May/June 2005
Deciding which knives, hammers, and other wear parts to buy for your shear or shredder—and when to replace them—doesn’t have to be a wearing experience. These tips can guide you.
By Lynn Novelli
Shears and shredders are the workhorses of the scrap recycling industry. Day in and day out, recyclers depend on them to maintain production and maximize quality, consistency, and profitability. Keeping this equipment operating in top condition is essential, which is why purchasing the right wear parts can be such a critical decision.
“Several factors come into play in the decision-making process, and there isn’t a single ‘right’ answer for everyone,” says one manufacturer of shear knives. “The customer can rate the performance of a part based on the parameters set forth by the original manufacturer, but it’s also a good idea to try a variety of [parts] manufacturers to compare performance and see which works best for a specific application.”
Remember, though, that buying parts from anyone other than the original equipment manufacturer will usually void the warranty on your shear or shredder.
This is the case even though most equipment producers don’t make their own wear parts; instead, they buy them from domestic or offshore wear-part manufacturers. With a few exceptions, equipment manufacturers don’t have the tooling or the back-end infrastructure to produce and maintain a full inventory of wear parts.
That’s one reason why most operators rely on a number of aftermarket sources for their shear knives and shredder hammers—probably the two highest-consumption wear parts—once the manufacturer’s warranty expires.
A knowledgeable parts salesman will be familiar with equipment brands as well as the OEM part numbers and will have specifications and/or drawings on file for most of the standard ones, notes one manufacturer of shredding equipment. “For custom parts, or in the event we don’t have a drawing or specifications to work from, we’ll send out our engineers to the site to take the measurements from the equipment,” he explains. “They’ll create the drawings, and we can cast the parts to the exact specifications.”
Metal recycling is one of the most demanding applications for wear parts. Operators have to consider the density, thickness, temperature, and metallurgical content of the metal being processed when specifying or purchasing wear parts.
How such parts are designed and manufactured will affect how they perform under the unique stresses that the equipment experiences in a scrap recycling operation. Wear-part performance ultimately affects efficiency and profitability, so it’s worth investing the time to learn how to make the best purchasing decision.
Staying Sharp
Shear knives are made of steel alloys that contain some mix of nickel, chromium, manganese, and carbon, notes one manufacturer. The alloy composition or chemistry is the number-one factor that determines the quality of a shear knife. The mix of alloying elements influences the steel’s stability, strength, overall performance, and resistance to cracking during heat treating and is basically what differentiates the various knife brands.
Consequently, “shear knife manufacturers are very protective of their proprietary material specifications,” says one blade vendor. “The percentages of each element within the alloy are critical, which is why they remain tightly held secrets. Change the percentage of any one of these elements within the alloy mix, and you dramatically change the performance characteristics of the knife.”
The ideal chemistry allows the blade to undergo heat treating sufficient to maintain edge retention while maintaining the blade’s hardness and preventing cracking or chipping. High-carbon steel alloys that have been designed to provide wear resistance and toughness combined with high strength make top-quality blades that command a premium price. Specialty elements in knives such as tungsten carbide also are available for unusual high-stress applications.
The heat-treating processes to harden and temper the blades are another factor that affects the quality of the finished knife. The temperature applied and the duration of exposure, temperature of the quenching solution, the quench rate, and the tempering heat and timing all make a difference in the knife’s performance characteristics, experts note.
Finally, the manufacturing process itself makes a difference in blade quality. To ensure that the blades seat securely in the shear during operation, they need to be machined precisely and ground to exact dimensions. High-quality, good-fitting blades cost a little more but yield the maximum performance and wear characteristics, sources maintain.
With manufacturers guarding most of the specifics about their products’ chemistry and heat treating, it’s up to the shear operator to determine which knife brand meets his requirements.
