Phil Bishop reviews the latest developments in lifting technology
It has taken a decade but finally another mobile crane manufacturer has decided that Tadano was onto something when it introduced its Lift Adjuster anti-sway system. Swaying loads are one of the biggest challenges that face crane operators. Good operators pride themselves on being able to ‘catch’ a load, to make use of the full combination of movements available – winching, slewing, telescoping, boom up, boom down – to keep load movement under total control. It is rather like a waiter sashaying across a busy room with a tray of drinks held aloft on his fingertips making continuous tiny adjustments to avoid any spillage. However, humans are imperfect and mistakes happen. Although it is not often that you will see a waiter drop a tray and it is probably rarer to see a crane operator crash their cargo against an obstacle, it is best to design-out risk where possible. And the ability to control sway is not just about avoiding collision; it is also about maximising the speed of operations.
Load sway is a particular issue for containerhandling cranes which are required to stack boxes precisely, one on top of another. These cranes can control sway by virtue of the fact that the containers are generally lifted by an arrangement of ropes and winches to create a four-point lift. Sensors can detect the relative position of each corner of the box and tell the winches to respond accordingly to keep the load level.
For mobile cranes, it is somewhat trickier because loads are mostly lifted at a single point. Even when complex spreader beam arrangements are used, there is a single sheave at the boom tip focusing all the forces on one place.
One of the load sway problems that mobile crane operators face is the tendency of a load to move relative to the crane as it leaves the ground. Modern telescopic crane booms are made of relatively thin high-tensile steel, making them light, long and strong. While this is mostly good, the downside is that they also flex like a fishing rod. The heavier the load and the more the boom is extended, the more they flex. And the more the boom flexes, the greater the tendency for the load to sway as it leaves the ground.
In 2004 Tadano of Japan came up with a solution for this load sway. If the crane operator switches on the Lift Adjuster system (there is a switch on the control panel) boom deflection is monitored and the hoist cylinder automatically adjusts to compensate for the change in radius caused by the deflection. Lift Adjuster ensures that the load moves in a true vertical path when lifted from the ground, without boom flex inducing deviation. It switches off automatically immediately the safe load torque is reached.
Tadano’s rivals appear to have largely ignored the issue of load sway, trusting in operators to continue to use their own good skill and judgement – until now, that is. More than a decade after the introduction of the Tadano Lift Adjuster, Liebherr has introduced its solution to the problem, which it calls Vertical Line Finder.
This control feature ensures that the load on the rope is hoisted in true vertical. Activated by pressing a button on the joystick, crosshairs on the monitor show whether the lift rope is in vertical position. If it isn’t, the boom and hoist winches and the slew motor work together to make it so. This function ceases when the operator releases the button or as soon as the maximum lifting capacity of the crane is reached.