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I think it might be a smart idea to 3D print the side plates, that way you don't have to use all your grant money machining something that might not work
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...in other news, this is really cool! Looks similar to some designs that Bryce2471 has posted in the past. |
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Remember, folks, 3D printing can be plenty strong, but if you 3D print plastic sideplates for gearboxes without redesigning from a design made for metal, you're probably going to shoot yourself in the foot. Gearboxes with plastic sideplates are few and far between as it is due to durability concerns. Now you're adding in that the sideplates would have a built-in weakness--the layer boundaries--and that they're not going to be all that filled in... I'd make sure to run the numbers prior to attempting a 3D print. |
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3D printing does not hold the required tolerances for work such as bevel gears, particularly plastic 3D extruders. The printers normally found in FRC are NOT suitable for work such as this, plus they have a "step" of 0.003" vertically; you will end up with a lot of slop in your gear train. You have to double or triple stack the tolerances due to the existence of side plates. Not to mention press fits with bearings is barely possible, as a decent press fit in plastic may weaken or destroy the frame during pressing due to the layering (depending on the printer). Plus, you still have to do heavily loaded tests to make sure that it can hold up due to the plastic and layering. Now, some actual questions: 1. What is the final weight of this gearbox without motors? 2. What is your reasoning for going with a bevel gear setup? 3. What advantages does this design hold over 192's gearbox design from 2014? I still haven't seen a shifting gearbox design that beats theirs in terms of weight or size. 20fps is too fast. You can limit the top speed in software for the driver, but your acceleration will be very poor with only 4 cims. Seeing as you are running bevel gears, would it be possible to add a 3rd cim sticking stright up? Your final gear reduction looks like it can be reduced to a much smaller/better ratio. Top speed for a 4 cim drive shouldn't hit above 17-18fps if you want to optimize distance/time. OC it depends on the game, but it would be a very rare game that requires 20fps on four cims. |
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I like the idea of 3-D printing the plates. I don't know why everybody is so against it, last year we (1885) used 3D printed parts to join an axle for our intake, it worked very well and only "broke" when we didn't check it and a shaft collar was loose. Then the axle would fall off and we would have tighten the collar up again.
Just use a high quality plastic and use a dense pattern while printing AND MAKE EXTRA'S!!!!! Better to get into 3D printing now while it is still being developed heavily, we all know it will only become more and more common, so this is a great foundation to build off of in the future. |
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Now, note this, I'm not against 3D printing, or even necessarily against 3D printing gearbox plates. I'm against 3D printing gearbox plates WITHOUT accounting for all the stresses and loads in the design, and "just because it's the next thing". If you think the plates should be 3D printed, please, by all means, print a set of plates for a given gearbox--just make sure to document the process and the failure. A coupler sees very different stresses from a gearbox sideplate. I've seen 'em used in very high-load applications when 3D printed, but as I recall that particular coupler broke about a month after it entered service, due to a combination of factors that were not necessarily directly related to the loads it was seeing, and was replaced by a machined part. Now, in case you need help reading a block of text, here's a list of reasons not to print the plates:
tl;dr: I wouldn't 3D print a gearbox plate unless I had specifically designed the gearbox for 3D printed plates, and specifically designed the plates to be 3D printed. This gearbox is, as far as I know, NOT designed for 3D printing. Therefore, it is a very BAD idea to 3D print this gearbox. |
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Gears (especially bevel gears) require very accurate positioning. If the gears are to close together, or the shafts aren't parallel, the gears wear much, much faster. Team 254 made some delrin side plates a long while ago and one of their mentors said that the flexibility of the plastic didn't keep the gears well aligned. The thrust loads from the bevel gears will only worsen the problem of the deforming side plates. Also, you can't really get a press fit for the bearing with a 3d printer. First of all, the printer isn't accurate enough to get the right size hole, and secondly, the plastic has a high chance of cracking if you try to press in a bearing. The side plates can be made on a mill and a bandsaw. The outer profile doesn't need to be accurate, so you could just print and cut out out a correctly scaled drawing, and trace it with sharpie on a piece of 0.25" thick aluminum to cut on the bandsaw. The mounting holes/bearing bores should be fairly easy to do on a mill. Personally, I wouldn't go with the ratios you have chosen for your shifter. I'd recommend using smaller wheels, or adding an extra stage of chain reduction first. 9 feet per second with 4 CIMs in low gear is on the fast side, and you'll experience breaker tripping in low gear. Also, 20 feet per second with 4 CIMs will have very poor acceleration, and will really wear down your batteries quickly. Which bevel gears are you using? |
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Using 3D printed plates instead of metal is somewhat like using zip tie instead of steel wire. Zip tie is nice, you can quickly fasten most things and reasonably strong, but if some applications will have so much dynamic forces they will just get ripped.
