EHS Spotlight: Controlled Failure

Jun 19, 2019, 16:53 PM
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May/June 2019

By Rachel H. Pollack

EHS-Spotlight-Controlled-FailureFor City Scrap & Salvage (Akron, Ohio), one unshreddable incident that created significant equipment damage very easily could have cost a worker his life. Neil Smith, City Scrap’s ferrous operations yard supervisor, and Steve Murphy, the plant manager, told attendees at ISRI’s September 2018 Shredder Operations Forum about this near-miss experience.

City Scrap’s two 1,500-hp Waukesha natural gas shredder engines connect to a belt-drive system that turns the drive shaft and shredder rotor. The shredder has a reject door, Smith notes, but even the first hit of an unshreddable object can cause extensive damage. On this day, he says, “a bad solid went through, and we rejected it, but I could sense a vibration. We shut the system down, and one of the pickers came around to see what was happening” in the engine room at the same time he did.

What they saw was a 1,100-pound piece of a shaft that had broken and was “bouncing around” in the engine room. “Seven of the 12 belts were broken, spinning at several hundred rpm, and sparking. The worker is looking at this, open-mouthed, and I told him to run,” Smith says. No one was injured, but “it could have been a fatality, and it could have shut us down for weeks or months,” Murphy says.

After that incident, City Scrap & Salvage installed a hydraulic torque overload coupling on its shredder drive shaft. Since then, Murphy says, it has “saved our shaft at least four times” when the facility has had an unshreddable event, and the operators have resolved each overload in about 15 minutes. “We could have saved a life by now, and we’ve saved considerable downtime and a lot of headaches,” Smith says. “I would not operate a shredder without it,” Murphy adds.

Scrap asked Jim Fraser, product sales manager, Americas, for Voith Turbo (York, Pa.), to explain this and other tools that can improve automobile shredder drive shaft safety.

What are the typical components in a shredder drive shaft? The shredder drive shaft, or driveline, connects the motor to the shredder to allow the transmission of torque, which allows the shredder to operate. Without the drive shaft, the shredder will not function. The typical components are the universal joint shaft combined with a motor shaft coupling and shredder rotor coupling.

Why is the drive shaft the “weak link” between the motor and the shredder rotor? The drive shaft is actually designed to be the weak link of the shredder driveline. This helps when there is an operational malfunction, namely when an unshreddable hits the system. When that happens, the drive shaft breaks or bends instead of allowing the torque overload caused by the unshreddable to damage the much more costly motor or shredder. Among the three main components, the drive shaft is the least costly.

How can a drive shaft failure result in injuries and equipment damage? When there’s an unshreddable, the shredder may stop, but the motor and the universal joint will still continue to rotate due to inertia in the driveline. Or, even worse, the motor may remain energized and deliver full motor torque to the universal joint shaft. When the universal joint shaft can’t deliver the torque to the shredder due to the malfunction, the universal joint shaft or the motor could catastrophically fail, leading to an uncontrolled release of stored energy. This creates an unpredictable situation that puts people and equipment in extreme danger.

What engineered solutions can reduce the hazard? A breakaway coupling installed in the driveline, which directly replaces the motor shaft coupling or the shredder rotor coupling, reacts automatically upon an unshreddable event to reduce the torque, thus reducing the likelihood of drive shaft failure and the injuries and equipment damage that could result. Another safeguard is a driveline monitoring system, which allows the shredder operator to predict the universal joint shaft’s operating life to avoid catastrophic failures.

How do breakaway couplings work? The design we offer, a hydraulic torque overload coupling, transmits torque via friction created by pressurizing hydraulic oil within the coupling. The oil pressure causes the coupling to expand and completes the mechanical connection between the motor side and the shredder side. These couplings are designed to release at a precise torque level that is lower than the designed torque limit of the other driveline components. When the torque exceeds that limit, the pressure will release and safely stop transmitting the torque from the motor to the shredder. This reduces the risk of operator injury and protects the complete driveline, ensuring unshreddables won’t damage the driveline or the motor. Operators can easily reset the coupling after a release by replacing a shear tube and then resetting the hydraulic oil pressure, minimizing downtime. You can adjust the coupling torque release set point within a predetermined range to use the perfect set point for each application. Standard maintenance requirements include periodically changing the lubrication oil in the coupling. And these couplings are customizable, making them easy to retrofit into existing shredders. We make the case that this one-time investment provides reliable, long-term torque overload protection that pays for itself every time it prevents a torque overload and minimizes the risk to people and equipment.

A less-expensive alternative is a shear pin torque overload coupling, which transmits torque via metal pins connecting the two halves of the coupling. The pins have been designed to break and sever the connection when they’re subjected to excessive torque. This achieves some torque overload protection, but we don’t consider them the best available solution. The release point can be inaccurate due to the inconsistent metallurgy of the pins and the uncontrolled combination of forces acting on the pins. We’ve found shear pin couplings are prone to false releases due to fatigue and wear, resulting in lost production time and requiring extra unplanned maintenance. When they do release, resetting them can take a significant amount of time, and torque adjustments when you install and reset them can be complicated and inaccurate. They require regular rebuilds with new shear pin bushings. And they come in a standard design that can be complicated and costly to retrofit into an existing shredder driveline.

