Rugged innovative gears and gearmotors along with other drive technologies rise to the challenge in demanding wood-processing applications.

By Oscar Lopez • Service manager — industrial gearing | SEW Eurodrive

Applications in the timber industry are varied and demanding — consisting of debarking, sawing, drilling, milling, and grinding operations. The machines used in these processes and the components and systems driving them must be dynamic and powerful enough to withstand harsh conditions involving high shock loads, changing speeds, and long hours of operation. Most challenging of all is the presence of copious dust, dirt, and wood shavings.

Not all drive technologies are up to the challenge. That’s why it’s important to know the design criteria that can make or break performance in these applications and specify components balancing versatility, material strength, and contamination resistance.

As we’ll explore, drive technologies suitable for timber-processing applications include helical and bevel-helical gears with invertible housing; large segmented girth gears made from austempered ductile iron or ADI; and gearmotors as well as screw conveyor drives with multiple sealing areas against contaminants. Recall from material-science fundamentals that austempering is a type of heat treating of high-carbon iron to produce high-strength and tough bainites.

More timber throughput with versatile gearing

Machines for wood processing, conveying, storing, and picking need powerful drives that are versatile and reliable. For a universal solution, certain industrial helical and bevel-helical gears having invertible housings that accommodate both left and right shaft positions. Such designs reduce gear variants needed and inventory costs. That’s because gearing that offers various mounting orientations reduces the number of spare drive packages stored onsite for maintenance.

How much torque does this design need? Beyond design flexibility and contamination, look for gear units that are high capacity. Gears capable of delivering up to 800 kNm can withstand the loads associated with many wood-processing applications, which typically need 175,000 to 475,000 Nm.

Savings are even higher if a given drive powers several applications in a facility or several facilities. For example, one application’s bevel-helical gear unit might cost $18,000. If the application needs both left and right drives, a single drive that can install in both orientations can save up to $18,000 — and reduce the use of valuable storage space.

In fact, the modular nature of some such gearing allows millions of different configurations to let design engineers arrive at a drive solution to best satisfy application speed, torque, footprint, and environmental parameters. Customizable features include numerous shaft styles and wide gear-ratio ranges — 6:1 to 400:1 for example.

Avoiding costly downtime in the timber industry

Gear options with top reliability are necessary in the timber and paper industries where even a few hours of downtime can lead to thousands of dollars in lost productivity. After all, downtime can cost $10,000/hr to $400,000/hr or more in certain pulp and paper-mill applications. These costs far exceed those associated with removing and installing a replacement gearbox — and the above are only readily calculable totals. Other costs of downtime include the payment of production personnel sitting idle and losses relating to a machine center that isn’t producing.

Many operations in the timber industry benefit from helical and bevel-helical gear units having the ability to bear high axial and radial loads. Some options include a single-piece block housing allowing horizontal, vertical, and upright assembly; optional motor adapters, backstops, sealing systems, shaft end pumps, and mounting maximize design flexibility. Where necessary, cooling systems engineered into the gearing solution can eliminate the need for a costly external cooling unit. Legacy products that have been in production for many decades (especially those with a large installation base) are often a suitable choice.

Benefits of austempered ductile iron (ADI) girth gears

Industrial girth gears are indispensable in wood processing applications, as they often drive large rotating systems. Typical applications include lumber trommels, debarking mills, and digesters. These operations typically rely on two to four machine segments that are assembled together.

The catch is that the large gearing that goes into such equipment is typically made and heat treated on large and expensive machines. In addition to these challenges, the sheer size of traditionally design girth gears (7 to 15 m) can degrade gear casting and heat-treatment quality.

Segmented girth gears avoid the drawbacks associated with traditional girth gears. For example, some girth gears include six to 14 identical segments that are relatively small and easy to handle, assemble, and install. Some segmented girth gears are also made from ADI mentioned earlier — a material defined by the International Organization for Standardization (ISO) and the American Gear Manufacturers Association (AGMA). Compared to traditional girth gear materials such as ductile iron or cast steel, ADI is technologically advantageous for many reasons. No wonder some gearmotor suppliers employ proprietary ADI formulas to manufacture girth-gear products. Standards specifying ADI mechanical properties include:

• AS A 897M-90 — the standard specification for austempered ductile iron castings.
• EN 1564:1997 — defining founding austempered ductile cast irons.
• ISO 17804 — classifying founding ausferritic spheroidal graphite cast irons.
• AGMA 939 — detailing austempered ductile iron for gears.

These gears incorporate a highly wear-resistant microstructure that achieves high tensile strength. ADI also features above-average contact fatigue strength due to its cold working properties. When combined with an appropriate girth gear size, these properties spur a lighter and more compact gear design compared to traditional materials. This lighter weight is important for handling and assembly purposes as well as the gear’s achievable (application-specific) circumferential velocity. Generally speaking, the latter falls between two and 10 rpm for the final rim speed.

Other technological advantages of ADI girth gears abound.

Tight gear tolerances: Due to their multiple smaller segments, individual girth gear components can be machined to tighter tolerances. The overall gear is therefore more precise and wear-resistant which in turn increases its power density.

High gear-pitch accuracy: Girth gears with many segments guarantee an initial pitch accuracy that meets AGMA 9 standards. Thanks to its high dimensional accuracy, these components keep vibrations to a minimum during timber industry processes such as debarking, sawing, and shredding.

Superior gear casting: Segmented girth gears’ multiple smaller segments lend themselves well to high-quality casting — reducing the amount of stress concentrations that can arise as the casting cools.

Small gear faces: Each segmented girth-gear component has a smaller face than traditional gears of comparable design for lower material costs. This also improves gear tooth contact, which typically becomes harder to maintain the wider a gear face gets. Once contact drops below 80% endusers must switch to using helical gears, which can drive up costs by 30 to 40%.

Easy handling and replacement: Handling gear segments is easy in the factory (during production) and at the gear’s installation site. There’s also no need to make any special transportation arrangements, as segmented girth gears can be transported in standard containers. If a segment is ever damaged, it can be easily exchanged without necessitating the dismantling of the whole gear ring.

These benefits render segmented girth gears a practical choice for woodworking and wood-processing applications as well as similarly heavy-duty industries such as mining, energy, pulp and paper, steel, and cement. With the correct dimensioning, load, and lubrication, ADI girth gears are virtually wear-free … even under the harsh operating conditions of these industries.

Gear-unit sealing to prevent contamination

As mentioned, timber-industry processes involve trying conditions such as outdoor operation and the presence of dust, dirt, and wood shavings. So, it’s important to select drive technologies that integrate multiple sealing features. For example, some gearmotors have reducers with an interlocking two-piece seal design along with a patented bi-helix fluoroelastomer (FKM) inner seal and double-lip nitrile outer seal. This creates a total of three sealing surfaces to keep out contaminants. The result is gearing for timber transportation lines and debarking machines that can withstand operation outdoors without adding anything to overall load.

It’s not just gearmotors that should sport advanced sealing features. Screw conveyor drives (used to convey sawdust and wood chips) should also feature such sealing. Some such drives include the following for extra protection against contaminants and abrasives:

• A double-lip seal in the flange
• A double-lip seal on the gear unit output shaft
• A labyrinth seal on the output shaft formed by the V-ring and flange
• A NILOS steel disc seal (LSTO) for extra protection against abrasive dusts

Finally, some screw conveyor drives include an expulsion port that provides a grease channel to the V-ring and oil seal, as well as a path for excess grease to purge freely during regreasing. That prevents high pressures that can destroy seals.

SEW Eurodrive | www.SEWEURODRIVE.com