AX2 transmission - gettin' the slop out

[mikebecher]

I've been really happy with my RC4WD AX2 transmission except for one problem - there is too much play between forward and reverse. This causes the rig to roll unexpectedly and lurch when crawling.

The cause of the play was easy to find. The 'shift disk' has three tabs that slide into slots in the high and low gears. These slots are about 2.5x to 3x longer than the tabs themselves. I figure this was done to make shifting smoother and to allow on-the-fly shifting.



The remedy I am pursuing is to partially fill in the slots on the low gear so they are just a bit larger than the tabs on the shift disk. This will likely make shifting into low more difficult and may require a full stop but I'm willing to make that trade-off.

I've given much thought to how to fill in these slots, from using an epoxy-based filler to adding some sort of set screw, to welding. I really don't want to permanently modify or potentially destroy the gear for this experiment. What I've decided to do is make a mold of the gear and try to cast small metal fillers that will occupy part of the slots.

Luckily, my wife has all kinds of crafting supplies laying around so I was able to make an impression using Sculpy clay. After baking in the oven I had two nice negative molds of the gear and shift disk. I don't know that I'll actually need a mold of the shift disk but decided to make one anyhow.



Tomorrow I'll pour a plaster of paris mold which will be the cast for the metal. I plan on using a zinc-aluminum alloy which is easily made from pennies, doesn't require any special equipment and is fairly hard when cast. Assuming all that works and I can extract the cast pieces (I have my doubts ), I can size and shape them as needed to fit the slots and fill the extra space.

I'll post an update on my progress in the next day or two.

[68_f100]

Same here. I have noticed quite a bit of slop in an otherwise perfect trans. BTW why you have yours apart can you tell me all the bearings sizes and how many? I want to replace mine with sealed bearings.

I would start off with some type of rubber that you could layer on and trim. That way if it comes off it doesn't wreck your trans. Maybe able to make some brass bushings to remove the slop.

[mikebecher]

Originally Posted by 68_f100

BTW why you have yours apart can you tell me all the bearings sizes and how many? I want to replace mine with sealed bearings.

Bearings are 8 each 5x11x4mm and 2 each 5x8x2.5mm

[mikebecher]

Originally Posted by 68_f100

I would start off with some type of rubber that you could layer on and trim. That way if it comes off it doesn't wreck your trans. Maybe able to make some brass bushings to remove the slop.

I have been thinking about the ramifications a slug coming loose in the tranny. I'm FAIRLY confident that I can secure them with some loctite, JB Weld, or solder if this crazy idea actually works. If one of the slugs does come out I think (hope?) it will just jam the shift disk since there's really nowhere else for it to go. For now at least, this is a "don't try this at home" situation.

[mikebecher]

My first attempt at creating the plaster mold was a resounding failure! I have no idea what I'm doing so this comes as no surprise to me. The plaster would not release from the clay negative even though I did it just like the Internet told me to.



This time I made the negative from wax so I can just melt or carve it out of the plaster. I'll let it dry overnight and check back in tomorrow.

[68_f100]

Originally Posted by mikebecher

Bearings are 8 each 5x11x4mm and 2 each 5x8x2.5mm

Thanks for the info

[mikebecher]

So using wax was a step in the right direction, but I need to make a few adjustments. Last night as I was explaining to my wife why I was making a huge mess in the kitchen she mentions to me "you know that plaster gets hot as it cures, right?" It turns out this was not a good combination with the low temperature candle wax I stole from her. Nonetheless, I melted the wax out of the mold and it came out sorta OK. You can see where the edges are not sharp, probably due to the wax softening as the plaster cured or perhaps air bubbles. I'll get some paraffin wax today and give it another go tonight.

I also learned this is an actual thing - Lost Wax Casting! There's tons of good info available so now I'm more confident that I can actually make a working mold.

[fyrstormer]

Get some fuel tubing (the duct-tape of the RC world), cut it in half, and make some rubber bumpers to fit into the sides of the slots on the high and low gears. Scuff the tubing with rough-grit sandpaper, and then hot-glue them in place. Trim the excess glue once it's cooled off. That should reduce play in the transmission and reduce driveline shock when you mash the throttle.

Pencil erasers might also work.

[mikebecher]

Aww, but then I wouldn't have an excuse to melt stuff!

I've just always had it in my head that the inserts need to be metal but I guess that's not necessarily the case. Anything that fills the gap and won't break or permanently deform would do the trick. I want to see this experiment through, so I'm going to keep trying for few more days and use that as plan B. I'm hoping I can use what I've learned about casting metal for other projects, R/C and otherwise.

