Wood Yard

The wood yard may be located at the saw mill, the paper mill, or at a satellite site. The wood yard prepares the wood for pulping. Just as there is no typical paper mill, there is no typical wood yard arrangement. Some paper mills only use logs of a specified length, which are called short logs. The use of only short logs eliminates the need for a slasher deck. Paper mills that receive whole logs and use chemical pulping must have the logs debarked, chipped, and screened subsequent to digesting. Paper mills that use mechanical pulping do not have the logs chipped. The logs are sent directly to a mechanical putper. Paper mills that receive only chips do not need the barker and chipper, although a small rechipper might be used.

Whether the mill received logs or chips, the source wood must be unloaded. Unloaders, which are sometimes called dumpers, use large hydraulic pistons (lifts) to unload chips from trailers or railcars. A log loader is used to unload whole logs from trailers or railcars.

Unloaders are exposed to the weather, heavy dirt, sawdust, and other abrasives. Lithium soap-based EP greases such as 3751 and 3752 are generally acceptable for most bearings in these applications. You will find that 6520 MONOLEC® Hydraulic Oil offers tremendous versatility for the hydraulics with its multi-viscosity weight (6110 MONOLEC® Hydraulic Oil or 6802 MULTILEC®) or 20 weight (6120 MONOLEC® Hydraulic Oil or 6803 MULTILEC®).

Wood yards that receive whole logs frequently use a slasher deck to cut the logs into usable lengths. Slasher decks consist of multiple circular saws that are mounted on a conveyor table. Dogged conveyor chains hold the logs perpendicular to the rotating saw blades and move the logs across the slasher deck. The multiple saws cut the logs to the appropriate length as the logs pass over the deck to the discharge conveyor.

Slasher decks operate under conditions that are similar to the other types of saws that were previously described. The saw bearings operate at high speeds and are subjected to heavy sawdust, dirt and silica. A light, premium~blended, anti~wear, high-quality paraffin hydraulic oil should meet the requirements of most oil lubricated bearing applications. A lithium EP (3751 and 3752 ALMAGARD®) gease that will withstand oxidation and contamination should meet the requirements of most grease-Iubricated bearings. The key is to keep your eye on the operating environment and climate as a less tacky grease may be more effective such as 1275 ALMAPLEX® Industrial Lubricant.

Once logs have been cut to usable lengths, the bark must be removed. The sulfite pulping process is ineffective at dealing with bark. The Kraft process can digest bark, but the presence of bark complicates the bleaching of the pulp. Most mills debark the wood subsequent to digestion; however, some mills use whole trees (wood, bark, branches, leaves, and all}.

Barkers remove the bark from logs while minimizing the amount of useful wood that is removed. Several different styles of mechanical and hydraulic barkers are used. Mechanical debarkers include drum barkers (often called a barking drum}, rosser head barkers, ring barkers, and multi-pocket barkers.

Drum barkers are large, rotating cylinders that sit at a slight angle. When the logs have been loaded, the drum barker is rotated. Rolling friction, created as the logs roll against each other, strips the bark from the logs. Metal lifters are often installed in the drum barker to minimize wear on the walls of the cylinder. An adjustable discharge gate controls the average length of time that the logs spend in the barker, which controls the degree of debarking.

Drum barkers are effective on both wet and dry logs. Water is sometimes added to the drum barker to loosen the bark and to speed the debarking process. Dry drum barkers are longer in length and rotate faster than wet drum barkers. Bark that is removed by a dry drum barker can be directly fired in a power boiler, but bark that is removed by wet barkers must first be dried.

Drum barkers can either be mounted on large pillow-block bearings or suspended on large chains that are supported by an overhead framework. Pillow-block, bearing-mounted drum barkers are driven by electric motors through a gear reducer. Suspended drum barkers are driven by an electric motor through a gear reducer, drive gear, and drive chain.

Drum barkers are the most cost-effective method of debarking and waste very little wood. However, drum barkers tend to splinter the ends of the logs, called "brooming," and any dirt that is on the logs can be driven into the wood. The dirt will subsequently be carried through the process and could affect the final product quality.

The rosser head barker consists of a rapidly rotating head that is mounted on a tensioning anti. The anti holds the head against the log. The Jog is rotated beneath the head by gears or rollers. As the log rotates, the rosser head removes the bark along the length of the log. Rosser head barkers are effective at debarking frozen logs but tend to remove more wood from the log than drum barkers.

Ring barkers, also call cambium shear barkers, consist of a set of tooled anT1s or rotating wheels arranged in a ring. Ring barkers work because of the poor bond that exists at the layer of cells between the bark and the wood. which is called the cambium layer. As the logs pass through the center of the ring. the antis or wheels ride against the log and rotate around the circumference of the tog. Ring barkers are not effective at removing bark from crooked or frozen logs.

Hydraulic barkers use a water spray to strip the bark from logs. The water is sprayed through nozzles at up to 1,000 psi. Hydraulic barkers are effective on large diameter trees. Hydraulic debarking is used mostly in the Pacific Northwest. Because hydraulic debarkers require large amounts of water and energy and the effluent water must be treated, hydraulic barkers are slowly being replaced by more efficient mechanical barkers.

