Clutch Drives High HP Grinding Mills

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

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

"High-speed motors using gearboxes orlow-speed synchronous motors have been available for grinding mills, butmismatched clutches resulted in a startup rate that was too fast, causingincreased stresses on mechanical systems," says Eaton's Don Keck, Airflexglobal market development manager - mining.

"The new VC clutch will enablelow-speed synchronous motors or high-speed motors driving through a gearbox toachieve full engagement at a slower rate not achievable with previous clutchsolutions, thereby protecting the power train by slowing the acceleration ofthe entire drive system."

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

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

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

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

Larger horsepower systems are also importantas customers look for higher throughput. The trend is larger mills and, ratherthan using concentrators, solutions that offer twice as much processingcapability.

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

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

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