Recalling the major power outage of August 2003, more than 50 million consumers and business owners from Ohio to New York and Canada would agree that a consistent power supply is vitally important, both to the overall economy and to the routine of daily life. And although an antiquated power grid was cited as the main cause of that outage, every element of the power generation system -- down to its smallest component -- plays a vitally important role.
About a year ago, a leading Midwest coal-fired plant was experiencing gear coupling failure in its coal pulverizer drives. Pulverizing coal results in fewer emissions and a more efficient use of the fuel as it is burned. The heat generated is used to create steam, which turns the generators, providing electric power. Depending upon its size, one plant may have up to 25 coal pulverizer drives.
Large electric motors drive the coal pulverizers by means of a worm gear arrangement, which also drives an exhauster fan that blows the pulverized coal to the furnaces. The shafts from the motor, worm gear, and exhauster fan must be connected mechanically. This is where the relatively small, yet vital component -- a coupling -- comes into play. If a coupling fails, the pulverizer can no longer operate, shutting down the whole system.
Outmoded Design
The power plant''s original design incorporated the use of "gear" style couplings, which allowed axial movement of the shafts, increasing wear on the gears and other components such as the sleeve bearings in the motors. Eventually, these components would fail prematurely.
Loss of production due to downtime was becoming a real issue; replacement costs were in the thousands per bearing. Preventative maintenance costs associated with gear couplings was becoming a burden: the plant''s rigorous preventative maintenance program required regular disassembly of the gear couplings for inspection, cleaning, and relubrication of the gear teeth. Visual inspection of the couplings was a judgment call unless extensive, time-consuming measurements were performed. In addition, heavy machinery was needed to move the motors for coupling replacement. In search of a solution, the plant manager contacted Lovejoy sales engineer Scotty Pucheu to analyze the situation.
Pucheu determined that the current gear coupling design was typical of the technology available when the plant was built, in the early 1970s. The gear couplings did not limit axial movement of the motor shaft; sleeve-type bearings were used in the motors. This, plus thermal growth of the shaft and its position at magnetic center, increased wear and shortened the lifespans of not only the motor sleeve bearings but the gear couplings as well.
"Increased sliding velocity of the gear teeth in the gear coupling creates a lot of heat and wear," says Pucheu. "It is like rubbing two sticks together. The motor moves a bit, the gear on the coupling moves a bit, and this motion could be repeated up to a million times over the course of the day, contributing to premature failure."
A possible solution would have been to install gear couplings with limited end float, but the maintenance issues would continue. Gear couplings are difficult to assess for wear, and inspection and replacement of the gear coupling still required removing the motors. Gear couplings would still have to be disassembled, inspected, cleaned, and regreased every time.
Switching to Disc Couplings
Unwilling to compromise on maintenance costs, the plant manager decided to invest in Pucheu''s recommendation to install Lovejoy SX disc couplings. Unlike gear couplings, the Lovejoy SX disc coupling design is inherently balanced, with less vibration as it wears, and performs limited end float function without special design considerations for the sleeve bearings. Maintenance-free operation is achieved because the Lovejoy SX disc coupling has no moving parts and requires no lubrication. This translates into long coupling life.
Preventative maintenance/inspection of the coupling''s flexible elements was now possible using a simple strobe light, even while the pulverizer is running, instead of expensive shutdown and disassembly. Replacement of the disc packs could now be done predictively and without moving any machinery or disturbing any original alignment.
Another bonus to the customer was that Lovejoy was able to provide, in the same space allowed by the original design, a heavier duty coupling than the gear type. Nothing had to be moved or modified for installation. From a cost standpoint, the disc component cost was comparable to those of a gear coupling.
"Until our disc coupling came along, it wasn''t practical to put a disc coupling in this system," says Pucheu, "but Lovejoy''s design allows for more capacity and misalignment capability than any other disc-type coupling available today."
Easier Maintenance
In addition, Pucheu points out that the disc coupling signals the maintenance staff when it needs replacement.
"With a gear coupling, it''s a judgment call," says Pucheu, "unless you take it completely off the machine and measure it extensively. No one can tell visually if the gear teeth are acceptable."
With the Lovejoy disc coupling, the outer leaf of the disc will crack before the entire disc coupling fails, signaling to the maintenance staff that it needs replacing. Then all that is required is to replace the disc pack.
"The coal pulverizer is critical equipment," says Pucheu. "If you''re not blowing coal, you''re not generating power and losing revenue."
The plant has been experiencing success for more than a year with the original installation. "Plant management approved the proposal to convert all gear couplings on 24 coal pulverizers to Lovejoy disc couplings," says Scotty Pucheu, "based upon the proven benefits and demonstrated cost savings."