AndyMark shifter Dog Gear screw problem - lower your pressures

[Andy Baker]

All,

This past weekend, we were informed of an issue by three different customers. The M2.5 screw within the 2010-2011 AM Shifter and Super Shifters are breaking at a rate higher than expected.

The initial fix for this is for teams to lower their shifting pressure from 60 psi down to 35 or 40 psi. Many teams already run at this lower pressure, to reduce wear on the shifting parts.

During the years of 2005-2009, we used a 3/32" coiled roll pin (heavy duty) to attach the dog gear to the shifter shaft. This past summer, we replaced this roll pin with a M2.5x20mm hardened screw. While our break tests proved that the same amount of force broke both types of pins, it seems that the screw does not handle fatigue as much as the roll pin.

The simple fix for this is to run your shifter at a lower pressure. A 3/32" hole can be added to the dog gear, in order to accommodate a 3/32" roll pin, if you wish to add this pin.

We will be doing some more testing on this, determining what exactly is going on. I just wanted to let folks know that this is going on, and they may want to lower their shifting pressure.

Keep in mind that the shifter shaft position should still be set up so that the small brass "pin saver" tube is in place so that the dog gear does not bottom out on either the full low or full high end of the cylinder stroke.

Once we know more about this, I will post the info.

Sincerely,
Andy Baker

[James Tonthat]

We found that running about 14 competition level matches breaks the screws. It happened to our practice bot at home, and then again at Lone Star. Our solution so far is to replace the screws (M2.5 x 20, McM# 91290A108) every three competition rounds and running at 45 psi.

Edit: The only thing is the shifters shift a lot more sluggishly at 45 psi.

[JesseK]

Hmm. We played 13 fairly rough matches in DC without issue from the gearboxes, though the driver admitted to not actually shifting as much as he could have. We mostly stayed in high gear. Perhaps I'll pull the gearboxes off during our fixit window to replace that bolt. I'm not sure I'd like to have to replace this bolt at all during competition, so perhaps I'll research some high-strength M2.5 bolts.

Keep in mind that the shifter shaft position should still be set up so that the small brass "pin saver" tube is in place so that the dog gear does not bottom out on either the full low or full high end of the cylinder stroke.

What does this mean? I vaguely remember seeing some brass something-or-other on the dog gear shaft, but we've never fully disassembled that shaft so it's probably still there.

Edit -- also, how long should the M2.5 bolt be?

[James Tonthat]

I posted the McMaster number for a similar replacement part, it's 20mm long, and they're physically the same as the AM ones.

Edit: It is a lot easier to replace the bolt before it has sheared than after. After it shears, it usually breaks off leaving the threading in the dog gear. For replacing the bolt, you just need to rotate it so the bolt faces somewhere you can put an allen key in (down for us.) It is literally a 2 minute job to replace the bolts versus a 30 minute swap of dog gear if it does shear.

[Bob Steele]

Originally Posted by JesseK

Hmm. We played 13 fairly rough matches in DC without issue from the gearboxes, though the driver admitted to not actually shifting as much as he could have. We mostly stayed in high gear. Perhaps I'll pull the gearboxes off during our fixit window to replace that bolt. I'm not sure I'd like to have to replace this bolt at all during competition, so perhaps I'll research some high-strength M2.5 bolts.

What does this mean? I vaguely remember seeing some brass something-or-other on the dog gear shaft, but we've never fully disassembled that shaft so it's probably still there.

Edit -- also, how long should the M2.5 bolt be?

I am curious as to what you mean by the "fix-it" window? Seeing that your are not from Michigan... I don't think you have one...
You could only do this repair during a competition I think

???

R

[thefro526]

Originally Posted by James Tonthat

We found that running about 14 competition level matches breaks the screws. It happened to our practice bot at home, and then again at Lone Star. Our solution so far is to replace the screws (M2.5 x 20, McM# 91290A108) every three competition rounds and running at 45 psi.

Edit: The only thing is the shifters shift a lot more sluggishly at 45 psi.

James, how many times per match are you guys shifting? If I remember correctly, we usually shift once or twice a match at most - and we've got at least 15 matches worth of wear on our transmissions.

Does anyone know of a way to make the transmissions shift more smoothly and quickly at lower pressures? From what I've seen, our transmissions have a noticeable (1-2 second) lag when shifting between high and low gear below 50psi.

