DeLaval Boiler Feed Pump Shaft

[87T2M]

Is anyone familiar with the failure mechanism on DeLaval multistage boiler feedpumps outboard shaft? We have had two failures of the outboard shaft, outboard of the thrust bearing at the location of attaching the stub shaft that would be used for speed pickup in our case, or for a shaft driven oil pump. I understand DeLaval is increasing the size of the shaft in this area by 1/4" (ours is 1-3/4"). The shaft is breaking right at the edge of the collar that is set screwed on the shaft, the collar is approx 2 inches in length and attached to it is a 3/4" diameter shaft that is approx 4 inches long for the speed pickup. It's the main 1 - 3/4 inch shaft that has broken, resembling a shear failure. The collar and shaft for the speed pickup are overhung on the pump shaft, and it's hard to imagine what would cause that shaft to fail with so little attached to it.

 

[JJPellin]

You are probably going to need to provide more details in order to get a meaningful answer. What is the shaft material? What sort of speed pick-up? A non-contacting proximity probe? What do you mean when you say it looks like a shear failure? What does the fracture surface look like? Is it a straight perpendicular break like one would expect from fatigue in bending or an angular break like one would expect from high torque? Based on the very limited information provided, I would be most suspicious of high cycle fatigue in bending. I would look for a side load on the end of the stub shaft that could produce the bending moment. I would look for a stress concentration in the shaft that could allow even a low stress to fatigue the shaft. I would look for a design or manufacturing error such as surface finish, keyway shape, etc. that could provide an initiation for a crack. We recently cracked shafts in two identical liquid ring vacuum pumps. The shaft broke across a 5 inch diameter in an area with relatively low stress. The cause was an improperly designed and poorly machined keyway. We have broken other shafts in lobe blowers because of a machining error that resulted in an extreme stress concentration in a very notch sensitive material (ductile iron). I have a number of other examples on my desk from shafts that failed with very low stress in high cycle bending fatigue situations. More information is needed. A picture of the fracture surface or a cross-section of the pump would be very useful.

Johnny Pellin

 

[Artisi]

OEM or pirate shaft?