In the mid-1990s, a gas productionfacility in Colorado sought new tech-nology for regular seal changes on its6000 hp (4474 kW) fleet of screw com-pressors requiring regular seal changes.The search began because of dissatis-faction with the process then in use.

For example, in order to access theseals, the coupling and the couplinghub had to be removed from the com-pressor shaft. Also, the hub size andequipment location made for a difficultremoval of the shrunk-on keyed hubs.And because open flame heat was re-quired to remove and reinstall the hubwhen changing seals, hot-work per-mits were a necessity. Furthermore, re-peated heating of the hubs graduallydeformed and ruined the hubs.

The solution the operator soughtwould permit easy installation and re-moval for the hubs without the use ofheat. In response to that desire,Couplings Corp. of America (CCA) de-signed and installed a new keylesshub with a mechanical interference fit.The new hubs allowed easy axial shaftpositioning, and installation requiredonly hand tools. The first hub was in-stalled in 1997 and hubs were later in-stalled on the remaining compressors.

Before the first hub was installed,it underwent a mechanical test toprove its torque carrying capability.After being subjected to five times itsrated torque, the test was halted forsafety reasons.

Since its original use, this keyless

device has been used in many differ-ent applications including gas tur-bines, pumps, fans, ship propulsion,and even wind turbine drives.

Many of these applications havenominal torques in excess of 1 millionlb.in. (113,000 Nm). There is oneplace, however, where users hadbeen reluctant to try this device —large reciprocating compressors. Manypotential users had been concernedabout the sizable torque spikes in thattype of application, and they deter-mined that the risk was too great onthat size equipment.

In 2006, a gas producer in Alberta,Canada, was experiencing disc cou-pling failures on some 1900 hp (1417kW) motor-driven reciprocating gascompressors. They hired Beta Ma-chinery Analysis in Calgary, Alberta,to help them determine the cause ofthe failure. Beta determined that thetorsional stiffness should be changedto avoid some torsional resonance inthe system. The easiest way for themto make this change was to increasethe size of the current coupling.

Meanwhile, the Calgary-based pack-ager who sold the systems elected totry a clamping hub in conjunctionwith the new coupling. The packager,working together with a local distribu-tion and engineering group, presentedthe idea to the end user. Despite theirhigher initial cost, the operator recog-nized the possibility of potential fu-ture savings in maintenance.

CCA designed clamp hubs for theapplication, and the flange of the hubwas made to match the bolt circle ofthe disc coupling. The compressorshaft size was 6.75 in. (17.15 cm) andthe motor shaft size was 6.3 in. (16.0cm). The motor was rated at 1900 hp(1417 kW) at 1200 rpm, for a nominaltorque of about 100,000 lb.in. (11,300Nm). Because of the severity of theapplication, the service factor was atleast three. Coupled with the need toincrease the size of the coupling fortorsional reasons, the service factorwas elevated even further.

The eventual torque rating of thedisc coupling was 813,780 lb.in.(91,940 Nm), with a peak capacity of976,536 lb.in. (101,030 Nm). Therefore,the clamp hubs were designed tomatch the torque capability of thecoupling. The peak torque capacityof the clamp hub was 1.63 millionlb.in. (184,165 Nm), or double thenormal torque capability. Eventually,the hubs were joined with the spacerand discs in Calgary before headingto the plant site.

Removing the existing hub re-quired heating, pulling and grindingto remove it from the shaft. The

This Anderson Clamp Hub, built for a 9000 hp (6710 kW) reciprocating compressor with a 10 in.(25.4 cm) shaft, on a test shaft just prior to shipping.

MARCH 2010 COMPRESSORTechTwo

HEAT-FREE HUB DESIGN REDUCES DOWNTIME

Clamp Hub Concept Demonstrates Advantages for Reciprocating Compressor Applications

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process took approximately 24 hoursand resulted in a gouge at the end ofthe shaft.

The clamp hub was slid onto theshaft with about 0.001 in. (0.254 mm)clearance between shaft and hubbore. The hub slid over the gougedpart of the shaft without any prob-lems. From crate to complete installa-tion on the shaft — and even thoughthe mechanics doing the work wereunfamiliar with the new design — thejob took only about 75 minutes. Withexperience gained from that first in-stallation, succeeding units took evenless installation time.

Two weeks after the initial installa-tion, the plant operators realized thatthe hub on the motor needed to bemoved slightly to accommodate a dif-ferent magnetic center. This processrequired removal and reinstallation ofthe hub. The design of the clamp hubhelped shorten that process by over aday, saving the plant about US$22,000in lost production and easily recover-ing their cost.

The concept of the clamp hub issimple. There are two main pieces,the hub body and the collar, bothmanufactured from hardened alloysteel. The hub body has a regular hubflange and a thinner sleeve sectionthat is partially slit. The hub body isbored to match the shaft, eitherstraight or tapered. CCA explainedthat the collar screws onto the outsideof the sleeve section of the hub bodyusing a patented asymmetric threaduntil it touches the back of the hub

flange. At that point, the collar comesinto contact with a set of load screwsin the hub flange.

