Typical electric Condra double girder overhead traveling crane, similar to the sea crane, but with motor drive on steel wheels instead of rack and pinion.
Condra will manufacture its first sea crane, an overhead maintenance machine with power-to-motion conversion achieved by rack and pinion drive instead of pure friction.
The order has been received and work has started. The delivery will take place shortly after completion in the middle of the year, well before the installation and commissioning on board, which is planned for November to December.
The crane will be used for maintenance work on helicopters and other equipment in a workshop at deck level, where the 2-ton, 4-meter, double-girder overhead touring machine will perform its duties both at sea and in port. In particular, maintenance is often done while the ship is sailing across the Atlantic, notorious for its storms.
Due to the customer’s need to be able to use the crane in moderately rough seas, the Condra engineers suggested driving through rack and pinion.
The engine must be able to operate while the ship is rolling or leaning through arcs of up to 30 degrees or 15 degrees on either side of the vertical. This oscillation would pose a challenge to conventionally designed long and transverse movements, in which motor power is applied to steel wheels and the friction between the wheel and the carrier creates movement. Vibration of the ship would cause the wheels of such a design to slip and the crane to run away.
So, to meet the design specification, Condra replaced all long and cross stroke wheels with rack and pinion arrangements that provide both traction and stability when the ship is moving.
Three phase 440 volt motors are used to drive the pinions along their racks, maintain a constant speed, and counter the varying component of gravity as the vessel rolls and tilts. When stormy seas force a deviation of more than a 30 degree arc, work in the workshop is interrupted and the crane is attached to a structural beam on the walls of the workshop via its floor block (hook) to prevent accidental movement .
Condra has already used rack and pinion drives in cranes that are designed for use in strong winds.
Condra managing director Marc Kleiner commented on the order and said that the design of the crane had proven interesting not only because of the need to use rack and pinion drives, but also because of the very tight installation parameters in the ship’s workshop, where ventilation pipes limit the space available and where careful design of the crane structure was required so that the hoist almost touches the workshop roof and thus offers maximum lifting height.
“The resulting design is an underground crane attached to the portal beam rather than the portal pillars. When the hoist sits astride the underground girders, we can give the customer the desired lifting height, ”said Kleiner.
“It will be a compact but very effective arrangement.”
After the sea crane has been delivered in a few months, Condra’s technicians will install and commission it in Durban at a much later time that is convenient for the customer, probably towards the end of the year when the ship returns to port.