Rubber-Tyred Gantry Crane

Overview

A rubber-tyred gantry crane, or RTG, stacks shipping containers in the storage blocks of a port without running on fixed rails. It is a portal: a steel frame that straddles several rows of containers plus a truck lane, with a hoist that runs across the top and lifts boxes down onto a road chassis or up onto a stack. Because it rolls on rubber tires rather than rails, an RTG can be driven to a different block as traffic shifts, which makes a terminal's yard layout flexible in a way a rail-mounted crane is not. A typical RTG spans six or seven rows of containers and stacks them five or six high.

The Gantry Structure is the portal frame, the Gantry Travel System system moves the whole crane, the Hoist Machinery raises and lowers the load, the Trolley positions it across the span, the Spreader grips the box, the Power and Drive system supplies energy, and the Control and Automation system coordinates everything from the Operator Cab.

Structure and travel

The Gantry Structure is a rigid portal of four Gantry Leg columns tied by Sill Beam beams at the bottom and two Top Girder girders at the top, squared by Tie Beam cross beams. What makes the crane mobile is the Gantry Travel System system: each corner sits on a Wheel Bogie that equalizes the load across four Crane Tire tires, sixteen in all, each driven by a geared Travel Drive Motor. To move from one block to another, a Steering Unit rotates the bogies ninety degrees so the crane crabs sideways across the yard, and Travel Brake units hold it in place while it works.

Hoist and trolley

Lifting is done by the Hoist Machinery. A variable-speed Hoist Motor drives a Hoist Gearbox and a pair of Rope Drum drums, which spool Wire Rope wire ropes reeved over Sheave pulleys down to the spreader. A fail-safe Hoist Brake holds the load if power is lost, and an Encoder on the motor reports drum rotation for height and anti-sway control. The Trolley carries the sheaves and runs along the top girder on Trolley Wheel wheels driven by Trolley Drive Motor motors, positioning the load across the span. A Festoon Cable System cable carrier feeds power and signals to the moving trolley.

Spreader

The Spreader hangs from the ropes and grips a container the same way the spreader on a handler does, with four Twistlock cones turned by Twistlock Cylinder rams into the corner castings. A Spreader Frame telescopes on Telescope Cylinder rams to fit 20 ft and 40 ft boxes, Guide Flipper guides funnel it onto the box, and a Landing Sensor at each corner confirms seating before the locks turn. Because the spreader hangs far from the machinery, a local Spreader Power Unit unit with its own Spreader Pump supplies the spreader hydraulics.

Power and drives

An RTG needs no fixed conductor rail, so it carries its own power. The Power and Drive system is usually a Diesel Generator Set with a Diesel Engine driving a Generator, though many cranes now add a Shore-Power Connection to run on grid power and cut fuel and emissions. Power feeds a Drive Cabinet of Variable-Frequency Drive variable-frequency drives that control the hoist, trolley, and travel motors. When the operator lowers a box, the hoist motor acts as a generator, and a Braking Resistor dumps that energy, or on a shore-connected crane it can be returned to the grid.

Control and automation

The Control and Automation system turns separate motions into a clean pick and place. A Crane PLC sequences hoist, trolley, gantry, and spreader interlocks, while an Anti-Sway System system reads a Sway Sensor and shapes the motion so the load does not swing as the trolley accelerates and stops. Because a rubber-tired crane has no rails to keep it straight, a Position System with Laser Scanner scanners and a GNSS Unit holds the crane aligned along a row and steers it accurately between blocks. Load Cell sensors in the reeving weigh the load. The operator works from a trolley-mounted Operator Cab with a downward floor window that looks straight at the pick.

Use and place in the fleet

RTGs are the standard yard-stacking crane of most container terminals because they combine high stacking density with the freedom to relocate between blocks. They lift full boxes that a Laden Container Handler handles only at the fringe of a block, and stack denser than any Empty Container Handler, while avoiding the fixed runway of a Rail-Mounted Gantry Crane. Their limits are the tire and pavement loads they impose, the diesel fuel burn when not shore-connected, and the need for accurate auto-steering to keep a tire-borne crane tracking straight. A terminal chooses RTGs when it wants dense stacking with a yard layout it can change, and switches to rail-mounted cranes only where the highest density and fully automated operation justify the rails.