Rolling rather than lifting
A change by some US Naval shipyards from suspending new parts from cranes to manoeuvring them into position by using wheeled transporters may benefit commercial yards.
The Navy is now using Wheelift’s transporters in some of its newbuild yards, as large ship sections can be accurately positioned and mated. While the introduction of these units was intended to find better ways to move the various ship components, avoiding the use of overhead heavy lifting gear and rigging, the transporters’ capability is giving the yards the ability to precisely mate components in a way not feasible with crane handling.
Avoiding hanging parts in the air also avoids some costly implications. A transporter can lift from underneath, without the need for pad eyes welded to the hull for rudder, propulsion unit and shaft removal. So, significant labour and the intricate procedures needed for hull integrity recertification can be eliminated. But possibly most important, it removes a lot of unnecessary stress from the components.
Though similar in principal to other multi-axle, self-propelled modular transporters, Wheelift's open-centre axle designs are very compact, with chassis and fixtures available with load capacities to over 500 tonnes and deck heights as low as 18".
To make sure of the structural integrity, all Wheelift transporters undergo a finite element analysis in the design phase to identify potential areas of high stress. Frame welds are 100% magnetic particle inspected. The solid urethane tire assemblies are particularly suitable for moving heavy parts across the typical, deteriorated floor conditions found in some yards.
Handheld controllers operate the units independently or in tandem, and each transporter has the ability to independently and automatically control a load's position by adjusting yaw, pitch and roll to accommodate variations in the surface over which the transporters move. It helps that the transporter’s omni-directional movement is teamed with a positioning accuracy of 0.005".
The final feature helps enormously with the Naval builds, as alignment of the Naval craft sections required simultaneously engaging three precision alignment pins that had only a few thousandths of an inch total clearance. Wheelift transporters performed the alignment with no scoring of the three pins, reducing the mating time dramatically.
“While we knew users could easily obtain relatively close tolerance accuracy, the Navy exceeded expectations for precision alignments in their typically dynamic and robust dry dock environments”, explained Mel Terry, Wheelift’s senior systems design and development specialist.
Wheelift transporters typically operate in hostile environments, including dirt and grit, high humidity and exposure to weather and salt spray with wind velocities of over 25 miles per hour. Ambient air temperature can range -18°C to +43°C.
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