rupture under head stainless steel bolt

[ecto]

hello to everyone, I would have a break to submit to the forum.
It is a coupling motor shaft with shaft duct, made by double 90° spindle, trees in aisi 304.
the motor shaft is full and is inserted into the shaft (tip of dia. external 60 mm), which introduces the holes specially paved to provide flat support to the bolt washers.
two bolts are then inserted at 90°, each consisting of:
screw m12 x 90 (accorciata in workshop at 78 mm) in aisi 304 class a2-70, flat washer under the head, flat washer at the other end and self-locking nut.
the system works for 8h, with about 10 start and stop. the speed is about 70 rpm.
bolt clamping is normally uncontrolled and should be around 100 nm.
It is clear that ideally instead of bolts two elastic thorns should be applied, so to approach the ideal case the two bolts should not be preloaded.
we come to break: about once a year the first bolt, the one closest to the end of the shaft, has the head breaking.
could be rotating bending due to the lack of straightness of the trees, or theoretical error or problem of bolt material (this last hypothesis would be to be set aside given the repeated breaks even with different screws)?
Thank you.

and everyone.

 

[cacciatorino]

without a design it is difficult to make any assumptions.

 

[TETRASTORE]

it would be useful to know also the torque, the spins of the shaft, the coupling tolerance between the motor shaft and the shaft (tube) conducted and have the images of the motor shaft, the type of fracture of the bolt head and see if the 4 holes on the de 60 tube are ovalized.

 

[meccanicamg]

It could be a hard break due to the non-alignment of trees. It would be interesting to have the close picture of the head of the screw to better assess the fracture, at least qualitatively.

 

[ecto]

thanks for the interventions. I attach an exemplary rendering of the coupling and a photo of the break, where the head is absent. Unfortunately the component is too far to be verified at the moment. the turns are written above, on the 70 rpm. the head of the screw was "mitten" by the machine that follows the one in question. the pair should be far from the maximum for this coupling. at the moment the non-alignment of the trees is the most plausible hypothesis. Thank you.

 

[meccanicamg]

with the fact that you have a cochlea you have pushes in all directions leading to a skeptical flexional movement... fatigue....crack.
little detailed photo but it looks like a fatigue break and there is partially a crown on one side.

 

[Marco F inox]

I think that in such a fitting, where it is possible everything that has already been reported, two narrow screws with nut, they only serve to try to block a system that always has oscillations.
I would put two free cylindrical plugs, with two caps at the ends to avoid the parade.

 

[TETRASTORE]

could be rotating bending due to the lack of straightness of the trees, or theoretical error or problem of bolt material (this last hypothesis would be to be set aside given the repeated breaks even with different screws)?

This type of coupling is the most in use in auger conveyors because simple and cheaper, however it requires a mounting accuracy that is often difficult to achieve.
the lack of alignment between the axis of the screw and that of the gearbox is the cause of the fatigue breaking of the head of the bolt; the application of washers and relative lamature has aggravated the situation.qui find described how to make the proper alignment of the screws to avoid breaking and qui Show it to you with a video.in quest'altro video you can see the type of screws to use and their correct assembly.