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Here's my 2nd gearbox design. It's geared at 13.81 fps in high, and 5.48 fps in low, but can be adjusted by changing the CIM pinion gear. All gears, shafts, and plates(except the CIM gears) are aluminum, the 54 tooth gear is lightened, and the 56 tooth gear needs to have a dog pattern added. The 40 tooth gear comes from vex with the dog pattern and hex bearing included. The gearbox is set up to use the shifting cylinder mount and coupling/bearing from the ball shifter/WCP DS gearbox. Without motors or pneumatic cylinder, SW predicts the weight to be 1.63 lbs. The gearbox also has the room to add a banebots rs-775 motor, which would have the 89% of the power a 3 CIM gearbox, but I can't find anywhere to buy .6 module gears.
My biggest concern is the shifting dog, which would need to have a diameter that is about .03" less, and I'm not sure what consequences of this would be. One of our previous dogs split on the really thin walls around the threaded hole, but I don't think this will be the failure point for the WCP dogs, which are much thicker.
A few more pictures and CAD files are available from here https://drive.google.com/folderview?...&usp=sha ring
Discussion
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06-06-2014 16:12
Andrew Schreiber
Re: pic: Gearbox
No, your gearbox is not geared to X fps.. It's got a reduction. With a certain sized wheel it would drive a robot X fps.
What's the reductions?
What size wheel is it designed for?
06-06-2014 16:40
rcmolloy
Re: pic: Gearbox
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Originally Posted by Andrew Schreiber
No, your gearbox is not geared to X fps.. It's got a reduction. With a certain sized wheel it would drive a robot X fps.
What's the reductions? What size wheel is it designed for? |
First Stage - 11 to 54
Low Stage - 36 to 40
High Stage - 20 to 56
06-06-2014 17:12
Jared
Re: pic: Gearbox
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Originally Posted by Andrew Schreiber
No, your gearbox is not geared to X fps.. It's got a reduction. With a certain sized wheel it would drive a robot X fps.
What's the reductions? What size wheel is it designed for? |
It is designed for a 3" to a 3.25" wheel. It can work with the 3" colson, and can go faster if the CIM gear is swapped to an 12, 13, or 14 tooth gear.
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Originally Posted by rcmolloy
3.25 Inch VersaWheel
First Stage - 11 to 54 Low Stage - 36 to 40 High Stage - 20 to 56 |
06-06-2014 23:10
TheHolyHades1
Re: pic: Gearbox
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Originally Posted by Jared
Sorry. I meant to post the picture with the 3.25" versa wheel DT so people could see how it mounts.
It is designed for a 3" to a 3.25" wheel. It can work with the 3" colson, and can go faster if the CIM gear is swapped to an 12, 13, or 14 tooth gear. The overall reductions are 5.45:1 and 13.75:1, which are a little fast for 2 CIMs driving a 3.25" wheel. I plan to add an rs-775 to the gearbox. |
Of course, I could also just be completely wrong here ... I'd love someone to confirm/deny what I'm saying with a better explanation.
07-06-2014 10:42
Jared
Re: pic: Gearbox
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Originally Posted by TheHolyHades1
I'm not sure adding an rs775 to the gearbox is a good idea. I'm pretty sure the rpm curve is different for the rs775 as compared to the cim (whereas I think the miniCIM is meant to have a very similar/the same curve). This could mean that the motors are trying to drive each other to some degree instead of all working together to drive the wheel.
Of course, I could also just be completely wrong here ... I'd love someone to confirm/deny what I'm saying with a better explanation. |
If you are interested, here's an example of a single CIM and one rs-775 gearbox, with free speeds not matched, then matched. To get these, you need to find what the rpm of each motor will be, use the motor data to calculate the torque at the shaft of each motor, then you need to find the torque at the output, which is the CIM motor shaft in all these examples.
Free Speed Stall Torque Stall Current Free Current (Amp) Power (W) CIM 5310 2.43 133.00 2.70 337.81 RS-775 (12V) 13000 0.78 86.67 1.80 266.60
CIM torque (Nm) vs. rpm
R/5310 + τ/2.43 = 1
τ/2.43 = 1 - R/5310
τ = 2.43 - R*0.000457627
RS-775 torque (Nm) vs. rpm
R/13000 + τ/0.78 = 1
τ/0.78 = 1 - R/13000
τ = 0.78 - R*0.00006
with same gear ratio, both at 12V, at 6000 rpm (this is possible)
τ775 = .78 - 6000(.00006) = 0.42
τCIM = 2.43 - 6000(0.000457627) = −0.315762 The CIM is providing resistance, and reducing availible torque
Total torque = 0.104 Nm
geared so that free speeds match at 5310 (2.45:1), at 12v, at 3000 rpm (nothing higher than 5310 is possible) The rs-775 will be geared down.
rpm of the 775 will be (2.45 X 3000) = 7350
τ775 = .78 - 7350(.00006) = .339 this is at the shaft of the motor
at the CIM, it will be 2.45 times more = .83055
τCIM = 2.43 - 3000(0.000457627) = 1.057119
Total torque = 1.888 Nm
It makes sense because when free speeds are matched, no motor ever has to push the other past its free speed, so the torque contributed by each motor will always be positive, and the CIM will always contribute more. The ratio between the contributed torque of the CIM and the contributed torque of the rs-775 will always be equal to the ratio of their power, if the free speed is matched. The ratio is about 79:100 for the rs-775 to the CIM.
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