I had a go at retensioning my three belts using @LiamN’s method, and went for a target of 125Hz, which on my SO3 seems to be the sweet spot (it will be interesting to hear what value others end up using)
280mm span on the right Y belt, using two #201 endmill shafts:
Previously, to tighten my belts I used to choose a given length of belt loop locking onto itself when the tensioner is fully tight against the plate, and then add or remove one teeth or two from the loop and re-tighten to adjust tension and have them “guitar string tight”.
I found out that one teeth can make a significant difference in tension using that method, and it did not allow me to reach exactly 125Hz (say, one measurement was at 110Hz, and just one teeth tighter got me 140Hz)
So I went for finding the loop length that gave me a value above 125Hz when fully tightened against the plate, and then slightly turned the bolt counterclockwise while measuring with the phone app, until I was at exactly 125Hz (+/-1 Hz). It takes very little untightening of the bolt, so I doubt to the bolt will go loose, time will tell.
after I tuned both my Y belts to 125Hz, I did the “home then manually jog to the front plate” test, and I now have my Y rails contacting the front plate at (almost) the same time, but more importantly with the exact same offset as the one I get when moving the gantry manually with the machine powered off (which means that the belt stretch is even on the left and right sides, and does not introduce additional shift). I still have a 0.5mm gap on the left side when the right Y plate is touching the front plate, but I chose to live with this tiny offset from perfectly square last time I shimmed my gantry (and 0.5mm over the Y travel is not much, I’m not even sure what the tolerances on the Y plates and their powder coating are…)
Thanks again @LiamN for opening the path to a much more reliable way to adjust belt tension, while keeping it very simple (it takes about a minute to grab two endmills and a phone to do the test).
Good question, that depends on the size of the blocks or bits and the length of the rail but it should be a reasonably small change in tension as you remove the blocks / bits. The smaller the block the better, all we need is to raise the belt far enough to provide a constrained length of belt which can freely twang without hitting the aluminium.
This is definitely a case of the observer effect, the act of observation very definitely changes the tension somewhat.
Assuming a short Shapeoko rail and 1/4" bits 280mm apart and equally spaced between the end-plate and the carriage it’s around 0.25mm additional extension. At the tensions we’re seeing the static extension is 1.5 to 2.5 mm (depends on the belt too) on a short rail. So that suggests that the measurement is adding between 10% and 17% extra tension. It’s considerably less on the longer rails of the XL and XXL.
The important thing is to make sure you do the two Y rails symmetrically (same distance from the end plate) so that you get the same additional extension & tension over the static extension and tension on the machine. That way when you balance the Y belts to be even, they’re even with and without the blocks in place and you get the same steps / mm on both axes.
Does that answer the question or did I miss the point?
Thank you for this WONDERFUL post. I spent 30 minutes retightening all my belts this morning, feel a lot better knowing there is some science and calculations behind my belt setup now.
For anyone else following down this path, here some additional info I found.
On my older iPhone SE, the measurements were (occasionally) all over the place. I kept at it until I got 4 or 5 nearly identical measurements. I also found that where I positioned the phone (in reference to the phone’s microphone) effected the results. So be consistent.
I initially was strumming the belt with my thumb. Then discovered I got more consistent readings if I used two fingers to actually lift the belt and then let go.
Burdened with the knowledge that my wasteboard was not really a 1.5" grid (based off my $100 and $101 recalibration) even though holding a ruler up to it I could not tell.
Followed @LiamN advice on this post and properly tightened by belts.
Went back to step 1 (it was still square).
Skipped to step 4, reset $100 and $101 to 40 and remeasured. Calculated 39.996 for X and a number even closer to 40 for Y.
Reset my $100 and $101 values back to the factory default of 40, and had a good laugh at my journey.
I really do appreciate all the knowledge you guys share on this forum. I took a very clumsy path but I like where I ended up and could not have figured this out on my own.
Yep, that’s my concern, but I don’t have enough data yet to really say anything about it.
I don’t have access to the specs for the steppers on the Shapeoko and I don’t know whether they were specifically selected for high shaft radial loads or not. I suspect not as typically a stepper rated for high radial load would have a larger (say 8mm) shaft and bearings, but I’m not a metallurgist so…
OTOH, the ratings I was able to find were for industrial quality steppers and therefore intended for motors with much higher duty cycles than an ‘average’ Shapeoko and so our machines are likely to see a longer elapsed time to failure just because they don’t work 8 / 24 hours a day, every day. Also, those ratings will be based on a minimum performance for the motors, not an expected average unit performance.
Like I mentioned above I currently see no reason why increased belt tension helps with backlash and it seems that the belt slippage in the clip may well be deliberate design intent to keep us at sensible tensions for the machine.
