pic: Plywood 8WD Concept

[Deke]

One thing to keep in mind is that the wheels don't have positive traction. So to calculate the force in the pinion gears you need to figure out the loading the wheels will apply before slipping. Adding up the stall torque of the motors and factoring in the gear reduction to the force applied on the pinion tooth is the front half of it. Traction and slippage is the back half. The pinions do take more loading than the usual frc gearbox.

I like the cim layout and have been playing with it around a year. I haven't been satisfied with the packaging to move forward with testing. Its neat to see how others approach similar problems with their designs.

2337 the enginerds have done a similar 8wd sheet metal layout with an integrated two cims and two speed gear box the past two years. I really like what they did. They have their 2013 cad online, you should check it out for design solutions implemented if your interested in this 8wd layout.

My concern would be the impact resistance of the 1/4 inch plywood. Metal bends before cracking better than wood in impact situations.

[magnets]

I can't see exactly what you're doing with your pinions, but 254's been running double loaded pinions in all their 3 CIM gearboxes.

[Deke]

Theoretically you can triple load a pinion gear as the drivetrain rocks on one set of wheels or the other as a worse case scenario. If both sides of the gearbox are loaded evenly, its more of a double loaded worse case scenario.

[Adrian Clark]

Originally Posted by Infinity2718

Theoretically you can triple load a pinion gear as the drivetrain rocks on one set of wheels or the other as a worse case scenario. If both sides of the gearbox are loaded evenly, its more of a double loaded worse case scenario.

The worst case load scenario would be during a high speed direction change in high gear. The pinion could see upwards of 6x stall torque.

-Adrian

[tcjinaz]

Wow, what responses for a summer post.
I was only thinking what wonderful designs two heads can come up with...

[Deke]

Originally Posted by Adrian Clark

The worst case load scenario would be during a high speed direction change in high gear. The pinion could see upwards of 6x stall torque.

-Adrian

Yes, thanks for the clarification. So in a typical gearbox that AM or Vexpro sells, that would be 2x the stall torque. So this layout would be 6x stall torque or triple of what the gear usually sees.

[Deke]

Originally Posted by tcjinaz

Wow, what responses for a summer post.
I was only thinking what wonderful designs two heads can come up with...

I enjoy these discussions, it encourages thinking, growing, learning, and corrections. Its always a humbling experience, but that's where the growing comes from.

[Gregor]

Originally Posted by Adrian Clark

The worst case load scenario would be during a high speed direction change in high gear. The pinion could see upwards of 6x stall torque.

-Adrian

Could you explain where the 6x number came from?

-Curious Student

[AdamHeard]

Originally Posted by Gregor

Could you explain where the 6x number came from?

-Curious Student

2x stall (as that's what occurs when you give full voltage and the motor is at free speed in the wrong direction, the back EMF of the motor is going the "wrong" way and adds to the supply voltage so you get double voltage, double current, double torque).

3 motors, as the last pinion transfers the torque of the other 2 motors.

That works out to 6x total, and really points out the danger of passing load down pinions.

[s_forbes]

Originally Posted by AdamHeard

2x stall (as that's what occurs when you give full voltage and the motor is at free speed in the wrong direction, the back EMF of the motor is going the "wrong" way and adds to the supply voltage so you get double voltage, double current, double torque).

This doesn't sound quite right... you see max current and torque at a stall condition (motor speed = 0 rad/s). Torque at free speed of a DC motor should be ~0. Are there by chance any experiments that confirm this is the case, or theory to back it up? Where's Ether?

I would expect to see very little torque on a motor pinion if the robot is going full speed and full voltage in the opposite direction was applied. Although, I've been wrong many times before...

[AdamHeard]

Originally Posted by s_forbes

This doesn't sound quite right... you see max current and torque at a stall condition (motor speed = 0 rad/s). Are there by chance any experiments that confirm this is the case, or theory to back it up? Where's Ether?

I would expect to see very little torque on a motor pinion if the robot is going full speed and full voltage in the opposite direction was applied. Although, I've been wrong many times before...

I design motors, it's right

I'll break it down a little more thoroughly. Regardless of how it got that way, if a motor is rotating at some speed, it will generate a voltage (the back EMF). So lets say you were going full speed one way, and then decide to go full voltage the other way. Right before the switch, the Back EMF opposes the battery voltage and you get a very small voltage differential to drive current (which is equivalent to the current required to generate the torque required to match the friction).

Let's say the battery is at 12V, and the back EMF is 11.8 (made up numbers), some current calculated by V=IR would flow.

When you switch the polarity, the 12V and 11.8V no longer oppose each other, you now have 23.8V across the motor. You once again find I from V=IR, and find that this I is ~ double the I that would be calculated at usual stall conditions. Since Torque is proportional to current, you will therefore see about double stall torque.

Another way to visualize this is to extend the classic torque/speed graph to include negative free speed, you'll see the torque there is 2x the stall torque.

Extending it past full speed positive shows that you need to apply external torque to get a motor going faster than free speed as well.

An important takeaway from all this is most FRC systems are capable of seeing 2x stall torque if they reach full speed one way, and negative full voltage is applied.

[s_forbes]

Originally Posted by AdamHeard

Another way to visualize this is to extend the classic torque/speed graph to include negative free speed, you'll see the torque there is 2x the stall torque.

This looks like a sensible way to demonstrate the point. I did some googling for a bit to try and find an extended motor curve that goes into the negative rpm region, but haven't turned up any good results yet. I'd be interested to see how the power and efficiency curves fit on the negative side as well.

[Deke]

I would like to crunch some numbers here to see what kind of loading the cim pinions usually take, i have not looked at the numbers before to see where they stand. It seems to me that the simple and conservative way to approach this is to use the stall torque at a steady state, but there are some dynamics involved here. Now a question would be how much of an impact do the system dynamics play into the tooth loading? When the motors reverse is there a ramp or instant loading? Do the wheels slip? I think if those parameters can be modeled, an accurate calculation can be made regarding the loading.

Does that logic make sense or am I missing something?

[AustinSchuh]

Originally Posted by magnets

I can't see exactly what you're doing with your pinions, but 254's been running double loaded pinions in all their 3 CIM gearboxes.

This is even more reason not to do it. 254 bends CIM shafts every now and then in the drivetrain. I didn't believe it was possible until I saw it myself.

[Deke]

Originally Posted by AustinSchuh

This is even more reason not to do it. 254 bends CIM shafts every now and then in the drivetrain. I didn't believe it was possible until I saw it myself.

That's interesting that the cim shaft failed before the gear teeth did.