Pump Shaft Failure 4

[djm883]

We recently had a pump shaft failure and I wanted to confirm my thoughts on the failure before we address the repair. I've attached the photo of the fracture surface.

Background: This pump was rehabilitated last year with a new shaft, bearings, packing, etc. The new shaft was made of 4140. The motor is directly coupled to the pump shaft with a pin and bush type flexible coupling. The failure occurred on the front side of the bearing (no shaft section change) on the motor end. The location was approximately 3" from the end of the shaft.

 

[3DDave]

They say a picture is worth 1000 words, but in describing the relationships between the components you are a few hundreds words short.

Maybe an engineering diagram of the relations between all the parts would help.

Other than that it looks like fatigue from misalignment.

 

[JJPellin]

What is the configuration of the pump? What type of bearing? I am not familiar with a pin and bush coupling. Please describe what this means. It looks like the fracture may have started as high cycle fatigue in bending and then failed catastrophically from over-torque at the last.

Johnny Pellin

 

[djm883]

See attached diagram of the pump. The motor is directly coupled to the pump shaft.

 

[desertfox]

Hi

Have a look at is article it might help

http://www.maintenancetechnology.com/2012/07/failure-analysis-of-machine-shafts/

The failure looks like torsional fatigue failure to me.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[metengr]

Send it to a metallurgical lab for proper analysis. There is collateral damage to the fracture faces from rubbing between the two halves of the pump shaft. The fracture does look flat in some areas but beyond this I can think of several failure mechanisms. Visual ain't going to cut it.

 

[Strong]

"motor" are you referring to an electric motor or a combustion engine? What motor size and speed? Was coupling in good condition? Was shaft alignment measured after the original repairs?

Walt

 

[djm883]

desertfox - Thanks for the link. Very helpful information.

Strong - It is an electric motor (15 HP @ 3600 RPM). One of the three coupling bolts had failed. The reinstalled parallel alignment was documented. Offset misalignment was 0.004". The gap between the coupling hubs ranged from 0.080" to 0.075". The coupling is from the 1950s so we don't have any information as to what is acceptable.

 

[Strong]

If motor and pump vibrations were fairly low during normal operation, then you can rule out unbalance; particularly with the coupling. I measure torsional vibrations with a battery-powered strain-telemetry system. A small machine like this may not justify the cost of torsional measurements. It is hard to guess what action to take without actual measurements or estimates from calculations. The shaft coupling from the 1950s should've had replacement rubber/elastomer bushings over the course of time. If you cannot update them with a higher or lower Durometer material, then I would replace the coupling to shift the torsional natural frequency above or below the excitation frequency. Typical excitation frequencies are at shaft speed, 2X line frequency, and at vane pass frequency. Poor material properties of the pump shaft may also have played a role in the failure. You may also want to rule out any foreign material in the pump that could have caused an abrupt stop.

Walt

 

[CouplingGuru]

djm883,

I would highly recommend tossing out that coupling and replacing it with something what will impart a much lower restoring forces and that has an independent relationship with transmitted torque and misalignment. Once that coupling starts to wear, it will most likely resist movement in the misalignment plane. This will be very hard to detect. That wear will impart a bending pulse with every rotation. This pulse is the restoring force that is required to bend the coupling out of alignment. This coupling in particular has a reverse leverage situation because of its dis-proportionally large diameter compared to the shaft (extremely low torque density). So a little force far out on the coupling diameter will produce a large restoring force at the shaft. At the speed you are operating at, these pulses will greatly shorten the fatigue life of the shaft. There are many options for couplings, but depending on your needs a long life disc coupling is specifically designed for higher speed applications like this one.

When it comes to couplings we are always here to help.

http://WWW.PSCCOUPLINGS.COM

 

[RolMec]

quote << One of the three coupling bolts had failed.>> unquote
Question is: Did that bolt fail before the pump shaft rehab one year ago or during the shaft failure event?
Imo (to be discussed): With a 3 bolt elastic / flexible coupling and one bolt failed, the coupling would at least give an asymmetric torque transmission response when reversing, and furthermore (torque transmission through 2 asymmetric pin-like elements) also a bending load by at least the elastic deformation of the coupling elastomer bushings.
Pls. see attached picture, with 3 bolts the forces would be equalized, but then with 2 bolts taking the full load the forces aren't equalized anymore.

I would hold that on the fracture surface there might be considered an influence of bending (3 to 6 o'clock)
Finally: What was the reason for the bolt failure, if perhaps this might be the root cause?
Regards

Roland Heilmann
Lpz FRG

 

[RolMec]

and here's the picture..

Roland Heilmann
Lpz FRG

 

[djm883]

RolMec - The coupling bolt after the pump was rehabbed during the shaft failure event.

 

[John2025]

Well, it sure sounds like the motor alignment was off. That would explain the coupling bolt failure due to repeated flex and the motor shaft, too. Did somebody replace the "coupling buffer" with something more rigid? I'd be inclines to replace the coupler with something modern, like Coupling_Guru said.

 

[electricpete]

There are a few unknowns to me.
1 - Do we have a theory why the bolt would have failed?
2 - What design feature assures that the bolts would share load during normal operation (without a failed bolt). I doubt the piloting is that precise and I'm pretty sure there are no clamped flanges in this design.
3 - How do we decide whether the coupling bolt failure was a cause of the shaft failure or a result of the shaft failure?

photo of coupling pieces might (or might not) provide some clues

=====================================
(2B)+(2B)' ?

 

[electricpete]

I found a presentation (attached) showing a pin and bush coupling in our plant in a vertical motor/pump application (pump has its own thrust bearing).
After refreshing my memory about the coupling, some possible failure modes come to mind.
If the holes become larger or the flexible sleeves wear, there will be increased stress on the bolts.
If the nut is not tightened to securely draw the bolt to the hub, that might be a problem.
If coupling hubs are too far apart, there is more stress on the bolts.
I also agree of course misalignment could be a cause of stress.


=====================================
(2B)+(2B)' ?

 

[CouplingGuru]

electricpete,

That is an excellent PDF reference. While there are mainly serviceable options for couplings in the market, unfortunately the OEM typically doesn't share the same motivation as the end user. Most often couplings are selected not off performance but off of cost and lead time. I will always contend that any coupling that transmits torque in the same plane it is required to misalign, is inferior for any continuously operational application. With these pin and bush style coupling, no matter the circumstance, those elastomer inserts will wear out. And at 3600 rpms that is probably quite rapidly. But how do you check for wear? You would have to check for some measurable form of creeping backlash which is very difficult, or you would just have to have those bushings on a PM cycle where they are changed out periodically. While nothing will last forever, I believe that any coupling wear component shouldn't expose your application to a catastrophic failure. The coupling should be there to transmit torque, take up misalignment and protect the equipment. These styles of couplings that put items in shear and rely on low friction movement of components are recipes for disaster when not properly maintained and monitored. If I was in an end users shoes I would much rather have a product that can be easily visually inspected. And in the off chance of failure, routinely isolates itself from other parts of the drive train. So a coupling failure will result in just a coupling failure.

A pin and bush coupling functions moderately well in angular misalignment, but in a parallel misaligned condition, its overall misalignment capability is significantly reduced because of its inability to flex in 2 planes. These style couplings along with jaw style are very popular because they are economical, but they both suffer from the same critical flaw. Torque and Misalignment in the same plane, never a good idea. The more toque you drive, the harder it is to misalign, which generates high restoring forces. These restoring forces can be so destructive, and a person won't know they are occurring until other items start failing prematurely.

When it comes to couplings we are always here to help.

http://WWW.PSCCOUPLINGS.COM