Quote:
Originally Posted by Andrew Lawrence
More like not enough material. Those thin pieces of sheet wouldn't hold up on their own even without the motors. At a minimum, especially for anything with a CIM attached to it, use 1/4" aluminum plate, or else your gearbox is going to snap before the first miliamp hits your motors.
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I will humbly add some things that we on 2168 have learned over the years about custom gear box design relative to gearbox plate material.
First, to re-affirm what others have already said. Both AndyMark and VexPro sell a number of successful gearbox designs based on flanged 1/8" sheet. You can also add 1114 to the great list of teams already mentioned that design custom gearboxes with I believe with flanged .090" sheet.
Second, our experience. I can't overstate that the following is simply sharing our (2168's) experience and opinions. We have designed both boxes and parallel plates for our transmissions, with more of the later. While we recognized that thinner material will suffice for the application, we always use 1/4" Aluminum. This is for two reasons:
A) Bearings (especially ones affordable on the average FIRST team budget) become less and less reliable the more they are point loaded. As an example, lets take the standard AM or VPro 1/2" hex bearing that so many teams used this year in gear boxes, axle load support, and on intakes. This bearing has a raceway depth of 1/4" (total thickness .312" - flange thickness .062"). Using .090" as my gear box plate, supports about 36% of my 1/2" hex bearing raceway. Following this a little deeper, because of course the bearing is installed with the bearing flange coincident to the plate, only a portion of the ball in the bearing raceway is supported by the .090" plate. As it turns out of course 1/4" plate fully supports the standard 'FIRST bearing' raceway. Based on our past experience this has been a contributing factor in bearing explosion on COTS transmissions we have used in the past. At the end of the day, as a number of the posts to this thread have already implicitly established, it really comes down to what your team has established as "best practices" and "acceptable risk".
B) The more CIMS you add to the standard parallel plate design, the higher the overall cantilevered load applied to the motor plate and to the output axle. The higher the cantilevered load applied to the axle, the higher the tendency for oscillation in the axle, which translates through the bearing. You can derivate the results from here. While not typically viewable to the naked eye, the cantilevered load creates small inflections the motor plate and ultimately "clocking" between the parallel plates, which become larger as your plate material becomes thinner. This can ultimately can be mitigated by a larger concentration of standoffs as well as creating recesses into both plates for your standoffs to shall we say "sink into". Ultimately both techniques lead to plate rigidity and remove the above mentioned tendencies. At the end of the day however, we are not designing a product with even a 1 year warranty, so it once again comes down to your teams "best practices" and "acceptable risk". Weight savings in your transmissions will buy you features else where. Many teams have used and will continue to use down to .090" sheet with satisfactory results.