Because barkers operate under dusty, abrasive environments that are frequently exposed to the elements, many of the lubricants that are used for barkers are the same lubricants that are used elsewhere in the wood yard. Due to the slower speeds and heavy loads. a lithium soap-based EP grease that resists corrosion and contaminants is generally acceptable for greased bearings. An industrial gear lubricant such as 1601 DUOLEC™ Vari-Purpose Gear Lubricant or 1606 DUOLEC™ Vari-Purpose Gear Lubricant is preferred for oil-lubricated bearings. Gear-type flexible couplings that show a tendency to leak with liquid lubricants are best lubricated with a special grease. sometimes a blend of thickener and polymer that practically eliminates oil separation due to centrifugal forces. A variety of Lubrication Engineers greases could be used such as 3751 or 3752 ALMAGARD® Vari-Purpose Gear Lubricant, 1274 or 1275 ALMAPLEX® Industrial Lubricant or 4701 MONOLEC® Industrial Lubricant.

Air cylinders may be lubricated with circulating oils or hydraulic oils that contain additives to give protection from rust, foaming. and wear. Hydraulic systems should use a light. premium blended. anti-wear. high-quality hydraulic oil of the appropriate viscosity range oil that meets the OEM recommendation for the speeds and pressures involved.

In order for a sawmill to operate efficiently and profitably. no part of the log should be wasted. Unacceptable slabs, scraps, and trimmings from the sawmill are processed into chips for paper manufacture. chip board. or burned as hog fuel. Another name for a chipper is a "hog.II Chippers use knives that are attached to a cylinder or a V-shaped spool, or chisel-type hammers that pass through spaced discs on a shaft to chip the wood. Chips are created when the Jog is forced between the knives and anvils of the chipper. Some manufacturers mount the knives on a rotor and the anvils around the periphery; other mount the knives around the periphery and the anvil on the rotor. Regardless of the design, the wood is forced between the knife blade and the anvil where it is cut to the appropriate size. A screen is installed in the chipper to retain the wood chips inside the chipper until the chips are small enough to pass through the screen. An infeed device that nonT1ally consists of toothed rolls regulates the wood feed rate. The toothed rolls grip the Jog to ensure smooth. continuous feeding with minimal kick-back. Most chippers are driven by electric motors. The output of the chipper is screened to remove oversized chips. The rejected chips are then rechipped.

Chemical pulping requires that the logs be chipped to the appropriate size. Wood chips between 3 to 6 mm wide and 10 to 30 mm long are acceptable for chemical pulping. However. the ideal size is 4 to 5 mm wide and 20 mm long.

The chipper rotor is mounted on a horizontal shaft supported by bearings on either side. For most chippers where temperatures will not exceed 2500F. a lithium EP grease (3751 or 3752 ALMAGARD® Vari-Purpose Gear Lubricant) should be used. For applications where the bearings may exceed 2500 F, a heavy-duty, high temperature grease may be required because this type of grease resists softening at high temperatures, has high load-carrying capability, and resists water wash-out. A light, premium-blended. anti-wear, high-quality oil should be used for hydraulic systems associated with chippers such as our MULTILEC® line which includes 6801, 6802, 6803, 6804, 6805, 6806 and 6807.

Screens are used to ensure that the chips are of the appropriate size. Usually, multiple sets of screens are used. The first set of screens, called primary screens. separate excessively large chips. The large chips. called "overs," are returned to be re-chipped. The chips that fall through the screen, called "accepts," are screened by the secondary screens. Secondary screens separate the accepts from the chips that are too small, which are called "fines." The fines are collected to be burned as waste wood. which is called "hog fuel". The accepts are sent on to the chip reclaim system.

Chip screens can be of the rotary disc-type or of the vibrating-type. Rotary disc screens use sets of discs that are mounted on a common shaft. The discs are intenneshed with similarly mounted discs on another shaft. As the discs turn, acceptable sized chips fall between the discs. Large chips that are not acceptable do not pass through the discs. The large chips are removed by conveyors to be rechipped or burned as hog fuel.

Vibrating screens use screens that consist of a perforated surface to separate the chips. The chips are placed on the face of the perforated surface. As the screen is vibrated. chips that are small enough fall through the screen. The remaining unacceptable chips are removed from the top of the screen.

Chip screens are subject to heavy shock and vibration. An industrial gear lubricating oil with excellent oxidation and thennal stability is recommended for applications that have oil-lubricated bearings. The tackiness and load bearing capabilities of the 1600 series DUOLEC ™ Vari-Purpose Gear Lubricants (1601, 1602, 1603, 1604, 1605, 1606, 1607, 1608 and 1609) are a natural for this application. Open chains can use any Lubrication Engineers lubricant with high tackiness to minimize throw-off such as 9102A SYNTEMP® Synthetic Lubricant or 5100A PYROSHIELD® or an appropriate tacky oil like 1601 DUOLEC ™ Vari-Purpose Gear Lubricant or 1605 DUOLEC ™ Vari-Purpose Gear Lubricant.