By Steve Bush 21st November 2019
The problem: folk are finding wear and gouging under the driven hobbed gear of a certain type of dual-drive extruder. In the photo right of a partially assembled dual-drive, it would be in the red surface under the bottom right of the hobbed gear (Extruder? – scroll to bottom to learn. ‘Hobbed gear’ is the imperfect name for the serrated filament drive cylinders of 3d printers.)
Most extruders are single-drive (photo page bottom), but some better machines have ‘dual-drive’ extruders where the idler wheel is replaced by a second hobbed gear (photo left – motor replaced by a stack of paper) – with a gear cut into the bottom of each hobbed gear transferring rotation from one to the other. The filament is driven by serrations in the two facing grooves.
Just recently, a cheaper form of dual-drive has appeared (from several manufactures) and I got one prompted by this Chuck Hellebuyck video.
Having just bought a dual-drive, I found out that lots of folk, including Chuck, are having problems with that wear and gouging under one lower edge of the driven hobbed gear – prompting suggestions that there needs to be a hardened surface or washer underneath.
Now, this may be true, but I think something else is causing most of the wear – as someone has been saving money in the second hobbed gear’s bearing assembly…
How the bearing is (right)
The second hobbed gear has a simple roller bearing in the middle consisting of eight rollers in a neat caged assembly. Through the centre of this there is a fully-threaded screw.
How it should be (left)
Where what should be running through the centre is a smooth (or even ground) 3mm shaft.
Even without wear, the threaded screw is only ~2.8mm in diameter, which allows the forces acting on the filament to skew the hobbed gear and shove its lower back edge hard into the aluminium below – causing the gouging.
It looks to me that whoever originated this design intended a special bolt to be made up with a smooth cylindrical centre section, but somewhere along the line this became simple M3 screw.
Also, the bottom mounting hole is threaded (M3), so doing the screw up tight pulls the top and bottom supports together, trapping the hobbed gear and jamming it – this is exactly what happens.
Also also, the top hole is ~3.3mm – not 3mm (or 2.8mm) – even further allowing the cylinder to tilt into its gouging positional (photo right), made even worse (could it get worse…?) because, as said above, the screw cannot be done up tight enough to hold it straight because the hobbed gear will jam.
3.3mm happens to be the pilot hole size for cutting an M4 (4mm) thread.
This, and other bits above, lead me to believe that the upper hole was supposed to be M4 threaded, and the lower hole was supposed to be a plain 3mm.
This theory (my dear Watson) is possible because the upper support is thicker than the lower (3.1 compared with 2.7mm, see photo) and 3.1mm is thick enough for an M4 thread.
Against my pet theory, there is very little metal left around the top 3.3mmm hole in which to cut an M4 thread, and the lower 2.7mm is plenty for an M3 thread.
Given an M4 top hole and a 3mm bottom hole, a special bolt with a short M4 thread just under the head and a long smooth 3mm shaft would screw nicely through the holes and both properly support the cylinder against gouge-inducing movement, and have no possibility of jamming by over-tightening.
Even had they made both holes 3mm it would be possible to drop a length of plain 3mm rod into the assembly.
Sadly, that pesky 3.3mm hole means a special bolt/axle.
To avoid the jamming issue, and not have to drill or tap the extruder metalwork, the best bet is probably to make up a special screw with no head that is almost all 3mm rod (at least as long as the hobbed gear roller bearing), expanded to 3.3mm at the top end and threaded to M3 at the bottom end.
The thread would have to be pretty short so that it didn’t extend far into the bearing, but long enough to hold in the lower 2.7mm thickness, and the parallel 3.3mm top bit would just slide up and down it the top hole – not pulling together the top and bottom supports. A tiny screwdriver slot would allow assembly it to be screwed in.
UPDATE – that is what got made.
By the way, my dual-drive extruder (top photos) is made by Zeelo and cost under £20, but looking at photos on the web suggests even the £40 Creality parts might have the same errant M3 screw – check for yourself as manufacturers might already be sorting this out.
