Atlas Shay Crank / Sheared U-joint Pins

[cbroughton67]

So, I have an Atlas Shay, and the drive shaft has come loose from the crank. The issue is the itty-bitty pins that engage the round "universal" joint are sheared-off. It's those pins that hold the shaft and crank together. As luck would have it, the crank (which comes with the offending part) is out of stock at Atlas. I've checked eBay, and no luck. Has anyone had this issue before and crafted a fix in the absence of replacement parts?


Chris


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[woodone]

Any chance that the side frame part with the u-joint might work?
Showed to be in stock.

[cbroughton67]

Any chance that the side frame part with the u-joint might work?
Showed to be in stock.

The shaft is a larger diameter than the crank, unfortunately. I did order a "female shaft" in hopes that I can somehow utilize the end as a replacement.
Chris

[Jim Starbuck]

Chris,

I checked my parts stash and I have a spare crankshaft but it’s just the shaft with no end connections on it.  Sorry
I hope the parts you have coming will get the little gem running again.

Jim

[mmagliaro]

I'm having a hard time understanding what is broken.  Is the one in your photo, circled in yellow, NOT broken?  Are the little fingers on that part the things that are broken off on your damaged part?

[cbroughton67]

The part circled in the photo is the damaged part. Here's the corresponding part on the other end of the crank that is the "good" part. It's the two "nubs" that are missing / sheared-off from the first photo.


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[wazzou]

I would think you could drill some holes and insert phosphor bronze wire of the correct diameter and secure them with Loctite.

[cbroughton67]

I would think you could drill some holes and insert phosphor bronze wire of the correct diameter and secure them with Loctite.

That's what I was thinking as well. That is, up until it popped out of my tweezers. I was trying to slide it onto a piece of styrene rod to hold it while drilling, then "poof." FML...

[Maletrain]

I don't have one of these and have never seriously looked at one, so I am just trying to relate to the photos in this thread. 

As I think I understand this assembly, the broken part fits inside the metallic ring and the (now missing) "nubs" protrude through the holes visible in the ring?  If I am correct, then that ring is also connected to two other "nubs" on the shaft coming out of the other side of the U-joint? 

If I am correct in my understanding, then it seems that the broken piece cannot simply be connected by putting a metallic pin completely through the ring plus the plastic U on the end of the shaft without locking the joint to the other part coming in from the other side? 

I am getting the impression that the U with the nubs needs to be squeezed to make it enter the ring, then let the U expand back to put the nubs into the holes in the ring.  If so, then 3-D printing of the part might not be a suitable solution unless it could be printed in something both flexible and tough.  Nylon, maybe?  I am guessing the original is Delrin?

[peteski]

I don't have one of these and have never seriously looked at one, so I am just trying to relate to the photos in this thread. 

As I think I understand this assembly, the broken part fits inside the metallic ring and the (now missing) "nubs" protrude through the holes visible in the ring?  If I am correct, then that ring is also connected to two other "nubs" on the shaft coming out of the other side of the U-joint? 

If I am correct in my understanding, then it seems that the broken piece cannot simply be connected by putting a metallic pin completely through the ring plus the plastic U on the end of the shaft without locking the joint to the other part coming in from the other side? 

I am getting the impression that the U with the nubs needs to be squeezed to make it enter the ring, then let the U expand back to put the nubs into the holes in the ring.  If so, then 3-D printing of the part might not be a suitable solution unless it could be printed in something both flexible and tough.  Nylon, maybe?  I am guessing the original is Delrin?

You are correct on all points. Not sure if it is Delrin, but it is one of those slippery engineering plastics.
 Hmm . . . using a long pin through one of the forks might actually work because there is nothing inside of the metal ring. The 2 forks just couple to the ring and there is open space inside of the forks.  Now having long pins through both forks would not work, but only on the single fork could work.

[mmagliaro]

I don't quite understand Maletrain's concern.
It looks to me now like you could just drill holes through the broken "fork" part where the nubs used to be, then insert it back into the cup part of the joint, and push a wire through the whole thing.  As long as you apply Loctite only on the *outside* after you push the wire through, and don't get any inside the joint, I don't see why this would not work.

[peteski]

I don't quite understand Maletrain's concern.
It looks to me now like you could just drill holes through the broken "fork" part where the nubs used to be, then insert it back into the cup part of the joint, and push a wire through the whole thing.  As long as you apply Loctite only on the *outside* after you push the wire through, and don't get any inside the joint, I don't see why this would not work.

Maybe he didn't visualize installing the pin *AFTER* the fork was located inside the ring. That is why he mentioned squeezing.  Heck the Anycubic Photon resin is quite strong and flexible.  I imagine someone could design and print exact copy of theis part in black resin, and it would work just as good as the original.

[mmagliaro]

I'm curious as to how those nubs broke off in the first place.  The design of the part looks like the "fork" fingers bear all the rotating stress and the nubs are just there to keep the fork from pulling out.

[Maletrain]

Just from looking at the photos in this thread, it looked to me like the other shaft (the one with nubs still intact and still installed in the metallic ring) has some material inside the ring that is different from the exposed shaft with broken nubs, and that it might be pierced by a piece of wire going through holes where the broken nubs once were.  That is why I kept saying "If I am correct."  If that is not correct, and a piece of proper diameter music wire could be passed through holes drilled where the nubs used to be while the U is inside the metallic ring without touching the other shaft, then the repair seems pretty simple.  Just put the U back in the ring, drill holes in the U with a bit that passes through the holes in the ring (to make sure they line-up properly), and then put a short piece of stiff wire through the assembly.  I would put an L in one end of that wire and a bit of epoxy over the L to keep the pin from coming out.

That should work fine, until the nubs break off the other shaft.  Then, it is back to the original question, because it would be difficult to get two pins through the center without one blocking the other.

So, if somebody can print these out of a suitably durable material, that would be my first choice.

[cbroughton67]

I'm curious as to how those nubs broke off in the first place.  The design of the part looks like the "fork" fingers bear all the rotating stress and the nubs are just there to keep the fork from pulling out.

@mmagliaro, it wasn't an operational failure. I'm not sure how or when it happened, but it occurred somewhere along the way during a decoder installation.