Fixing Centrifugal Pumps: A Step-by-Step Guide

Published on April 29, 2024

Fixing centrifugal pumps

At PerformancePro Pumps, replacing a mechanical pump seal and motor bearing is one of the most common service requests once warranty has expired. After all, they are the only wear items in the pump system. While this article provides a step-by-step guide to fixing self-priming centrifugal pumps, the same will hold true for flooded-suction external pumps.

Aqua UV

Before beginning any pump and motor service, please disconnect all power sources.

Required parts include: ¼-inch and 5/16-inch nut drivers, a 9/16-inch wrench, a 3/32-inch Allen wrench, a flat-head screwdriver, a dead blow mallet, a bearing puller, replacement seals and bearings, a bearing installation tool, and replacement O-rings, as necessary.

Tear Down

remove the failed bearing with a bearing puller
Use a bearing puller to remove the failed bearing and increase the life of the pump.

Self-priming pumps typically have bolts attaching the seal plate to the housing. These give us easy access to the pump internals. Remove them, and the seal plate will separate from the pump housing with very little effort. Locate the O-rings on the seal plate and the diffuser, inspect for any wear or distortion, and replace if necessary. Otherwise, do not misplace the O-rings.

On the motor side of the assembly, you will see the internal diffuser. Remove the non-rotating diffuser to gain access to the impeller. Two bolts located 180 degrees apart hold the diffuser to the seal plate. Remove them to gain access to the impeller.

The impeller requires removal to gain access to the seal. This is a rotating item and is threaded on the motor shaft. Many, but not all, will have a reverse threaded set screw and O-ring holding the impeller to the shaft.

Hold the motor shaft in place while you rotate the impeller off the shaft. Motor manufactures vary in how they lock their shafts, but most will have a plastic insert located in the center of the Opposite Drive End (ODE) bracket or fan shroud. Removing this piece gives access to the shaft. Insert a screwdriver to help hold the rotor/shaft and allow unthreading the reverse set screw and then ultimately the impeller itself. If there is a set screw, it is a left-hand thread and will require clockwise rotation in order to be removed. The impeller has a standard thread direction and rotates off in the standard fashion (counterclockwise). Remove both the set screw and the impeller.

What is left is a motor assembly attached to the seal plate with four bolts. Remove and detach motor from seal plate.

Replacing the Seal

Around the brass thread of the impeller will be the rotating section of the seal. This seal face is held tightly in place by a rubber boot. Using a flat-head screwdriver, pry this face from the impeller.

At the center of the seal plate will be the non-rotating portion of the seal, which is simply pressed into the plate. To remove, press out the seal in the opposite direction of the motor. Typically, there will be a small O-ring (or sealant) in the seal plate bore. Again, please inspect and account for this O-ring. Clean all parts of residual debris.

Install the New Seal

Use caution to not touch the stationary side of the seal during installation.

Press the rotating face of the seal into the impeller making sure not to damage the face. Install it squarely to the impeller. This can sometime require some effort with a soft material to protect the face during installation.

On the seal plate side, ensure the small O-ring is installed in the center bore. Press the non-rotating seal face into the plate in the direction toward the motor.

Motor Teardown

motor tear down
Figure 1: Housing is removed from the motor; the bracket exposes the diffuser. Figure 2: The diffuser and impeller are removed, exposing the mechanical seal. Figure 3: The motor is disassembled and fan cover removed, exposing thru bolts. Figure 4: The rotor shaft assembly is removed from motor.

Use a bearing for the following steps. Please make sure you have access to one.

Four thru-bolds hold the motor together. Access these bolts from the ODE of the motor. If the motor is fan cooled, the fan shroud will need to be removed in order to see them.

Removing these bolts will mechanically separate the motor brackets from the stator. A mallet applied to the motor shaft while firmly holding the stator housing will break the motor apart, exposing the rotor and the two bearings. There will be a locking tab or retainer on the Drive End (DE) of this assembly. Please remove it and set it aside. Remove the preload washer from ODE bracket. Clean and gently remove any rust of foreign materials from the bearing seats.

Install New Bearings

Remove both bearings from the motor shaft using puller. Clean off any excess rust or debris from the shaft and bearing journals.

Place new bearing near the bearing shoulder of the shaft and orient the rotor and shaft assembly vertically on the bench. Place the properly sized bearing installation tool around the shaft and flush to the new bearing. Using a hammer, seat the bearing to the shaft. When it’s tight against the shoulder, you know when to stop. Additional hammer blows can damage the bearing, so be careful.

Once the first bearing is installed, repeat for the other end.

Reassembly

Place the DE bearing into the DE bracket and re-install the locking device.

Set the stator vertically over the ODE bracket. Visually inspect the stator bore for stray wires to ensure nothing gets caught while installing the rotor. Install bearing preload washers into the bearing seat of ODE bracket. Lower the DE bracket (with rotor installed) down into the stator until the bearing sets into the ODE seat. Re-orient the motor to a horizontal position and re-install the thru bolts. Cross-pattern tighten the bolts in several steps to ensure you do not bind the bearing. Install the fan and shroud, as required.

Place the motor shaft through the seal plate and mount with the four motor bolts. Ensure both seal faces are very clean before assembly. Thread on the impeller and reverse set screw.

Install the diffuser with the (2) mounting bolts. Slide assembly into the housing, making sure both the diffuser O-ring and seal plate O-ring are installed. Complete assembly with seal plate bolts and washers.

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