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How tight should i screw?

Hi,

I would like to know some reference to tighten end caps and penetrators to my ROV, has anyone used a torque wrench or something similar to determine empirically how much should things be tightened on the ROV?

Torque wrench reference

Are they O-ring sealed parts? I think the correct way to tighten O-ring fittings is to tighten it until it’s snug, and then another little bit (1/16 - 1/8 of a turn). I have been criticized a few times in the past since I have a tendency to overtighten O-ring parts and then the O-ring gets extruded or is crushed.

Hi @tguajard,

There are various screw tightness indications in our BlueROV2 assembly guide, although they’re based on the specific parts, or the tools that come with the ROV kit. As @btrue mentioned, the advice there for penetrators is

It’s worth noting that our new WetLink penetrators do actually have tightening-torque specifications, and since the part that mates with the end caps is the same (material + size + o-ring) then that specification (3.5Nm) likely applies for our M10 potted penetrators as well.

I brought this question up internally and was told the end-cap bolt tightness is likely just the relevant one for stainless M3 bolts, but the relevant design engineer will be able to confirm next week. The best resource I could find for that is this tightening torques table. The closest match there was DIN 7984 - Hexagon socket thin head cap screws, which applies to the A4-70 bolts that we use, so that suggests a max tightening torque of 0.6Nm. I’ll try to get back with confirmation next week on whether that’s appropriate.

Note that the screw tightness for the various frame components is determined by things like whether the screws are indenting into the frame (as covered in the guide I linked to), so those will likely have different torque specifications that are more difficult to estimate effectively.

Thanks for the answer, if you could confirm those numbers it will be great.

Regards

I personally wouldn’t worry too much about the torque specs as long as your enclosure(s) pass your vacuum test. In my limited experience, It’s not going to leak if it passes the vacuum test that is roughly comparable to the operational depth.

We had a leak some weeks ago, and we don’t know why as everything seemed tight enough, we think it may have been o-ring damage from too much force. That’s why we want to make some kind of manual with references and values for everything.

I’ve spoken with the relevant engineer and been told that those recommendations (3.5Nm for the penetrators, 0.6Nm for the end-cap bolts) should be fine.

On this front,

It’s a face seal o-ring, so too much compression is impossible
You’d break the screws as the aluminium bottoms out

My understanding is that o-ring over-compression is more of a design consideration than an installation one - even more so in face-seal cases, where the components being connected are meant to be solidly pressed against each other, rather than having a gap.

When installing it’s important to ensure the o-rings and surfaces are clean and correctly lubricated, and in the right position (it shouldn’t pinch on a corner - gradually tightening the bolts in a star pattern can help avoid this). If a leak occurs it’s very likely that it’s from some dust or hair or a small cut or gouge in the o-ring that has compromised the seal, rather than an issue with the amount of compression (which has been designed for).

If it’s any further peace of mind, there’s a blog post here which covers o-ring squeeze ratios, and recommends 25% squeeze for static applications of NBR70 o-ring seals. Our face o-ring is listed as AS568 size 154, which according to here has a cross-sectional diameter (CS) of 2.62mm \pm0.08 mm. In the provided CAD model our 4" Flange has a gland depth of 2.06mm. That gives

\begin{align} \%_{squeeze} &= \frac{CS - d_{gland}}{d_{gland}}\cdot 100\%\\ &= \frac{2.62\pm0.08 - 2.06}{2.06}\cdot 100\%\\ &= (27.2\pm3.9)\% \end{align}

which is very close to the recommended 25%, and also fits well within the ‘rule of thumb’ presented here for 10-40% compression of static seals.