How does a driveline monitoring system work? It consists of electronic sensors measuring torque on the universal joint shaft and vibration on the motor and shredder rotor bearings. The sensors provide readings via a Windows-based industrial PC with an SQL database and ring buffer. The system analyzes the torque and vibration data and generates trend reports, alarms, and an estimate of the shaft operating life. This feedback and long-term documentation of motor and shredder condition can result in increased safety for operators and workers. It also can reduce maintenance costs by allowing condition-based maintenance, planned downtime and repairs, and fewer maintenance stops. The monitoring system is modular and can be accessed via computer remotely within or outside the facility.

Do you recommend both the overload coupling and the monitoring system for each shredder? Yes, both technologies are very beneficial for optimizing the performance of shredders. In safety terms, the breakaway safety coupling is the fail-safe device protecting against unavoidable unshreddable events, whereas the monitoring system helps prevent catastrophic drive shaft failures by predicting operating life.

What do these technologies cost, and how does that compare to the cost of a driveline failure? The cost of the breakaway hydraulic coupling is comparable to the price of a universal joint shaft replacement, which makes for easy budgeting since it will limit the facility’s need to replace that shaft in the future. The same is true for the monitoring system. But preventing just one unshreddable-created failure event and the injuries and equipment damage that it can cause should more than justify this investment.

How common are these technologies in North American scrapyards? Both technologies are new to the industry, although some shredders have been operating very successfully with hydraulic breakaway couplings for over 10 years. The acceptance of new technologies, even when well-proven and documented, can be slow in a mature industry like auto shredding. However, once shredder operators begin using them, they will be very reluctant to work without them in the future.

Rachel H. Pollack is assistant vice president of publishing and strategic foresight and editorial director of Scrap. Reach Fraser at 717/767-3294 or, or visit


Other Tips For Driveline Safety

What’s the best way to prevent an unshreddable from wreaking havoc in your shredder? Prevent it from entering the shredder in the first place. Thorough material preparation and inspection are the first line of defense against driveline failures caused by unshreddables, according to shredder, motor, and driveline equipment providers. “Never run bales or logs from unknown sources without first breaking them apart,” says Joe Kowalkowski, vice president, large drive division, of Quad Plus (Joliet, Ill.). “Look in trunks of cars” and other places where unshreddables might be hidden. “Good inspections can drastically reduce unshreddables.”

Unshreddables are not the only cause of driveline failure, Kowalkowski notes. Mechanical torsional resonance is another concern. “All rotating equipment has a natural frequency that it vibrates at,” he explains. “When the frequencies of this equipment are excited because of incorrect drive shaft design or changes in the mass of the system, they can become very destructive.” He urges equipment manufacturers and end users to “take the necessary engineering steps to ensure the mechanical system, including foundations of the machine, won’t introduce a resonance.”

A breakaway coupling can reduce the effects of mechanical torsional resonance but can’t detect or solve the problem, Kowalkowski says. For detection, Quad Plus strongly recommends that shredder facilities conduct a geotechnical survey, first to establish a baseline for the soil conditions around the mill “after the shredder has been running for some period of time,” with additional surveys “every few years or after a catastrophic unshreddable event.”

He explains that “unshreddables can cause damage to the foundation or can change the interactions between pilings in the foundation and the soil. This … changes the friction and ultimately the mass of the system,” which can result in harmful resonance. “We’ve seen this happen on machines that have run successfully for years,” he says. After an unshreddable event, “from that point forward [they] have problems bending or breaking motor shafts or damaging drive shafts.”

Wendt (Buffalo, N.Y.) offers an integrated drive shaft restraint, “a heavy-duty enclosure that surrounds the drive shaft and prevents injury to personnel or damage to surrounding equipment in the event of a catastrophic failure,” explains Ethan Willard, business development manager. “By fully enclosing the drive shaft, [it] also prevents access to the moving parts while in operation. This restraint comes standard with all of our shredders and is mounted directly to the shredder itself.” The company offers additional technology to improve driveline safety, “from belt drive systems on our smaller shredders to fluid couplings and torque limiting couplings on our larger shredders,” he says.

“The best way to ensure a safe environment around a shredder drive shaft is to have a strict safety plan in place for all employees,” Willard adds. Train all employees to stay away from the drive shaft and the shredder while it’s in operation, he says. Keep the drive shaft guard locked in place at all times during operation, and follow lock-out/tag-out procedures if you ever need to remove the guard.

When a 5,000-pound drive shaft on an automobile shredder breaks, severe injury and equipment damage can follow. Breakaway couplings and driveline monitoring systems are two approaches to controlling this hazard.
  • 2019
  • May_Jun

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