When is a drum chipper a better choice?

ONEOFTHEMOSTASKEDQUES-tions about usingdrum chippersin general sawmill applicationsis, where in the mill is a drum chipperthe best option? The answer is couchedin terms we all know too well: the formof the wood to be chipped, chip quality,capacity, power requirements, maintenance access and cost, durability andover-all cost of ownership. As with anypiece of industrial equipment, a drumchipper is a tool to get something done,and how well it performs that functionmakes all the difference.

To begin evaluation, start with theform the wood is in when it needs to bedealt with.

Running the length of the primarysaw-line is a vibratory waste conveyor.It collects the downfall from every processing centre above it, includingedgers, lumber scanners, end trimmers,circular and band saws, canter and profiling heads, and others. The wood thatfalls down is in the form of sawdust,chips, slabs from the sides of logs, lumber edgings, rejected lumber pieces, trimblocks and even log ends. All this woodis destined to become chips, to be loadedinto trucks and sold for pulp chips orperhaps used to generate power in theboiler, to run the sawmill and dryingkilns.

In North America, the historicalmeans to render this wood into chipshas been a disc chipper. But disc chippers have a few inherent design featuresthat cause problems in sawmills. Vertical disc chippers have a spout floor thatis quite long on one side, meaning shortpieces have to be pushed from behind inorder to reach the disc. The chips wewant to make are created when thewaste wood pieces are chipped perpendicular to the wood grain or, put another way, chipped in the sameorientation a long log would be chipped.When chunks and pieces are pushedinto the disc, there is no assurance theywill be orientated in this “correct” way,so that instead of chips, slabs andchunks are produced. And in disc chippers, the slabs and chunks are immediately shot out of the back of the disc andonto the collection belt below withoutfurther processing. There is no screening inside the disc chipper and no second chance at chipping these big chunksinto more chip-like pieces.

True, longer pieces like slabs andedgings are correctly chipped and discchippers produce excellent chips fromthem. But any chipper can make goodchips from slabs and edgings: At least intheory.

The first key difference when usinga drum chipper is in the spout. A drumchipper has a series of rolls across thewidth of the bottom and top in-feedareas. These rolls are powered withsmall horsepower gear-motors whichhave the effect of moving the smallerchunks and pieces towards the drumwhere the knives are ready to chipthem. The hinged upper roll case is hydraulically powered to lift up when alarge chunk enters the spout or when asurge of material is present. In this waynothing jumps around in the spout but,rather, all the wood pieces are held securely and fed properly into the chipperto make just the right chip length.

The lower rolls have a second advantage in that they pass sawdust and reallysmall materials down and away fromthe chipping area, reducing the wearrate that sawdust causes in chippers. Inmost drum chipper designs there arecollection screws that catch this smallerfraction and mechanically convey it tothe back of the chipper where it can bedeposited into the over-all chip collection conveyor. In larger drum chippersthe collection conveyor situated belowthe chipper’s discharge is large enoughto catch the sawdust directly.

The second major difference is inherent in the basic design of the chippers themselves. A disc chipper mountsknives radially (mostly) on a steel discwhich is supported on a shaft thatpasses through the center of the disc.Bearings on the front and back of thedisc (typically) support the shaft anddisc and it is through this shaft that thepower of the motor passes, turning thedisc. The shaft is usually shorter on thefront of the disc but longer on the backin order to pass through the bearing andstill leave enough space to mount therear driven pulley.

A disc chipper has two types of anvilin the spout. One is the bottom anvil;the tolerance between it and the knivesis not especially critical. The side anvilknife tolerance is critical and methodsof moving the disc towards and awayfrom the side anvil have been developedby sliding the support shaft inside thebearings as well as using adjustableanvils themselves. Setting the toleranceto the “high knife”, which is the knifethat sticks up furthest from the disc surface, means that the remaining kniveswill have larger gaps.

A drum chipper’s design is quite different. The rotor is cylindrical, like adrum, with knives mounted directly intothe drum. They span the width of thedrum, so the in-feed spout is rectangular,and a drum chipper’s size is usually described in terms of the dimensions of theopening. The knives themselves are typically much thicker than disc chipperknives, both owing to the forces generated in chipping and to provide greaterresistance to damage from foreign objects entering the chipper. The curvedknife clamps conform to the shape of thedrum and hold the knives securely, supported from below by a reversible wearbar that acts like a counter knife. Thedrum is fixed onto a large-diameter shaftusing ring-feeders on both ends, makingchanging a shaft or drum a simple matter. A pair of large-diameter bearingssupports the shaft with drive pulleysmounted on one or both sides, depending on drive requirements.

The anvil unbolts and slides out ofthe drum chipper housing for sharpening. In many designs the extraction ofthe anvil is accomplished using a hydraulic cylinder. The anvil is easily accessed and can be sharpened manytimes. The knife-to-anvil tolerance ismaintained through adjustments of theknife width at the time of knife grinding. The shaft and bearings are not involved, and a simple measurementmade from outside the housing providesthe information needed to make theknife width adjustment.

Inside the drum chipper housing is acurved screen of wear-resistant steel.This screen is there to be sure anychunks or pieces generated by mis-oriented wood pieces are retained insidethe chipper until they are reduced insize and are similar to typical woodchips. A primary anvil is situated at thebottom of the spout and is mounted toa hinged carrier with shear bolts thatcan kick the anvil back and out of theway, if needed. A second anvil, at theback of the housing, traps the largerchunks for a second chipping event, ifthey need it. The hole-sizes used in thescreen are selected with the finishedproduct size requirement in mind, as isthe chip length.

Drum chippers come in many sizes,from small desktop models, to whole-logchippers weighing 25 tons and more.Most sawmill chippers are in the smallto mid-range in size and capacity andcan be shipped in 20- or 40-ft containersat a reasonable cost.

For sawmill waste-wood applications, a drum chipper cannot be beatenin terms of chip quality, flexibility, durability, maintenance access and cost containment, ease of installation anddischarge options. An increasing number of companies are making the switchfrom disc to drum chippers, and enjoythe savings in maintenance costs andincrease in chip quality.

Bruks Siwertell, www.bruks-siwertell.com

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