Clutch Drives High HP Grinding Mills

Eaton Corp. announced it has expanded its Airflex^® clutch line by creating the world's largest clutch for the mineral processing industry. The Airflex 76VC2000 dual clutch system transmits rotary torque from high-speed electric motors that drive gearboxes or low-speed synchronous motors to high-performing mills up to 14,750 hp per pinion. The clutch's 76-inch-diameter friction drum enables it to transmit adequate torque for large grinding mill-drive applications.

The clutch offers a new motion control solution for higher horsepower grinding mills, which have been driven primarily by clutchless low-speed motors, by offering a very slow startup rate that keeps component wear to a minimum and reduces power consumption.

"High-speed motors using gearboxes or low-speed synchronous motors have been available for grinding mills, but mismatched clutches resulted in a startup rate that was too fast, causing increased stresses on mechanical systems," says Eaton's Don Keck, Airflex global market development manager - mining.

"The new VC clutch will enable low-speed synchronous motors or high-speed motors driving through a gearbox to achieve full engagement at a slower rate not achievable with previous clutch solutions, thereby protecting the power train by slowing the acceleration of the entire drive system."

Keck says Eaton has gradually increased the size of clutches over the years as application requirements have grown, and the availability of new gearing has enabled systems offering larger horsepower. Until recently, for example, a single pinion was limited to 9,000 hp across the gear faces, but with the advent of new gearing technology and grinding alloys, systems can handle up to 15,000 hp per pinion.

Working with GE's Large Motor Division, Eaton is supplying systems that often use two large synchronous motors and two pinions to drive a common bull gear. This approach saves the end user on power consumption because a low-speed synchronous motor offers a power factor where the system can actually put power back on the grid.
"With the availability of 15,000 hp per pinion and dual pinion drives, systems can produce a combination of 30,000 hp which is the current state-of-the-art," says Keck. "Our idea was to make the existing 76VC1600 wider to increase torque capacity by 25 percent per element."

Because low-speed synchronous motors need to start "no loaded" when coming up to speed, a key advantage is very low power consumption at start-up compared to mills that are direct drive and have very large power consumption on start-up. Keck says that nearly all of the power companies in the U.S. charge based on peak power rather than average power, so power spikes can cost millions of dollars per month.

"With a low speed synchronous solution, we use 70 percent less power just by providing the low-speed start," says Keck. "Start-up times can be extended over seven, eight, nine or 10 seconds which allows the gearing, motors and mill to come up-to-speed and keep the power spike much lower. Until now, solutions were to the 9,000 horsepower range but now we have made a quantum leap and moved into the 14,000 to 16,000 horsepower range depending on system speed requirements."
The clutch technology also offers benefits in terms of protecting the power train by cushioning the impact of the motor. The clutch is the "fuse" in the system and, even though the clutches are costly, motors and gearing are multi-million dollar pieces of equipment. The clutch functions as a torque limiter to protect the bearings and gearing, and also protects customers from excessive downtime due to equipment damage.

Larger horsepower systems are also important as customers look for higher throughput. The trend is larger mills and, rather than using concentrators, solutions that offer twice as much processing capability.

"The key to the clutch technology is that it is ultimately simple," says Keck. "There are no wear adjustments required, and no significant downtime is required to repair and replace the unit. The only moving parts are the actuating air tubes that inflate as we supply air from the air tank group."

Advancements in auxiliary systems include complete air tank systems and electrical controls. The latest innovation is a "slip detect" control which monitors rotating components and feeds information back into a PLC. The system can be programmed for an optimum start time, for example, so if the customer sets the equipment to start too fast, the system will flash a light indicating to slow the system down. If the system is set up to start too slowly and exceeds the pre-programmed limit, it aborts the clutch rather than letting it slip.

"It monitors the system during rotation so that if something goes wrong in the mechanical or electrical system and we see a speed differential between the motor and gearing, the system also abort the clutch to protect the clutch, motor and grinding mill," says Keck. "The 76VC may be the world's largest clutch but it is also the world's simplest. The beauty is in its simplicity, and it's been this way for many years."