Drive train questions
 

We are looking at doing a 6WD system this year and had initially looked at having 2cims direct drive the center wheels and then use belts or chains to power the outer wheels.The design has evolved into using a single cim on each center wheel that is chained to the front and using another cim to direct drive each rear wheel. Does anyone see any issues with the evolved setup.I am concerned about matching speeds between wheel sets and a potential loss of power, but I may be worrying for nothing. Thanks for your thoughts.

Brian Holmgren
Team 1723

Re: Drive train questions
 

As long as the gear ratios are the same there should be no issue and the motor should share the load just fine.

If you had different motors then you would have needed to (closely) match motor speeds using different gear ratios but since both sets of motors are the same, all you need to do is have the same gearboxes and you'll be fine.

Re: Drive train questions
 

The matching of speeds may or may not be an issue depending on the type of wheels used. The added weight of extra gear boxes would be a bigger concern for me.

Re: Drive train questions
 

Note: this only applies when you are driving two wheels of a different size that you can have different ratios!

Just make sure that the center wheel is 1/8in lower than the outside wheels or else you drivebase will not turn unless you are using omni or slick wheels on the outside wheels.

Re: Drive train questions
 

Direct drive with a CIM doesn't sound good to me. The native speeds of the CIM's are in the high 1000's of rpm. Unless you are planning to use tiny wheels, you need a gearbox to gear this down to a manageable speed in order to able to control the robot.

Re: Drive train questions
 

There would still be an advantage to chaining the rear wheels with the center wheels in that in the case where the rear wheels are elevated, the front and center wheels would still be powered by 2 motors instead of just 1, allowing for double the power in this case.

This could also be the case if the normal force is unevenly distributed among the wheels, e.g. center of gravity is more toward the front of the robot or the center wheels are dropped. This would mean the rear wheels would have less load on them because there is a lower frictional force on them, so you would get more out of your motors by chaining the wheels together.

Worth noting that these two reasons are both conditional, however.

Re: Drive train questions
 

When you say CIMs direct driving your wheels, I hope you mean off a gearbox. A CIM motor spins at 5300 RPM under no load and it won't have enough torque for a drive train. You're going to be trying to get a 120lb robot moving and it will take forever to get up to speed. Not to mention the fact that you'll be getting pushed all over the place.

If you are in the market for gearboxes, using the CIMple box from the KOP isn't a half bad idea. The ~5:1 ratio is good for the CIM. I am personally a fan of the AM ToughBox which is 12.75:1 if you don't play with the gearing.

Re: Drive train questions
 

I'm not a fan of ToughBoxes (though I do like the name). Our team once used them regularly, but not anymore due to consistent problems and low performance. The ToughBox is very heavy, and has too much (IMO) gear reduction for most uses. Maybe it's a good ratio for very large, high-traction wheels, when you value torque over top speed. The worst part is that it is too easy to put something together incorrectly in the ToughBox (encoder mounts, etc.), and not know it until further down the assembly process. Then it's a lot of work to disassemble, fix, and re-assemble. That, unfortunately, happened to us more than once.

The CIMple gearbox in the KoP has been much better for us.

Re: Drive train questions
 

The ToughBox can have a gear ratio of anywhere from ~6:1 to ~14:1. You can buy various gears and change the ratio around to fit your needs.

Re: Drive train questions
 

6WD must be the thing this year We are doing this as well. What problem are you trying to solve by moving the cim motor off of the center?

I think you are better combining the motors as you then always have the cumulative effect of the 2 motors and the KOP box is prebuilt for it so it isn't difficult.

Re: Drive train questions
 

I'm not sure why we moved the one motor, but it looks like we're moving it back to a single CIMple box in the center. Yes, definitely using a transmission with a hex shaft to drive the center and then chain up the outside wheels. Thanks for the inputs!