Belt Tensioning (Travel Distance)

I’ve seen a few threads covering belt tension and a few ways to do this from the setup video showing how to put the belt into the clips and adjust until the screw just grabs onto the clip to using tuning apps on a smartphone. Now I thought I was OK with my belt tension on my machine but recently I’ve started hearing a strange noise from what seems to be coming from the left side Y stepper motor. The sound is only present on rapids in the Y direction (ie. using the jogging buttons to move from “Front Left” to “Back Left”). While running jobs everything sounds “normal” so it’s only ever present during faster movements on the Y axis. Now I’m thinking eventually I may have to replace a left stepper motor and this had me rethinking my belt tension so I went through the threads again covering this topic.

I tried the tuning app to compare both belts (on the Y axis) but didn’t feel in my case I was getting consistent readings or I had to pluck many times to get one reading to register so I thought this may not be the best approach me. So my next thought was that the machine is designed with very specific stepper motors and gears with belts of a specific pitch with default values in GRBL $100 = 40.000 and $101 = 40.000 meaning 40 steps to travel 1mm. Would it not be safe to assume that if I leave these settings at their default values and I move the machine forward in Carbide Motion (ie. 750mm) and the traveled distance measured between the left side of the gantry and left front plate as well as the right side of the gantry and right front plate travel the exact same distance of 750mm (EDIT: that my belt tension is just right)? If either of the measurements taken on the left or right side of the machine are higher or lower then instead of tuning the $100 and $101 settings why not adjust the tension of the belts instead. This may result in a small gap between the belts end clips and the Y extrusion front plate but just add shims to get a snug fit between the clip and plate it mounts to. The the same approach can be taken on the X belt tension by comparing movement in Carbide Motion and the actual distance traveled.

I don’t know how accurate that would be, but since you would be doing those adjustments by simply jogging the machine in the air, I suppose you could still end up in a situation where both belt tensions are equal (=good), but actually both belts would be too loose to cope with an actual cutting situation where (much) more resistance is put on the travel? You would probably also have to verify whether there is not too much backlash when changing direction (i.e. again both belts could be equally loose, the gantry could travel exactly the requested distance in one direction, but not in the other?)

Before the phone app trick, I used to have a must simpler rule that with the gantry all the way to the back, when I lifted the middle of each belt I could slip my pinky finger under the belt, but only barely. This was on the standard size Shapeoko3 with the shorter/narrower belts, so that specific criteria may not be good for a Pro XXL, but you could similarly have a given un-scientific reference. I think I was able to tell if one belt was looser/tighter than the other by just plucking them in the middle like that.

If I remember correctly there is a luggage-scale technique in @LiamN’s belt tuning thread, that you could try in case it works better for you than the microphone approach ?


I’d say the luggage scale technique is even harder to get consistent results with than the belt twanging.

In terms of getting the tension even between left and right Y belts, yes, the travel distances should be equal if the belt tensions are equal, assuming the belts are undamaged and from the same batch in manufacturing.

Getting exactly the target distance does not imply that you have the correct tension, there’s substantial variability in the tension modulus of the belts and you extend the belt in order to get to the target tension, by exactly how much depends on the belt mould, the reinforcement material, the belt width and the age & condition. This is why we measure deflection or frequency, not length, to get a tension measurement.

If you’re trying to balance left and right, if you can hear the twang tone you can set up both belts with blocks in the same place and just compare the tone you get between them by listening.

As for the weird noise from your machine, I would not start with the belts or steppers as a suspect unless you’ve had really high tension. If there’s no perceptible knock on the stepper shaft when the belts are removed it’s probably still OK.

I’d suggest that the noise is either something loose or something binding.

Something loose may rattle at the step frequency of a rapid, I’ve had a few things do this, including clamp handles rattling on a rapid and spent ages hunting the loose bolt on my machine.

Something binding, likely the V wheels being over-tensioned or the Y rails not quite being parallel, would cause a different resistance to motion across the axis movement and the faster the steppers go the less torque they have. It’s possible you have something binding a little which only shows up during a rapid.