They used to use Wantai sourced steppers+motors. The technical data should not be difficult to find, i’ve got the data on the BLDC motors for the nomad (but never bothered with the steppers). I suggest a quick email to them - they seem to be friendly enough to supplying data. http://www.wantmotor.com
(The nomad was shipping with the Wantai 42BLF for the spindle)
I for one could use some more talk about how you get consistent tension with the clips. The belt almost always slips a couple of teeth when tightening. The clips don’t put enough downward force on the belt to hold the teeth together until it’s almost fully tightened, and finger strength down on the belt while also trying to bring it in range of the bolt isn’t the best either.
I honestly think the belt system is the next big thing due for an upgrade on the machine now that the old belt Z-Axis is gone. It’s easily the source of the most deflection in the entire build, the gears on both Y axis motors effectively extend the shaft increasing the stress on the shaft even further, and setting belt tension is error prone. Half the time when you’re trying to tension the belt, the clip with slip down a couple teeth and you’ll be left with a loose belt.
Since I got the machine late last year I’ve had the original belt Z axis slip so much I had to upgrade, then I had both Y motor shafts snap from a combination of tension and a sudden stop, and most recently I noticed my Y accuracy had deteriorated and tearing things down found the Y belts had started failing. In 8 months every problem I’ve encountered with the shapeoko has been linked to belts.
I think @Julien uses blue tape to help get the belt position in the clip consistent.
However, I don’t think you need very much tension in the Shapeoko belts, certainly no more than the holding force of the steppers - 60N, or less, that’s breakaway torque, not actual usable during motion torque.
My motivation in trying to measure the tension quantitatively was to figure out what sort of forces we were actually achieving and running at so that we could;
Set the tension to a known value
Check if these values are “reasonable” compared to other machines, the Gates spec book, stepper motor specs etc.
Be able to go back and test if it had changed due to belt wear, stretch or some other effect.
I only realised part way through as I did the math(s) that matching Y tensions was important for squaring & calibration.
It currently seems to me that the slipping of the belt clips may well be either smart design of happy accident and that slippage is there to avoid us cranking the belt tensions up to the point where we fatigue the stepper shafts and break stuff. I’m considering going back to mine and backing off the tension, when I’ve done a few more measurements.
I have found, since measuring the tension with my phone that I can get a couple of teeth worth of extension in the clip with finger pressure holding the clip down, if it jumps before the clip gets cammed down by the bolt I’m probably over-tightening it.
I agree with you that the belts are a pain, but they’re much cheaper than ballscrews (which need more motor torque and therefore bigger stepper drivers in the controller etc. etc. ), they’re at least the equal of a leadscrew for backlash, on the regular size Shapeoko probably better and their material is actually pretty good at vibration damping, this is all by design at Gates who designed the GT2 & GT3 belt profiles.
That’s not to say I’m not trying to work out ways to improve them but I think I’ll be focusing my dial gauge on the X rail to Z carriage V wheels first as many others have before me (thanks for posting the CAD Dan).
Has anyone else tried the “measuring belt frequency” method ?
(@ColdCoffee you mentioned you did, did you make a note of what frequencies you got ?)
I’m in the process of including that tip in the ebook, and it would be nice if we could crowdsource a reasonable frequency range for each machine. On my SO3 at this point I feel like [110-140Hz] is a good target range.
I’m going to soon. I just finished the X-axis linear rails and I’m switching my belts to steel core so I’ll be following this guide… maybe tonight (EST).
Currently, working on a drag chain support since I rotated the extrusion so the v-wheel rail is on the back side.
Haha… Well… I didn’t have to take off my “SHAPEOKO” and machine serial number stickers off. It’s easier to use the flat side to establish a datum with, say, 1-2-3 block when spacing the rails. Nothing particularly better, but slightly more convenient.
I Have an XXL with stock Y belts from 3 years ago and a new steel core belt on the X from a couple of months ago.
The Y consistently cuts within .004 over the total Y travel and has a frequency of 58.
The X is off about .002 for each 10-12 inches over it’s travel and is at a frequency of 73. This using the Gates Carbon Drive App.
A small update on belt tension targets, in this vintage Winston Moy from 2015;
He describes the target GT2 6mm belt tension as between 10 and 15 lbs force on the belt and the “hang the machine by the stepper motor” method of adjustment.
At 15 lbs force tension in the belts that would give ~ 135 Newtons belt tension. That’s already on the heavy side for a regular NEMA 23 stepper with a 6mm shaft but gives an indication that the belt tensions are designed in this sort of range.
Another interesting element is that the current belt tensioning clips were used simply as belt anchors in that generation of machine and the belt tension was applied in a more normal way by moving a pulley to set tension.
I was looking for 125Hz, which I picked because that is what you used
Once I found the “tooth” that got me close, I then used torque on the bracket screw to dial it in. It wasn’t easy, took 30 minutes of trial and error until I got it right. The shorter Y belts (I have an XL) were much easier than the longer X belt. Those with the XXL will probably need to budget more time.
and I’m switching my belts to steel core so I’ll be following this guide… maybe tonight (EST).