Ever wondered what happened to that stove you got rid of? Or the car that was totaled and towed? Most likely, they were trucked from a scrap yard to a recycler – and fed to a monster.Known in the metal recycling industry as a shredder, this monster is a scrap-eating behemoth with a voracious appetite for anything metal.
In less than 20seconds, a shredder can chomp a car into walnut-sized pieces. Smaller morsels such as washers and stoves are chewed up and spit out considerably faster.
   Keeping the monster fed is a fairly simple matter of filling its belly with a steady diet of scrap. Keeping a shredder shredding takes engineering designs and heavy-duty components that can withstand the impact of constant crunching and smashing shocks.
To assure trouble-free digestion, one major shredder manufacturer has turned to SKF bearings and condition monitoring.

No mere machine
To talk about a shredder as a machine is like referring to New York City as a “town.” A shredder is way bigger, far more complex – and much more exciting to people who like things big and bad.
   At Texas Shredder, one of the world’s leading shredder manufacturers, Vice President Jim Schwartz refers to the shredder system as a “plant.” “A shredder plant is really a number of systems,” he notes. “It combines conveyor systems, the shredder itself and a complex scrap separation process. The whole plant can measure more than 375 feet from one end to the other.” The price tag? Between $1.5 and $5 million, depending on the size of the system.
   Texas Shredder sells shredder plants (and retrofit components) to customers worldwide. These customers are recyclers who make their money selling scrap metal to steel foundries. With the enormous price tag for a shredder system, most recyclers have only one plant. And that puts tremendous pressure on shredder engineering and maintenance professionals to keep the system up and running. “If a shredder goes down,” notes Schwartz, “a recycler could lose tens or hundreds of thousands of dollars in productivity and parts replacement.”

Shock and pressure
Shredding cars is rugged work. “There’s an extremely high shock load,” notes Schwartz. “If something doesn’t want to be shredded, the whole machine shakes like crazy. This constant shock and vibration, along with the contaminated environment inherent in this operation, can easily lead to component damage.”
   The most likely place for a problem to occur is in the shredder itself. The biggest shredders, powered by a 7000horsepower electric motor, operate through a fairly straightforward – but very large – system of main components, including automated infeed rollers, a 200-ton shredding box with a horizontal rotor (with multiple hammers) and SKF double spherical 420mm bearings. When operating, this combination of components can, at top speed, shred a complete automobile with the motor turned off, simply through the weight of the rotating mass.
   This never-ending cycle of vibration and shock also puts pressure on the engineering and maintenance staff responsible for keeping the systems up and running. At American Iron and Metals, a Texas Shredder customer situated in Montreal, Canada, eight hours of every day are spent feeding their shredder – and another four hours are spent on maintenance. “This is a tough operation,” notes Production Manager Pierre Paquet. “Everyday, we have to inspect, repair and grease our liners, hammers, conveyors and other components. It’s never ending.”

SKF: In the belly of the beast.
To enhance and assure reliable performance of its shredders, Texas Shredder uses SKF spherical roller bearings exclusively for the main rotary shaft. With bore measurements of up to 420mm (16.5inches), and outside diameters of up to 620mm (24.4inches), these bearings are large in both size and value. To help recyclers protect their investment, Texas Shredder has begun incorporating an innovative SKF enveloped acceleration monitoring system on those bearings. “The bearing is critical,” notes Schwartz. “If there’s a problem in the bearing that goes undetected, a recycler could end up with a catastrophic failure that could put them out of business for weeks.”
   Unlike other systems that monitor multiple machine parameters, the SKF enveloped acceleration monitoring system is focused only on bearing vibration. The system uses an advanced signal processing technique that filters out all machinery vibration except that occurring in the bearing defect frequency range. Once the bearing frequency range is isolated, the system looks for higher harmonics of repetitive, impulsive signals that could signal the early stages of a defect. Shredder machine operators are warned of possible problems through an alarm system and are able to review the vibration trend on readouts. If the trend indicates possible damage, the bearing can be inspected during the next scheduled maintenance downtime.

An essential system
American Iron and Metals’ Pierre Paquet views the SKF bearing monitoring system as absolutely essential. “If we end up with a damaged bearing, we lose days of production, at a cost of $2,000 per hour. A new bearing can cost as much as $20,000. A crane has to be brought in to lower the 40-ton rotor onto the ground. It becomes a major operation and a major loss. The SKF enveloped acceleration sensor prevents us from getting to that point.”