The application of various types of cranes in the field of wood processing

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The production chain for any wood product begins of course in the forest where the trees grow. In industrial-scale forestry the wheeled or tracked machines that grasp, cut and stack or load the logs for onward transport to the sawmill would be classified more as cranes than as hoists, which come into play at the sawmill or panel plant; but the attachments that grasp and lift the round logs are common to both. They are, in logging world language, known as grapples. They can be remarkably specialised. Scandinavia, as might be expected, is a world leader in equipment for forestry operations. Swedish-based Cranab produce three different designs of grapple, optimised for working with round timber in stacks, with wood for fuel, and for handling single logs.

One of the most important properties of a grapple, says Jonas Järnö, marketing manager for the off-road business at Cranab, is how it rolls the logs in when it closes. The timber needs to roll smoothly up against the underside of the grapple cradle without catching or blocking the movement. Since, as we have said, logs are never perfectly round and have branchstumps sticking out at irregular intervals, this is not a given. This is particularly true if the grapple is picking up a dozen logs at a time, as with a gantry crane or hoist at a sawmill or chipping plant; the logs must roll against each other, the outside logs making the inner ones rotate and compounding the likelihood of jamming.

It is important for the grapple to be able to penetrate down into a pile of timber without damaging it. Spiked jaws, though better for picking single logs from the ground, may stick into an outer log in a pile of many. Spade-like ends to the grapple-jaws work better here, but require more power to penetrate the pile; Cranab adjust the design of claw tips and the force in the grapple cylinder to allow smooth picking from the timber stack or from a particular size of load. Cranab also offer a particularly Heath- Robinson-looking attachment for their grapples in the form of a chain-saw that attaches onto the end. The grapple saw is operated remotely from the driver’s cab, and while the grapple is lifting the roundwood to its destination it cuts the end off, so that all the logs have a uniform length.

After the forest the next stage is the logyard, which may be at a saw-mill, where it is turned into planks and beams, or a panelplant, where it is chipped to produce woodbased panels such as MDF or, increasingly nowadays, oriented strand-board known as OSB or by the trade-name Sterlingboard. Rough-sawn planks are, obviously, rough, which makes it not easy to grip them for lifting. As they dry they are also prone to bending out of true, sometimes to a considerable extent. Aerolift offers vacuum pads that can handle bent planks.

“Wood is a natural product which changes after sawing,” says Pia Mörike, marketing manager at Aerolift. “When storing, the wood still swells and bows. Simple standard suction plates can no longer lift it.”

Aerolift’s device consists of two slim suction plates for rough wood that are hinged to each other and adjust their angle to fit the curved load. With it, they say, even highly deformed wood can be transported easily, quickly and precisely.

At panel plants, large flat panels pose their own lifting and handling problems. They are big, wide, and long, and they flex. A recent issue has arisen through changes in customer demand, driven in part by digitisation and Industry 4.0. In the past, panels from a press were all identical, and would be delivered in batches of 20 or 200 or 2,000 to the customer. Today, customers demand their own individual mixes of thicknesses, sizes, weights, densities and the like. Pressures and times within the press are varied accordingly, and the panels coming out at the end will be in stacks of, say light, medium, and heavy grades; but the customer may want a delivery of five light panels, 27 mediums, and one heavy. So picking and sorting from the production-line stacks is necessary; panels have to be sorted for delivery into mixed batches that can contain from a few hundred of each type to a few dozen to, sometimes, just one. This is so-called ‘Batch Size One’ delivery.

But panels, even moderately-sized ones are awkward to lift, as well as heavy. Hand-picking large flat items from several different stacks each of different specifications, and piling the unique selection into a single stack for delivery to a specific customer, is a task that normally requires many employees.

For the problem of Batch Size One sorting and delivery Eurotech industries have a solution. They claim it as the first customer-specific Industry 4.0 vacuum lifter. It will have many applications, but Eurotech have designed it for panels and other large, flat-surfaced products, and are demonstrating it on wood-based panels. “Customers are increasingly calling for individual production and faster deliveries,” says Eurotech CEO Thomas Schultz.

“Production and storage departments have to keep up with these demands. Orders of ever-smaller batch sizes require efficient, cost-saving production and storage management.”

The system consists of multiple pallet positions, side by side, each containing panels of a particular specification. The lifting itself is by multiple vacuum cups on a lifting frame. A trolley with the vacuum lifting frame moves above the pallets, picks a panel, and moves and deposits it on the customer’s pallet. An employee operates the user interface of the system to specify source and target stations and picking volumes.

To start its fully-automated picking cycle the machine requires information about which vacuum circuits to activate for the different loads. These data are stored in a database or a bar code. Picking then takes place automatically.

The system is equipped with multiple sensors to prevent errors. For example, it compares the weight of each lift with previously-registered data. If the weight is a match the machine carries out its cycle. If there are deviations between the source and target weight, the cycle is interrupted and the machine produces an error notification.

Such an error would occur if the vacuum lifter raised two panels at once. Known as adhesion, this is a problem that can frequently occur when picking wood-based panels from a stack: large, flat panels piled one on top of another do not easily admit air between them, so that if the top one is lifted, the one below may come with it, held to its upper neighbour by partial vacuum above and air pressure below. Eurotech have incorporated a solution into their order picker. Lifting up the corners of the load allows it to flex and let in air. Eurotech’s lifters do this by using suction bellows cups at the corners of the lifting frame.

It is good practice also to create delivery stacks that are well-aligned, not merely for neatness but to protect sensitive surfaces and edges. As an option at an expansion stage of the order picking system, a floating vacuum lifting frame can make up for misalignments of up to 50mm in the customer’s stack. Integrated linear actuators adjust the position of the floating frame using the edge of the previouslystacked panels as a reference. A maximum misalignment of 2mm is claimed for the system. With the fully-automatic order picking system, users can effectively pick small and single-unit batches, says Schultz; it also eases the physical work involved and creates safer conditions for employees.

The application of various types of cranes in the field of wood processing