I had a similar experience. Used the O-rings supplied in this kit from LabCrafter. (bought december 2025)
The one in the X position I got in the first try. When trying to do the Y I first broke 2 O-rings. (in the kit there is a bag with extra parts - luckily 2 extra O-rings). Then I got an O-ring at Y. And then at Z. So I started with 5 O-rings and have used them all now.
I am not sure if it comes down to technique or what. I was quite alert about not making any cuts to the ring with the tool. I can only suspect that the force it takes to apply the O-ring is very near the breaking limit. And sometimes it breaks and sometimes it doesn’t.
When the rings wear down and goes bad I think it will be reasonable to change them all. Even though only one will break. In that case I might buy more than just 3. And maybe try to look at something that has just a little less tension. A few % might make a big difference.
@rabuk thank you for commenting on this thread. These types of failures are very hard to pin down because they are inconsistent. I think that there is certainly partly technique involved. With the same stock of o-rings I have seen some people break several o-rings and others break none. However it is not clear which part of the techique is important, or what needs to be explained differently to make it clear.
It is also the case that the o-rings are working towards their limit of stretch at the top of travel, but also getting close to slack at the bottom of travel. We are currently looking in detail again at the design range of motion for the stage, which has also brought up questions about the bands. Taller feet and longer bands would give some extra margin of error.
Interesting video. It shows some of what happens. Still I am a bit puzzled about the plastic pin that is put in the middle. I did put it in - but not sure what it did.
I assembled the mechanical parts following this video
I noticed that the big gears could be spinned so that the hole would be more or less open.
I did like in the video. And as mentioned. With X it went fine. And with Y 2 rings failed. I then adjusted the gear so that it got just a little under the level. Meaning the hole is still completely open at the bottom, but closing a little in the top. My idea was to make it possible to mount the ring with less tension.
I haven’t got enough rings to test if it worked by chance or if it is indeed a good idea. But after I did it Y and Z went without broken O-rings.
I might have shortened the level by 1 mm. And since the O-ring is applied in two sides and turning around I can multiply this by approx. 4 mm. That is actually quite a lot. And I can adjust the height afterwards when I get the actuators running. I hope. Fingers crossed.
Is there a good reason not for doing this already in the instructions?
I agree that the exact placement of the band, and the twist and hence the tension at every point, etc. and differences in the bands themselves (even room temperature) makes a lot of variables. But making the distance a little shorter might make the process a little more forgiving.
The plastic pin does two things. First, for the gear / cog on the top, it holds the brass nut in place in the correct position, so it gets pulled into a hexshape when you tighten it initially. I messed that up and tightened it when it wasn’t “edge” towards me, but “flat” and had to reprint the body, since the nut would spin. After that, the pin holds the actuator in place. The oring pulls the actuator to the bottom of the main body, when the screw pulls it up when you tighten it. When you assemble it, it prevents the actuator from “running away” from you when you put force on it to get the orings to fit.
I’ve seen similar results to you, when it’s lower, it’s easier to get the oring to fit. I put the pin in, turn the gear a bit (this puts it at an angle), then put the oring on. I also don’t put the oring around the foot initially, but I loop it around the tool, holding the middle myself and push it in. So far ~3 microscopes and no broken orings since I’ve started doing this.
I have just watched that video again, and looked at the build instructions.
In the video there is a very clear step making the band even on both sides of the foot: https://youtu.be/vwuFgYbz_MM?t=1295s . This is important, and is not actually mentioned in the instructions.
The band overall should be well within the maximum elongation, but during insertion it is folded so that it stretched in four separate sections. The band does not slide over the prongs of the insertion tool, or over the bridge in the foot. If the band is not distributed evenly between the two sides of the foot, and also evenly each side of the prongs on the tool, then there can easily be a lot more stretch needed in part of the band, that could cause it to break.
When I put the oring on, I naturally do this, since I put it on the tool first and hold the middle, see pic. I also have the foot already mounted up to the microscope, it seems like it’s a slight snap fit. Might be easier than having the foot be put on with the oring already on the tool and in the foot? Never had it slip from the prongs doing it this way, only issue I had was sometimes I had to guide the oring into the foot channel, but that’s not a problem