Once axial positioning of the hub iscorrectly tightened, the loadingscrews push the collar away from thehub flange. The more the collar ispushed, the more pressure is built upin the threaded interface between thecollar and hub. The thickness of thecollar resists the internal expandingforces, and the pressure is translatedinto interference between the huband the shaft.

In order to determine how tight thehub is on the shaft, CCA utilizes aspecific procedure. Depending on thedifference between the hub bore andthe shaft diameter, there could be dia-metrical clearance between them ofup to about 0.002 in. (0.0508 mm).Obviously, the tightening procedureneeds to take into account anyamount of clearance between huband shaft.

The first step of the process is totighten each loading screw approxi-mately a quarter turn in a sequentialpattern rather than a standard starpattern. Once the pattern of loadingscrews has been tightened once, thetorque capability of the hub is testedby twisting it by hand on the shaft. Ifit slips on the shaft, the loading

screws should get one more roundof tightening, and the process re-peats until the hub cannot be movedby hand.

Once the hub has reached thisstate, the small gap between the collarand the hub flange can be measuredwith a feeler gauge. The first measure-ment is considered the “starting gap.”For every specific hub size, a prede-termined collar movement is neededto achieve the correct pressure be-tween the hub and the shaft. Theamount of the movement is usuallybetween 0.02 in. (0.508 mm) and 0.1in. (2.54 mm), depending on the sizeof the hub and the shaft.

The amount of movement is alsogiven as a range, where the lowervalue is the minimum movementneeded. By adding the starting gapamount to the predetermined collarmovement amount, the final gapamount is determined. Here is an ex-ample of how the numbers are used:

Knowing the final dimension target,the loading screws can begin to betightened using the same sequentialprocess. The collar should move ap-proximately 0.002 in. (0.0508 mm) foreach complete pass of the loadingscrews. Once the final gap dimensionis reached, the hub is ready to handlethe designated torque.

For removal, the process is basicallyidentical except in reverse. The load-ing screws are loosened sequentiallywith very small movements. Once thecollar has retreated to its original posi-tion, the hub should be loose on theshaft. In the case of the collar gettingstuck in the tight position, the hub isequipped with three puller holesalong with a lip on the OD of the col-lar. By placing the bolt heads over thecollar lip, the other ends of the boltsprotrude from the hub face, and thennuts can be used to pull the collarback to its original position.

According to CCA, the mechanicalshaft/hub interference created by theclamp hub is usually significantlylarger than that of a heat-shrunk hub.A normal interference for a heat-shrunk hub could be between 0.0005and 0.002 in. per inch of shaft diame-ter (0.0323 and 0.1290 mm per cm ofshaft diameter), depending on severalfactors, including whether or notthere is a keyway. A typical interfer-ence using the clamp hub is 0.002 to0.004 in. per inch of shaft diameter(0.1290 to 0.2581 mm per cm of shaft

MARCH 2010 COMPRESSORTechTwo

Mechanic installing an Anderson Clamp Hub with a low-profile flange on a 6 in. (1.52 cm) ta-pered shaft.

Starting gap A 0.015 in. (0.381 mm)Predetermined collar

movement range

Final gap dimension (target) A+B 0.075 to 0.080 in. (1.905 to 2.032 mm)

B 0.060 to 0.065 in. (1.524 to 1.651 mm)

CT411.qxp 3/31/10 4:10 PM Page 2

diameter), although that number canchange depending on the situation.

In terms of the torque rating for the

hub compared to other hubs, a similarmethodology is used. Usually the hub israted for the same nominal torque value

as the rest of the coupling. In mostcases, however, the hub should not bethe weakest point of the complete cou-pling. The flexible element should bedesigned to fail before the hub slips onthe shaft. Therefore, even though thehub has the same nominal rating as therest of the coupling, it has a highersafety factor. In a typical application,the clamping hub can be expected tocarry four to five times the nominal ap-plication torque before slipping.

The hubs were installed in April of2007 and the compressors have beenrunning for the majority of time sincethen, CCA reported. The end user haspurchased more hubs for similar ap-plications despite their premium pricebased on savings realized by reducedmaintenance expense. Because hot-work permits are not needed, the hubservice does not require unnecessaryshutdown of adjacent equipment forsafety reasons.

CCA said other benefits of using theclamp hub are the elimination of shaftdamage from heat-shrunk hub re-moval and installation, the ability toremove hubs with simple tools andwithout the need to ship a compres-sor shaft to a shop to have the hubremoved, which is especially benefi-cial for remotely located units. A

End view of an Anderson Clamp Hub showing the slits in the hub sleeve.

REPRINTED FROM MARCH 2010 COMPRESSORTechTwoCopyright Diesel & Gas Turbine Publications

Printed in U.S.A.

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