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Motor Shaft Failure (Insights for Root Cause Analysis Needed)

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mtomechanical

mtomechanical

Student
Mar 29, 2024
1
Hi Everyone,

Recently we had a failure on a motor shaft (Photos attached of both the shaft end in motor and the rest of the shaft from the coupling side)
The shaft is connected horizontally parallel to the pump via Jaw Coupling (Photos Attached of Jaw Coupling Found to be Broken)
After the failure, we confirmed both motor shaft and pump shaft are rotating freely.

Some info about the shaft is:
Speed = 1490 RPM
Shaft Diameter = 80 mm

This pump is used to transfer Thermanol 66 (Heat Transfer Fluid). When the motor shaft broke, the vapours of this liquid caught fire immediately and caused a explosion in the pump area.

I am still trying to do RCA/FMEA/Report for broken motor shaft. Kindly share your views regarding the following incident (especially regarding the type of failure that resulted in shaft failure). I would really appreciate your help.

Condition_of_Shaft_from_Motor_Side_l3fpg5.jpg
Condition_of_Shaft_from_Motor_Side_2_pml0ka.jpg


These two above photos show the condition of shaft failure from the motor side​

Shaft_from_Coupling_Side_mhmnta.jpg
Condition_of_Shaft_from_Coupling_Side_Pump_Side_cwywnr.jpg


These two above photos show the condition of shaft failure from the coupling side (pump side)
​

Photo_showing_that_coupling_of_motor_side_flew_and_hit_the_outer_casing_of_steam_generator_wacvga.jpg


The above photo shows that after the shaft failure coupling broke and hit the Steam Generator equipment next to it.​
​

Coupling_Female_Side_Pump_Side_qinieu.jpg
Coupling_Male_Side_Motor_Side_ovt1ck.jpg


The left side photo shows the coupling from pump side (Female Side) and the right side photo shows the coupling from motor side (Male Side).
​
 
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Hi
There is a lot of damage due to the explosion however I can see a ratchet mark on one of the photos similar to that of the one shown in this article So I am leaning towards rotational bending due to misalignment, also I suspect but it’s hard to tell that a crack might have been initiated at the bottom of the keyway on the shaft.
How long as this been in service?

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein
 
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Looks like a classic fatigue fracture. If that is the pump shaft still in place it should also be checked for fatigue fracture. Check alignment for the next motor and coupler installation as well as ensuring the coupler is balanced. I'd also suggest a substantial cage over the coupling area for both containment in the event it happens again and to keep people away from the entire shaft and coupler installation. It might be worthwhile to detect when a sudden substantial change in the amount of power happens to cut power to the motor as well as any over-speed condition.
 
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