Pacific Northwest mills seeing less downtime with BRUKS drum chipper

Snohomish, WA - What if a sawmill maintenance manager could schedule a chipper knife change every five days instead of when the wood stops feeding in the current disc chipper every eight to 12 hours? And, at the end of the five days of continuous run-time the knives would only need a light touch-up regrinding since the edge is only slightly affected? This is exactly what mills in the Pacific Northwest are experiencing when they replace disc chippers for sawmill wastewood with a drum chipper from BRUKS.

Several design features make this reduced maintenance schedule possible. The drum itself plays a critical role. The curvature of the drum surface puts the knife into a circular path where the tip of the knife pulls the wood into the chipper due only to the shape and rotation of the drum. The chipper is not relying on the grind angle being particularly sharp, or the pull-in angle being large enough to allow the wood to advance between knife cuts, as they must be in disc chipping. (Disc chippers stop feeding when these angles are lost due to wear, and they must be refreshed in order for the wood to be correctly pulled into the disc. Such a disc chipper signals when to change the knives because it stops feeding.)

As seen from the side, the knife path through the spout’s opening starts at a point just below the centerline of the drum, and it progresses downwards towards the bottom of the feed spout. As it falls, the compression face of the knife engages the wood in the spout at an angle which is mostly vertical in the beginning, but quickly changes to the horizontal as the knives near the bottom anvil.

The anvil (or counter knife) is located at about 5:30, if we are looking at the side of the spout as a clock. It is not at the complete bottom of the stroke, but just above it where the knife is best able to pull the wood into the chipper. As sawmill wastes are generally piled two to eight inches deep in the bottom of the vibrating or belt conveyor that is feeding the chipper, the feed rolls and the action of the drum ensures that the wood is quickly and efficiently pulled into the final cutting zone.

Only occasional surge feed conditions or a large log end will need to use the upper reaches of the rectangular spout opening.

Chips that have been cut are held in a curved pocket in the drum as it spins, and pass the anvil without needing to interact with it (and make pins and fines). Rather, they exit the pocket once past the anvil and then pass through the chipper’s internal screen. Screen opening sizes are selected to pass acceptable chips, but also to retain chunks and pieces that might have been created when trim blocks were cut sideways to normal orientation, for example, or if a short stick of lumber was turned at an angle. In disc chipping these chunks and pieces need to be screened out using a separate shaker screen, but in drum chipping the screen is included.

Most drum chippers use a knife with grinds on both sides of the cutting edge. The longer flat grind is used to create the “sharpness” of the knife, and a shorter small-angle back grind toughens the knife edge and acts as the compression face. The flat grind edge faces the wood as it enters the chipper. The knives are adjusted for width after grinding using double- nutted screws at the back of the knife, or with sliders held in place with setscrews.

One of the neat design features of drum chippers is their ability to change chip length settings, if needed. For example, local market conditions may make standard pulp chips attractive to buyers during some seasons, but microchips are a better choice at others. By adjusting the upper and lower feed roll speeds, the internal screen, and some aspects of the knife setup it is possible to make some substantial changes in chip length without costly major hardware changes – costly in terms of hardware components, and in downtime to make the change. It is also possible to make small changes to the chipper to meet specific chip quality specifications, through size distribution optimization.

The greatest advantage of a drum chipper is its ability to process the wide assortment of materials that are dropped onto the waste conveyor at the bottom of the mill. The lower feed rolls pass sawdust between the rolls just like a disc screen (and there is a collection conveyor below the rolls to catch it). Trim blocks are propelled towards the drum to within just a few inches, with no dead space for them to sit. Scrap lumber and edgings feed smoothly across the full width of the chipper, taking advantage of any open space for chipping by sliding gently sideways, as needed. And the powered upper roll case holds the wood between itself and the lower rolls, creating a positive feed condition for everything that comes down the line.

In disc chipping there is a fundamental geometric problem in forces generated when a knife sweeps across a spout opening towards an anvil. There is a point where the knife line and the bottom anvil will be parallel, using a slant disc wastewood chipper as an example. If the knife line first crosses the anvil near the center of the disc then the scissors effect will push everything towards the rim of the disc. If the knife line first crosses the anvil near the rim, then there is a force generated that will push wood towards the center. The key is to get the knife to chip an average piece of wood with the knife line parallel to the anvil and near the mid-thickness of that piece at the same time, where in the forces are balanced. When that ideal condition happens the wood will be stably chipped and won’t move to the left or right. A large chunk will always be pushed to the rim of the disc for chipping. Thin strips will always be pulled to the center. That is basic disc chipper geometry in action.

There is no tendency for a drum to push the longer pieces to the right or left in the spout due to scissor action, however. The knife line is parallel to the anvil at all points of travel, and there are no right and left forces generated. Everything gets chipped exactly where it first hits the drum. This feature of drum chipping shows well in on-line drum chipper videos, where trim blocks and log ends 20 times their size are being chipped simultaneously, neither piece moving left or right.

The toughness of a drum chipper is evident when one considers overall weights, the thickness of the wear plates, the heft of design, and durability under stressful conditions. Many mills watch surges that amount to more than double the design feed rate disappear as solid wood into the drum chipper spout only to reappear as chips flooding the takeaway conveyor. The only evidence that something has happened is that the upper roll case has gracefully lifted to allow the wood entry.


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