I wish we could find a better solution than grub screws for attaching pulleys to shafts in 3D printing. Small-scale requirements make it difficult to use other industrial systems.
Grub screws often loosen under the effect of vibration even with Loctite. They also introduce eccentricity. And the tip of the grub screws marks the shaft, making it difficult to remove the pulley.
Any idea how we could improve this?
#3dprinting #3dptech #3dprinting_tech #pulley #motion #belt #grub_screw @3dprinting
Here is a video showing the most common assembly types of timing pulleys:
https://www.youtube.com/watch?v=wzjbxlDdkGA
Taper lock would be the best but we don't have enough room.
@bear_lab Taper lock relies on deformation of the sleeve to grip the shaft, right? Printed parts are nothing if not deformable. I could see the taper lock being integrated into the pulley body, the whole pulley being printed, and feasible around a GT2 20 tooth size. Maybe?
Better, how about compressing a rubber O ring inside the pulley? Rubber has better friction coefficient than PETG or ABS.
@kbob I think a taper lock would need more teeth, I need to test some CAD design.
For the o-ring I am not sure it would be strong enough over time but the idea is interesting.
@bear_lab I wouldn't have the name for it exactly, not my area of expertise. We use hexes for Allen wrenches, drill bits and so many other situations of relatively high precision though. Can that not be applied to the end of a driveshaft?
I'm not thinking the whole length of the thing needs to be machined or anything, just enough to fit the belt pulley.
@gooba42 @bear_lab I would use a spline rather than hex, because it preserves the circular reference. I was thinking keyway, but a spline is basically a bunch of fixed keyways. Essentially the same idea.
I think the robust tooling you are referring to is a broach? (I've been designing inexpensive rotary broach holders lately.)
There could be a mashup of your and my ideas here: Use retaining compound to hold a male spline section on the motor shaft, then ream and broach the matching female spline on the pulley. It would then still be removable and adjustable, but couldn't really slip.
Commercial spline rotary broaches are hundreds of dollars, and a cheap commercial rotary broach holder is $500, most over $1000.
This could be fine with either rotary broaches or linear arbor broaches, and would be easy because it would be made with aluminum.
@bear_lab @3dprinting If you really want it to be permanent, use loctite / thread locker on the motor shaft.
I haven't had blue loctite (242) on the motor shaft give up, but you can use permanent red loctite (271), or if you really want to go all-in, retaining compound like R-638. R-638 will fill a 0.25mm gap, and is fairly high viscosity, so put it on without grub screws to limit eccentricity. Its working time is limited to 4 minutes.
The 600-series retaining compound has a wide variety of max gap, viscosity, and initial cure time. They all are 24h full cure time though.
@bear_lab It will be more concentric than a set screw... Just don't load it while it sets and it will be closer to concentric than anything else that comes to my mind
It won't be adjustable, so get pulleys that are much wider than the belt. Misaligned flanged pulleys would certainly create artifacts. Could even go with non-flanged pulleys on the motors and flanged pulleys elsewhere. But I don't know about availability of non-flanged pulleys, haven't looked.
The classic solution for this problem is of course close tolerance holes and shafts with keyways and keys. But at this size that's kind of hard!
Hmm. Given my recent fascination with rotary broaches, I could imagine making a pair of broaches (male and female) to cut matching 1mm keyways in motor shafts and pulleys. 🤔
@mcdanlj yeah a lot more concentric than grub screws for sure.
Yes for flanges it is not an issue, I already use a wider pulley than a belt and make sure everything is aligned.
Thanks for all your comments, very interesting points.
Hexagonal or other-shaped shafts that don't let the pulley turn on the shaft, and then a through-hole with a retaining pin - my favourite is a linch pin, can be re-used indefinitely and is easy to put on or take off - that just keeps the pulley from sliding axially.
These shafts seems too small for a key.
@bear_lab @[email protected] If you're cool with mating the pulley to the shaft forever, there are adhesives like Loctite 638 that are designed to bond them together.
I've also found that there are big differences in how the various threadlockers work. Some are *much* better than others for this application.
Finally, I've never seen a grub screw work long term in a driven application without a flat (or better yet, a slot) on the mating shaft.
@PKL @3dprinting Agree for the flat but in general those pulleys have two grub screw so one will be on the flat and another one on the rounded side.
I am not ready yet for a definitive assembly for reparability purpose but clearly a cheap option. Might still need to be careful with the shaft tolerance that could vary from manufacturers and so introduce eccentricity. Would be low but still.
@mcdanlj @chrishuck I am using a shaft with a flat but those pulleys have a second grub screw that is tightening in the round shape. Also, even with a single grub screw you will create eccentricity unfortunately.
Also, let's make it clear, I hate grub screws 😅
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