MAINTENANCE-FREE DISC COUPLINGS
The traditional paper machine drive train for fixed rolls consists of a motor connected to a reducer with a close coupled gear type coupling. The reducer is then connected to the paper machine roll with a floating shaft gear coupling (sometimes referred to as a jack-shaft). These couplings transmit the motor torque to the roll while accommodating any shaft misalignment through sliding friction between the coupling teeth. The resultant sliding friction between the coupling gear teeth will require coupling lubrication and re-lubrication on an ongoing basis. In the increasingly competitive global paper market, there is a need to reduce maintenance cost and improve machine reliability. With that in mind, the elimination of lubricated couplings on the paper machine drives will definitely provide a maintenance cost reduction – not to mention reducing unscheduled machine downtime when lubricated couplings fail.
Gear Coupling Maintenance Challenges:
The traditional gear couplings are plagued with a number of maintenance issues:
The gear coupling accommodates shaft misalignment through the sliding of the hub teeth across the sleeve teeth as the coupling rotates. This tooth sliding occurs twice per revolution. Lubrication, generally in the form of grease, is required to reduce this sliding friction. In addition to the initial greasing, couplings have a tendency to lose lubrication so regular relubrication is required.
As the gear coupling rotates there is a centrifugal force created within the coupling. This centrifugal force forces the coupling grease past the “O” ring seal in the coupling sleeve. The expelled lubrication is thrown inside the coupling guard and onto the plant floor causing an environmental concern which must be cleaned up regularly (Figure 3).
Figure 4 is a gear coupling with a broken lubrication fitting. Here you can see the lubrication coming out of the broken lubrication fitting and being sprayed inside the coupling guard. At some point the coupling will run dry, gear teeth will fail, and unscheduled downtime will result.
As mentioned before, there is a centrifugal effect within the coupling caused by the rotation. (Figure 5) As the coupling teeth wear, very fine wear particles are in suspension within the lubrication. These particles have higher specific gravity than the lubrication and under centrifugal force are thrown to the outer diameter of the coupling lubrication cavity. Coincidentally, the coupling teeth are located in this same area. The wear particles are then lodged around the coupling teeth.
When lubrication is added, the new grease generally stays in the center of the coupling and the old grease with particles is not purged out. (Figure 6) Through time the wear continues; the wear rate begins to increase based on the increase of wear particles. The end result is premature coupling failure - even when new grease has been added on a regular basis.
To correct this situation, the couplings must be disassembled and the old grease washed out. In many cases, a very difficult task - the equipment shaft is not long enough to slide the outer coupling sleeve back and expose the working teeth for washing.
There is a backlash between the hub tooth and the sleeve tooth. This backlash is designed to allow the hub teeth to bend between the sleeve teeth, to provide for angular misalignment between the connected shafts. It is interesting to note that when the alignment is good the amount of backlash is at its highest level. In most facilities, the operating speed of the paper machine has increased over time. Subsequently, older D.C. drives are being replaced with newer AC drives with digital controls. These new drives and controls sense roll speed change in milliseconds and adjust roll speed to maintain tension. The accuracy of these drives can sense the backlash in the gear couplings and try to correct for it. This is referred to as drive hunting and in many cases, the drive needs to be de-tuned to reduce the hunting.
C. Heat & Humidity
In the dryer section of the machine, one end of the floating shaft coupling is often located inside the dryer hood. Inside the dryer, the hood is additional environmental challenges, including high heat and humidity. The high heat coupled with high humidity has an adverse effect on the coupling seal and lubrication properties, resulting in coupling failure of the roll end coupling - while the reducer side coupling is still functioning.
Perhaps the most significant challenge with gear couplings is that the condition of the teeth cannot be determined without shutting the machine down and disassembling the coupling.
In many cases, the short equipment shaft makes it impossible to slide the sleeve back to expose the coupling teeth to check for wear. Inspection of gear coupling is very labor-intensive and is not always carried out on routine bases. Subsequently, the end result is coupling failure and unscheduled machine downtime.
The Solution: Kop-Flex® Disc Couplings
A Kop-Flex disc coupling (close-coupled on the motor or floating shaft design to the roll) has several advantages over a gear coupling in paper machine drives. These advantages include:
- Lubrication is not required.
- They are dynamically stable.
- They have no backlash.
- They are not adversely affected by heat and humidity.
- Perhaps most importantly, the disc coupling’s condition
can be inspected while the machine is running.
The Kop-Flex disc coupling utilizes a unitized disc pack assembly. The disc pack consists of a number of thin stainless steel “scalloped” discs, the design of which reduces the bending force on the connecting shaft bearings when misalignment occurs. The discs are assembled on a bushing and held together on each end with a beveled washer.
Unlike a gear coupling that accommodates misalignment through friction, the disc coupling accommodates misalignment through flexing. Therefore there is no requirement for lubrication and the disc coupling is truly maintenance-free.
The Kop-Flex disc coupling components are manufactured on modern CNC equipment. The various bolt circles which connect to the disc packs are held to a very tight concentricity tolerance to help ensure that when assembled all coupling components run concentrically to improve the coupling dynamic stability. Kop-Flex disc couplings are manufactured to meet or exceed AGMA Class 9 Balance Standard as manufactured.
During operation, the coupling disc pack can be inspected with the use of a strobe light or a digital camera without shutting down the equipment. This allows maintenance staff to inspect couplings on production days, not just down days.
Warning: Do not operate equipment without guards in place. Be sure to adhere to all applicable local safety requirements.
The disc pack can also provide an indication of load that the coupling is transmitting and the misalignment condition the coupling is working under.
Figure 7 is a disc coupling actually operating at 3,800 RPM on a blower application. In this image the disc pack is perfectly straight which suggest the alignment is very good.
In Figure 8 we can see that the disc pack is flexing which would suggest there is misalignment present and it should be checked at the first available opportunity.