Using 3D parts for lighter loads as in to feed a ball is different than gear box or any structural parts in a drive system. May be just for prototyping and quick test it may be ok, but for competition robot... its not just worth the risk. |
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I like the gearbox-- it looks like some nice work. I'll add my voice to the people that would recommend against printing this for actual use on a robot-- I think the others have done a pretty good job of explaining why that's a bad idea. I can say firmly from experience that it isn't nearly as good an idea as it first seems.
That being said, printing certainly has its place in the design of an all-metal gearbox if you want it to. Last year I had a decent amount of success doing a high shell count and drilling/reaming essential holes, and it's always nice to test the geometry in practice as well. With a little bit of adjustment and a small amount of work, you could probably do a manually powered version to print and test before you made a metal version. I'm not entirely sure how beneficial that would be, but it would certainly be cool/interesting to show off. |
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Alternatively, you mention that you're getting an inhouse mill in your first post. Do pre-season testing on a variant without all the fancy lightening and contouring, and then use your external resources and funds to do the full, lightened version for the season.
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This argument shouldn't be about why to 3D print/not 3D print...it should be about why this design is a good use of resources to make at all.
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This gearbox likely won't be cheaper than an off the shelf wcp ds or ball shifter gearbox, but it doesn't hang low to the ground, and I'd bet you could direct drive a 3.25" wheel with the gearbox, something that can't be said about most off-the-shelf gearboxes. Its unique shape also frees up the space where motors would normally go in a typical configuration If you think that this is the best way to improve your team's robot (this totally depends on your team's unique situation), then it's a great idea to try to build one in the offseason first. |
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Anyway, my post above asked a few questions about this gearbox. EDIT: The questions from ym last post: 1. What is the final weight of this gearbox without motors? 2. What is your reasoning for going with a bevel gear setup? 3. What advantages does this design hold over 192's gearbox design from 2014? I still haven't seen a shifting gearbox design that beats theirs in terms of weight or size. 20fps is too fast. You can limit the top speed in software for the driver, but your acceleration will be very poor with only 4 cims. Seeing as you are running bevel gears, would it be possible to add a 3rd cim sticking stright up? Your final gear reduction looks like it can be reduced to a much smaller/better ratio. Top speed for a 4 cim drive shouldn't hit above 17-18fps if you want to optimize distance/time. OC it depends on the game, but it would be a very rare game that requires 20fps on four cims. |
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Curious about the questions above.
And, How you're dealing with the thrust loads from the bevel gear and pinion? If they gear isn't supported with thrust bearings you'll probably destroy the radial bearings you're using. The cim shafts can move a bit in and out, do you have a way of making sure the pinion is in the correct place? |
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I wouldn't say it was anything specific more than the combination of many smaller issues compounded. We had a couple of Replicator 2's that we were just starting to get familiar with, and there wasn't a ton of experience designing for printing on the team then. We printed in the wrong orientation, broke a couple of plates trying to fit in gears, and once we had it all together it made sounds I hadn't heard come out of a gearbox before or since. I think it probably could have worked with some more effort (and experience), but when there's so many nice COTS options out there it just didn't seem to be worth the effort. In retrospect, I wouldn't necessarily say it was a bad idea, just that it wasn't nearly as good of an idea as we thought it was at the time. We learned a ton about designing for printing from that gearbox though. |
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Moving the motors out of the way isn't a good enough reason for most teams, IMO. What is all that extra work and potential compromise of reliability really buying you? An extra 8" in the interior of your robot that you probably don't really need anyways? 192 had the benefit of doing something similar (with worm gears) to OP's design in 2012. They didn't do it again after that. They have at least 4 revs of their 2014 gearbox (as I recall they made 2 prototypes in the 2012 offseason, plus the 2013 gearbox, then the 2014 gearbox). There are so many better obstacles for most teams to tackle than making custom gearboxes. If 254 were starting a new team right now, I highly doubt we would make custom gearboxes. Maybe custom sideplates to get the right ratio, but that's about it. The stuff that's out there now is so high quality that if you have any question about your ability to solve every other aspect of the game challenge, you really shouldn't be going custom. |
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I could vary well be biased, but tend to think that building something unique and ambitious in the off season is almost always a good idea.
