Category Archives: High Misalignment

No More Bearing Failures After Coupling Change

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A power plant had six identical feed pumps powered by 3500 HP motors. The equipment was initially specified and connected with gear couplings. The plant personnel had grown accustomed to performing the maintenance on these gear couplings by greasing them at regular intervals. Unfortunately, they had also become accustomed to replacing pump bearings as well. Every nine months to a year, the bearings were cracking due to fatigue, resulting in the need for frequent replacement.

After some research, plant personnel realized that the misalignment between the motor and pump was about 0.035”, much higher than the coupling was rated to handle. They determined that the bearing failures were occurring because of high load stemming from high coupling forces. The plant desired to improve the alignment; however, since the majority of the misalignment resulted from thermal cycles in the system, there was no simple way to accomplish this.

There was some resistance among plant personnel to make any changes due to bearings failing on a regular, consistent basis. They looked at it as the “devil you know” versus the risk of unknown results. Eventually, the plant decided to make a change, starting with upgrading the couplings to allow for the higher misalignment.

Coupling Corp was called in to design a coupling that could handle the high misalignment without adding bending loads to the bearings. A close-coupled FLEXXOR coupling was designed to fit the application and was capable of handling an offset misalignment of about 0.060” without any maintenance. More importantly, the spring rates of the new FLEXXOR coupling were roughly 1000 times lower than the original gear coupling.

Flexible Couplings Extend Seal and Bearing Life

Close-coupled, Reduced-moment FLEXXOR coupling

Close-coupled, Reduced-moment FLEXXOR

The newly installed couplings immediately yielded positive results. Despite this, the site mechanics continued to check the bearings every few months to look for cracks as they had for so many years during routine maintenance checks. Eventually, they realized that the couplings were doing their job very well, and the system was running smoothly without any bearing failures.

 

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Pelletizer Coupling Success

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A plastics manufacturer came to CouplingCorp about 10 years ago with an interesting problem.  On their plastic pelletizer machines, the cutting knives were powered by a 120 HP motor.  The machine used a gear coupling to transmit torque between the motor and the knife drives.  While this coupling application seems to be very simple on the surface, there was a major complication.  During the process of operating the pelletizer, the knives wear down as they constantly cut plastic.  The amount of this wear can vary for different size/model of the pelletizer, but it is a significant amount of movement that changes the length of the coupling as it wears.  For example, when new knives are installed, the Distance Between Shaft Ends (DBSE) might be 10.75 inches (273mm).  By the time the knives have worn to the point of being replaced, the DBSE could be down to 10.06 inches (255.5mm).  This means the coupling must accept an axial change of 0.690 inches (17.5mm).  Even for a gear coupling, this is a large amount of movement.

The user wondered if it was possible to replace the gear coupling with a maintenance-free, non-lubricated coupling.  But, they worried about whether a flexible coupling could handle the large movement without putting too much axial force on the hydraulic knife positioning system.  In other words, if the coupling was too stiff, the axial force generated by the coupling would be larger than the hydraulic force used to position the knives to cut the right size pellets.

CouplingCorp looked at the situation to determine if such a coupling could be produced.  After doing an analysis of the axial forces found in the gear coupling during operation, it was determined that the FLEXXOR coupling would have a lower axial force than the original gear coupling did.  The final hurdle was a large axial movement, and with the use of some high-misalignment diaphragms, the problem was solved.  The coupling was designed to handle +/- 0.384 inches (9.7mm) of axial travel which is a total range of 0.768 inches (19.5mm).  With this axial capacity, the user was able to stretch the coupling to accommodate the new knife position, and then gradually the coupling would move through the neutral position to a compressed position as the knives wore down to their lower limit.

The user installed a first coupling, and it worked as predicted.  After a trial phase, the same coupling was implemented on all pelletizers at the plant.  The coupling has been running successfully for many years without any maintenance.