Recently, we've seen some hex trucks breaking, others bending.
And still others run without an issue for miles and miles.
What's the mystery here, and can we shed some light on this?
I don't have any official CAD model of any of the trucks, nor official loads.
I'm just estimating and comparing here, so feel free to throw my observations out of the window.
Numbers and Assumptions
We know these are made out of 5/8" stock hexagon, and are advertised as 6061 Aluminum, the length is 240mm.
This should give a head start with some assumptions to complete the picture:
- The geometry is modeled from pictures available online.
- I'm going to play with half hanger with symmetric boundary conditions.
- The bushing seat and the bolts will be taken into account by constraints.
- The load is my standard 120kg guy (30kg/wheel) and additional 30kg of motor drag.
- I've found this, so I'm assuming it's 6061-T651 Aluminum with the following properties:
% to break=12%
This version has the bolts countersunk into the hanger and the thread is in the bushing seat:
We'll start with the basics, the displacements seem normal and within something one would expect from a stiff truck:
Now for the stresses.
The overall picture seem ok, with low values:
Let's zoom into the bolts area:
The red area on the edge there? Not good*
This is the edge of the bore (very small fillet), and also in tension (bottom of the hanger).
These two create a stress concentration area, which might be tolerated once or twice, but is not sustainable on the long run with repeated stresses.
Just look again at this picture and see if the breaking point is familiar?
*Notice the stresses are higher than the Ultimate value. Obviously this cannot be real. The reason is that the material is past Yield (plastic), but the analysis is still linear.
Being plastic with moderate loads is not a good place to be.
I don't go deep into plasticity and fatigue here (maybe future posts?), but the point is clear - this isn't a sustainable design and should be avoided.
A step in the correct direction - removing the countersunk part, but otherwise maintaining the design as is: the head of the bolts is on the bottom and the bushing seat is threaded.
Just look at the difference:
The stresses are lower (and in the elastic range). But the stress concentration point was not removed entirely. This might result in fatigue failure, or more benign permanent deformation (once the hole becomes oval)
A point to remember here: I'm using "smooth road" loads. Once there's a pot hole or any real world road really, this point can go past yield quickly.
Let's compare to Surf Rodz TKP
(Known for reliability and no breaking reports)
The design is slightly different, with the bolts going from the bushing seat, the hanger is threaded. Still the same hex profile.
**The hanger is shorter at 177mm. To match the root moments, I added a piece of axle.
When looking at the equivalent stresses, at first it doesn't seem different at all:
The difference becomes obvious when looking at tension/compression, and it's important.
Positive - tension, negative - compression.
So, the stress on the edge of the bore, it's compression.
What they did here is removing the stress concentration point from the tension side (bottom).
The hanger compresses towards the bolt, and it doesn't matter, because things don't fail in compression.
The arrows mark the edge of the bushing seat.
Other than that?
Nice, uniform stress distribution on the bottom.
What's the bottom line?
Try to avoid stress concentration points in your design. Especially for something that's aimed at repeated loads
v2 (of anything) should (at least) have different problems than v1.
FEM is cheap and readily available, use it.
What can be done with this design?
The obvious choice is to run the bolts from the top.
Other option is to make the pivot seat bigger. Widening the support even by 10mm from each side decreases the stresses significantly.
If possible, take the bolts further away from the middle, towards lower load area.
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