Couplings Bring Paper Machines Under Control

By eliminating coupling backlash, disc-pack couplings make paper machines faster and less troublesome

Paul T. Harkness, Esco Couplings NV.

Gear-type couplings have long been used in paper machinery drives. But today's emphasis on faster speeds and more accurate control, combined with higher temperatures and a reduced work force, tend to magnify problems with gear couplings. As a result, coupling failures are said to be the most common reason for unscheduled downtime in pulp and paper mills.

Paper machines must maintain roll surface speed to ensure constant web tension. Otherwise, speed variations can cause flaws or tears in the paper. Early machines were driven by line shafts, with tapered cone pulleys to adjust roll speed. Operators simply set the speeds as close as possible with dial tachometers and didn't change then during operation. Gear couplings, though susceptible to wear, performed well under these conditions because wear related changes to system parameters occurred slowly.

Upgrading the machines to direct drives with digital controls accelerates wear of these couplings. Direct drive machines with modern digital controls are also more sensitive to backlash in the drives. Paper mills that retrofit the machines with disc couplings, however, eliminate the backlash and boost control precision, thus helping to ensure high product quality.

Up to speed
Modern paper machines generally use multiple drives with synchronous speed motors, each one driving a separate roll through a gear speed reducer. Digital controls sense roll speeds within milliseconds, making continuous adjustments to maintain optimum web tension. This precision tension control helps the machine turn out higher-quality paper by reducing the major cause of flaws such as wrinkling and tearing.

Each drives contains a high speed coupling -- generally a close-coupled, double flexing gear coupling between the motor and speed reducer, plus a long span gear coupling or universal joints between the reducer and the roll. The high-speed coupling typically operates at the motors synchronous speed, between 1,180 and 1,500 rpm.

On the low speed side, rigid hubs normally are mounted to the reducer and the roll. The coupling then becomes a jackshaft, mounted to the rigid hubs with a flexible hub on each end. These couplings typically operate at around 300 rpm, and even slower on pulp machines.

Coupling clearance compounds problems
A gear coupling requires clearance or backlash between the meshing gear teeth so it can slide while mis-aligned. As the coupling wears, this backlash increases.

But backlash works at cross-purposes to the precise speed control required for paper machines. As the digital control senses this backlash, it constantly tries to compensate for it. However the resulting false signals make it difficult to control speed accurately, causing imperfections in the paper. Furthermore, the couplings operate in a hot, humid environment that not only corrodes unprotected steel components, but also causes the lubricant and seals to break down, which shortens coupling life. Batch lubricated couplings typically used on paper machinery must be torn down, inspected, and re-lubricated every 6 to 12 months.

Another problem with gear couplings in high-speed service is thrust lock-up, a condition in which fictional forces prevents the coupling components from sliding freely during axial shaft motion.

Gear couplings can be visually inspected only by stopping the machine, disassembling the coupling and housing and cleaning off the lubricant. Then the teeth are inspected for wear and the coupling replaced if necessary. Replacement typically takes at least eight hours. Long span couplings add to the difficulty because they have heavy solid steel intermediate shafts that require two people with a hoist to perform an inspection. So most mills have reduced their coupling inspections to once or twice a year. But this means gear-coupling wear is likely to progress further making the couplings less reliable.

Disc Pack solution
These problems have been overcome with disc couplings designed to fit the needs of paper mill applications. These devices have replaced gear couplings in a large number of mills world-wide. They have no wearing parts, no backlash, and require no lubrication. Moreover, corrosion resistant stainless steel disc packs provide long service life.

Paper machines use a close-coupled disc coupling for the high speed side, between motor and speed reducer, and a long span jackshaft version for the low speed side between reducer and roll.

The high-speed disc coupling replaces a close coupled gear coupling. Its diameter normally fits within the same space as a standard gear coupling. An axially split centre member lets you replace the disc packs without moving the connected equipment.

On the low speed side, a long span disc coupling bolts to gear coupling hubs already mounted on the reducer and roll. The centre section is a hollow tube, which is much lighter and easier to handle than the solid shaft of a gear coupling, and it reduces weight and load on the connected equipment.

Disc couplings require closer axial position adjustment than gear couplings, and this is accommodated by adjustment shims. The long span couplings can also be supplied with slip splines that allow more axial movement.

When properly installed and aligned, disc couplings provide long service life without maintenance, in some cases 30 to 40 years. Excessive misalignment can distort the flexible disc packs, leading to eventual fatigue fractures in the outer part of the disc. Periodic visual inspection lets you detect such failures without disassembly. And the discs can normally be replaced quickly and easily during a scheduled shutdown.

Recent paper machine upgrades to new generation variable speed DC motors has lead to uncharacteristic bearing failures in normally reliable gearboxes. Further investigation has revealed bearing fluting due to stray electrical discharges from the DC motors tracking to earth. Electrically insulated disc couplings have successfully been installed at many locations to isolate venerable machinery from these dangerous effects.