[68_f100]

I'm starting to think it would mess with the shifting the more I think about it.

[fyrstormer]

Originally Posted by mikebecher

Aww, but then I wouldn't have an excuse to melt stuff!

I've just always had it in my head that the inserts need to be metal but I guess that's not necessarily the case. Anything that fills the gap and won't break or permanently deform would do the trick. I want to see this experiment through, so I'm going to keep trying for few more days and use that as plan B. I'm hoping I can use what I've learned about casting metal for other projects, R/C and otherwise.

If you want to make metal inserts, buy some brass rod, cut it to the appropriate thickness, and hot-glue it in-place.

However, the overarching problem with narrowing the slots for the dog-clutch to engage the low and high gears is that there really isn't any room for a servo-saver in the Wraith's transmission tunnel once you've filled it with a bigger transmission *and* a shift servo, so you need the vehicle to be able to roll freely a little bit so the dog-clutch can move into place without straining the shift servo for too long.

[SSD]

I wouldn't advise messing with it. You may find it is impossible to get into gear after narrowing the shift window.

[68_f100]

Originally Posted by SSD

I wouldn't advise messing with it. You may find it is impossible to get into gear after narrowing the shift window.

You could probably still get it to shift after closing the window a little. It probably wouldn't shift on the fly like it does now but that may not be an issue for some. The only way I can see to do it is machine up some brass inserts with a smaller window. I wouldn't want to weld onto the gear since it's heat treated.

[mikebecher]

Agreed, these are issues I have already taken into consideration...

Originally Posted by mikebecher

The remedy I am pursuing is to partially fill in the slots on the low gear so they are just a bit larger than the tabs on the shift disk. This will likely make shifting into low more difficult and may require a full stop but I'm willing to make that trade-off.

I've given much thought to how to fill in these slots, from using an epoxy-based filler to adding some sort of set screw, to welding. I really don't want to permanently modify or potentially destroy the gear for this experiment. What I've decided to do is make a mold of the gear and try to cast small metal fillers that will occupy part of the slots.

I'm only doing this on the low gear. I really don't care about slop in high and I've noticed that I rarely try to shift from high to low on-the-fly anyhow.

If the whole experiment is an epic failure I've not made any changes that I can't undo.

[mikebecher]

I'm definitely getting better at this. The cast I made last night came out OK. There are a few bubbles and one of the through-holes broke on the wax negative. Overall it looks pretty good, nothing a little grinding of the final piece won't fix. After this one dries for a while I'll clean it up and fill it with metal to see what happens.

[84yoda]

This is a cool idea, wonder how its going to turn out. Would be nice if I could do this with some losi plastic gears and make some metal gears since no one makes metal gears for it anymore.

[mikebecher]

My plaster mold needs to dry for a few days, especially since the humidity here has been running 80-90% daily, ugh! Instead of sitting around watching plaster dry, I decided to see what would happen if I just made the inserts out of the same polymer clay as the first mold. The stuff is pretty hard after baking and the inserts are supported on three sides so it could feasibly work. I don't know how durable it will be but it sure was a lot easier than trying to make a mold for casting metal. I made sure to cut the notches so the shift disk would be against the gear itself in forward and against the clay inserts only in reverse. That should reduce the overall stress on the inserts. I guess the next step is to put it all back together and see if it works.

[fyrstormer]

You need to leave a little more room than that for the dog-clutch pins to engage. Each one is a slightly different size and shape, even if you can't see it, and if the remaining space is custom-fitted to each of the three dog-clutch pins, then the dog-clutch won't engage properly unless it just happens to be in the correct orientation when you hit the shift switch.

If you look at the upgraded aluminum dog-clutch that LEM made for the Traxxas Summit's transmission, you can see that there's still some room for free play as the truck rocks back and forth -- it is reduced to a small fraction of what it used to be, but there is still some there. That is necessary for dog-clutch shifters to work. Even race car and motorcycle transmissions have free play in their dog-clutch shifters -- heck, even synchromesh shifters have a little free play, which is used to lock the synchronizer in-place when there's torque being applied to the transmission.

[mikebecher]

There is about 5-10° of play in it now (down from 60°!) and the shift disk slides in smoothly from any orientation. I actually used that disk to create and lengthen the holes before baking. I should have time tonight to re-assemble the transmission and go for a test run.

[68_f100]

I like the direction your going. You get something I can cast it for you at work in a sand cast.