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 Ok, I am geting flustered since I can't perform trigonometric functions with the tools FIRST provides. - jweric [more] |
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CGX-115 is a special type of counterweighted cycloidal gearbox with a 900:1 reduction off of a miniCIM, for a speed of 6.49rpm (2.5 seconds to move the arm 90*).
Normally, in a cycloid drive, an off-center camshaft causes the rotation of a cycloidal gear, which has large holes around it to drive the "carrier". However, this type of design can be somewhat lacking due to the massive vibrations caused by the off-center gear rotating at high speeds.
The easiest fix for this is to use a second cycloidal gear of the same size on a 180* phase offset caused by a second offset in the opposite direction on the shaft, which balances out the two off-center masses. This method ends up being bulkier and requires more intricate machining and more parts than I like, although it definitely gets the job done.
CGX-115 overcomes both of these problems by putting a large lightening pocket on one half of the cam, as well as making it out of bronze and making the gear out of aluminum. This balances out the off-center mass of the gear and allows the use of a single large gear. It is also easy to stack a second stage onto that gear to square the reduction, in this case from 30:1 to 900:1, in a very compact package.
Note the presence of only a single fixed plate. No plate is necessary on the other side, however, the 1" shaft with 4 holes in the end of it is supposed to remain fixed, preferably to something like a 2x1 or solid block, and has to be very strong as it almost totally supports the slightly cantilevered load of the 2x1 "arm". If 2 of these gearboxes is used (one per side of the robot) then the cantilevering issue becomes less of a problem.
CAD is available here in the 2015 offseason release: https://workbench.grabcad.com/workbench/projects/gcBvYbFAI7TQYHiKMOq7jiSQVQxuTYPyTAXwhy5IZUFIGK#/space/gcvLTQUiZ-c9-szjJsPH3WvpxdeKOB9B1Z2b-tQWAq3gUh
Despite it being 2016, I like to keep my files in one spot.
Questions or comments are appreciated.
Discussion
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23-10-2016 23:24
Scott Kozutsky
Re: pic: CGX-115, cam-counterweighted cycloidal drive
As I understand this mechanism, the cyclodial gear nearest the cim drives against the mounting plate of the cim, which would be fixed against the main structure of the bot. The other cyclodial gear is driving against another plate which is mounted to the arm.
Because both cyclodial gears advance in the same direction, wouldn't this gearbox just provide a powered zero output?
BTW, great work, it's actually because of you that I know that these gearboxes exist at all.
23-10-2016 23:32
asid61
Re: pic: CGX-115, cam-counterweighted cycloidal drive
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Originally Posted by Scott Kozutsky
As I understand this mechanism, the cyclodial gear nearest the cim drives against the mounting plate of the cim, which would be fixed against the main structure of the bot. The other cyclodial gear is driving against another plate which is mounted to the arm.
Because both cyclodial gears advance in the same direction, wouldn't this gearbox just provide a powered zero output? BTW, great work, it's actually because of you that I know that these gearboxes exist at all. |
This is the order of motion:
1. The miniCIM drives the camshaft
2. The wobble gear rotates around the CIM axis, but it itself turns once for every 30 rotations (the first 30:1 reduction)
3. The second wobble gear, bolted to the first one, rotates once for every 30 rotations as well.
4. Due to the eccentric nature of that rotation, the arm is pushed 1/30th of a rotation for each rotation of the second gear about its axis, creating the second 30:1 reduction.
It's a bit easier to see if you have the cad model working. The arm is forced to rotate, essentially.
Don't thank me, thank s_forbes! He provided the CAD model for the first cycloidal gears and the spreadsheet to calculate the gear ratio and dimensions.
23-10-2016 23:51
Scott Kozutsky
Re: pic: CGX-115, cam-counterweighted cycloidal drive
That's really clever. I had the cad open on the grabcad site but the explosion was clear enough to get me to (mostly) figure out what was going on.
Just clarifying; because of the difference in gearing between the two cyclodial gears this goes from a powered zero to moving the difference between both "stages" every revolution.
The gear ratio should be the same as if this were a standard 2 stage cyclodial gearbox with the same number of teeth, as I understand it.
24-10-2016 00:08
asid61
Re: pic: CGX-115, cam-counterweighted cycloidal drive
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Originally Posted by Scott Kozutsky
That's really clever. I had the cad open on the grabcad site but the explosion was clear enough to get me to (mostly) figure out what was going on.
Just clarifying; because of the difference in gearing between the two cyclodial gears this goes from a powered zero to moving the difference between both "stages" every revolution. The gear ratio should be the same as if this were a standard 2 stage cyclodial gearbox with the same number of teeth, as I understand it. |
24-10-2016 00:19
Scott Kozutsky
Re: pic: CGX-115, cam-counterweighted cycloidal drive
Wouldn't it be a 28*29:1 = 812:1 gearbox then? The output 'stage' has 30 pins therefore 29 lobes and the input has one less lobe.
