Re: Sonic Shifter - recent feedback?
 

Sorry for the thread jack, here is my two cents.

My personal opinion regarding AM's shifting products is that they are very robust and depending on your driving style worth the extra weight.

Steel gears are heavier, but stronger. This may not be warranted in some parts of a gear box where speeds are higher and torque is lower, but I believe it's a nice feature in the final reduction stages where you see the highest dynamic loads and you're transmitting the most torque.

In my FIRST career I've only had one type of failure from an AM dog style shifting gear box and that was due to mis-use It's important to regulate the pressure available for shifting down to the stated spec because that will damage the linkage connecting the pneumatic piston to the shifter shaft. It would appear this issue has been mitigated with shorter throw cylinders at this point.

Winch mechanisms circa 2010 are an example of how much abuse the mechanism will take. Many users were disengaging dog gears with several hundred pounds of force in play.

In comparison, I do like ball lock style shifters, because there isn't a force acting to disengage the shifter and shifting seems smoother to me.

These factors would drive the decision in my world.

Cost: Within 50 dollars of one another.
Safety factor: AM seems to have higher safety factors while VP designs are lower.
Weight: AM gearboxes are heavier while VP designs seem to be lighter.

I have my horror story. I've done a thorough analysis of why that happened and I'm confident in my understanding. Do I think most teams would encounter the same issue? No.

Also, I have some inherent bias. I have a number of good friends who are a part of Andymark, so no doubt it feels good to spend money there.

Ultimately you've got to use whatever gives your team the competitive advantage and fits your need.

Please implement automatic shifting regardless of which transmission you use. With the worry of brown-out and observations during 2014 with people blowing 120 amp breakers I think it's a must. Drivers generally don't use manual shifting when they should. It's hard to teach and takes a ton of experience to learn to use without hesitation. Ultimately in an intensive enough competition situation they will forget.

Re: Sonic Shifter - recent feedback?
 

We used 4 of them on our 2014 robot in a Mechanum setup. We haven't had any problems with them in 2 district events, district champs, 4 offseason competitions and a handful of demos. We haven't checked them for signs of wear but there are no signs of them breaking or showing age.

Re: Sonic Shifter - recent feedback?
 

I want to point out for folks who don't spend a lot of time looking at gearboxes that this is a very aggressive gearing with one of the highest CoF wheel tread materials. My quick calcs say this generates ~747 pounds of force at the wheel patch with 6 CIMs, well over the traction limit of 185 for a maximum weight robot in 2014. Meaning it is an extreme case and may not be the best one to guide the decision of an average team.

Overall this is great feedback. I have been wondering how the 3rd stage on the 3 CIM works at high loads. I always thought the sheet metal with standoffs 3rd stage looked a bit rickety. I am curious if you ever subbed out the 7075 aluminum gears for 4140 steel gears in the third stage of the 3-CIM shifter? Sounds like it was a misalignment issue more than a material strength issue. Did you ever try stacking on a second Vex third-stage plate to add stiffness to the third stage bearing? This would pickup more of the bearing race and possibly prevent angular deflection.

I am assuming the output of the gearbox was direct driving your center wheel in a tank drive. How do you think a 2 stage 3-CIM shifter would work if offset from the wheel axles and with a #35 chain reduction between the gearbox and wheel axles in lieu of the Vex 3rd stage?

Was there a particular reason for adding this "stop on a dime" feature? Did it work as you had hoped? Would you recommend it? It does seem like a recipe for gear teeth shearing.

Based on photos and gearing - I agree - you win!

-matto-

Re: Sonic Shifter - recent feedback?
 

I second this.

Though that said, we used the same gearbox in 2014 with a 26.04:1 low gear and a 7.08:1 high gear on 4" wheels with the same tread. :rolleyes:

I don't think the issue is the plate stiffness, but the fact that the small output gear (the one with the missing teeth in the photo on the last page) is on a cantilevered shaft. If one was to modify the plate so that another bearing could be used on this shaft, there likely would not be any problem.

