pic: 20's IRI Carnage

[apalrd]

If anything, the code would have more effect, since it's actively reversing torque immediately, while the electrical brake requires the motor to be overdriven to begin producing reverse torque.

But a forward-reverse slam initiated by the driver would put way more shock load on the dog than this would.

I'm saying that this part likely sees extremely high repeated shock loading, so the analysis of it is more complex. This poor little part has such a huge job to do.

[KrazyCarl92]

Originally Posted by apalrd

If anything, the code would have more effect, since it's actively reversing torque immediately, while the electrical brake requires the motor to be overdriven to begin producing reverse torque.

But a forward-reverse slam initiated by the driver would put way more shock load on the dog than this would.

I'm saying that this part likely sees extremely high repeated shock loading, so the analysis of it is more complex. This poor little part has such a huge job to do.

My thought was that electrical braking would not even begin to approach the load demands of a full forward-full reverse transition, so the shock loading from that would be negligible in comparison. That's just a thought experiment though, not supported by any evidence.

[Travis Covington]

I think Cory forgot that we actually did break 2-4 of these on our practice bot this year in a similar manner. They were also from WCP but modified to be 1" OD. We had run the same size dog since 2007 without failures, so I am pretty confident that the changes implemented in the WCP ones, along with the additional CIM motor (we had 3 per gearbox) caused the failures for us.

In the past, we had either used a #3-48 screw (which failed a few times) or a 3/32" roll pin (which never failed). I believe the larger diameter hole of the #4-40, as well as the inherent stress risers that exist with there being a thread, combined with the additional torque of our 3 motor gearbox caused our failures. As some have mentioned, the cyclical/shock loads that this part sees are very high, and the relatively thin cross section at that threaded side of the dog does not provide much, if any, factor of safety.

We lucked out because we never had a failure on the competition bot, but did have drilled (non-threaded) dogs as backup. Our kids got plenty of practice and were able to swap a broken dog in under an hour if they had to. I am glad we never had to do that between matches though!

I think for the future we will use non-threaded dogs, as well as possibly increasing the thickness of that section of the dog at the expense of an ever so slightly wider gearbox.

Adam may be able to provide more insight in to their circumstances, but I think 973 may have just lucked out on not breaking their parts considering they had very similar loading characteristics to us. It may have had something to do with the CG of their robot versus ours, and the way it decelerates, but might also be because their dogs are the standard 1.125" OD (I believe?). We broke almost all of our dogs when stopping or changing directions, or when going fast over bumps/metal plates under the carpeting in our lab. Given the failure, those loading conditions make sense.

Hope this helps!

Edit: On second thought, 973's larger plastic wheels might have dampened some of the shock-loading as well. Who knows.

[AdamHeard]

Originally Posted by Travis Covington

I think Cory forgot that we actually did break 2-4 of these on our practice bot this year in a similar manner. They were also from WCP but modified to be 1" OD. We had run the same size dog since 2007 without failures, so I am pretty confident that the changes implemented in the WCP ones, along with the additional CIM motor (we had 3 per gearbox) caused the failures for us.

In the past, we had either used a #3-48 screw (which failed a few times) or a 3/32" roll pin (which never failed). I believe the larger diameter hole of the #4-40, as well as the inherent stress risers that exist with there being a thread, combined with the additional torque of our 3 motor gearbox caused our failures. As some have mentioned, the cyclical/shock loads that this part sees are very high, and the relatively thin cross section at that threaded side of the dog does not provide much, if any, factor of safety.

We lucked out because we never had a failure on the competition bot, but did have drilled (non-threaded) dogs as backup. Our kids got plenty of practice and were able to swap a broken dog in under an hour if they had to. I am glad we never had to do that between matches though!

I think for the future we will use non-threaded dogs, as well as possibly increasing the thickness of that section of the dog at the expense of an ever so slightly wider gearbox.

Adam may be able to provide more insight in to their circumstances, but I think 973 may have just lucked out on not breaking their parts considering they had very similar loading characteristics to us. It may have had something to do with the CG of their robot versus ours, and the way it decelerates, but might also be because their dogs are the standard 1.125" OD (I believe?). We broke almost all of our dogs when stopping or changing directions, or when going fast over bumps/metal plates under the carpeting in our lab. Given the failure, those loading conditions make sense.

Hope this helps!

Edit: On second thought, 973's larger plastic wheels might have dampened some of the shock-loading as well. Who knows.

We actually didn't shift this year. The highest loading we've put on dogs was 2011 w/ 4 cims and 2 775s. They were the full 1.125" diameter though.

[KrazyCarl92]

Originally Posted by Travis Covington

I think Cory forgot that we actually did break 2-4 of these on our practice bot this year in a similar manner. They were also from WCP but modified to be 1" OD. We had run the same size dog since 2007 without failures, so I am pretty confident that the changes implemented in the WCP ones, along with the additional CIM motor (we had 3 per gearbox) caused the failures for us.

Did these dogs fail in the drive shifting application or the PTO application? And what would be the maximum powered torque that the dog would be transmitting in the given application?

[apples000]

Originally Posted by KrazyCarl92

My thought was that electrical braking would not even begin to approach the load demands of a full forward-full reverse transition, so the shock loading from that would be negligible in comparison. That's just a thought experiment though, not supported by any evidence.

While I agree that this makes sense, we actually found the opposite. When we had a drivetrain where the wheels moved in an arc up and down (to go over a bump), we found that we bent aluminum axles/joints/#25 sprocket teeth way more often when we were in break mode. This makes no sense to me, but it's what our team experienced.

This is just a thought, but we may have implemented a ramping effect on the drive motor outputs to limit acceleration and prevent tipping, which may have made the full forwards to full reverse a little bit less drastic. We may have also had a linear approximation of drive voltage vs speed, and would limit the drive output to being plus/minus 40% of the approximation. This made sure that if we were being pushed forwards at a high speed, the motors could only go down to 60% power, instead of 100% reverse. It also had the effect of maintaining traction in low gear.

[Travis Covington]

Originally Posted by KrazyCarl92

Did these dogs fail in the drive shifting application or the PTO application? And what would be the maximum powered torque that the dog would be transmitting in the given application?

Drive. PTO wasn't shock loaded nearly as bad/at all. I do not think motor-provided torque was the issue. I am almost certain the failures were induced by external shock loads exceeding nominal torque.