Replacing cutters on a shear shredder regardless of number of shafts is a significant expense and affects most operation’s bottom line if not done timely. Whether using hours, shear gap, or tons processed, most businesses know over time what works best for them and these decisions are predictable. Thus, preparing ahead of time is feasible and can reduce total downtime if done correctly.
First, review the overall wear characteristics and ensure the materials being processed have not changed and adversely affected the cutters. Cracks, chipped areas, severe rounding, flaked hardfacing, or large shear-gaps may indicate another alloy should be used. First, consult the shredder manufacturer before making a call as they usually have good insight from their customers and order history on what cutter alloy has the best track record. This is also an opportunity to see if the cutter thickness, hook height, and pattern/number of hooks is optimized for the materials processed. Talk to the operators or review PLC logs to review shredder reversals per hour and decide if the number is excessive. Reversals reduce throughput and put additional stress on shredder drive systems.
Next, review the other parts that may need to be changed due to wear including splash plates, seal retainers, bearings, and other wearable surfaces. Most shredder manuals have a diagram with a bill of materials that should be used to put the initial list together. If in doubt on whether the part will need to be changed, error on the side of caution if the part is a long lead time item and could derail the cutter change timeline. Of course, ask the manufacturer if they agree or see a need for further parts, but starting this on your own will help when reviewing the parts quote and cost. As a good practice, order extra seals and each type of extra fasteners in case they are damaged or lost during install.
Special tools may be needed for the cutter change depending on the make of shredder. Large fastener specific spanner wrenches are often required to tighten large shaft nuts. Tools for installing seals correctly may also be called for. Some manufacturers offer a special clamp to allow the initial first cutters placed on the shafts to be tensioned for a quick check of the shear gap before all the cutters are stacked on. Shaft protectors put over the shaft ends after the end plate is removed are common and keep the shaft from becoming damaged as cutters are removed and replaced. Besides the specialized tools, conventional types should be bought prior as well. These tools may include sledge hammers, feeler gauges, honing stones, lifting straps, shackles, port-a-power type jacks, ratcheting chain hoists, swivel eye bolts, air tools, torch set, and appropriate sized sockets and wrenches.
Consumables will be easy to acquire prior but do save time being on site during the job. Oil usually has to be removed from the main gearbox and should be on hand meeting the manufacturer’s recommendation and your climate conditions. A large amount of rags and clean up supplies should be on hand. Grinders, flap wheels, emery cloth, honing stones, penetrating lubricant, and appropriate solvent to remove cutter anti-rust oil or coatings should be ready. Even renting a parts washer if one is not already present can be a large timesaver. Cleanliness: consider buying or renting carpet runners for the duration of the project to be used in the office or areas where flooring could become soiled. This will help avoid any damage and frustration.
Safety is the most important part of the preparedness efforts. A risk assessment by a safety professional should be done prior that covers the rigging plan, chemicals used, access, elevated work, and lift equipment. Isolating the area from staff will be required to ensure a safe work zone. Safe shredder chamber access plans and fall protection for the blade change team need to be included in the safe work plan. Using forklifts, telestackers, or other powered lift equipment for all heavy parts movement can prevent lifting injuries. Avoid standard lifting eye bolts if possible since they significantly weaken if any off vertical loading is placed on them – swivel types with an actual lift rating when properly installed are superior. Many shredder manufacturers make a special tool to lift the cutter for use with forklifts and small cranes which should be purchased. Appropriate labeled containers should be on hand to receive any hazardous wastes generated from the blade change and cleaning activities.
Once the materials arrive, the next phase of preparing begins: verification and cleaning. Prepare an area large enough for the unpacking and isolate from normal plant traffic. Open all crates and check the cutters and parts against the purchase order and packing slip. Include checking quantity of the different types of cutters (single hook, double, etc). Consider using a micrometer to measure all cutter and spacer thicknesses and write the measurement on the outside of each with a paint marker. This measurement will come in handy as the blades are stacked on later to keep track of progressive error. Depending on the brand and cutter stack design, shear gaps can range from 0.010” to 0.030”. If the first few cutters on the stack are slightly larger, the error can add up to enough of a difference to cause the cutters to rub further down the stack which would require unstacking the machine and double work to remedy. At the same time, the cutters can be cleaned especially if an anti-rust coating is present. Once complete, cover all materials and keep them free of dust and dirt. If your manufacturer already marked the measurements, then perform QC on a percentage of them you feel comfortable with.
Depending on the shredder size, pre stacking the cutters and arranging them based on the order they will be installed can same time later. The manufacturer supplied stack sheet will have instructions on the orientation of each cutter to make sure every hook ends up in the correct pattern – this is critical to follow otherwise the shredder performance could be compromised by too many or few cutters engaging at once. Painting a temporary line down the stack of cutters is a simple QC measure for verification of correct blade orientation once on the machine if the pre-stack approach is used.
Staff and skillset: For some facilities, a blade change may be a new activity for the maintenance team. Besides studying the manual and procedure, consider enlisting the manufacturer’s service team or another millwright or shredder specialist company. Although sometimes costly, an ROI can be achieved by a shorter downtime duration and minimal errors. The decision will come down to risk. Having an experienced party on hand for the cutter change is insurance against re-work or a longer than planned downtime event.
Preparing for a cutter change prior is a wise investment and usually yields a return. As the team undertakes this best practice, each time it will become more routine and lead to faster cutter change cycles.
I hope you enjoyed this short article and find it helpful to your business. If you would like to learn more about our company, Converge Engineering, please follow us on LinkedIn. We are experienced shredding and materials recovery solutions engineers and develop plants with an operator’s point of view and would enjoy speaking with you about your shredding or recovery challenges.