Some reasons: Building something unique tends to get students excited about off season work. Building something ambitious will force to team to expand their resources. The experience wI'll improve students' CAD and machining skills. There is very little risk of failure. |
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A custom gearbox can be cheaper than an off the shelf gearbox, especially for a shifting one. Aluminum to make side plates and bearing blocks isn't that expensive. |
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-2.5lbs less total -Extra bellypan space (especially with the smaller size this last year) -Easy motor/gearbox removal (no need to pocket the bellypan as much to pull out the gearbox) -Money. For a WCP gearbox it's $300 without cims. A non-COTS option with shifter shaft might be half that. Maybe 254 doesn't need the space, but 115 would have absolutely adored a few extra square inches last year. Disadvantages: -Manufacturing time. If designed properly, this can be reduced to a couple hours on a mill early in the season. For our team, it's not a problem to quickly churn out a couple custom gearbox plates on our mill and machine 2x1 sides in a few days while the drivetrain is deisgned. Plus, we would have to wait a couple days for COTS gearboxes anyway, so instead we can just wait for shifter parts. I open-source all of my designs, so it doesn't need to be redesigned each year. I'm remaking my 192 gearbox clone to be easier to machine and use COTS shifter parts right now. However, any custom gearbox should be deisgned and tested pre-season. It's too dangerous for many teams to do otherwise. While 192 made several revisions, it is relatively easy to copy their design because they've done all the hard work in the basic design. The hardest part of a design IMO is coming up with the overall design first. After that, it all falls into place in CAD. |
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I think the OP should, at the very least, make one gearbox as a prototype to test the design. He'll learn a lot more from seeing how his design performs. Quote:
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Was gone for most of the day today after posting this last night and was very shocked to see how popular this was. I am first going to 3D printing, all of that discussion was all for not because my team does not even have access to 3D printing and would be cheaper to mill one in house then to pay to have one printed. Now to asid61's questions
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2. We mount our electronics on the belly pan of our robot and constantly ran into issues because the motors were in the way. Also I started this team last year and a vast majority of the team is graduating this year including me. We are working to get a permanent workspace, machinery, etc. so we can establish a program that continues after the founding members leave. We want to create a robot this year that will set a standard for members that follow us that we strive for creativity (bevel drive) and quality custom. I know this can be done in an off season project, too much risk, etc but in the end it is our team and this is what the team wanted to do and that is what it comes down too. I know there are many reasons not to do it but think of what we will learn doing this! In the end it is not about a robot but what has been learned during the process of building the robot. TL:DR Nicer form factor and it is what the team wants to do 3. It does not, that is a BEAUTIFUL gearbox but it is not ours and this is the design fits much nicer in a drive base and works for us. As for our issues with gear ratios what really stopped us from gearing it further down was I thought that 9fps was way to slow already but after being told that is pretty fast and we will have breaker trip issues I will play with the ratios more. I will shoot for 16fps High and 5fps Low. Also here is a link to a better picture of the bevel gear setup should answer some of your questions. The bevel on the CIM is bored and keyed for the CIM then just slides on but a retaining ring will be added but no thrust bearing. The other bevel has a bearing on its 0.75" hub but also has a 3/8" hex shaft that goes through its bore to the rest of the gears. Both don't have thrust bearings. Hope that all makes sense http://i.imgur.com/Vbxs3Sb.jpg There was a lot of questions and comments and I tried to cover them all but what I really want to hit on again is that this is what the team decided as best and we will be making one during the off season using in house CNC equipment. Please let me know if you have any more questions or comments. EDIT: The bevel gears are also lined up exactly to the manufacturer's specifications so they should mesh perfectly and are made of steel. |
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THAT BEVEL GEAR MOUNTING IS AMAZING!!! Tons of potential here. If you change the ratio of the bevel gears to 1:3 or 1:4 rather than 1:1, then you can eliminate the final stage of gearing. Then, because of the size of a 4:1 bevel gear, you can put the first set of shifting gears right next to the face of the big bevel gear and only require two plates. That would slim down the gearbox a ton and make the weight much lower. The width would increase though, but not by much. The output shaft could be a vex shifter shaft then too. EDIT: sorry for being pushy, but I'm used to trying to optimize gearboxes. Many times when I see a team's bevel gear setup, they use 1:1 in favor of 1:3 or 1:4. Generally this is because the Vex bevel gears are 1:1, but in this case I don't see why not go for a larger ratio, as it would optimize many things. |
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That looks pretty good, with the miter gears being in the first stage I'm not to worried about the thrust loads. However that also means high speed so wear is a big concern.
If you can get your gears from martin sprocket instead of Boston gear, should be the same specs but the former are case hardened (may have been doing this already). I would also make sure the gear on the cim won't move, it may be fine how you have it but just a possible concern. Can't wait to see it running :) |
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1678 geared our drive for a theoretical 22/8fps high/low. We measured our top speed at about 18fps on our practice field. It was very fast and I will admit we ran a 6 CIM drive train. It should be said that each gear ratio should be tested under load to find it's ACTUAL output speed. But at this gearing and motor to speed ratio you are unlikely to hit the max potential of the CIMs, you will be accelerating for a large portion of if not all of the distance of the field. It takes us about 1/3rd of the field to get moving at full speed with 2 horsepower backing our drive train. |
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I just finished re gearing the 3rd stage and it now has a predicted 17.10fps High and 7.54fps Low. I like the idea of adding some reduction in the bevel gear but the whole box was designed around those gears. I would have to pretty much start from scratch in order to do that. Thank you for all the encouraging words.
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From this render I can't see but did you in-close the side plate where there was a gap previously to allow access to the bolt for cim? I have been meaning to send you my cad on it, just been super busy.
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