Shaft Boot Loading

[ks11]

I have a shaft connected to a piece of equipment, with a protective covering welded between the piece of equipment and the shaft to protect the hardware the shaft is attached to the piece of equipment with from corrosion. As the equipment rotates, the shaft bends, pulling on the boot and eventually the weld between the boot and the shaft ends up cracking. Running an FEA on the boot, and we see the highest von mises stresses on the sides of the boot, where the boot/weld would be mostly in shear. Since it's shear, there is no negative value associated with it like there is with tensile stress, but its happening in opposite directions on the left and right side of the boot, so, should I evaluate the stress as cycling from X psi to -X psi, or from X psi to Y psi (where Y is some lower, non-negative value)?

 

[Tmoose]

I'd spend some time looking at nice close-up pictures of the fracture surfaces of boot welds before firing up the engineering computer.

What is the goal of this analysis?

What are the "highest von mises stresses on the sides of the boot.." ?

What is the configuration of the boot ? A simple cone, convoluted/accordion, some thing else entirely un-guessable due to due to essentially NO up fornt information.

If it is (now) known that the shaft "bends" in service the boot must be quite flexible, lest it inadvertently become shaft reinforcement.

 

[ks11]

FEA is what I was directed to do, and I don't have any pictures of the cracked weld to look at anyway.

The ultimate goal is to alter the design of the boot in a way that increases the lifetime of the boot, hopefully to the point that the weld never cracks. The point of the FEA was to compare the currently used design with a different design.

Peak stresses shown are around 9.5-10ksi.

The boot is a flat round sheet 3/16" thick, approximately 45" OD, 15" ID, with a small 1" radius outward flare at the ID so that instead of the boot coming in perpendicular to the shaft, its closer to a 45 degree angle.

Yes, the shaft bends in service, perhaps .010" from the undeformed centerline at the point the boot joins to the shaft. I have already changed the design of the shaft mount to reduce this significantly for future equipment, but the issue is with a piece of equipment that was designed before my time.

 

[btrueblood]

"The boot is a flat round sheet 3/16" thick, approximately 45" OD, 15" ID, with a small 1" radius outward flare at the ID so that instead of the boot coming in perpendicular to the shaft, its closer to a 45 degree angle."

Wow. Lotta words, hard to follow. If only we could see a picture of the part like you can...

mumble, mumble...pictures...something, something...thousand words.

 

[ks11]

Ok, let's ignore even talking about a boot since y'all seem to be hung up on that part. If I have a weld in a straight line "X" inches long, and I apply a load "Y" parallel to the weld axis, so the weld experiences shear, then I remove the load, and then I reapply the load in the opposite direction, and this process is repeated for "Z" cycles, would I treat this as a stress range from 0 to Y, or from Y to -Y?