Rexroth standard range." He points out that service was also an important consideration. "Bosch Rexroth has a worldwide service organization and already has a service support center or its own subsidiary practically everywhere we go."

The PTC III is the fifth twin-ring crane Mammoet has designed and built in cooperation with Huisman-Itrec, Scheidam, Netherlands. It weighs 2100 t, so it can lift more than 3 /4 its own weight.