Hello Chief Delphi,
I am on the verge of completing the CAD for my 6 Wheel WCD. The final task on my list is to pocket the 2-speed gearbox sideplates. What is the agreed upon standard for the amount of material that should be left around any given hole (strut width).
After I finish, I shall post my complete CAD for further criticism and opportunities to improve the model before fabrication.
Thanks
I don’t think there’s any real rule of thumb for this. The amount of material you need to leave depends on a number of factors, including the thickness of the plates, gearbox layout, torques involved, etc. I would suggest picking a value that looks “about right,” and performing finite element analysis on your model to identify trouble spots and locations where you have more material than you need. Take this information, and iterate to an acceptable solution, keeping in mind that you’ll eventually reach a point where gains from this type of pocketing become pretty trivial.
we’ve found that its better to just leave it as a solid plate but bring thickness down to 1/8 to save weight and preserve strength and use machining resources efficiently
Our previous gearboxes have worked just fine with 1/8" of material around bolt/motor holes, with 3/16" thick material connecting them. This is with 1/4" thick plates. Check them out in the cad link in my sig.
It’s funny that you mention this, since your sideplates are already currently open in my SolidWorks. I will use this advice on our current gearbox.
Thanks
Just for curiosity’s sake, are there any resources on the internet that neatly describe how to effectively use FEA on a model? I’ve tinkered around with it a bit in Solidworks, but it’s hard to estimate the applied forces that will be experienced by this gearbox.
Generally speaking, when pocketing .25" Al, I keep a consistent .2" thickness around everything. However, as others have mentioned, there is no standard here and you can remove as much as you think is necessary.
I haven’t found very much either (a reason that I’m very much looking forward to my finite element analysis class at college next year). But you can get some pretty good results through free body diagrams. For example, if you know the applied torque at a CIM motor’s pinion, you can calculate the force experienced at each bolt. The same goes for gears; think about how they’d move if there was no material present, and what kind of reaction forces the sideplate is providing at the bearings. Be methodical about it, and you can get a pretty accurate picture of most forces generated internally by the gearbox.
Finite element analysis is quite sensitive to incorrectly set constraints and forces. My stress analysis professor gave us some advice on how to verify that a finite element model is set up correctly. Simplify the geometry of your part (in this case, probably a non-pocketed plate), and perform classical stress analysis. Model the same simplified geometry, set it up for finite element analysis, and verify that your results are in line with the hand-done analysis. If they are, your forces and boundary conditions are accurate. You now know that as long as these remain the same, your finite element model will now provide accurate results, no matter how complicated the part geometry. Add in the pocketing, and rerun the analysis, and you should get accurate results.
The more difficult forces to estimate are those created by impulse forces (ie, the bending moment experienced from the shock of a robot flying off the bump and hitting the ground). When in doubt, overestimate. Unknowns like this is why everything is engineered with a saftey factor. Remember that this is an application where you’re shaving mere fractions of pounds…it’s probably best to be on the safe side with such a critical part of the robot.
Another thing to remember is that finite element analysis is good not just for showing material displacement under loading, but also stress flow. Even if your force magnitudes are off, if you get the force distribution correct, you will get a pretty accurate picture of where high stress and low stress areas in your gearbox are. You can then beef up the areas that show up as stress concentrations, and feel safe with minimal material where there isn’t much stress.
Another thing to consider in gearbox pocketing: I’m suprised that this style of pocketing hasn’t caught on much more. Leaving a very thin “skin” in the pockets dramatically improves strength at a miniscule weight cost.
I know Inventor has a fairly good tutorial system that I learned FEA from. I don’t know if the package of SolidWorks you are using includes an FEA add on (I think it’s called Simulation). It’s very possible that there are good tutorials for SolidWorks out there.
I would recommend leaving more material than you think is necessary. While it may be tempting to try to shave every gram possible out of a gearbox, there are many easier (and safer) ways to cut weight (aluminium gears, shafts, thinner plates, etc.). I would go with a safe half inch.
Also, keep in mind, there are alternatives to pocketing. You can chose another type of machining feature that can cut more material while maintaining strength, such as webbing.
Thank you all for your hasty responses. I am going to bring this discussion up to our technical mentor, and see what he has to say.
And of course, I’d rather be safe than sorry!