SUBJECT : Troubleshooting rub marks in a centrifugal pump 5-05

When a centrifugal pump is disassembled there are a couple of things visible to the trained trouble shooter. He can see either corrosion, or evidence of rubbing, damage or wear. Whenever a rotating piece of hardware hits a stationary piece it leaves a mark that is clearly visible and capable of being analyzed for cause.

This type of rub mark should never be confused with the dull appearance we see on a piece of metal that has been rotating in an abrasive slurry. In strong corrosive applications the rub mark may not be visible. The contact will cause an increase in the metal temperature causing rapid chemical attack. This condition is easy to identify because the corrosion is localized at the rubbing location.

Shaft fretting is another common rub mark that should not be confused with the rub marks we will be discussing in the following paragraphs. Fretting is visible between the dynamic elastomer in the mechanical seal and the sleeve or shaft that the elastomer is sealing against. You will also observe this type of damage immediately under the grease or lip seals that we find being used to seal most bearing applications.

There are five possible rubbing combinations that can be seen:

You should look for the rub marks on those pieces that normally come in close contact. Common sense will dictate that the further the hardware is located from the bearings, the more likely the contact will occur. Here are some likely candidates for rubbing when the pump experiences shaft deflection, or any other type of radial displacement. Look for contact between :

In the following paragraphs I will list the observations, explain the causes and where practical list some of the conditions that can initiate the problem with centrifugal pumps. If you would like to learn more about how to trouble shoot the rubbing marks we normally find in ball bearings, please refer to the bearing paper in this series

Observation - All around the rotary, one spot on the stationary. The shaft is being deflected from its true position or the hardware surrounding the rotating piece is being forced into the rotary unit.

Observation - All around the stationary, one spot on the rotary.

Observation - The mark is all around both units.

Observation - One spot on both the stationary and rotary units.

Observation - One spot on the rotating unit.

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