Pump Dynamics (Part 3): Shaft Dynamics and Troubleshooting

Welcome back to part 3. In Parts 1 and 2 we discussed Industrial Pump Fluid Basics and Fluid Hydraulics . Today, we’ll discuss shaft dynamics, examine some scenarios that might need troubleshooting and address why these problems are occurring. 



Shaft Whip occurs when the shaft shifts 180 degrees from its centerline in every rotation creating a cone shape. This is usually caused by an unbalanced impeller on some side of shaft.


Shaft Deflection occurs when a radial load or mechanical imbalance causes the pump shaft to bend downward when it is in one position. When the shaft is rotated 180 degrees, it still bends downward in a similar way.


Shaft Deflection is a function of four factors:

  1. Radial force in the impeller
  2. Length from impeller to radial bearing
  3. Shaft diameter (the smaller the diameter, the more deflection)
  4. Material properties – different materials have a different flex


Here’s a quick review:

  • Whip
    Shaft-end rotates in a manner that generates a cone shape.
  • Deflection
    Bending of shaft due to a constant load in one direction.
  • Run-out
    Caused by a bent shaft or eccentricity between the shaft and shaft sleeve.


— It is possible to have all three events occurring simultaneously —


Now that we’ve reviewed shaft dynamics, let’s examine some problem scenarios that may require troubleshooting.



No liquid coming from the pump: If the plate is full of green oil, pitch or tar, no fluid can flow in because it plugs the impeller completely off and thus nothing will be pumped.


Suction pipe not opening: The pipe is either not submerged enough or the suction lift is too high. 25 feet is the maximum lift for water before it starts to boil.


Pump is not producing rated flow: This creates an air leak. When this happens, spray it down with soapy water and see if the water goes into the stuffing box. It will go in if there is a pulling vacuum.


Pump starts then STOPS pumping: It’s improperly primed and it won’t suck in any air.


Bearings run hot: This may occur due to improper alignment, improper lubrication, lube cooling, etc.


The motor requires excessive power: The liquid is too heavy, requiring more horse-power to pump.


The pump is noisy/vibrates: This is caused by improper alignment or cavitation.


Well, that’s it for Pump Dynamics. Hopefully, you now have a better understanding of the machinery you operate and are more capable of facilitating repairs when machinery goes down.


By educating our customers about the machinery they rely on for their day-to-day operations, C&B Equipment can help increase uptime, while decreasing the need for costly off-site repairs. That’s what we call Uptime Solutioneering™.


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