Although gear-type couplings have long been used on paper machinery drives, new factors that have developed as these machines are upgraded to individual direct drives are causing accelerated coupling …
Although gear-type couplings have long been used on paper machinery drives, new factors that have developed as these machines are upgraded to individual direct drives are causing accelerated coupling wear and high maintenance costs. In many paper mills, replacing gear couplings with disc couplings designed for the application has reduced both downtime and wear while also helping to maintain high product quality.
One of the operating requirements for paper machinery is to maintain roll velocity, and therefore web tension, as constant as possible throughout the process. Variations in speed caused by torque fluctuation can result in flaws in the paper or tears in the web. In earlier machines powered by line shafting, tapered cone pulleys were used to adjust the speed at the roll drives. Speeds were simply set as closely as possible with dial tachometers and not changed during operation, other than during maintenance. Gear couplings performed well under these conditions, as changes to system parameters occurred slowly through wear.
Today, a highly competitive market dictates higher output and better quality with less downtime. This translates into faster machine speeds and higher temperatures. Reduced staffing levels are also a present day fact of life. As a result, the problems that can occur with gear couplings are magnified.
The trend is toward using multiple drives that incorporate synchronous speed motors, driving each roll individually through a reducer and a long jackshaft. To maintain optimum web tension, digital controls sense speeds to within milliseconds and continuously adjust the speed. This precise tension control enables producers to turn out higher quality paper by reducing the major cause of many flaws, such as wrinkling and tearing.
Most modern paper machines include both a high-speed motor coupling, which is generally a close-coupled, double-flexing gear coupling, and a long-span gear coupling between the reducer and the roll that operates at lower speed but higher torque. The high-speed coupling typically operates at the motor’s synchronous speed, between about 1180 to 1500 rpm. On the low-speed end, the rigid hubs of the gear coupling are mounted to the reducer and the roll. The gear coupling then becomes a jackshaft, mounted to these rigid hubs with a flex hub on each end. These typically operate at about 300 rpm, and even slower on pulp machines, which may run as slowly as 50 to 60 rpm.
While the large paper machines are anchored to bedrock, the motors, reducers and related power train components are mounted to the floor, rather than the frame of the machine. This results in relative movement between the components due to expansion caused by factors such as seasonal temperature changes.
Clearance compounds problems
A gear coupling requires clearance in the gear mesh, for both installation and efficient operation. This clearance works at cross-purposes to the fine control of drive speed that has become a requirement in the industry. As a gear coupling wears, this clearance or “backlash” increases. Both the rate of wear and backlash will vary from one drive position to another, depending on how well each coupling is aligned and how its lubrication is maintained. As the digital control of a drive senses this backlash, it constantly tries to compensate for it. However, the resulting false signals make it difficult to control speed accurately and can cause imperfections in the paper surface.
Lubrication is critical for gear couplings, yet it poses several problems. Batch lube couplings typically used on paper machinery must be torn down, inspected, and re-lubricated every six to 12 months. Some extended-life batch lube couplings claim longer running times but have not been available long enough to fully confirm these claims.
Higher temperatures also affect lubricants and seals, which can shorten coupling life.
Additional problems that can develop with gear couplings in high-speed service include thrust lock-up, and loss of piloting. In thrust lock-up, frictional forces cause the gear coupling to resist motion, making it unable to slide freely under load during axial shaft motion. When the close fit necessary to centre the coupling components relative to each other is lost due to wear, the result is loss of piloting. Centrifugal force during operation can cause wear and allow the sleeve to “grow” a greater amount than the hub. In this case, the centre member may shift and create an unbalanced condition.
Further, the couplings operate in a hot, humid environment, which not only corrodes unprotected steel but will cause the lubricant to break down. The result can be premature coupling failure and possibly an unscheduled production shutdown. Only when the machine is stopped, the coupling sleeve disassembled and the lubricant cleaned off can the gear and grid couplings be visually inspected. Then, the teeth can be inspected for wear and a decision made on whether replacement is required. Replacement typically takes a minimum of eight hours downtime and, therefore, is not approached lightly. Long-span couplings are a special problem, since they have solid steel intermediate shafts and are heavy enough to require two people and the assistance of a hoist or forklift to perform an inspection. Obviously, this is not a simple matter, so most mills have reduced coupling inspections to once or twice a year. Since gear couplings are a high maintenance item, they are certain to become less reliable with reduced maintenance.
The disc pack solution
All these problems have been overcome with two configurations of disc couplings designed to fit the needs of the industry. These are available in a close-coupled design for the primary (high-speed) side, between the motor and gear reducer and a long-span jackshaft design for the secondary (low-speed) side, between the gear reducer and the roll.
Gear coupling failures have been identified as a common reason for unscheduled downtimes. Even without considering the cost of unscheduled downtime and possible production loss, the switch from gear couplings to disc couplings can save an estimated 300% of the cost associated with gear couplings over a 10-year period.
Both maintenance costs and downtime can be lower with disc couplings. Since they can be inspected visually, even while they are running and without disassembly, there is no downtime or cost associated with inspection. Disc pack failures, if they do occur, are gradual. Because the flex pack is a laminated, multi-disc assembly, misalignment cracks propagate from the outer sheets of the lamination, so broken discs are easily visible. Therefore, if discs are inspected periodically, unscheduled shutdowns can be eliminated. Installation and service can be performed, on average, in one-fifth the time for lubricated couplings, most often during short stoppages. When disc packs must be replaced, the entire operation can be performed quickly, usually in less time than it takes just to inspect a gear coupling.
Disc couplings are replacing gear couplings in a growing number of mills worldwide, resulting in substantial savings in every case. Savings include the lower cost of replacing disc packs instead of entire couplings, elimination of inspection and lubrication costs, and drastically reduced spare parts costs. With disc couplings, only one parts kit and two disc packs are needed per coupling size, while a much higher inventory must be stocked for gear couplings.
David Hendrickson is an industry specialist, Thomas Coupling Division, Rexnord Corporation, Warren, PA. This is an edited version of an article that appeared in Power Transmission Design magazine.
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