Operators generally track knife performance per tons sheared or hours per edge during normal usage, with the goal of achieving the maximum life per edge at the highest efficiency and maximum blade quality at the most economical cost. To make valid comparisons, the operator should consider service life (how often edges need to be rotated or blades need to be changed due to normal use) and failure rate (breaking or cracking). They should also follow bolt-torque specifications to optimize blade life and get a true indication of the wear parts’ performance.
If I Had a Hammer
Shredder hammer manufacturers owe a debt to Robert Hadfield, an Englishman who, in 1896, developed manganese steel, the primary alloy used today in the production of shredder hammers. By definition, Hadfield manganese steel contains between 11 and 14 percent manganese and 1 to 1.5 percent carbon.
Hammer manufacturers today tweak those basic specifications to meet their own manufacturing and heat-treating requirements. Though each hammer manufacturer’s exact recipe is proprietary, “the closer we can be to 1.2 percent carbon, the longer the hammers last,” one manufacturer notes. Make any change in the chemistry—by adding molybdenum, for example—and all the elements must be adjusted to ensure an optimal carbide level.
When carbon levels approach the ideal 1.2 percent, the carbides all go into solution during the eight to 12 hours of heat treating and realign on the faces of the grain. At carbon levels greater than 1.2 percent, too many carbides remain on the grain boundaries, which ultimately will cause the hammer to break during service.
Carbon levels, the percentages of other carbide-producing elements in the mix, and the temperatures applied during heat treating and quenching all play a role in determining hammer quality or grade.
Heat treating partially hardens Hadfield manganese hammers, and they undergo work hardening up to Brinell hardness of 500, caused by the deforming of the grain due to impact. Low-alloy steel castings, appropriate for some applications, achieve their maximum hardness in heart treating and do not achieve a work-hardening effect.
Commonly used parameters to estimate the efficiency of shredder operation include the cost per ton for casting usage and replacement as well as the number of tons produced over a hammer’s service life. The goal is to maximize the ratio of castings purchased to the amount of castings discarded, which will depend in part on the efficiency of the hits throughout the hammer cycle, according to a hammer manufacturer. Though every manufacturer has its own preferences for the most efficient shape of hammer design, most hammers are variations on a bell shape.
In addition to the cost of castings per ton of shredded steel produced, shredder operators also track the consistency of casting quality, which determines casting life and the frequency of replacement. Consistently producing the same quality of castings depends on the manufacturer’s ability to keep the alloying agents within narrow ranges. Some shredder operators have reported consistency problems with castings produced offshore, but the major U.S. manufacturers generally produce castings of consistent quality and chemistry, sources note.
Up, Up and Away
Scrap recyclers everywhere are feeling the effects of higher prices for wear parts. Parts prices are always related to quality, but right now outside influences are exerting price pressure on parts across the board, at every grade.
Scrap industry fundamentals exert a strong influence on the price of wear parts, says one parts manufacturer. “Right now the cost and availability of high-quality manganese steel scrap and standard charge scrap are contributing to [parts] price increases,” he explains.
The availability and price of ferroalloys are also exerting upward price pressure, he says. As an example, ferromolybdenum was roughly $9 a pound in January 2004. By April 2005, the price had gone as high as $38 a pound. Manganese prices are also rising.
Oil prices are another influence on the cost of castings. The benzene-based sand binders used in the casting process are petroleum derivatives, so their price jumped in concert with skyrocketing oil prices in the first quarter of this year.
Everyone in the business is struggling with how to cope with these huge increases in raw material prices. “There’s no way for us or any manufacturer to control these factors,” says one vendor. “We’re dealing with the situation by adding a surcharge to our base prices, but we don’t know where [prices] are going to go.”
Still, some manufacturers are trying to hold the line. “We try to hold our prices for a full year,” says one blade producer. He considers price stability a way of differentiating himself in the market, though it can be a challenge. “The steel market volatility pushes the price of blades, so there’s some degree of prognostication that goes into our pricing strategy,” he adds.