Other than the M3 screw issue, and one other minor thing, the Zeelo part is a well-made extruder, the fit of the other parts is good – particularly of note is the gap at either end of the hobbed gear when installed – which can only be a few dozen microns – most impressive.
On a ‘fused filament’ 3d printer, the filament ‘extuder’ comes in two parts:
The ‘hot-end’ which melts printing filament and delivers this to the thing being created
And something that forces filament into the hot-end, which tends to get called ‘the extruder’
By the way, the hot end and drive part can be bolted together (‘direct drive’) or, to save moving weight, can be separated by a flexible but longitudinally-rigid tube (‘Bowden‘ drive).
The drive part of the extruder (which I abbreviated to ‘the extruder’ above) generally has a stepper motor that drives a cylinder (dubbed the ‘hobbed gear’ – toothed brass in the photo on the right), and the filament is forced against this cylinder by spring pressure, allowing rotation of the cylinder to force the filament (black in photo) along (from left to right in the photo).
The extruder in the photo on the right is ‘single-drive’ because spring pressure is transferred using an idler wheel (silver). In this case the extruder is mine, an ‘all-metal’ replacement for the standard plastic Creality type.
The standard plastic single-drive Creality type is the one Chuck Hellebuyck identified slippage problems with.
As it happens, when driving PETg, my metal single drive unit did not slip. In the 100mm filament test, mine varied over only 100.2mm to 97.0mm when varying the hot-end across 240-195°C – so it didn’t need replacing anyway. If I used PLA in my machine (as Chuck did in his) my results may have differed because PLA is harder and more likely to slip.
BTW, I realise all of the above is crying out for diagrams or annotated photos, but time is short right now.
A bit late, but Aliexpress sell a version with a modified bolt and copper gasket.
https://www.aliexpress.com/item/1005001431887581.html
I solved the problem by drilling out the m3 tread with a #32 drill. I took a piece of 3mm OD SS Capillary tubing and cut to 20mm. I cleaned and oiled the bearings an pressed the tubing into place. I now have a nice smooth hard rod on which gear runs.
I believe that the m3 screw threads cause the bearings to follow the threads and run up against one side of the aluminum mount. Since I put in the tubing my problems are gone.
Another fix will be a shoulder bolt. Shoulder bolts are available at mcmastercar. M3 thread 3mm body 17mm long is what I was going to order for my winsin… I’m looking for a decent left hand…
Simple fix is to go to a hobby shop and get some 2.89mm brass rod. Cut off a piece slightly longer than the original screw. Using a m3 die, a vice and a slow drill , tap ~4mm of thread on one end. Put 2 small flats on the other end of the shaft so you can tighten it with pliers. Then shim the bottom with a small piece of transparency film with a hole for the shaft. Don’t lose any of the other parts. The bearing will run 10 x better on the brass.
I have one of the ubiquitous $15 dual gear extruders which undoubtedly all come from the same factory but are sold under several names. Mine is WINSINN purchased January 2020.
It is almost identical to yours except:
– The inner bore of the driven gear is 5mm – The M3 bolt has 2 little roller bearing cages with outside diameters of about 4.6mm
The two roller bearing cages extend the full length of the driven gear which prevents the problem of the gear tilting. However, the threads on the bolt are slowly wearing away.
More seriously, there is nothing to prevent the ends of the steel driven gear from rubbing the aluminum at each end of the shaft. I see grooves into the aluminum at each end. At the toothed end, the groove is just over 1mm deep! This happens despite having carefully aligned the filament grooves of the drive and driven gear.
I’m trying to find something better.
I’m experiencing the same problems right now. My extruder has been bullet proof. I ran a print the other day and the tube fitting failed. I put a new fitting in and now I can’t get anything to print. Leveled the bed several times, I just can’t get it to extrude the right amount any longer.
When I run the square test….I get a good left square first and then the extrustiion gets worse and worse.
Kind of stumped. About to order a new extruder…
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