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SRAM 12t X-Sync pulley for 1x drivetrains

by cmh Dec 5, 2017
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hey nice work! what type of plastic is sram using on it´s pulleys if we could get that to be 3D printed that would be an awesome resource! but though this is an awesome contribution I think we could use it for reference so it can be milled in delrin or some other hard bad ass plastic.

Not sure the plastic SRAM is using, anything I'd say would just be a guess. I can say that I just pulled off the original red PLA pulley and it's in amazing shape, with only a partially chipped tooth and minor wear. I'm realizing that the lower pulley only provides tension with no side loading, so as such it's going to be much more durable. The upper pulley is responsible for guiding the chain during shifts, so is probably subject to side loads.

One design issue I've found with my design is the center bore is too large -- every version I've run for any amount of time winds up with play on the bearing, I think I need to shrink the hole so it is more of a press fit, or incorporate the lips at the edge that SRAM has designed into its pulley. The red PLA pulley I removed had gotten so loose on the bearing that the bearing basically fell out.

I've reprinted two ABS split pulleys and glued them together with acetone on both halves, and am just starting to run those now. I'm considering recreating the pulley in Fusion360 as it'll give me more flexibility in adjusting the design compared to SketchUp, and then I can address my early findings.

So to answer your actual question, I don't think the plastic itself is going to prove to be as crucial to the design as getting the design right, which I can see I still need to work on.

Comments deleted.

How've these held up?

I've had an idea in my project idea queue for a little while...designing/milling an alu mold to anneal [/or acetone vapor treat] printed parts (especially w/ geometries/dimensions that don't lend themselves to a freestanding, oven-annealing process).
It might actually be overkill for derailleur pulleys, but also might extend their lifespan--esp as their teeth wear, and as the pla degrades..?

Not sure,... but wondering if hobbyist/appropriate-tech/open-source hw production of those same types of parts might currently lend itself more towards a multi-step 3d printing/post-processing approach, than setting up the table/fixtures/etc to mill them on a hobbyist/open source 3 axis cnc.
...hoping to test it out (or at least read about it, heh) over the next year or so.

I did have an issue with durability. The one I printed in ABS - what folks always say to choose if you need durability - broke completely leaving me with a naked bearing. Amusingly enough, the derailleur still shifted reasonably well but not great. Noticing the shifts weren't quite right is what clued me in to look at the pulley wheels, and notice the ABS one was gone. In the meantime, the PLA one on the bottom was still there and still working. Now I'm pretty sure the top pulley is subject to harsher forces with shifting loads, where the bottom pulley is only subject to the loads of chain tension, so no surprise that the top one broke, but I took one of my PLA pulleys and put that on the top and it's been working fine for a while.

So an ABS pulley didn't work and disintegrated, whereas the PLA has been okay. I'm wondering if the ABS wasn't a great print, as it should have been more durable, not less so. I'm currently waiting on a new printer, and hopefully that one will work well with PETG and ABS, so I'll try those when it comes in.

Now as for a mold for heat treating, that definitely sounds like overkill to me. Definitely an interesting concept, but I think these could survive conventional open-oven annealing (something I still haven't tried) with decent results, and having an aluminum block milled to the exact shape of the pulleys, at that point I'd just want to mill replacement pulleys out of aluminum and be done. Is the purpose of the mold to reduce warping during heat treatment?

Regarding wear, my thought is it's such a simple print, as long as the pulleys aren't falling apart (again!) then I would think they could be swapped out as necessary, no need to stretch them out like you might be inclined to do if it were a $30 set of pulleys.

Yea, from the bit of experimenting w/ annealing I've done and have read/watched, the part and its geometries can become distorted/deformed depending on the temperatures and rates/duration of the heating and cooling, as well as how a specific part itself heats up due to differences in its volume throughout it.
Thats what orig led me to thinking about a milled mold that could prevent or limit deformation w/in acceptable tolerances, and also distribute heat more evenly/rapidly.

With annealing:pulleys, I was wondering if the teeth might be prone to drooping, and if thatd interact w/ the risk of axial/bearing/bolt features&dimensions shortening/going out of round.
On the one hand, the wide/narrow tooth profile seems like it might sorta provide supports during annealing, but looking at your pics of the broken pulleys...I wonder if w/n profiles can cause weak/vulnerable spots in the pulleys under shifting loads/movements?

My preferred way of printing the pulleys is the split version, which I then superglue together. I find very short support material like you get when you print this thing in one piece is quite annoying to remove, so I like the better results of the split pulley. Printed split, it would be pretty easy to oven anneal the parts laying flat on a cookie sheet. I guess there's a risk of the teeth curling up, I dunno. No experience with annealing parts at all, I've watched a couple videos on it, but that's it.

Honestly, I'm not certain I see the point of the narrow-wide pulleys. Narrow wide works on chainrings to help keep the chain from jumping off. I've been running 1x drivetrains for over 10 years now (long before narrow-wide or clutch derailleurs were a thing) and I have never had a chain drop off of a jockey pulley in any riding situation. Still, SRAM puts it on the pulleys in their 1x drivetrains, so I guess they have some reason for it. Dunno.

Regarding the aluminum mold for annealing - if you've got a mold, you could just use it as a mold to form parts with molten plastic, no?

Yea, to an extent. I'm armchairing my brainstorming for now... Probably depends on the plastics thermal properties and viscosity the more towards injection molding it moves?

Yeah, gets into a realm I don't know at all... I do know that injection molding machines are usually gigantic things that have to handle a boatload of pressure, I imagine even molten plastic requires big pressure to get it to flow properly into all the crevices, so that could be tricky to accomplish as DIY. Completely out of my skills/knowledge at that point!

Word, robotdigg has some like di2 or eagle gruppo level injection molding stuff but I'd probably spring for the former, heh..

Yea...if I can get around to being able to build a 3 axis cnc or getting membership at a maker spacejym here, I need to try to cut some bb adapters for my ss Fairdale commuter too...it uses a bmx bb/crankset and id love to be able to convert it to using mtb crankset types to use my on hand chainrings. Of course if I can do that, I could just try making my own chainrings too. OTOH alloy cranks would probably save me well worth their weight in makerspace membership costs, tho......

Maybe N+1 holds true for project idea queues?
At first I think I thought open source hardware and consumer fabrication stuff would help me simplify/avoid compatibility issues and complexities, but maybe it just facilitates/enables them and closes some loops??