Clearly I still have to flesh this out quite a bit, but you get the idea. Additional bonus is that these rockers will be stupid easy to machine, even after I add another mounting point for a F-ARB. Would be nice if I could get the force vectors of the pushrod and damper a little more in line so there isn't as much of a net force (and moment!) on the single shear rocker mount. I'll have to play around with it. At this stage of the design it's more roughing things out in terms of what space they'll have to occupy.
As an aside, you may note that when I put in blind (as opposed to 'through') weight relief pockets, I like to fillet the bottom to mitigate any stress risers. In this case I have R0.060" fillets at the bottom of the pockets on the rockers. At first, this may seem dumb from a DFM perspective - how are you going to get a DIA 0.125" ball endmill down in there without it chattering and running slow as hell?
Luckily, God gave us 'bullnose' endmills - they are clutch for this sort of application. Think of it as a flat endmill with a small radius on the end. A hybrid between a flat and ball endmill. Allows you to pocket things out with a healthy diameter cutter for efficient metal removal while leaving a nice rounded edge at the bottom. Additionally they tend to be slightly more robust. Since the cutting edges don't come to a small sharp point they are less likely to chip off. Believe they also leave a slightly better RMS surface finish. What's not to like?!They are also known as 'corner radius' end mills. Can find them over at MSC or most places with a good selection.
2 comments:
Hey, it's good to see you back posting again!
I meant to ask this when you were previously talking about it but forgot, so since this has reminded me, what's the deal with resolving the forces through the dampers into that mount? Are you just biting the bullet and making it stiff enough not to deflect, or does is it going to be triangulated/integrated with the chassis further than it looks in the previous post?
It's just, i'm considering damper placement at the moment in our fsae car and the rear is a bitch - but one possible solution is to have the dampers vertical and feed to loads into the chassis. I'm just a little loathed to do that because I know I'll get slated in design!
Ed
Kind of becomes a two part question. Maybe four part. Stress and deflection, mount and underlying support.
The mount I'm not worried about (isolated and presuming it's bolted to an infinitely rigid support structure). It's a stout little piece of alloy steel, which I'll probably redesign to be in double shear.
The underlying support (aluminum bulkhead) is going to see some substantial bending moments in multiple directions. I'll have to run some FEM's on it for sure and evaluate a few options. Might be able to work on its own with some smart pocketing and I-beam sections, from both a max stress and deflection standpoint. If not, wouldn't take much to put a small compression link between the rear bulkhead and the front roll hoop (rules permitting), or another element - even the front bulkhead. Again, for the time being I'm mostly fleshing out space requirements.
In design judging for FSAE / FS (incidentally I may do some judging next year)... you get dinged if you don't have good reasoning behind your decisions. As much as we try for elegant, clean, smart designs.. you can't win it all. Need some compromise. If you can put loads through the chassis in a way that they can be mainly resolved through axial tube loading.. or at least not giving much up in terms of un-necessary stress and deflection.. then I'd say go for it.
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