Pricing issues have filtered down even to the hammer pin manufacturers. Pins are produced from steel bars or plate, which means that pin manufacturers are equally at the mercy of the steel mills. “The key factor in pricing is the cost of the bar,” says a pin vendor. “As the mills increased their prices based on supply and demand, we’ve had to raise our prices.”
With everyone’s prices rising, wear parts manufacturers are looking at other ways to differentiate themselves in the market. “Customer service is key in this business, including parts availability and warranty,” notes a wear parts salesperson.
Some manufacturers offer same-day shipping, and most offer continuous availability on all standard parts. For specialized applications, many manufacturers have the capability to custom-design and temper parts to the customer’s exact specifications.
In the current pricing environment, there’s also a big emphasis on product differences such as cast vs. forged blades or through-hardened vs. surface-hardened pins and hammers. Even among similarly priced parts, there can be differences in metallurgy or chemistry, so parts buyers need to do a little research before making a decision, the experts recommend.
Improving Service Life
With prices going higher, it makes sense to try to maximize the service life of wear parts. The best way to do that is through regular maintenance of the shear or shredder in accordance with the original equipment manufacturer’s guidelines, including attention to the wear parts. Ensuring that knives are sharp and in alignment and that hammers are positioned accurately in the rotor, for example, can prevent breaks, maximize service life, and reduce the stresses on the shear or shredder.
Properly adjusting shear blades takes time but pays off in longer parts life, say blade manufacturers. Settings should be adjusted to maintain the correct cutting-edge tolerance specified by the manufacturer based on the gauge and metallurgical properties of the metal being processed.
“The operator must be able to recognize the symptoms that indicate it’s time for maintenance or blade replacement,” says one blade producer. Symptoms to look for include excessively worn or dull cutting edges, gouges in the cutting edge, galling, tapering, or uneven edge wear.
Blades are designed to be rotated to a fresh edge when one surface is worn, and delaying that rotation beyond the recommended life of the edge can reduce the blade’s ability to seat properly, causing instability, the blademaker adds. Proper seating also relies on the tightness of the knife bolts: They should be tight enough to keep the knife stable but not so tight that the threads stretch and the bolt loosens.
Like blades, shredder hammers must be matched to the application for optimal service life. Metal scrap today contains a higher proportion of heavier material, so it’s important that the hammers be cast strong enough to withstand the impact. Ensuring that scrap is as clean as possible—free from contaminants and nonshreddables—when it goes into the shredder will also extend hammer life.
Another point is that hammers must be positioned for the optimal clearance with the wearing castings. The correct internal clearance varies depending on the type of shredder, so the best advice is to follow the manufacturer’s directions. Also, operating a shredder at a lower rpm will extend the life of the hammer but at the cost of productivity, explains a shredder manufacturer. “Operating at higher rpms yields greater strike force and better density, so productivity is higher, although higher rpm hits cause more wear,” he notes.
Predicting the service life of knives or hammers is virtually impossible because wear depends on the type of metal being run, the size of the equipment, the size of the knives or hammers, and the frequency of use. As one source notes: If the blades are run down too low, the blade can crack or break and the shear will be down until new blades come. One operator estimates that every day his shear is down costs him $10,000. That’s why the operator needs to either order new blades ahead of time or have a backup set of blades on-hand to eliminate downtime.
As a general rule, manufacturers recommend that knives should be rotated or changed when the cutting edge becomes worn to 1/8- to 3/16-inch radius from the original cutting edge, with a 1/4-inch radius showing excessive wear. Hammers should be replaced when they have less than half of their original weight remaining. This usually allows for even hammer replacement. But if just one hammer needs to be replaced due to a problem, remember that replacing the opposite hammer as well will help keep the rotor in balance.
Likewise, when you have to replace a wear part, finding the right balance—between production demands and maintenance requirements, costs and quality, among other factors—can make the whole process a bit less wearing on your business.
Lynn R. Novelli is a writer based in Russell, Ohio.
Deciding which knives, hammers, and other wear parts to buy for your shear or shredder—and when to replace them—doesn’t have to be a wearing experience. These tips can guide you.