FEM allows you to focus exactly on what you want. This time it's the shoulder bolt. This completes the machined hanger calcs we did in the past.
The bolts have interesting contact interface with the hanger. While intimidating at first, the precise dimensions and excellent material properties make it extremely capable.
Truck in analysis
The CAD model was generously provided by Kevin from Boardnamics. Go check them out, they have precision machined trucks and other cool drive-train components for your next esk8. The quality is top notch with reasonable prices.
The hanger is machined out of 6061-T6 Aluminum. It's 220mm long, enough to mount 2x larger motors. The 36mm deep 8mm diameter bores from each side allow for 80mm shoulder bolts to be screwed in into M6 internal thread.
FE Model
This is where the FE flexibility comes in handy. I'm interested only in the shoulder bolt and the surrounding hanger area. So, why not use half model only? (With appropriate boundary conditions)
This assumes symmetric loading from both sides, which is fine for our purposes.
Loads
We'll start with the classic 120kg rider spaced evenly between 4 wheels. Each side will have it's own 30kg of vertical force on 50deg inclined truck.
Results
It's always a good idea to look at the displacements and vet out any weird things. In our case nothing unusual with ~0.5mm of nominal deflection.
And maybe a small sanity check with previous results. Recall the values from Caliber analysis: 28MPa against the current ~30MPa. Slight variations are allowed because the geometry is different and we're playing with boundary conditions.
Here it becomes interesting
Shoulder bolts can't be in contact on all sides, especially under load. Unlike the thread, which is nicely engaged, with generous pretension and thread-lock.
This happens under load:
Naturally the contact areas will be loaded (and free edge at the separation):
What about the bolt?
The elevated stress areas are right where it meets the hanger at the edge:
This is one of the cases when looking only at Von Mises stresses might not tell the whole story.
The upper area is in compression, and so doesn't really matter. The same goes for the hanger at the contact.
Showing the 1st principal stress reveals the tension areas which should drive the design (negative=compression, positive=tension)
What's the bottom line?
These are Alloy Steel bolts with Minimum Failure Stress of 140000psi, or ~970MPa.
The Yield Stress of 6061-T6 Aluminum is 276MPa
To calculate safety factors, we'll use only the stresses in tension.
30MPa for the hanger, and 140MPa for the bolt.
This gives safety factor above 9 for the hanger and above 6 for the bolt.
We used "classic" loads that might seem low.
How about x4 times the load? High enough?
Just scale the numbers, this hanger still won't be the first thing to fail.
Great product Boardnamics!
Enjoy,
Dani
P.S.
I always get push back with something like "the dynamics loads are much higher", "what about the vibrations", or "impact loads", or something like this.
True, the above analysis is static and the dynamic effects aren't considered directly.
And, with safety factors above 4, no one cares.
You can always get the "equivalent of dynamic loads" and use it with static analysis. Let me know if you want a post about it.
It's usually no more than 200%, and we're way over this..
Enjoy,
Dani
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