O-rings and microscope resting position

Hi everyone,

I recently had to change an o-ring on one of my microscopes (~1 year old, some 50 - 100h of use) after noticing that the mobility on the x-axis was not quite proportionate. At first I did not think it was an o-ring issue since the o-ring was still intact.

After cutting off and replacing the o-ring, proportionate x-axis mobility was restored. When comparing the old o-ring with a fresh one, the malfunctioning o-ring had gotten ~2cm longer.
This extension leads to a loss in “pulling”-power would explain the lack of mobility in that direction (compressed actuator, negative x-axis movement), if I understand everything correctly.

This made me think. To minimize tear and extension of the o-rings, would the optimal resting-position of the microscope (when not in use) be when there is the least tension on the o-rings? This would then be when the actuators are compressed (negative direction). Until now, I try to leave my microscopes in a middle position (leveled actuators), to minimize tear on the plastic. But this might be completely unnecessary and also seems to be sub-optimal with regards to the o-rings.

So I wonder, is there a recommended position for when leaving the microscope resting taking both plastic and o-ring tear in consideration? Excuse me if I have overlooked some existing recommendations in the documentation or here in the forums.

Best regards



This is really interesting to know.

Thanks for this @Pelle. I agree that storing the microscope with minimal tension is ideal. I think this also feeds into some conversations that we have been having in design review around maintenance and verification:

  • How often should people replace the bands
  • How can we test if a microscope is degrading

This is a really nice data point for this indicating that we need to replace the bands at least yearly.

It’s also worth checking: was this a Viton O-ring, or another polymer? I’ve had one or two go brittle and disintegrate, but unfortunately I wasn’t able to ascertain whether they were properly-specced Viton or some of the Nitrile or EPDM O-rings we tried at one point.

you’re very welcome. I am happy to contribute with whatever I can to this amazing tool.

I measure and note the mobility of my microscopes now and then (4 microscopes to date). From now on I’ll try and do it once a week to get some more reliable data. I’ve also started to leave one of the microscopes in the x/y negative position, for minimum o-ring tension.

I’m happy to share the data of the mobility once some time has passed, maybe in a few months or so.

The O-ring was viton FPM according to the reseller (swedish), but that is about all the information there is. I have not contacted them for further specifications.

I’ve had issues mounting the viton FPM o-rings (as many others it seems), and am now using NBR-version which mount without any issues.

That’s interesting - do you know what it is about the NBR ones that helps (are they more slippery, or perhaps more abrasion-resistant)? I’ve never loved the O ring solution - but I’ve struggled to find a better alternative. Speccing/sourcing springs is surprisingly tricky, and I never found time to figure out if magnets would work…

The NBR ones require a little less effort for that extra tension when mounting them on the actuators. With some of the failed FPM o-ring mounting I noticed gashes in the rubber near the two top ends of the loop. It almost looks like they are pushed into some sharp surface inside the actuator (of which there aren’t any) creating a gash. It could be that they are less abrasion-resistant, and the gash is the start of the breaking. Anyway, this has not been an issue with NBR so far.

According the the seller they have slightly different ultimate tensile strength.
FPM: 150-200 Kp/cm2
NBR: 100-250 Kp/cm2
Initially this does not seem to play any role, with regards to mobility, but we’ll se how it turns out in the long run.

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On a weekly basis during the last couple of months I’ve measured the x and y min/max boundaries on my microscopes to see what impact the resting position might have.

I started out by leaving the microscopes in a resting position where the o-rings were under maximum tension (when the stage is as far away from the dovetail as possible). Already after 2 weeks they lost ~10% movability on both x and y axis. For both x and y axis the lost mobility was when the stage is moving towards the dovetail (positive coordinate values). As mentioned previously this could be explained by the fact that, with time, the o-rings are stretched and thus lose “pulling” force. Removing and measuring the o-rings I could confirm that they had stretched ~7mm.

I mounted new o-rings and have since left the microscopes in a resting position where the o-rings are under the least amount of tension (when the stage is as close to the dovetail as possible). What’s interesting is that contrary to the expected minimizing of movability loss over time they’ve actually gained ~10% movability (mostly in the first couple of weeks). For both x and y axis the gained mobility was in the direction when the stage is further moving towards the dovetail (positive coordinate values). I think this might have to do with the plastic main body (ladder structure) being slowly stretched and accustomed over time, allowing for further movement. I don’t know if this is healthy for the stage structure in the long run but so far, I have not noticed any breakage.

So, my conclusion so far is that leaving the microscope in a resting position where the o-rings are under the least amount of tension (when the stage is as close to the dovetail as possible), is preferable when it comes to minimizing o-ring tear and thus movability loss.

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Hi @pelle, that’s really interesting. I suspect what you are seeing is “creep” in the plastic, as you say it will gradually deform so taht the position where you leave it is the position it naturally adopts. If that position is the one where the O rings are least stretched (i.e. stage next to dovetail), then you’d gain some travel at the bottom of the actuators.

I don’t think this would damage the stage - unless by doing so we increase the stress when the stage is at the other end of its travel; as designed, the flexures are pretty close to their yield stress when they are bent, and I guess you’d potentially double that if the resting position became one end of the travel. My guess is you’d probably be OK, but it would be interesting to hear if moving the stage all the way to the end of its travel and back results in anything breaking.

I think this is evidence that would favour swapping to springs - but that’s a decision for the future I think.