VEXPro Ball Shifter Shaft Malfunction

[Kevin Sevcik]

Originally Posted by Al Skierkiewicz

John and Paul,
Do you have any suggestions for a slipping 36 tooth encoder gear? I added some double sided tape but not sure that is a good fix.

We had the same problem of the press fit between the output shaft and the encoder gear not being tight enough. We cleaned up the shaft, bearing, and encoder, then added a dab of super glue in the gap between the shaft and the inner bearing race before we pushed the encoder into the bearing. It seems to hold well enough now.

[fox46]

I am not familiar with these gearboxes but could you remove the components from the box, assemble the two pieces and pin them together with a 1/16" roll pin or hardened dowel? It appears you wouldn't need anything too substantial since all the torque is carried by the hex but a pinned connection would guarantee it wouldn't release. If that stub shaft is driving anything with a side load, the slightest play or flex in the material will almost guarantee that it will eventually wiggle out of its hole. Make sure you use the "green" locktite retaining compound 680 or similar, not the threadlocker types. JB weld could also be a wise choice. I would imagine the brittle nature of superglues (cyanoacrylates) would crack and debond.

[Al Skierkiewicz]

Fox,
The encoder gear presses into a counter bore in one end of the shaft. The gear is delrin or some type of plastic. The counter bore is located in the end of the shaft supported by a bearing. I am inclined to use super glue but I fear the long term effect on the gear as I do not know the exact material it is made from.

[fox46]

Sorry Al, I was referring to the OP's issue of the output comming loose. Here's an illustration of what I was referring to with a pinned connection to keep the shaft in place-

I think superglue would be a great choice for the encoder issue. There is such little load on that gear anyway.

[BrendanB]

Originally Posted by fox46

Sorry Al, I was referring to the OP's issue of the output comming loose. Here's an illustration of what I was referring to with a pinned connection to keep the shaft in place-

I think superglue would be a great choice for the encoder issue. There is such little load on that gear anyway.

Thank you for the suggestion we might try that solution as a precautionary measure after we get a replacement shaft!

[Jimmy Nichols]

Originally Posted by BrendanB

Thank you for the suggestion we might try that solution as a precautionary measure after we get a replacement shaft!

That is what we did after the 4 we had failed. The new ones that were shipped were drilled and pinned before ever seeing the robot and drive time. We have not had any issues since.

[fox46]

Good to hear!

[fox46]

After a second look at that picture, I've noticed if you have that drive sprocket in the picture on the shaft and that nylon spacer is too long, when you tighten the bolt in the end of the shaft it will put a compressive force on that sprocket. The result will be that the bolt in the end of the shaft will be pulling the hex out of the transmission almost like a gear puller. This could certainly break any adhesive bonds and pull the hex stub shaft out of its press fit. Make sure you have the appropriate sized spacers on the end of that hex shaft and leave some space - like 0.010" for the drive gear to slide back and forth to ensure you do not put any tension on the stub shaft and pull it out of the bore.

[Kevin Sevcik]

Originally Posted by fox46

After a second look at that picture, I've noticed if you have that drive sprocket in the picture on the shaft and that nylon spacer is too long, when you tighten the bolt in the end of the shaft it will put a compressive force on that sprocket. The result will be that the bolt in the end of the shaft will be pulling the hex out of the transmission almost like a gear puller. This could certainly break any adhesive bonds and pull the hex stub shaft out of its press fit. Make sure you have the appropriate sized spacers on the end of that hex shaft and leave some space - like 0.010" for the drive gear to slide back and forth to ensure you do not put any tension on the stub shaft and pull it out of the bore.

That.... is a very astute observation, sir.

Anyone in this thread that's had a hex shaft come out: Are you retaining your sprocket with a bolt at the end of the shaft? Is it possible that you're tightening the bolt so much you're literally pulling the shaft out of that coupler?

[BrendanB]

Originally Posted by Kevin Sevcik

That.... is a very astute observation, sir.

Anyone in this thread that's had a hex shaft come out: Are you retaining your sprocket with a bolt at the end of the shaft? Is it possible that you're tightening the bolt so much you're literally pulling the shaft out of that coupler?

I will check tonight when I can get a close look at the transmission. It is possible but IIRC after inspecting the completed drivetrains those hex sprockets had a little bit of play in them.

Very good observation though!

[Jimmy Nichols]

Originally Posted by Kevin Sevcik

That.... is a very astute observation, sir.

Anyone in this thread that's had a hex shaft come out: Are you retaining your sprocket with a bolt at the end of the shaft? Is it possible that you're tightening the bolt so much you're literally pulling the shaft out of that coupler?

We are using the timing belt pulley's which were far enough from the end of the shaft that the bolt tightened into the end of the shaft with some slop left on the pulley.

[Nemo]

Has anybody intentionally removed these press-fit shafts in order to install longer hex shafts?

[tribologist]

Originally Posted by Nemo

Has anybody intentionally removed these press-fit shafts in order to install longer hex shafts?

It is very easy to remove. The anodization, left over schmutz and grease makes it a week bond. Clamp the shaft in a vise and tap it off with a brass drift (or that screw driver you don't like).

When you make the new shaft you need to drill out a hole at the end so that the spool is not blocked. Measure the hole on the old shaft.

Use Loctite 603 and be very careful to clean everything before bonding. You are best off lubricating after assembly.

Ulf