On a related note, our team actually did this recently with a pair of VexPro 2 CIM Ball Shifters that we had inadvertently ordered without the 3rd stage (and apparently the output shafts on the 2nd stage are longer when order it like this). So we made a pair of replacement plates so that the smaller 3rd stage gear could be retained by an additional bearing instead of having to remove the shaft and lathe a snap ring channel into it.

Re: Sonic Shifter - recent feedback?
 

Wow I had never noticed that the output gear is unsupported on the 3rd stage for both the 2-CIM and 3-CIM ball shifter. For the 3-CIM I saw the hole in the plate and assumed it was 1.125, but it is only 1.000. Perhaps Vex thought the extra bearing would over constrain the shaft? A rare miss by Vex. The being said, the gear separation is 84T, so you could add some of my new favorite part, the Face Bearing Mount.

I am curious if you mounted your wheel right the to the output shaft or if the shaft was supported? I would take the 3rd stage output shaft and put two chain sprockets on in, then pass it thru a VersaBlock and put the wheel on the other side of the tube for a WCD setup.

The more you learn ...

Re: Sonic Shifter - recent feedback?
 

BoilerMentor,

Were you guys using the old ball shifter shaft without the pin in it or the new shaft?

I ask because the press fit in the original shaft would allow the deflection you speak of but the new shaft would most definitely not.

Also, a cantilevered shaft with the proper bearing spacing behind it is a perfectly legitimate design strategy especially with the cantilevered gear so close to the external bearing face.

Additionally, this is the first example of a failure like this that we have seen with the 3 CIM shifter so I really would like to get more information from you.

PM me if you would like me to email you.

Again, this failure mode is not normal in the typical 3 CIM ball shifter use case (even with your ratios you are within our normal use case).

Thanks,
Paul

Re: Sonic Shifter - recent feedback?
 

The decision was based not on pushing power, but a time to distance calculation that considered a two speed transmission. That pair of ratios minimized field crossing timed on a tool that was written up in Excel taking the two speed gearbox into account. People perceived 1747's robot as being tippy that year, specifically because of the brutal acceleration this setup generated. It was the one thing we didn't take into account and actually down-regulated our peak acceleration. No doubt the robot was quick across the field though. I personally think time to distance is an important consideration most teams neglect, but there are certainly some games where it's not a useful piece of data i.e. 2015.

We ended up machining a replacement for the shaft with a 1/2" round to sit in a bearing in a plate opposite the face of the plastic housing. The 3rd stage mounting plate wasn't used. The hole pattern was transferred onto our drive train plate. We haven't had a problem since implementing the modified shafts, which lends a great deal of credence to the theory that it was deflection in the cantilevered shaft that caused the failure.

The axle was supported with a bearing on the opposite side of the drive pod from the gearbox, so as long as the mounting of the gearbox was adequate the bearing in the face of the gearbox and the outer bearing we added should have picked up the load. I did forget to mention that we machined custom shafts from the get-go for the output of the gearbox. We needed a longer output shaft to span our drive pod. These shafts went to 4140 steel, instead of aluminum. Also, out of center might be a concern with a custom machined hex shaft. I verified they were centered prior to installation.

The automatic shifting code was very simple. Outside of handling for turning cases, there were two shift thresholds based on gearbox encoder feedback. I do not remember what drove the specific value used for down shift, but I think it was based on the highest speed that allowed enough separation between values to prevent oscillation between gears. The reason that the downshift occurred before full stop was in consideration of a number of logical conclusions about robot-robot interaction. The downshift feature did have one drawback. We had to have a specific case to handle direction changes, because the robot would end up on its back otherwise.

Just to clarify, my responses haven't been about "winning and argument" I'm just try to establish that we covered all of our bases as far as a well put-together investigation of the cause of the failure. It was actually a great exercise to be able to work through with my students. You can imagine in a competition setting there's a great deal of anguish caused by a failure of this magnitude in the student's eyes. It's a great feeling for them to be able to say, "This was not a flaw in our design, it was a failure of an input part. Here's why:"

-Charlie

Re: Sonic Shifter - recent feedback?
 

