O-ring problems

Hi,

I just got my Viton O-Rings, 30x2 as indicated, and trying to install them I already broke two of them – so: am I the only one having problem with them? (I had printed TPU rings in there before, so I think I know how to correctly install them).

The order (from ebay) and the bag they came in definitely say “Viton”, and they feel very hard and non-stretchy – have I been sent crap?

Anyways, I’m gonna reprint the main assembly now because I broke the flexures with all the force it took to break the O-rings, and I’m gonna try simple household rubber bands next…

regards,

christian.

Oh dear! It does take some force to get the bands in, they need to be tight to give a positive action. It should not break either the body or the band though.

Viton is a well-defined material which is why it has been chosen here. Your example clearly does not follow that expectation.

Early OFM used household elastic bands which work fine. You will need to work out how many loops to get the right tension. They can also be tensioned by wrapping excess around the legs. Unfortunately they perish quite quickly.

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Same happened to me and I replaced Viton Orings with Nitrile ones and it work perfectly.

As I managed to get one of my printed rings out unharmed, I was able to measure it, it had stretched to about 38mm of diameter.

That has been printed with “Fibrology Fiberflex 30D”, which apparently is not stretchy enough for this application.

What TPU brands have people used successfully?

cm.

I also had the problem with viton O-rings and as @Bogdan, I am using Nitrile now. The only time I succeed to put in place the Viton Orings, the motors were not strong enough to move the stage. I came back to Nitrile and it is working nicely.
The only disadvantage to nitrile is the need to change them regularly as they distend a bit and even break after a few months of use.

This is really interesting feedback with the O-rings. We have been using Viton due to the quality and ability to specify. It would be interesting if you have a lint to the bands you purchased. I am worried whether they specify the OD rather than the ID, or if they are not genuine Viton.

Either way this is something we need to understand and “fix”. I put fix in quotes because it may just need to be a documentation fix.

Mine are these: https://www.ebay.at/itm/30mm-ID-x-2mm-C-S-Viton-O-Ring-30x2-Choose-Quantity-New-Metric/292446776040?ssPageName=STRK%3AMEBIDX%3AIT&var=592002220183&_trksid=p2057872.m2749.l2649
The 30mm are given as the ID.

Thanks @rabbit this is really useful. What was interesting is the spec (FKM80 ) afterward the term Viton. So what I have learned in the last 13 mins is:

  • Viton while a registered trademark of a manufacturer is also used as a generic name for similar products by other manufacturers
  • There are a number of similar fluoroelastomer rubbers that are quite different that use the same “Viton” name
  • Viton comes in different “durometer” values specifying the hardness. What you have is 80 what I have here is 75. I will have to see if I can find out what the lab has. 80 and 75 sound close…

What I need to work out is whether the scale is linear, how easy it is to make a test for it, how accurately companies provide the correct hardness, and what value is best for the microscope.

Thank you so much for raising the issue on the forum. We learn so much when people give us feedback and ask questions. :grin:

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Durometer/shore hardness might be the value we can get at for O-rings, as it might be the one relevant for their sealing properties – but it is not what we’re actually interested in, right?

What we actually want is elastic modulus and maximum elongation – and I’ve seen quite a range of those quoted for “Viton”, and I have no idea if they correlate with shore hardness.

As an example, my printed rings from FiberFlex 30D would translate to about 85A, so it should be even harder than my O-rings, but they didn’t break, they did deform plastically instead.

This is true. My hope is that for FKM materials that the durometer value is related to the Young’s modulus. But I suppose a testing rig where we stretch them is the true test. I’ll have a think.

The basic problem is that o-rings are not specific for use as a tension spring.
For basic use of o-rings, sealing, you need only a few percent elongation for the o-ring.
Technical information you can find here:
https://issuu.com/eriksbv/docs/eriks_o-ring-technical-handbook, see page 16 for mechanical properties.

I use NBR O-rings: 30 mm ID and 2 mm rope, hardness 70 shore, and they work well.

They spec FKM at 100-500% elongation NBR is specced at 100-650%. If we check Eriks for their Viton75 it is 224%, and the RS brand Viton75 is specced to be 175%.

Doing a back of the envelope calculation the microscope should be stretching the bands about 75%, so well within the elongation for Viton75 or indeed the range Eriks put on KFM in general.

Both @WilliamW and I have just done tests on extending the bands until they rupture. We seem to be getting about 175% to 200% (Large uncertainty as I am eyeballing the distance with a steel rule). The ruptured band is now plastically deformed and is about 3% longer than an unstretched one that has been snipped. Elongating to to about 120% before snipping gave 1% deformation and to about 75% gave no measurable deformation.

It is good to know that manufacturers do spec elongation. We should probably devise a test to test the bands.

In the elongation there is a noticeable change in the feel of the stretch at about 100% elongation. It suddenly goes very stiff and takes a lot of force to pull out the next 75% to breaking.

It would be really useful to have some tests of the bands Forumites are using. @rabbit @Bogdan @LeaBog @jvm if you have spare bands could you try this for us:
Take a band and two pencils. Unstretched it will be about 4.5 to 5cm distance to the outside of the pencils.

Then pull the pencils apart until harder resistance is felt. It may help to go in and out a bit. For this band it is 9cm to the outside if the pencils when it becomes stiff.

Then pull far enough to break if you like, but I think the stiff point is probably more useful here.

Let us know how you get on, with any details you have if the bands tested.

Thanks for your measurements. Mine only goes to 100% elongation before it becomes too hard to pull, but it takes some back and forth to get there.

It might be a good idea to add pre-stretching to the instructions… I’ll try that the next itme I take apart my stage and I’m pretty positive this will allow me to get my viton rings installed without breaking…

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If the microscope is 75% elongation and the bands happily stretch to 100% then things should be fine. I worry about “pre-stretching” as this may take the band beyond the elastic limit. I think stretching 1 (or a few) from a batch to test quality is a good idea, but I think these would be sacrificial.

Out of curiosity are you blocking the actuator column with the nut tool to keep it in place when you insert the band?

I have applied counterpressure on the gear, and also tried extending the screw so as to minimize the distance the band has to be stretched.

Regarding prestretching – my rings, after a few cycles, are much easier to stretch, and the permanent elongation from my trials is maybe a millimeter… might depend on the exact material…

Interesting. We will need to investigate some o-ring properties then.

Counter pressure on the gear normally works though the hex bolt can try and work it’s way up out of the gear.

I did some test with the O-rings I use.
See attached screenshots.
The excel files I could not include to this reply, otherwise it was probably interesting if you would calculate more options.

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That is a nice test setup. I also picked up a nitrile o-ring from work this afternoon. It is 3×30mm not 2×30mm, but that should only affect the force not the maximum elongation. It broke at nearly 100% elongation.