Shoulder bolt in bending?

[tetwin11]

I've got a scenario where I would like to use a shoulder bolt (M8 threads, 10m diam 60mm long shoulder) threaded into a block (material to be chosen) as a small wheel axle. Because a wheel will be mounted on this bolt/axle, there will be a load perpendicular to the axis of the bolt. Worst case scenario, 940 N at the head of the bolt. I'm not concerned with the bolt breaking (should I be?) but more the threads ripping out of the block. Any thoughts/suggestions about what might concern you and/or how you may remedy it? See attached

 

[Cockroach]

There are several threading in this forum regarding fastener performance. I don't think you have an issue though, I would recommend the following.

Clearly your Shoulder Bolt is subjected to shear. You can check for strength and size the shank of the fastener according to your external loads, since they are so well defined. The threading simply supports the loading as a cantilever.

What I strongly suggest is that you counter bore the shank into the block by a small amount, thereby relieving the threads from bending. In other words, do not butt the shoulder of the shank to the block, instead have it mate a touch below the face. You can easily see this simple change provides for a much stiffer assembly, the threading now is asked to provide axial load and not bending.

Hope this helps. Good luck with it.

Regards,
Cockroach

 

[rb1957]

if you wanted to check the threads in bending, i'd calc a bending stress assuming a circular section, dia = mean thread, then apply that as axial load to compare with the bolt allowable. else the bolt allowable moment comes to mind.

mind you, you'll install so there's pressure on the shoulders, driving the axle into the block maybe using an installation torque ?
and so some of th emoment will go into the block across the shoulders.

is teh block material the same as the axle ?

 

[drawoh]

Cockroach,

The only way the threads will be loaded in bending is if the screw comes loose. This is, of course, a possible failure.

If the OP works out the maximum possible load on the wheel, some kind of impact no doubt, he can select a clamping force that grossly exceeds that. Use lubricating threadlocker, and specify wrenching torque.

--
JHG

 

[drawoh]

On second thought, I am just working out the clamping force of an M8 alloy steel screw. The contact stress of the 10mm diameter at the base of the screw is quite a lot. At the very least, the OP will not be able to use the maximum clamping force of the shoulder screw.

--
JHG

 

[rb1957]

agreed that the moment is reacted by the shoulders and the thread ... assuming all is carried by the thread is conservative.

the shoulders could react bending by relaxing the preload contact pressure, or (if gapping by a compression load on the corner of the shoulder and a tenion load on the shank.

 

[SnTMan]

Without getting into technicalities, I would point out that even the least expensive lawn mowers use essentially this arrangement, except nutted to the sheet matal, rather than tapped into a block. It can be done

Regards,

Mike

 

[tetwin11]

Thanks all. It looks like there are no problems in shear or bending on the bolt. I do like the counterbore idea though!
It looks like the bolt can be torqued down to ~40% of yield strength on the threaded area without over loading the shoulder. 780 MPa
The maximum tensile stress created by the bending moment is only 350 MPa. So it looks like the bolt can be torqued down enough to hold the load in the shoulder.
Thanks!

 

[Cockroach]

I was thinking more along the lines that if the shank is butted up against the face of the block DrawOH, then a vertical load in shear would put the threads into a tensile state on one side and a state of compression to the opposite side. This would always be the case even if the shoulder screw were tight against the face of the block.

See the attachment. Pts A acts as the pivot point regardless of the shoulder bolt shank being loose or tight against the face of the block (Case 1). By counterboring the hole in the face (Case 2) and allowing the shank to shoulder into the block provides a much better seat to mitigate shear, hence, reduce thread failure by bending.

Thats's all I'm inferring.

Regards,
Cockroach