I agree with you that this can be an important parameter and we do calculate it, but it is game dependant. Sometimes that few tenths of a second gets you a game piece or allows you to avoid a defensive hit. There is always a lof of discussion on free speed, but I think time to X feet is a more relevant metric. Simbotics talks about it in their videos on strategic design and drivetrain design.

Thanks for the details on the design and fixes. Since we have never geared that aggressively, it is good to know some of the pitfalls in case we ever want to.

-matto-

Re: Sonic Shifter - recent feedback?
 

It is a legitimate approach, but will have more deflection than if the end of the shaft were supported. Is the difference in deflection sufficient to cause a problem? Sounds like it was in a few cases.

Based on your reply (and the comparison here) it sounds like there were a lot of design changes in the v2 Ball Shifter Shaft. Under what circumstances do you recommend teams replace this shaft? We just bought the upgrade, but have not installed it.

Re: Sonic Shifter - recent feedback?
 

I was wondering if someone from Vex would drop by this thread. Glad to hear this issue is an unusual/unique failure mode. Makes me feel better about our teams continued use of Ball Shifter Gearboxes (which as I've said already, we've been very happy with so far). ;)

Re: Sonic Shifter - recent feedback?
 

I would use the upgrade. The upgraded shafts are much better and allow for the use of the ThunderHex bearing without any additional machining.

By my math, the deflection difference is not enough to cause their problem. I believe it was related to a loose fit between the hex and shifter shaft in the v1 version of the ball shifter shaft combined with their increased load case.

Re: Sonic Shifter - recent feedback?
 

I bought several v1 Ball Shifters Transmissions when they first came out which are still unopened. We have yet to use them in our drivetrains, and used some of them instead for other robot features such as our winch for the ball launcher in 2014.
I better have them checked to possibly get the upgraded shafts.

Re: Sonic Shifter - recent feedback?
 

In 2013, we selected your team at the Boilermaker regional because we saw 1st hand what a 3 CIM drive could do on defense. After that 1 match where our robot almost got destroyed, I said we needed this team on our alliance.:)
Its nice to hear about your continued success in using the same setup in subsequent years.
However, I would respectfully disagree on the automatic shifting. We tried it in 3 different seasons and will never ever go back to it. In every instance, it failed/or started to wear heavily on some parts causing shifting problems. We got tired of the constant checking and paranoia that it would fail in a match.
We are aware of some of the issues that was discussed in this thread due to personal experience.
Modifications we have done in-house the past 2 seasons to our AM Supershifter, are using some VEXPro Aluminum gears and the pancake shifters instead of the ones that come with the AM or WCP ones.

Re: Sonic Shifter - recent feedback?
 

2 related questions as we consider the sonic shifter.

A.
One of the decisions related to AM super shifter or sonic shifter (vs. options of other vendors) is the different pneumatic actuator for shifting.

We've used both types of pneumatic actuators over the years and have not seen one to be more effective or more reliable than the other. Both types have worked fine for us.

Is there any advantage of pancake shifter (compared to typical cylinder required of AM shifters) other than more compact form factor?

B. Steel Gears vs. Aluminum

Aluminum saves weight and that can be a critical consideration. However, a bit of savings in weight is less important to us than robustness.

If weight is taken out of consideration, are the wear characteristics of aluminum gears (available by the typical robotics vendors) close enough to steel to be left out of the comparison pros and cons?

I'm not talking about outer-edge use cases, just a typical FRC shifting drivetrain for a game that has some pushing. (no automatic shifting). We've used both aluminum gears (in VEX products) and steel gears (in AM products) in past.

Re: Sonic Shifter - recent feedback?
 

The form factor, including the mounting, is the only real difference. If you use a COTS shifter, use the recommended cylinder. If you're building your own, managing space is the deciding factor.

It depends on how much pushing you're doing, and (more importantly) how hard the collisions are. The case where you're most likely to need steel is on the bull gear (gear nearest the wheel) and the gear that engages it. It would take a truly extreme case to require steel gears in the earlier stages, or even on the bull gear for simple pushing without collisions.