[James Tonthat]

We would shift about the same amount of times per match, we'll be doing it more at Champs since we're getting more comfortable but we don't want to break screws.

I'd like to know about the shifting too, we're running at 45 psi, and our shift times are about 1-2 seconds as well.

[JesseK]

Originally Posted by Bob Steele

I am curious as to what you mean by the "fix-it" window? Seeing that your are not from Michigan... I don't think you have one...
You could only do this repair during a competition I think

???

R

Hmm, no you are correct. I mis-remembered what I read. I thought we had some 6-hour window somewhere due to bag-n-tag, but that's wrong.

[balloons]

Originally Posted by James Tonthat

We found that running about 14 competition level matches breaks the screws. It happened to our practice bot at home, and then again at Lone Star. Our solution so far is to replace the screws (M2.5 x 20, McM# 91290A108) every three competition rounds and running at 45 psi.

Edit: The only thing is the shifters shift a lot more sluggishly at 45 psi.

Are those ones from McMaster sheering too? Those are class 12.9 bolts...Or have you not actually experienced any problems with them and are changing them out for "just in case?"

[artdutra04]

The shifting cylinders on both our practice and competition robots are fed off of a secondary regulator that's set around 10 PSI (as low as the pressure gauge displays). Neither robot has had any problems shifting.

[Raul]

To allow the shifter to work more smoothly and reliably at lower pressures (and just in general):

What I have always found with the AM shifters is that you need to leave the cylinder somewhat loose/floating in the mounting. In other words, do not completely tighten the nut holding the cylinder to the mounting plate. What this does is allow the cylinder to easily adjust to any slight misaligment of the dog shifting shaft and the cylinder and mounting holes.

Use threadlocking lock-tite to hold the nut in place. Tighten the nut all the way and then back off about a 1/4 turn to allow everything to float and align itself how it wants.

To fix the screw breakage problem more reliably (especially if you do not want to reduce the pressure):

Do not just replace the M2.5 screw with another screw. The threads on the screws inherently act as a stress concentrator. Instead, drill out the one side that is threaded with a 3/32 drill (use Anchor Lube to make this easy) and press in a roll pin.

Hope this helps.

[martin417]

We have had no problems with ours, and since we have implemented automatic shifting, we shift a LOT. We are shifting at 60 PSI. We ran many many practices, and ran 16 matches at Peachtree. I want to be sure we don't have troubles at St. Louis. I am not sure if ours is the older version or the newer version (we bought them from another team that had used them in an off season project). Where is the pin/screw in question? Can they be checked / replaced without dis-assembling the transmissions?

[DonRotolo]

Thanks for the tip Andy!

Perhaps the threads are a stress concentrator? Fatigue works differently from impact load, but I suspect you know that already...

[Brant Bowen]

Originally Posted by martin417

Where is the pin/screw in question? Can they be checked / replaced without dis-assembling the transmissions?

Andy is on the road headed towards the Smokey Mountain Regional right now so hopefully I can answer a couple of the questions. The screw that we are talking about is in the dog gear which is between the two gears on the output shaft. This is a picture of the AM Shifter, so ignore sprockets on right. This shows the dog gear with the pin in it (the old way we used to secure the dog gear to the shifter shaft.) Take the black housing off your Super Shifter and you should be able to easily find the dog gear on the output shaft. It sounds like the one you have is older and should probably have the pin, not the screw, but you might want to check it to be safe.

Yes, the screw is a M2.5 x 20 socket head cap screw.

[martin417]

Originally Posted by Brant Bowen

Andy is on the road headed towards the Smokey Mountain Regional right now so hopefully I can answer a couple of the questions. The screw that we are talking about is in the dog gear which is between the two gears on the output shaft. This is a picture of the AM Shifter, so ignore sprockets on right. This shows the dog gear with the pin in it (the old way we used to secure the dog gear to the shifter shaft.) Take the black housing off your Super Shifter and you should be able to easily find the dog gear on the output shaft. It sounds like the one you have is older and should probably have the pin, not the screw, but you might want to check it to be safe.

Yes, the screw is a M2.5 x 20 socket head cap screw.

Thanks! We'll check it when we un-crate the robot in St. Louis.