Wood works

15 March 2019

Wood, and products made from wood, can be strong, sustainable, and good-looking, and are increasingly in demand for everything from medium-rise buildings to flat-pack furniture. It is, though, awkward to handle. Julian Champkin looks at lifting kit for wood and timber.

Wood is a natural product. That means it is irregular, in its shape and in its behaviour. If you are dealing with roundwood logs, they are not actually round; they have sticking-out bits where branches have been cut off and no two of them are the same size. If you are dealing with sawn or planed timber it flexes, it bows, it changes its weight depending on how much moisture there is in it. If you are dealing with wooden panels such as plywood or MDF sheets, they are inconveniently large, have nowhere obvious to grip them, and if they are stacked on top of one another then lifting the uppermost one tends to bring the others with it through suction, as air cannot enter in between the individual sheets. Equipment for lifting wood has to be able to cope with the sometimes contrary and individual nature of the beast.

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.

Further again down the wood processing and production chain is the user of wood-based panels, such as furniture factories. Here the panels need not only to be moved, but manipulated: tilted to the vertical or other angles, or fed into CNC cutting machines, with sometimes-high degrees of precision. Again vacuum lifting, of various kinds, is the preferred solution. TAWI produce vacuum grippers able to move wood sheets weighing 500kg or more when they are hanging horizontally and up to 250kg when vertical and providing a large range of motion and precision. These loads would otherwise require two employees to move each panel individually. With the vacuum gripper, one person can handle the panels, tilting them if necessary a full 180°. Handle controls are ergonomically designed and fully integrated with the hoist; with a button push, the operator can activate the vacuum, lift up or down, control the vacuum level, or tilt and release the load. Sheets and panels of all densities and dimensions are handled equally well, says general manager Mike Wooster.

The SSP vacuum suction spider from Schmalz has suction cups that are specially designed for the wood industry. A special soft sealing foam on the cups handles workpieces without damaging them or leaving marks, so that they can be coated later on without surface imperfections. Schmalz have a different solution to the problem of adhesion. When handling porous or semi-porous workpieces such as MDF or particle board, an integrated separating function ensures that the vacuum between the boards is halted by restricting the flow of air from the suction cups. In this way, only one workpiece is picked up and held.

A double sealing lip on the inside of the suction cup is designed to give a very tight seal on rough and smooth surfaces. The special structure of the bottom surface of the suction cup allows for extremely short cycle times and highly dynamic handling. The vacuum spider is protected by check valves on every suction cup. If only some of the suction cups are covered, the uncovered ones are switched off. This makes it possible to handle wooden boards with many different dimensions. “The vacuum spider is made up of a number of individual components that are perfectly tailored to one another, which makes it possible to adjust the device individually for every process,” says Andreas Dolker, manager for corporate communications at Schmalz.

One user is a manufacturer of prefabricated housing, who has to load long, heavy wooden boards into a machining centre. A Schmalz vacuum suction spider is used to achieve high reliability in this automated process. Schmalz’s SSP suction spider consists of six area grippers. The wear-resistant sealing foam on the grippers is extremely durable and can stand up to workpieces with rough surfaces. Flexible spring plungers compensate for unevenness and differences in height so that the suction spider always grips reliably and with high precision. Two powerful blowers are installed directly in the gripper and provide the necessary vacuum.

The gripper is highly adaptable, which allows it to easily fasten to wooden boards and beams of different shapes and sizes. The gripping system is attached to the hoist and can cover a very large work area. The hoist is controlled from a control panel on the machining tools.

The system helps to optimize the handling process and shorten production times. Vacuum tube lifters are frequently used with vacuum cups. Schmalz has supplied tube lifters to a Swiss-based manufacturer of high quality wooden doors. In its semiautomated production process the heavy raw wooden panels used to be moved by hand between a number of different process steps, needing sometimes two employees for the task. To make the process more ergonomic and efficient, the company now uses the Schmalz JumboErgo vacuum tube.

The tube lifter is equipped with a flexible four head suction pad. For maximum flexibility, the suction pad on its cross beam can be moved to adapt it to the size of the workpiece. An optional swivelling unit allows workpieces to be turned by 90° for further processing. Vacuum lifting, says the company, has reduced the numbers needed and protected the health of staff. There is a growing emphasis on wood as a carbon-friendly replacement for concrete and steel in construction, as a replacement for plastic in throw-away items, and as a good-looking and versatile material in its own right. And though it demands its own solutions it can be lifted without too much difficulty, as long as you add a little ingenuity and thought. 

Cranab’s grapple-saw simultaneously cuts logs to length while lifting them to the stack.
The grapple jaw to the left is designed for penetrating log-piles without jamming, with the jaw on the right designed for single logs.
Aerolift’s hinged vacuum lifters are designed for bowed or distorted rough planks.
Eurotech’s pick’n’mix system for Batch Size One sorting.
TAWI’s vacuum lifter can handle sheets that bend…
…and can rotate them to any angle including vertical.