"8" CIM drive?

[jman4747]

I have been working on a couple of random drive train ideas over the summer just as design/CAD practice and I came up with this 4 wheel tank drive module with the vex 5.33-1 3 CIM gear box and 2 miniCIMs. This would (in theory) give a robot with two of these modules the overall power of 8.6 or so normal CIM motors. I'm wondering if it is actually worth it with all the extra gearing/chain drive necessary and the added friction and such. I also wanted to know what people thought of the overall design (CIMs mounted vertical to save space inside the frame, miniCIMs inside module etc.). I've also included a picture of a more complete version with 4 CIMs and unnecessary suspension.

*note this is incomplete (missing some shaft collars small spacers etc.)

Current draw - Current draw everywhere.

[T^2]

You can fit 4 CIM-sized motors with 12t pinion gears around a single 60t gear (20dp, ofc). Food for thought.


...But you may need a few more batteries.

[1683cadder]

Current draw is you problem. Try a 6 cim drive, there really isn't much of an advantage of using more motors past 4 cims really. If you're looking for more torque, just have a lower low gear and higher traction wheels.

[Joe G.]

Aside from current draw problems, traction is a limiting factor. All wheels break traction under a certain torque. Adding more motors increases the speed at which this occurs. Beyond 6 or so CIMs, with a reasonably efficient drivetrain this starts to happen at speeds beyond those which are reasonable to drive an FRC robot at for most games. Adding more motors beyond that really doesn't serve a purpose, and in fact the inefficiencies generated in the required gearing will bring you back down some.

I like many of your space saving ideas though, these could come in handy for other reasons, such as a manipulator that needs the space a gearbox typically takes up.

[jman4747]

So from what I can see the better option is to take advantage of the space savings but go with more standard motor and gearbox options?

[1683cadder]

Originally Posted by Andrew Lawrence

Current draw - Current draw everywhere.

Originally Posted by jman4747

So from what I can see the better option is to take advantage of the space savings but go with more standard motor and gearbox options?

yep, though a mini cim and 2 cim gearbox is worth a try.

[Chris is me]

From what I can tell *on paper* so far, the main benefits of 6 CIMs are better acceleration at very high top speeds and a faster traction-limited low gear. So if you're geared at like 15+ FPS, or you want to run a low gear faster than 5.5 FPS, you'll want those extra CIMs.

I'm hoping my team will do some prototyping with a 6 CIM system this fall. The results could be quite interesting.

Even if we don't get 6 CIMs again, there may be an argument for 4 CIM 2 MiniCim in 2014. Depends on the game.

On paper, it doesn't really seem like the jump from 6 to 8 CIMs helps more than it could potentially hurt (tripping the 120A breaker, draining the battery too fast). There are diminishing returns on additional motors.

[AdamHeard]

This is really a case where modeling can be very useful. JVN's older drive calc is perfect here.

One can take the same drivetrain, and model it with different gear ratios and # of CIMS.

You can then compare whatever you're concerned with;

-Time to reach a certain speed
-Current draw at slip, during acceleration, at top speed
-Top Speed
-Total power used during acceleration (current draw * voltage)

You can then compare with actual (modeled) data which beats speculation. You'll start to notice some trends (that are partially a matter of opinion) like after 16-18ish fps, acceleration tanks with a 4 CIM drive, and at about 10-12 fps adding a 3rd CIM just isn't a huge performance gain aside from requiring less current per motor to slip wheels.

Don't take my word for it though, model away!

[jwallace15]

As is, how much does one side weigh? (as in one transmission w/ the wheels and frame)

[jman4747]

I believe the wheght would be about 25 or so pounds I can't say exactly at the moment.

[Pault]

Originally Posted by jman4747

I believe the wheght would be about 25 or so pounds I can't say exactly at the moment.

My first reaction was that there is no way each side weighs 25 pound. But then I remembered that all those motors will rack up ~13.5 pounds on each side, and also those side plates look pretty thick, so maybe its true. Well, thats another downside to so many motors.

[HumblePie]

My first reaction, from a strictly mechanical perspective, is that all this extra power could be wasted if the wheels break traction before maximum torque is reached. From a durability standpoint, I'm concerned that the vertical hex shaft may flex under load, causing stripping of the miter gears. Consider a mechanical support for the free end of the hex shaft, or use a worm/worm gear arrangement, like in the AM Raw Box.

BTW, we had great fun at Peachtree with you guys!

[KrazyCarl92]

Originally Posted by HumblePie

My first reaction, from a strictly mechanical perspective, is that all this extra power could be wasted if the wheels break traction before maximum torque is reached.

In many cases, the wheels slipping before maximum torque output of the motors is actually preferable. At first, I can see why it might seem counter-intuitive, but it is simply the best alternative.

Something is going to be the limiting factor in the drive train's ability to transmit force to the ground. It can be, among other things:
-Motor torque
-Mechanical strength
-Circuit breakers
-Electrical safety conditions
-Traction

I would rather have my wheels temporarily slip on the ground as opposed to my motors stalling, electronics frying, wheels breaking, shafts bending, gears shearing, chain snapping, belt skipping, circuit breakers tripping, battery dying, or wires catching on fire.

It is not a waste to have more torque available than needed to break traction, it is actually a very good practice in FRC.

[JamesCH95]

Originally Posted by KrazyCarl92

In many cases, the wheels slipping before maximum torque output of the motors is actually preferable. At first, I can see why it might seem counter-intuitive, but it is simply the best alternative.

Something is going to be the limiting factor in the drive train's ability to transmit force to the ground. It can be, among other things:
-Motor torque
-Mechanical strength
-Circuit breakers
-Electrical safety conditions
-Traction

I would rather have my wheels temporarily slip on the ground as opposed to my motors stalling, electronics frying, wheels breaking, shafts bending, gears shearing, chain snapping, belt skipping, circuit breakers tripping, battery dying, or wires catching on fire.

It is not a waste to have more torque available than needed to break traction, it is actually a very good practice in FRC.

This. All of this post, actually.

Also what AdamHerd said.

Model your drivetrain to see what motors you'll need to hit your target speeds and what gearing you'll need to help ensure that you can still slip the wheels.

As discussed in a few other threads a little bit of slip during acceleration can actually increase your effective traction. Yes, your HS physics teachers were not completely honest with you!

I do like the mitre gear design and the general construction and compactness of your design. It might need some revisions that others have pointed out, but it's got a good fundamental design and offers some distinct advantages over traditional designs. Good work.