24-10-2016 00:32
asid61
Re: pic: CGX-115, cam-counterweighted cycloidal drive
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Originally Posted by Scott Kozutsky
Wouldn't it be a 28*29:1 = 812:1 gearbox then? The output 'stage' has 30 pins therefore 29 lobes and the input has one less lobe.
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24-10-2016 16:14
nuclearnerd
Re: pic: CGX-115, cam-counterweighted cycloidal drive
I'm a bit confused, this picture shows a big old mounting flange, but others show the versaplanetary mounting pattern. Did you draw both? I ask because we used 3 stage versaplanetaries this year at 250:1 reduction. We found we were really pushing it with the strength of the hex shaft, and especially the holding power of the two #10 screws. If you've got such a big reduction, consider engineering it to use another output and mounting arrangement to bolt directly to frame members and/or sprocket bolt circles.
24-10-2016 16:57
asid61
Re: pic: CGX-115, cam-counterweighted cycloidal drive
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Originally Posted by nuclearnerd
I'm a bit confused, this picture shows a big old mounting flange, but others show the versaplanetary mounting pattern. Did you draw both? I ask because we used 3 stage versaplanetaries this year at 250:1 reduction. We found we were really pushing it with the strength of the hex shaft, and especially the holding power of the two #10 screws. If you've got such a big reduction, consider engineering it to use another output and mounting arrangement to bolt directly to frame members and/or sprocket bolt circles.
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I'm seriously considering designing a better VP output stage or output shaft that can hold a larger shaft and maybe output to a 1.875" hole pattern. I totally agree that the mounting options and hex shaft are limiting factors here.
26-10-2016 13:31
Mike Schreiber
Re: pic: CGX-115, cam-counterweighted cycloidal drive
For anyone struggling to visualize this (as much as I hate to refer someone to Wikipedia) There's a good animation here.
https://en.wikipedia.org/wiki/Cycloi...idal_drive.gif
26-10-2016 13:41
asid61
Re: pic: CGX-115, cam-counterweighted cycloidal drive
So apparently the radial loads on the camshaft (which is attached to the CIM shaft in this case) is equal to Torque/(eccentricity * gear ratio), which assuming you want to have a 200ft-lb (2400 lb-in) torque on this maximum comes out to
2400/(0.05*900)
or about 53 pounds of pure radial force on the CIM. A CIM at stall on a 12t pinion experiences about 22lbs of pure radial force. At the moment I have a pair of 8mm bearings in the output shaft, so it's probably safe, although on smaller gearboxes like the Versaplanetary version where this is not the case there could be savage failure modes. This is mainly a problem for cam-counterweighted cycloidal drives, as they don't have 2 wobble gears on 180* phase offsets to cancel out the forces.
26-10-2016 14:05
Cothron Theiss
Re: pic: CGX-115, cam-counterweighted cycloidal drive
This all looks great! A few quick things before I dive back into your CAD:
You're missing a few parts in your GrabCad folder for this assembly, like the small bearings around the first wobble gear, or the countersunk screws bolting the two wobble gears, and just a few small parts here and there.
Also, where did you get the 50mm ID 65mm OD bearings? Just wondering what supplier sells those.
And how did you determine the dimensions for the lightening pocket on the camshaft?
26-10-2016 14:41
asid61
Re: pic: CGX-115, cam-counterweighted cycloidal drive
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Originally Posted by Cothron Theiss
This all looks great! A few quick things before I dive back into your CAD:
You're missing a few parts in your GrabCad folder for this assembly, like the small bearings around the first wobble gear, or the countersunk screws bolting the two wobble gears, and just a few small parts here and there. Also, where did you get the 50mm ID 65mm OD bearings? Just wondering what supplier sells those. And how did you determine the dimensions for the lightening pocket on the camshaft? |
Recently I discovered the 6700 series bearings, and less recently the 6800 series bearings. These are very thin-section bearings, with the 6700 series being a mere 4mm wide and usually having a difference in OD between 6-7mm, so you can get a 30x37mm bearing. These are crazy useful for a lot of designs, and cost just a few dollars on Ebay or Aliexpress, as they are standard sizes. I believe I am using 6810 bearings in this design. There's an amazon supplier for 6800 series bearings that offers Prime, but they are vastly overpriced and usually cost $10-50 each compared to $1-5 for Aliexpress.
To get the lightening pocket just right, I first drew the maximum size pocket on the camshaft, in this case somewhere around a 0.09" outer wall and 0.125" thick material between the front and back pockets. I then made an assembly consisting only of the cam and the off-center rotating components including all the the gears, screws, and bearings. I mated the center of the cam to the origin and took Mass Properties readings to determine the center of mass, and trial-and-error adjusted the depth/wall thickness of the pocket accordingly. In this case, I ended up with a 0.09" wall and a 0.187" wall between the two pockets, and under 0.0001" of theoretical concentricity.
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