Knife Holder Advancements Cut Impact of Vibration

New holder construction and side load control can maximize knife life, no matter what your running speed.

By Dave Rumson, National Sales Manager, Dienes Corporation

In a perfect slitting world, converters would have total control over any given knife set-up, no matter what materials or speeds they were running. Top and bottom knives would locate perfectly. All rotating knives and shafts would have unchanging zero- dimensional run out. The shear angle of top-to-bottom knives would be ideal, and knife blades would last forever, never getting dull. External effects such as vibration would not affect slitter performance.

The Utopian operator would always produce every single roll perfectly, regardless of all technical issues, for the next process. Perfect width, slit edges, dust limits and efficiency for perfect profits.

In reality, we know it's simply not that easy. From operator to operator and shift to shift, knife set-ups vary. With time, most knife holders wear and loosen, and knife axial and radial (dimension) run outs worsen, resulting in shorter periods between knife replacements. Slit quality lessens, and the operation becomes dustier.

A quick way to test knife holder integrity is, while wearing protective gloves, to grasp the extended lower knife head and try twisting it back and forth. Any movement that results may be equated to lost profits.

Insidious Damage

Not often considered, machine vibration inflicts insidious damage to knife blades. Running at higher production speeds, even low vibration levels can cause cyclical impacting of the top knife to bottom knife, resulting premature metal fatigue.

For converters that strive for those Utopian slitting goals, a new generation of knife holders is available to enable them to slit with better quality and lower expense. Advancements in holder construction and knife side-load controls can maximize knife life, no matter what the running speed may be.

One example is the Dienes DF-145 pneumatic shear knife holder (photo to left). This component had dual, preloaded linear ball bearings surrounded by a rugged 35-lb steel frame. Its benefits include these features: no slip fitting, no key ways and no wearing components. Mounted on dual linear rails and a stiff crossbeam, the design has no loose-fitting parts to be worn down by vibration.

Another recent holder development is the Dienes DF-150 Constant Side Load pneumatic knife holder. Knife holders typically use mechanical return springs to push the top knife away from the bottom knife when the operator is stopping the slitting process. Air pressure shuts off, and the horizontal cylinder collapses under the return spring force.

If the horizontal-stroke, starting location distance of the top knife varies from set-up to set-up, the amount of top knife to bottom knife contact side-load force will also vary. This is relative to the amount of force applied by the return springs. Forces could be from 3 to 25 lbs at 30-psi input, depending on which supplier manufactured the knife holder.

The DF-150 removes the return-spring force factor. IT allows only regulated air pressure to set and control how much mechanical force is developed when contacting the top knife to the bottom knife. Because converters can now control the level and consistency of knife to knife contact force, knife life can be maximized.

Damping down vibration

This new arrangement makes the effects of vibration and run-out on slitter performance negligible. With the return spring not impacting side-load force, the quicker responding air cushion acts as an efficient damper to prevent the top knife from leaving the bottom knife.

Mounted on a dual linear rails and a stiff crossbeam, the DF-145 design has no parts to be worn down by vibration.

Such "constant side load force" control can help move the slitting operation closer to that distant Utopian goal of perfection. Converters can empirically establish the optimum force level for a given material and significantly reduce knife-replacement costs.

Dave Rumson is national sales manager for Dienes Corporation, Spencer, MA. He can be reached at 800-345-4038 ext. 227, Fax: 508-885-3452, email: drumson@dienesusa.com

Photo: Constant side load force

Constant side load force control can reduce vibration on knife holders, cutting metal fatigue and extending knife life.