Delta-Stage O-rings size

Hi guys,

The build guide for Delta and Microscope both say the O-rings are ID30mm.
But the Delta stage is 4.5mm higher so shouldn’t it be aprox. ID36mm?

This is a good question. I am looking to build the delta and block stage versions, and I was confused by the o-ring specifications. I had looked in the block stage BOM. The link was broken, and the specification abbreviation wasn’t clear to me.

Not having previously designed with o-rings, I hadn’t initially realized that O-rings are commonly specified by inner diameter (ID) x cross-section (CS), and that outer diameter is implied as ID+2*CS.

OpenFlexure lists o-rings for the various models:

Here is an example of a 30x2mm viton o-ring fully specified via a commercial supplier.

I do wonder whether the delta o-ring size is correctly specified, but I haven’t verified/tested whether the delta axes are actually different lengths.

My experience with natural rubber elastic bands is that they rapidly degrade (particularly in labs), and I completely agree with the openflexure project moving away from them toward other solutions.

O-rings are designed to be installed with a 0-5% stretch, so that they’re compressed into a groove to resist flows of liquid or gas. I’m used to seeing material selected to be unaffected by the fluid they are sealing against, and so that the working temperature and pressure doesn’t cause leakage in the designed lifetime. When they’re being stretched far beyond the nominal design, they exhibit permanent deformation and “compression set”.

Given questions about the longevity of the viton o-rings (from various posts, it seems that they deform after about 1 year). I wonder if viton is the ideal material?

Common o-ring materials include (based on this material guide and this selection guide):

  • Silicone
    5-80 Durometers Shore A
    excellent flexibility retention, low compression set, and fatigue life
  • CR (Neoprene, chloroprene)
    50 to 80 durometers Shore A
    relatively low compression set, good resilience, abrasion, and are flex cracking resistant
  • NBR (Nitrile Butadiene Rubber, Buna-N)
    hardness from 40 to 90 durometers shore A
    low compression set, high tensile strength
    plasticizers can migrate out and cause problems with certain plastics
  • EPDM (Ethylene-Propylene)
    hardness from 40 to 90 durometers shore A
  • Viton (fluorocarbon)
    50-95 Durometers Shore A

Given that silicone is reported to have good fatigue properties, neoprene has low compression set and resilience, and nitrile is very commonly used, I’m wondering if anyone has tested those materials in openflexure designs?

Thanks in advance!

@Hades_Corps the O-Ring size in all of the Openflexure actuators is I believe the same. The lugs that hold the O-Ring are inside the actuator, so other dimensions of the stage are not necessarily an indication of the distance the O-ring needs to span. The current Deltastage version and instructions were recently updated, @samuelmcdermott would be able to confirm.

The instructions for the microscope V6.1.5, as the current version on the main website, are not as thorough on some of these things as the new version in development - currently V7.0.0-Alpha2. The actuators have not changed, so the new instructions for assembling the actuators may help, particularly the cut-away renders.

@few Viton O-rings are certainly not the ideal tensioner, but it fulfils some important needs. For an open project the band needs to be a part that can be defined precisely and is widely available. O-rings fulfil that, and are actually quite frequently used as holding bands, not just as seals. We have found Viton to be better than nitrile for durability. There is indeed a considerable ‘set’ when the bands are stretched, but that does not matter particularly, as long as the bands still provide tension over the elongation required.

There is another thread on O-rings and microscope resting position

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Thanks @WilliamW, I had seen the thread on resting positions, and my thoughts on o-ring material were based on the reported fatigue issue.

Perhaps I’ll try a silicone o-ring. They’re lower durometer (70A vs 80A). Should I opt for a slightly thicker one (2.5mm cross-section, instead of 2 mm CS)? Also, do you know if there is room in the actuator assembly for a slightly larger o-ring?

I’ll likely buy them from McMaster-Carr, as amazon has a limited selection (like this 29 mm ID x 1.5mm CS one).

The actuator column on the delta stage is indeed 5mm taller than the microscope. However, we decided to specify the O-rings the same for both devices for consistency. It probably is true that the delta stage would benefit from a larger o-ring (it’s certainly tighter to insert them), but that is not something I have explored yet.

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Thanks @samuelmcdermott, am I correct in thinking that the extra 5mm size would nominally use a 50mm ID o-ring (4 lengths increased by 5mm) because of the folding pattern? I realize both the 30 mm and 50 mm likely work in practice, with the 30x2 o-rings being under much greater tension…

@few Not as much as that. The circumference increases by the extra diameter times pi, so you only need to go to 36mm ID to get an extra 20mm in initial circumference. In the installed position, but at rest, the bands are considerably stretched, and the new bit of o-ring needs to be 20mm once it is stretched to the same extent. So that means the required initial diameter is smaller than 36mm, with the stretch of ~75% in the microscope it comes to about 33mm.

Ah- of course. Thank you for the correction.

The silicone O-rings are softer, would a slightly thicker (2.5mm CS) one still fit?

I was thinking of testing 35x2.5mm silicone o-rings for the Delta stage and 30x2.5mm silicone o-rings Block stage. I’m guessing that with the ~50% greater cross-sectional area, that would give the 2.5mm o-rings roughly 50% more tension (to make up for the lower hardness). The silicone should be closer to an elastic band, versus the viton/nitrile which is similar to an eraser. Buna-N has the same hardness as silicone… so the results should be similar, though it’s available in many more sizes.

These are the currently available sizes of silicone o-rings at McMaster-Carr (likely the supplier I would use):

I’m digging into this because the applications I have in mind wouldn’t work well if the user was expected to regularly perform stage maintenance.

I think that it is really going to be about testing some different things. The bands need to stretch by 75% to the mid position of the stage, and then not break as the actuator goes up further, and not come slack as the actuator moves down. Using a supplier like McMaster-Carr is important as they do give all of the specifications so that you do know what you have got - although they don’t seem to have a datasheet giving elongation for silicone. Other suppliers give 150% specification 197% typical elongation, which should be enough.

I have not found anything on here abut anyone trying Silicone, but someone may well have.

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@few did you get round to trying the silicone O-rings?

I used wide vegetable elastics for my delta stage since my o-rings will take a while to arrive. They’ll have a short lifetime, but they work and they’re free*.
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