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ENGINEERING PRACTICE

Mission-critical couplings

HOW TO SELECT,OPERATE AND MAINTAIN THEM

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Examples of unbalance

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RANSMISSION Balancing a coupling is critical for the long life of equipment in almost all turbomachinerytrains. Centrifugal forces from unbalance lead tovibration. Too much unbalance causes excessive vibration on its own,or it may be that the forc-ing function excites a train-lateral resonance.Since the force from unbalance grows withthe square of the running speed,the higher the running speed,the more important balancingbecomes. For example,an unbalance of 2 g-in(50.8 g-mm) at 3,000 rpm exerts a force of 1.1 lbs,while the force from the same unbalance at10,000 rpm is 12.5 lbs. Also,a well-balanced coupling runningexcessively out to the center of rotor (shaft)rotation has the effect of an unbalanced cou-pling. For example,a coupling balanced to low levels —a 50 lb coupling balanced in a single plane to 0.28 g-in —runs out to thecenter of rotation 0.0005 inches (Total Indicator Reading) when mounted. This“error”may be due to the eccentricity of therotor mounting surface,a runout on the cou-pling mounting surface,or a combination of the two. This shift of the coupling from the

Mission-Critical Couplings - 15937 C

ouplings are a critical part of a turbo-machinery train. They transmit thetorque of connected equipment whileaccommodating the inevitable mis-alignments —angular and parallel offset —and axial displacements that occur as machineshafts move relative to each other.There are two types of couplings used inturbomachinery applications —general-pur-pose and special-purpose,also known as high-performance couplings. General-purpose cou- plings are found on process pumps and otherlow-speed (less than 3,600 rpm) equipment.High-performance couplings are found on mission-critical trains,such as high-speed(5,000 - 20,000 rpm),gas turbine-driven,cen-trifugal compressor trains. They are relatively more expensive than general-purpose cou-plings and are engineered specifically for theapplication. They usually operate continuous- ly for five years,often in corrosive environ- ments at temperatures up to 500 ºF.Turbomachinery users should know the cor-rect selection,operation and maintenance of couplings. They can learn a lot about couplingsat meetings such as the Texas A&MTurbomachinery Symposium held in Houston, Texas. This symposium hosts discussion groupswhere users express their concerns in the pres-ence of peers,experts and manufacturers. At the 2004 Turbomachinery Symposium,the discussion session on couplings and align-ment was attended by over 50 turbomachineryusers. There was a lively exchange of ideas and views. Below is a summary of the Q&A at thesession that will help you understand the issuesrelated mainly to high-performance couplings. Figure 1: Once the coupling assembly is placed on the balance machine it can be assemblycheck balanced or assembly balanced Coupling Speed Less Sensitive machine More Sensitive Machine 0-1,800 rpm No balance Component only 1,800-3,600 rpm Component only Component only 3,600-5,000 rpm Component only Component and assembly check 5,000-10,000 rpm Component and assembly check Component and assembly balance Over 10,000 rpm Component and assembly balance Component and assembly balance Table: Guidelines for coupling balance
www.turbomachinerymag.com January/February 2005 • Turbomachinery International 17