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AUGUST 2005 www.pump-zone.com PUMPS & SYSTEMS

Bearings in a centrifugal pump serve threemain purposes:

I They absorb the radial and thrust loads trans-mitted through the shaft.

I They keep the rotating element in position andin correct alignment with the stationary partsof the pump.

I They permit the shaft to rotate with the leastamount of friction.

In the horizontal end-suction pump, both bear-ings locate on one side of the impeller so that the im-peller mounts on a cantilevered extension of the shaft.

Most multistage pumps have one bearing oneach end of the shaft with the impellers locatedbetween them. The radial bearing in these pumps isnormally situated at the coupling end of the shaft,with the thrust bearing at the outer end.

The same is also true of the double-suctionpump, though this style is considered in axialhydraulic balance because its inlet flow impactsboth sides of the impeller.

But, lets get practical. This balance can be detri-mentally affected by unequal wear on the wearrings. Or, the flow of liquid into the two suctioneyes may differ due to an improper suction pipingarrangement. Consequently, a thrust bearing is stillrequired to accommodate any possible loads on thedouble-suction pump.

Bearing LoadsBearing life depends upon the loads being car-ried and the rotation speed of the pump. In addi-tion to the physical forces involved, major loadsresult from the hydraulic forces acting in the pumpcasing at the impeller.

The axial thrust created in a horizontal end-suction process pump is predominantly from thehydraulic forces acting on the front and back of theimpeller. The magnitude and direction of the resul-tant axial thrust depends on the impeller design andflow conditions.

In most instances, the resultant axial thrust istowards the pump suction and results from pressureon the back shroud overcoming the pressure on thefront side. Closed impeller pump designs oftenemploy back wear rings to reduce the axial thrust.They may also use axial balance holes to connect

the high pressure at the rear of the impeller with thesuction pressure at the eye of the impeller. Thisreduces the axial thrust towards the suction. Theresultant axial thrust can be reversed with high suc-tion pressures.

Open impellers often include pump-out vaneson the reverse side of the impeller to reduce thepressure behind the impeller. This reduces the resul-tant axial thrust towards the suction.

Lets Get Practical

The Function of

Process Pump Bearings

Ross Mackay, Contributing Editor

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Figure 1. Drawing of shaft and impeller with thrust loadsindicated.

Figure 2. Closed impeller with back rings and balanceholes and thrusts.

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PUMPS & SYSTEMS www.pump-zone.com AUGUST 2005 9

The radial force from the impeller acts at rightangles to the shaft and creates a radial loading onboth bearings, but primarily on the radial bearing.Other factors also affect the radial loads, such asrotor imbalance, shaft misalignment and the weightof the rotating element.

The Anti-friction Ball BearingsThe modern centrifugal pump depends pri-

marily on using anti-friction ball bearings whichcomprise inner and outer hardened steel rings sepa-rated by a number of steel balls. The inner ringmounts on the pump shaft while the outer ring fitsin the housing. A separator spaces the balls aroundthe bearing and helps reduce the amount of fric-tion. These components are manufactured toextremely tight tolerances. It is essential to hold theshaft and housing to the same degree of machiningtolerances during repair and overhaul.

A typical end-suction process pump permitsthe outer ring of the radial bearing to slide slightlyin the housing to accommodate any expansion orcontraction of the shaft length. The thrust bearingis fixed in the housing and accommodates the axialthrust acting along the centerline of the shaft fromthe impeller.

The single row, deep-groove bearing is the mostwidely-used bearing on centrifugal pumps. Besidesits ability to handle the radial load, this bearing typeis designed with a close contact between the ballsand the deep continuous groove in each ring, mak-ing it capable of handling an axial thrust load ineither direction. In typical process pumps such as

the ANSI or API designs, it serves as the radial bear-ing, while the thrust bearing design most frequent-ly used is the duplex arrangement of the angularcontact bearing.

Angular Contact BearingsA single angular contact bearing is designed to

support a heavy thrust load in one direction only.Typically used in the duplex arrangement, threealternative arrangements are possible and the bear-ings must be properly arranged for the anticipated

thrust loads.

In the Tandem

duplex arrangementthe load lines are par-allel and the pairaccommodate axialloads in one directiononly, with the loadingbeing evenly dividedbetween the two bearings. This arrangement is onlyused when the pump design guarantees the resul-tant thrust in one direction only. It is rarely used ina single-stage process pump.

In the Face to Face arrangement the load

lines converge as they approach the bearing center-line. This arrangement accommodates axial loads inboth directions, but by only one bearing at a time.

It is interesting to note that, in this arrange-ment, the second bearing transfers the load fromthe inner race, through the ball to the outer race,and ultimately onto the bearing housing.

With the Back to Back arrangement the loadlines diverge as they approach the bearing center-line. This one also accommodates axial loads inboth directions. Here, the first bearing transfers theload from the inner race, through the ball, to the

outer race.This arrangement is normally used in processpumps to provide greater angular rigidity. It is bet-ter for accepting tilting moments that may resultfrom shaft misalignment difficulties.

So, lets get practical. When purchasing areplacement bearing, it is extremely important toreceive the exact bearing as was selected for the orig-inal pump design.

Unfortunately, this is not as simple as it mightappear. Within the same 7000-series of angularcontact bearings, there are a number of optional

series and preloads. Other variables within thesame-size bearing include cage style, materials, andthe contact angle. Though numerical suffixes can becross-referenced from one bearing manufacturer toanother, the alphabetical prefixes and suffixes fre-quently differ. The buyer may order with a differentbearing with unfortunate results. P&S

Ross Mackay is the author of The Practical PumpingHandbook and specializes in helping companiesincrease their pump reliability and reduce operatingand maintenance costs. He can be reached at 800-465-6260 or through www.practicalpumping.com.

Figure 3. Open impeller with pump out vanes and thrusts.

Left: Figure 4. Face toFace angular contactball bearing.

Below: Figure 5. Backto Back angular con-tact ball bearing.